I. I I I I I I I I I I I I I I I I _} THE UNIVERSITY OF MICHIGAN COLLEGE OF LITERATURE, SCIENCE, AND THE ARTS Department of Astronomy Final Report ORBITING CORONAGRAPH Richard G. Teske 0rren C. Mohler 0RA Project 07270 under contract with: NATIONAL AERONAUtiCS AND SPACE AD_SWRATION CO_CT NO. NASr-54(09) WASHINGTON, D.C. administered through: OFFICE OF RESEARCH ADMINISTRATION ANN ARBOR May 1966 i https://ntrs.nasa.gov/search.jsp?R=19660020689 2020-03-16T20:54:25+00:00Z mI. I I I I I I I I l I I I I I I I I TABLE OF CONTENTS LIST OF ILLUSTRATIONS Io OBSERVATIONAL OBJECTIVES OF AN ORBITING CORONAGRAPH A. Corona B. Chromosphere and Limb C. Disk D. Recommendation II. DESIGN OF TELESCOPES TO MEET OBSERVATIONAL OBJECTIVES A. Optical Designs 1. The Schupmann System a. Objective Lens b. Internal Occulting Disk c. Field Lens d. Mangin Mirror e. Relay Lens f. Folding Mirror g. The Telephoto Relay Lenses 2. The Compact System 3. The Large Spectrograph 4. The Small Spectrograph 5. Direct Photography a. The f/42 Focus b. The f/3.6 Focus B. Performance 1. Exposure Times a. Direct Photography of Corona in the Emission Lines b. Photography of Coronal Spectrum c. Direct Photography of Chromosphere in Emission Lines d. Photography of Chromospheric Spectrum 2. Resolution 3. Spectral Range 4. Polarization a. The Schupmann System b. The Compact System 5. Remarks C. Comparison of Telescope Designs 1. Performance iii Page v i i 2 3 3 4 4 4 6 8 8 9 9 i0 i0 12 15 19 2O 2O 20 2O 2O 22 23 2,5 23 23 24 24 26 26 29 29 29 I TABLE OF CONTENTS (Concluded) 2. Structure and Guidance IIio ENGINEERING THE TELESCOPES IV. CONTROL AND GUIDANCE OF TELESCOPES Ao Disturbance Torques Bo Pointing and Guidance V. THE ROLE OF MAN Ao Telescope Maintenance B. Mode of Operation VI° SPECIAL PROBLEMS A. Micrometeoritic Hazard B. Cleanliness of Environment C. External Occultation / VIIo RESUME A. Optical Design Bo Mechanical Design C_ Control and Guidance D. Platform E. Orbit APPENDIX A_ CALCULATION OF EXPOSURE TIMES B° SUBCONTRACTOR'S REPORT (Evaluation by the Bendix Systems Division of the Bendix Corporation) iv Page 29 31 32 32 32 34 34 34 36 36 36 36 48 48 48 48 48 48 49 53 .I I I I I I I I I I I I I I I I I I I • I. I I I I I I l I I I I I I l I I I Table I. II. III. Figure 1. 2. LIST OF ILI/JSTRATIONS Schupmann Optical System Compact Telescope Optical System Exposure Times for Instruments Optical scheme of Schupmann system. Optical scheme of Compact system. 3. Optical scheme of modified Czerny-Turner spectrograph. 4. Optical scheme of modified Bowen spectrograph. 5. Polarization of light after reflection, lO ° angles of incidence: degree of linear polarization and degree of ellipticity. 6. Polarization of light after reflection, 45 ° angles of incidence: degree of linear polarization. 7. Polarization of light after reflection, 45 ° angles of incidence: degree of ellipticity. 8. Geometrical relations used in calculation of rate of drift of occulter relative to telescope. 9. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration A. lO. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration B. ll. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration C. ]2. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration D. V Page 6 i0 21 5 ll 13 16 25 27 28 39 40 41 42 43 Figure 13. 14. 15. LIST OF ILLUSTRATIONS (Concluded) Rate of drift of occulter relative to telescope in two different orbits, for initial configuration E. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration F. Rate of drift of occulter relative to telescope in two different orbits, for initial configuration G. vi Page 44 45 46 I I I I I I I I I I I I I I I I I I I m t I. i I I I I I I il I I i I I I I i I I I. OBSERVATIONAL OBJECTIVES OF AN ORBITING CORONAGRAPH Observation of the solar corona from a ground station is limited by scat- tered light in the terrestrial atmosphere and in the telescope. While it is possible to observe the inner E corona, it is not possible to observe di- rectly the K corona. Of the two sources of the scattered light, the domin- ant one will be the coronagraph objective lens, given a suitable mountain ob- serving site and an unusually good observing day. Thus, total elimination of atmospheric scattering of light will not by itself make possible direct ob- servation of the K corona. The advantage to be enjoyed by an orbiting coronagraph lies in the in- creased angular resolution _hich it affords through elimination of the prob- lem of "astronomical seeing," and in the increased opportunity for observa- tion which is permitted by its independence of weather. The name coronagraph is applied to an instrument which is capable of ob- serving the solar chromosphere and such structures as spicules, prominences, surges, flares at the limb, etc., in addition to the lower E corona. One of the present objectives of solar astronomy is to examine these features with a high angular resolution, and as free from scattered light as possible, in an attempt to define their physical structure with greater precision than hereto- fore. Studies which might be carried out with a coronagraph operating at vis- ual wavelengths are the following. A. Corona (i) (ii) (iii) Study of the chromospheric/coronal interface at high resolution. Rarely have eclipse or coronagraphic observations provided de- tails of structures at this level of the solar atmosphere with high resolution. Some work (e.g., the 1930 eclipse plates by Marriott) suggests strongly that such observations be carried out. Desiderata are spectra and direct photographs. Refinement of model of lower corona. Observational desiderata are coronal spectra at all position angles around limb and at available distances from limb. Refinement of model of coronal condensation. Observational desiderata are coronal spectra. 1 (iv) Determine physical nature and extent of coronal changes which accompany changes in solar activity and which accompany active events such as flares and eruptive prominences. Observational desiderta are coronal spectra and direct photographs of corona in emission lines. (v) Test theories of polarization of coronal emission. Observa- tional desiderta are polarization measures in emission lines. Observation of the corona in the ultraviolet emissions of highly-ionized atoms, using grazing incidence techniques, affords theunique advantage of viewing the corona against the sun's disk. There is no doubt important information to be gained from such observations. However, there is in the visual portion of the coronal spectrum sufficient physical information to permit further strides in the study of coronal structure, given observations of sufficient quality and continuity. Further it will be difficult to achieve, with grazing incidence observations, the angular resolution which is possible with a well-designed telescope operating at visual wavelengths. B. Chromosphere and Limb (i) (ii) (iii) (iv) (v) Obtain data on dimensions and life-times of spicules, prominence threads, flare filaments, Ellerman bombs, etc., with high angu- lar resolution. Desiderata are direct monocrhromatic photographs, obtained at intervals suitable for the studies being pursued. Refine models of structures such as spicules, prominences. De- siderata are stigmatic spectroscopic observations with high an- gular resolution and high dispersion. Observe magnetic fields in structures at solar limb. Spectro- scopic observations of high dispersion required. Study polarization of emission at solar limb. Polarization measures in emission lines are desired. Observe active events at the solar limb and the interrelations of chromospheric and coronal phenomena associated with these events. Desiderata are time-series of monochromatic direct photographs ar rapid cadence. 2 I .I l l I I l l l I l I I I l l I l l •I. I I I I I I I i I I I I I I I I I C. Disk (i) Contrast of granulation at the limb. (ii) Sunspot structure at the limb. (iii) Distribution of specific intensity in the continuum across the limb. Achievement of a significant increase in angular resolution demands a large instrument and great expenditure of effort and funds. In our opinion A _r=_ _i_ u_ one uf _ _ _ a ...._"_ .... _ pos- sible in justification of these expenditures. D. Recommendation The proposed instrument should incorporate a spectrograph of speed and dispersion suitable for studying the coronal emission lines. Provision should be made for spectroscopic observation of brighter features above the limb and of the chomosphere itself. The telescope should have a station or stations for direct monochromatic photography of the corona in its emission lines and for direct monochromatic photography of the chromospheric spicules, of promi- nences, etc. At these stations the filters should be interchangeable to per- mit observations in different wavelengths and to permit polarizing optics to be employed. The instrumental contribution to polarization must be minimal if polarization studies are to be carried out successfully. In its application to some physical studies of the corona, the tele- scope's spatial resolution is not a critical design factor. However, a reso- lution of at least a few tenths arc-seconds is highly desirable for study of limb phenomena, the chromosphere, the interface between chromosphere and corona and coronal structures which are associated with spicules, prominences, etc. Every effort should be made to obtain diffraction-limited resolution at all foci of the instrument. If the telescope can be designed for observation of the solar disk, the scope of its programs will be vastly enchanced. If external occultation can be provided, a great extension of its contribution to study of the corona will result. 3 III. DESIGN OF TELESCOPES TO MEET OBSERVATIONAL OBJECTIVES A. Optical Designs Two preliminary optical designs have been examined. These were sketched out by us at The University of Michigan and submitted to the Bendix Systems Division of the Bendix Corporation for verification. This subcontractor's re- port of his optical evaluation forms Appendix B of this report. A detailed and complete ray-tracing analysis has not been undertaken. Both designs incorporate an f/18, 10-inch diameter objective lens, a large spectrograph of the Czerny-Turner type, a small, fast spectrograph con- taining a Schmidt camera, and two stations for direct photographymone at an f/42 focus, one at an F/3.6 focus. Each of these componentsmthe two spectro- graphs and two direct-photography stations--is desired to satisfy the observa- tional needs discussed in Section I. Their physical arrangement, suggested here, is dictated by our beliefs that the cameras should be located close to- gether to simplify access for changing of film reels_ and that to locate them near the center-of-mass will have some advantages in the dynamics of the tele- scope. The design which folds the light path at nearly normal incidence incor- porates a Mangin corrector_ and the subcontractor refers to it variously as the "Unfolded System" or the "Straight-Through System." In this part of the report we will call it the Schupmann System. The design which folds the beam with 90 ° reflections is referred to by the subcontractor as the "Folded System," and by us in this part of the report as the Compact System. i. THE SCHUPMANN SYSTEM A schematic diagram of the optical system is shown in Figure i, and a listing of the optical elements and data pertaining to them forms Table I. 4 I I I I t I I I i I I I I I I I I I -0 a) 0 r-t u 0 TABLE I SCHUPMANN OPTICAL SYSTEM Element f, Diam, Separation, cm Cm cm Objective lens 457 25.4 Occulting disk - 4.4 430-460 Field lens 172 9.0 33-3 Stop - 13.0 274 i Mangin mirror 140 14.0 492 Relay lens 136 ii.0 102 Folding flat _ 9.0 87 Telephoto 1-218 7.0 L + 18.7 7.0 54 Folding flat 7 f/42 focusj - 294 a. Objective Lens Since scattered light and the multiplicity of extra, spurious solar im- ages due to internal reflections must be stringently minimized, a single- element lens is superior to any other ty_e. We have assumed here that the lens is BSC-2 glass. Newkirk and Bohlin± have investigated the scattering and fluorescing properties of various materials for coronagraph lenses; BSC-2 ap- pears to be of useful quality in these respects• TRANSMISSION PROPERTIES OF BSC-22 (i0 mm thickness) _.(_) 2,960 cut- off 3,600 76.0 3,800 95.0 4,000 99.5 10,000 94.5 20,000 85.0 iNewkirk, G., Jr., and David Bohlin, Applied 0ptics_ 2, 131, 1963. 2Smithsonian Physical Tables; Ninth Ed., 1951, p. 512. 6 I I I I I I I I I I I I I I I I I I I I. I i I I I I I I I I I I I I I I I Preliminary calculations suggest a lens with the following geometry: rI = r2 = qD = 258.4 cm -2782 cm +0.83 for minimim coma in the yellow. 3 Spherical aberration of a spherically-surfaced lens results in a focal dif- ference of 3.6 mmbetweenmarginal and paraxial rays. The amount of aspheric figuring needed to remove this abberation amounts to a few wavelengths de- parture from the spherical figure. For an object _ degrees from the optical axis, the longitudinal displacement of the focal point from the paraxial focal point is given in the following table: Af (astigmatism) 2 ° arc 0.6 cm 10' arc 0.1 mm 5' arc 0.05 mm It is thus desirable to maintain the place on the sky which is being observed as close to the optical axis as possible. As will again be mentioned, the optimum field of view of the Schupmann system is also very small (~5' arc), and therefore an on-axis system is mandatory. Hence the coronagraph's inter- nal occulting disk must be movable and servo-controlled to follow the sun's disk. G t F 10-INCH DIAMRNER BSC-2 LENS, fD = 15 FEET Theoretical Radius of k, Angular n' f, Solar Re solution, cm Image, arc-sec am 5ooo o.3o - - - 3500 - 1.54o: 436 2.03 43o7 - i.327o8 448.5 2.09 4861 - 1.5_6_ 432.3 2.lO 5ooo 0.50 - - - 5890 - 1.517oo 457.2 2.13 6[363 O. 66 1.31642 437.8 2.13 80oo 0.79 - - IOO0O - 1.31o: 462 2.15 12000 1.19 3The optimum shape factor is a matter for more careful consideration. We here choose the value in the yellow for illustrative purposes. 7 The chromatic aberration is large_ requiring a considerable longitudinal motion of the internal occulting disk. The Mangin correcting mirror will how- ever remove almost all the longitudinal and lateral chromatic aberrations. Internal reflections in the objective lens result in formation of a real solar image, diameter 6 mm_ at a distance of 64 cm behind the objective. b. Internal Occulting Disk The internal oceulter must be freely movable laterally to permit bringing solar features under observation to the optical axis of the telescope. Design and selection of a mechanism for carrying the internal occulter is a critical factor in the telescope design. In Appendix B the subcontractor discusses his conception of a possible design for a servo-controlled device. In this design the disk automatically seeks to position itself laterally and is driven by the observer-astronaut to the correct longitudinal position cor- responding to the wavelength region of interest. An override_ or partial override_ of the servo-system will be necessary. The occulting disk_ somewhat larger than the solar image_ will obscure regions very close to the limb. To observe these_ it must be decentered from the solar disk a bit. If the telescope is to be used to observe the photosphere_ the internal occulter must be removed from the light path. The occulter is to carry or consist of a mirror to direct unused photo- spheric radiation out of the telescope. c. Field Lens The field lens must exactly image the first nodal point of the objective onto the vertex of the Mangin mirror surface. 4 This lens must be achromatic, since in principle it is the only source of chromatic aberration in the image formed by the Mangin. Co Mangin Mirror Employment of a Mangin mirror in a coronagraph is dicussed by Rush and Schnable_5 as well as by Baker. 4 Its primary attraction as a principle in eoronagraph design is that it permits practically perfect achromatization of the optical system. Requirements in its employment are: (i) use at equal 4Baker, J. G._ Astronomical Journal, 5_29,74, 1954. 5Rush, J. H. and G. K. Schnable, Applied Optics, _, 1347, 1964. 8 I I I I I I I I I I I I I I I I I I I i • I. I I I I I I I I I I I I I I I I I object and image conjugates to eliminate coma; (2) use as close to the optical axis as possible to avoid large astigmatism; (3) an aspheric figure; and (4) restriction of the working field of view to minimize field curvature and the off-axis astigmatism of (2). It must be admitted that while we are interested in using the advantages of this system, we do not consider ourselves experts in the design principles of a Schupmann system. A more careful assessment of its optical intricacies is needed before a final design can be attained. Rush and Schnabie 5 indicate that their coronagraph attains diffraction-limited resolution. The Mangin is to have an unaluminzed central spot which will transmit ..... _rm_ _ _ _e_c _ in thelight ±±um the spurious solar i_= - __ r _ .... _ ^ _^_. objective lens. e. Relay Lens The subcontractor 3 following Bush and Schnable, 5 suggests that the relay lens be figured aspheric to avoid the necessity for aspherizing the Mangin. This field lens must also be achromatic. Its function in the design is to provide a final image of such size as to match the resolution of the telescope objective to the resolution of photo- graphic emulsions, for which we have assumed a 20 -30_ resolution element. f. Folding Mirror Particular engineering attention should probably be paid to the mounting of the beam-folding flat. It is to be rotatable so as to direct light to three stations: (i) the Czerny-Turner spectrograph; (2) the fast Schmidt- type spectrograph; and (3) the f/42 direct-photography position. In its position at the sunward end of the telescope, its mounting may be particularly vulnerable to the effects of solar heating, either by direct conduction or by the secondary effects of radiation by the telescope's pro- tective cover. The mounting must permit the accurate positioning of the mir- ror in any of the three directions. This accuracy is specified by the follow- ing considerations: By requiring that the accuracy of "aiming" at the f/42 direct-photography station be _ 20 arc-seconds on the sky, or _ 900 microns on the focal plane at a distance of 294 cm from the folding flat, the angular accuracy of positioning of the flat is + 900 microns 294 cm x 10 -4 cm/micron _ _ 3 x 10 -4 or _ _ i arc-minute. 9 Similarly, to align the telescope axis with the optical axis of the large spectrograph, permitting a _ 5_ deviation in illumination of the collimater requires an angular accuracy of positioning of the flat of about + 0.05 x 8.5 lO. 4 -- ~ + 7.1 x or ~ + 2 arc-minutes. 596 cm -- During an exposure, the orientation of the flat must be maintained to within a value given by half the resolution of the telescope 0.15 arc-second x 45 microns/arc-second x 1 294 cm x 10 -4 cm/micron = ! 2.3 x l0 "6 or ! 1/2 arc-second g. The Telephoto Relay Lenses An inverted telephoto system is employed to provide an f/3.6 focus to feed the fast spectrograph of the focus for direct coronal photography. The negative element of this pair of lenses collimates the light beam, and within the collimated portion of the beam may be inserted broad-band or narrow-band filters. 2. THE COMPACT SYSTEM A schematic diagram of the optical system is shown in Figu&re 2, and a listing of the optical elements and data pertaining to them forms Table II. Less effort has been expended on analysis of this system than on the Schup- mann system. TABLE II COMPACT TELESCOPE OPTICAL SYST_ Element f, Diam, Separation 3 cm cm cm Objective lens 457 25.4 430-460 Occulting disk - 4.4 Field lens 54 9._ 33-3 45 ° flat - 14.0 _ 61 stop - 3.3J _ SecondobJective 122_145 4.0 54.o 12o Telephoto L + 11.2 4.0 ....................................................... Second obJectiveq 127 f/42_oous J lO I I I I I I I I I I I I I I I I I I I I I I I I I I I I I !- ;_I _ _ _ - I I I "Y'.. I I_ I i t c_ I .m 0 i I I I l I I I I I I I I iI CO 4-_ U r_ q-, 0 m ,-I c) ..r-i 0 G) -,-.i &. (i) Objective lens. This element is conceived as identical to the objective in the Schul_ann system. (il) (iii) (iv) Internal occulating disk. Conceived as identical to that in Schupmann system. Field lens and 4_ ° prism. The field lens must be overcorrected chromatically to provide an achromatic image. It is mounted in- tegrally with a 4_ ° prism to minimize light losses, the prism de- viating the light path through 90 ° towards a mirror which provides a second 90 ° deviation. 4_ ° flat mirror. The prism and this mirror fold the light path through 180 ° . (v) Second objective lens. An achromat, this lens has an opaque spot on its forward vertex to intercept light from the small solar image which is produced by internal reflections in the main objective lens. Since it was found difficult to illuminate both foci and both spectro- graphs with a singly-folded light path, we have suggested that these foci be placed at different azimuths around the telescope optical axis. To feed light to each, the field lens-prism, the 45 ° flat and the second objective lens may be structurally integrated into a housing which rotates about the telescope's optical axfsmthat is, about the axis through the center of the field lens. Calculations show that angular orientation of the optical paths at the three azimuths neededmust be within 1 arc-minute of true. _vlJ ph ....... Thi pai - _ -_$^_ ÷_Tne teie o_o relay ±_a_ s r uf o_o _o _ focal ratio of the light beam to the focal ratio (f/3.6) of the collimator of the fast spectrograph. ....±_j also _r_^ _^v_ the f/3.6 beam for direct coronal photography. It should be noted that in Appendix B, containing a draw- ing of this system, the telephoto combination is incorrectly located. The data in Table II of this report are preferred. 3- THE LARGE SPECTROGRAPH A schematic drawing is shown as Figure 3. Dimensional data are given there. Coma is minimized by the over-and-under Czerny-Turner arrangement sug- gested here_ a system which is achromatic through its use of all-reflecting optics. Spherical aberration and astigmatism are present, however, the latter 12 I I I I I I I I I I I I I I I I I I I I- _ ! I I L _ I I I I I I i I I o m, N I::::: :=3 ;,.,) u'_ c_ eu E_ q::r ;]-3 E..._ E E E (.) ¢.) ¢._ bO 0 rj CO I N q-I 0 rll r-I 0 4_ 0 ,g, being improved by keeping the collimator and camera mirrors as close together as possible. Aberrations of this system have not been fully explored by us, but we feel it is a spectrograph worth careful consideration. For example, the spherical aberration of the camera mirror: if a photo- graphic plate is placed at the axial focus, 280 cm from the mirror, marginal rays parallel to the optic axis (5.25 cm from the axis) are focussed hi (5"25)2 am = 0.123 mm 8f = 8x280 h3 in front of the plate. These rays intersect the photographic plate _f2 = 2.3 microns from the optical axisj well within the resolving power of the emul- sion. Assuming the grating has 600 grooves per millimeter, the following table describes the dispersive characteristics of the spectrograph. CZERNY-TURNER SPECTROGRAPH Milliangstroms per k Order Dispersion, Photographic A/mm Re solution Element 2 2.9 87 mA 4,000 3 1.8 54 4 1.3 39 2 2.7 81 6,000 3 1.6 48 4 i.o ]50 i 5.7 171 8, ooo 2 2.5 75 3 1.3 39 i 5.5 165i0,000 2 2.1 63 The half-widths of chromospheric lines are thus resolvable in the second and higher grating orders, and of photospheric lines in the third and higher grating orders. While a Doppler displacement at 6000 _ which corresponds to 14 I I I I I I I I I I I I I I I I I I I I. I I I I I I I I I I I I I I I I I 0.i km/sec is only 2 mA (roughly 2 microns or 1/15 resolution element), Doppler shifts of this magnitude should be observable, since the whole line shifts. Auxiliary devices for use with the spectrograph will be essential. (i) A means of guiding the spectrograph slit upon the solar re- gion of interest by means of a TVmonitor is suggested by the subcontractor. The TV monitor, which provides the astro- naut-observer with an image of the reflecting slit jaws, should be illuminated through a narrow-band filter set upon H_, or perhaps Ca II K. (ii) Filters should be provided at the spectrograph entrance slit to eliminate overlapping grating orders. An indexable wheel containing several filters might be employed for this pur- pose. The wheel may be placed behind the spectrograph en- trance slit. At the spectrum focal plane the length of spectrum observed is limited by dispersion, by vignetting and by film format. The usable spectrum height is about 1.3 cm, corresponding to the presumed diffraction-limited field of view of 5 arc-minutes. The length of unvignetted spectrum depends upon the diameter of the camera mirror. A 10.5 cm mirror subtends 0.0389 radian at the grating, so that the entire length of the spectrum, including the vi- gnetted portion, is about i00 _ in the IV order at k6000. A collimator di- ameter of 8.5 cm and camera mirror diameter of 10.5 cm permits an unvi- gnetted spectrum length of 2 cm = 20 _ in the IV order at _6000. The height of the spectrum is approximately 1.3 cm, its (vignetted) maximum length about i0 cm. Hence it may be possible to photograph the spec- trum on 16 mm film which is transported parallel to the dispersion or on i00 mmfilmwhich is transported perpendicular to the dispersion. This spectrograph is intended to be used for high-dispersion studies of the solar chromosphere and near-limb phenomena. Its _peed will be discussed in Section B.I. 4. THE SMALL SPECTROGRAPH A drawing of the optical system is in Figure 4, which also contains di- mensional data. (In this figure, the radii of curvature of the mirrors are not to scale.) In securing observations of the coronal emission lines close to the limb, and in work where measures of line profiles are not important, a fast spectro- graph is desirable. Such a spectrograph is incorporated in our design. The focal ratio of the collimator, f/3.4, is matched by using an inverted tele- 15 :1.6 I I. I I I I I I I I I I I I I I I I photo system in the telescope beam. The spectrograph camera focal ratio is f/2.1 in the first order. O_rconceptual spectrograph employs principles of design in use else- where.O'V A Schmidt corrector plate is placed nearly in contact with the grating and parallel to its surface. Light is admitted to the collimator mirror through a hole in grating and corrector plate, directed back to the grating and dispersed. The spectrum is formed by the 27-cm diameter camera mirror which is placed on the grating normal. Since the corrector plate is twice traversed by a light ray, its figure must be modified from the conven- tional figure. 7 We have in mind a camera which mounts film to be transported past the focal surface in the same manner as is done for the Baker Super- Schmidt satellite-tracking telescopes. The intention of this design is to provide a very compact instrument which could be fit within the sunward end of the telescope body. There are, however, three severe design difficulties which will require careful atten- tion in detailed studies of the optical system. (i) The telephoto system must be strongly corrected for spherical aberration as well as for chromatic aberration. (ii) A spherical collimator mirror will produce, in the present configuration, large spherical aberration (the lateral ex- tent of the circle of confusion is 150 microns) Hence a nonspherical figure must be computed. These problems may be lessened by increasing the focal ratio of the beam delivered by the inverted telephoto, but which would require a relocation of the spectrograph. Its present physical location has been suggested by our de- sire to locate all cameras close to the center of the telescope boty and, in the case of the Schupmann system_ to minimize the angle of reflection from the beam-folding flat. (iii) The spectrograph camera is to be used in two orientations: at angles of 27 ° and 40 ° with the optical axis (telephoto- collimator axis). (See below.) The correct curvature for a twice-through corrector plate is cos @ times the curva- 1 + cos @ ture for a once-through corrector, 7 where @ is the angle 27 ° or 40 ° . The value of the correction favor is 0.471 (@ = 27 °) and 0.434 (@ = 40°). If the average correction factor is em- ployed, the @ = 3474, and a decrease in definition from the optimum will be experienced at the extreme ends of the spectra 6Bowen, I. S. Ap. J., ll6, l_ 1952. 7Bowen, I. S. Telescopes, eds. Kuiper and Middlehurst (Chicago: University of Chicago Press), 1960, Chapter 4. 17 • ! observed. The circle of confusion at worst will be ~ 45 microns, corresponding to_ ~ 12 arc-seconds on the sky. With the above difficulties in mind 3 we review the characteristics of the conceptual spectrograph (Figure 4). The collimator axis is maintained in coincidence with the telephoto-slit axis. To photograph the entire spectrum from the objective lens cut-off to the near infrared (2900 _ - 12500 _) the camera must be moved to two posi- tions. The camera axis is maintained parallel to the grating normal_ and the assembly (camera plus grating plus corrector plate)_ rotated about an axis in the face of the grating which is parallel to the grating rulings. The two re- quired positions are G = 27°_ _ = 40 °, where _ = angle between camera axis and collimator axis. Thus _ = i angle of incidence of collimated beam on grating. We have adopted a camera field of view of 12°_ a camera focal length of 30 cm and have assumed a 600 groove/mm grating. Hence the angle of disper- sion of included radiation is -6 ° < @ < +6 ° . DISPERSIVE CHARACTERISTICS OF FAST SPECTROGRAPH Angstroms per Grating k, Dispersion_ Photographic @ Order _ A/mm Resolution Element, = 27 ° -6° 1 585o 55 1.6 -6 2 2925 27.5 o.8 0 i 7590 56 1.6 o 2 3795 28 0.8 +6 i 9320 55 1.6 +6 2 4660 27°5 0.8 = 40° -6 i 9000 55 1.6 -6 2 4500 27.5 0,8 0 i 10720 56 1.6 0 2 5360 28 0.8 +6 1 12500 55 1.6 +6 2 6250 27.5 0.8 18 I I I I I I I I I I I I I I I I I I m • I I I I I I I I I il I I I I I I i In Appendix B, engineering drawings show the film camera for this spectro- graph to be located 21 ° off the mirror axis. Objectionably large aberrations will be encountered in such an arrangement. It is desirable to place the film at an on-axis focal surface, as in Figure 4. Focal ratios are f/2.1 for _ = 27 °, and f/l.8 for _ = 40 ° . Bowen 8 provides data and equations which indicate that to cover the entire range 2900 _ - 12500 _ a fused quartz correcting plate is perhaps required, and that the camera focal ratio for our application should not be less than f/l.4. This limit is set by consideration of the chromatic aberration introduced by the correcting plate, and is met by our camera. An f/2 Schmidt camera with a 12 U field of view produces images for which the maximum radius of the circle of confusion is 8.5 microns, below the as- sumed resolution of photographic emulsions. The system discussed here is geometrically practical. The maximum length of spectrum to be obtained is 6.5 cm. Thus a 70-mm film may be employed, the film being transported perpendicularly to the direction of dispersion (the height of the spectrum is about 0.65 mm) between exposures. At the focal plane of the camera there is about 7.3 cm clearance between the camera optical axis and the edge of the collimator beam. Since 1/2(7.0 cm) = 3.5 cm is needed on each side of the camera axis for the film, there is adequate room for rilm tracks to be mounted as well, without interfering with the collimator beam. Separation of orders in the sepctrograph may be achieved by using glass filters as, for example, a Schott 0G-I to isolate the first order and a Schott BG-18 to isolate the second order. Selectable insertion of these into the beam will be discussed below. 5. DIRECT PHOTOGRAPHY Two cameras for direct photography are included in the design. They are located at: (i) an f/42 focus for photography of the chromospheric structure and of prominences and other supra-limb features of sufficient surface bright- ness, and (ii) an f/3.6 focus for photography of the corona. Generally, di- rect photography is to be accomplished through narrow-band filters centered on the important emission lines such as H_, H_, Ca II K for the chromosphere and at k 5303, k 5694 for the corona, to provide phenomenological data and data on spatial structures of features. White-light or wide-band photography of the disk and of the K corona will be possible at these foci if the internal ther- mal characteristics of the telescope warrant and/or if external occulation can be accomplished. 81bid. 19 a. The f/42 Focus Here the image scale is i arc-second = 45 microns, permitting photographic resolution of features down to 0.5 arc-second. The diffraction-limited field of view can be about 5 arc-minutes diameter. This field is included on an area 13.5 mm in diameter, and calls for application of a 16-mm film camera. A filter wheel_ which will mount those narrow-band filters desired plus polar- izing filters_ should be placed in front of the cmaera_ the wheel to be con- trolled by the astronaut-observer. The solar beam is directed to the filter wheel and camera by positioning the beam-folding flat. b. The f/3.6 Focus At this focus the image scale is i arc-second = 4 microns, permitting photographic resolution of features down to 6 arc-seconds. Such resolution is not necessarily unsatisfactory for those coronal features which have a smoother spatial structure than chromospheric features. The 5 arc-minute image spans a diameter here of 1.2 millimeters, suggesting application of an 8-mm film camera. Again a filter wheel mounting filters and polarizing op- tics should be in front of or part of the camera. The subcontractor has supported an arrangement in which both this camera and the fast spectrograph grating are mounted on an arm which swings one or the other into the light path from the inverted telephoto system, while the collimator and camera mirrors of the spectrograph are separately mounted. If this suggestion proves practical, then filters for direct coronal photography and for isolating orders in the spectrograph should be mounted separately from the camera or spectrograph. A useful place for a filter wheel would be between the elements of the inverted telephoto system, in the col!_mated beam from the negative lens. B. Performance i. EXPOSURE T_S Results are collected as Table III. Appendix A. Calculations are reproduced as ao Direct Photography of Corona in the Emission Lines Data given by Wlerick et al. 9 yield an intensity of the coronal green line 9Wlerick, G., J.-P. Dumont, and J.-C. Perche, Th___eSol____arCorona, ed. J. Evans_ Academic Press, 1963, p. 177. 2O I I I I I I I I I I I I I I I I I I I I o I. I I I I I I I I I I l l I l I I-I I-I IM cr_ r._ I--i 0 M CO o d o _M z_ F_ (D o o ! -,-I A .,-I E_ _d o o 0 -r-I 0 C_ 0 0 0 Od 0 J 0 _o kO OJ _H ° _h zi 0 L_h v I {D @ O b0H 4-_ -r-t -. r--I -r_ ._ O v q-t 0 c;J ,-t 0 t_ ,--I t'- 0 o _ oJ • --. = ed O_ Ctl ["-- OJ J_ o II -M 4_ .M cG _ 4-_ © o o O _ q_ o _D • OJ O bD O 21 co oJ C_ 8 O t-- .-° O oJ O c_ -,0J o o o (1 (D r-_ OJ O -M O O II 4_ L_k OJ O Oh .-o.. _--O o oJ _J cO OJ c$ c_ O O o oo,_ o o 0',.o q-_ i_ • oJ o _'_ --_- II _ 0 _-_ _+_ o ._ ( 5305) of 62 ergs cm -2 sec -I steradian -I, measured at the earth, at p = R/R@ = 1.2. Assuming telescope losses of 50_, filter transmission of 30_ and a photographic exposure of 0.i meter-candle second (fine-grain_ high- contrast emulsion) the exposure time for a direct photograph of the corona in the green line and at p = 1.2 is 0.7 second at the f/3.6 focus and 8.2 seconds at the f/42 focus. Van de Hulst I0 gives for the average ratio of intensities (yellow line)/(green line) = 0.3/20, so that a direct photograph of the corona at p = 1.2 in the yellow line will in principle require an ex- posure time of about 50 seconds at the f/316 focus. Whether this can actually be accomplished will depend upon the filter employed in isolating the line, and especially upon the scattered light in the telescope. It is highly prob- able that the corona will be unobservable, to this instrument, in such weak lines. b. Photography of Coronal Spectrum Assuming a spectrograph slit width of 50 microns, which projects to a width of 28 microns at the spectrum focal surface_ and referring to the table of dispersions for this camera, we see that the spectrum lines will be essen- tially images of the spectrograph slit. Therefore spectrum exposure times for optimum density will be similar to exposure times for direct photography, de- creased for absence of a dense filter (we adopt a glass filter transmission of i00_), and decreased again for focal ratio. Assuming telescope losses of 50_, and a grating efficiency of 30_, the f/2 focal ratio will permit an ex- posure time for the green line of 0.2 second. To reach the faintest available lines will in principle require exposures of only a few seconds. It is of in- terest to determine the faintness of lines which can actually be observed by this instrument against the background of scattered light present in the coronagraph. Assume for purposes of this discussion that optical perfection and relative freedom from UV fluorescence of the objective keep scattered light in the instrument to 5 x 10 -6 the average disk brightness. Then the green line is observable to 4 arc-minutes from the limb and the red line to 2 arc-minutes from the limb. Other coronal lines are generally much fainter han these. The table below presents an estimate o_ lines available to the pectrograph_ assuming a scattered light of 5 x i0-- B®. Therefore the cor- onal spectrograph, which is desirable for the information it can provide re- garding relative intensities of some spectrum lines, will attain a wider use- fulness if external occultation is available to decrease the instrumental scattered light. I I I I I I I I I I I I I I 10Van de Hulst_ H. C., The Sun, ed. G. P. Kuiper_ University of Chicago Press_ 1953, p. 268. 22 I I I I I II. I I I ! ! I ! ! I I l ! I I l ! Can Be Observed To Lin____e (arc-minutes from limb) 10747 Fe XIII 4:5 10798 Fe XIII 4:5 5303 Fe XIV 4' 3388 Fe XIII 3'.5 7892 Fe XI 2' 6374 Fe X 2' 5694 Ca XV ~ i' (active regions) Others < l' c. Direct Photography of Chromosphere in Emission Lines Data given by Dunn (see Appendix B) I permit calculation of exposure times needed to register spicules in H_. At the f/42 focal plane, the spicule H_ radiation over a band-pass of 3.4 _ produces an illuminance of 0.045 meter- candle cm -2, if we take into account 5_ telescope losses and a filter trans- mission of 30_. Assuming an exposure of O.1 meter-candle-second, the required exposure time is 0.2 second. d. Photography of Chromospheric Spectrum The high dispersion at the spectrum focal plane reduces the illuminance to very low levels. According to the calculations of Appendix A, the ex- posure time to obtain a good image of H_ in a spicule is about 71 seconds, for a slit width of 25 microns. For optimum performance of the spectrograph in chromospheric studies, the slit may be widened to 50 microns or more, thus reducing exposure times to below 30 seconds. 2. RESOLUTION Optical design of all components except the small, fast spectrograph should be directed towards diffraction-limited resolution. The theoretical angular resolution of our designs is about 0.5 arc-second at k 5000. 3. SPECTRAL RANGE The accessible spectral region is kk 3000 - > 12,000 _. Capability to utilize the range kk 3000 - 12,500 _ has been designed into all components discussed here. A requirement for the infrared is for photographic emulsions which are sensitive there. 23 4. POLARIZATION Of especial astronomical interest is the polarization of light from the solar limb, chromosphere, and corona. The only way to measure polarization with high precision is to eliminate all optical surfaces. Since this is im- practical, we are interested in the amount of spurious polarization intro- duced by our instruments. A full assessment of telescopic polarization at this stage is neither practical nor necessary, and so we have confined our attention to nonnormal reflections in the instruments, since these will be the major source of spurious polarization. a. The Schupmann System To minimize polarization introduced by nonnormal reflections_ the sys- tem was designed for small angles of incidence. The maximum angle of inci- dence is about 8 ° arc, occurring at the beam-folding flat. We have adopted, for purposes of exposition, an incidence angle of i0 ° arc and have calculated the instrumental polarization for one reflection and for two reflections at this angle. We assumed that reflections occurred at aluminum metal. It was assumed that linearly-polarized light is incident upon a mirror at i0 ° arc. The Stokes paramrters I, Q, U, and V for the reflected light were calculated assuming that the plane of the electric vibration makes an angle _ with the plane of incidence (90 ° < _ < 0°). This was done for three wavelengths k 4000, k 7000, k 9500. The degree of linear polarization (Q2+ u2)l/2 and the degree of ellipticity V Pv - I were computed for the reflected radiation. Figure 5 presents the results for a single reflection. (For perfectly linear polarization, p = i, Pv = 0.) The maximum degree of ellipticity is about 1.5_ in the blue for _ = 45 °. We then computed the resulting polarization for two reflections at an- gles of incidence of i0 °. The results, also given in Figure 5, are in terms 24 I I I I I I I I I I I I I I I I I I I I 0.9998 - _ 0.02 I °*_'I -I°'°* I 0.-_-- o....,,_,_oo .oo,°o,,o.,,oo_;_---o I I I i Two reflections, lO°angle of incidence I _.oooo !o.o, ' 0.9998 X4000 0.02 I 0.9997 0.01 , 0.9996 OO090 ° 60° SO°! o I Figure _. Polarization of light after reflection, 10 ° angles of in-cidence: degree of linear polarization and degree of ellipticity. I 25 of P and pv defined above. In this case_ the maximum degree of ellipticity is less than 3_ (2.73_ at _4000 for _ = 45°), and the degree of linear polariza- tion remains above 99.95_. In the Schupmann telescope design presented here_ the spurious instrumen- tal polarization will be less than that computed_ and we tentatively conclude that the degree of ellipticity introduced by the reflections at the Mangin mirror and at the beam-folding flat will be of small importance. b. The Compact System Two reflections at angles of incidence of 45 ° arc are required in this telescope. We assumed that both reflections occurred at a vacuum-to-aluminum metal surface. We have not checked to see whether this assumption will ade- quately approximate the actual polarization introduced by the prismatic inter- nal reflection plus mirror. However the results for even one 90 ° reflection are enough inferior to the results for the Schupmann telescope to suggest that the Compact system will introduce a serious amount of polarization. As before_ the Stokes parameters for reflected light were computed_ assum- ing linearly-polarized light to be incident on the first mirror. The plane of the electric vibration makes an angle _ with the plane of incidence. Figure 6 presents values of p and Figure 7 presents Pw as defined above_ for one reflection. The degree of ellipticity is about 30_, and the degree of linear polarization about 95_, for blue light and for _ = 45 ° . Results for two reflections are also shown in Figures 6 and 7- In blue light_ for _ = 45 °, the maximum degree of ellipticity approaches 60_ and the degree of linear polarization approaches 80_. From these theoretical data_ we conclude that significant polarization observations could probably not be made with our Compact telescope. 5. REMARKS For a telescope orbit plane arbitrarily inclined to the earth-sun direc- tion_ there will generally be a (changing) component of telescope velocity towards or away from the sun. It may be necessary to take this into account in using narrow-band filters for direct chromospheric photography_ depending upon the narrowness of the band-pass and the desired criticality of wave- length tuning. In general_ band-passes of the order of a tenth to a few tenths of an angstrom are desirable. In operating the large spectrograph_ the observer-astronaut will select the wavelength regions of interest by rotating the grating to the necessary 26 I I I i I I I I I I I I I I I I I I I One Reflection I ReflectionsWO I 1.00 . \ i _ / I \\, _ I/ I 0.95 \ \\ ____1 II, . I I /I I X9500 \ \ xs_ \ / / / PI I I I I I I I i I l \ \ \ \ \ \ \ \ \ /\ \ / \ / \ / / / / / / / / / / / / / I I 90° 60° 30 ° 0° C! Figure 6. Polarization of light after reflection, 4_ ° angles of incidence: degree of linear polarization. 27 0.60 O.PO 0,40 0.30 0.20 0.i0 One Reflection Two Reflections 90° 60° 30 ° 0 Ot Figure 7. Polarization of light after reflection, 4_ ° angles of incidence: degree of ellipticity. 28 I I I I II • I I I I I I I I I I I I I I I I angular orientation. To do this he will need either (I) a command system into which the selected angular setting is fed, or (2) a monitor read-out which in- forms him of grating angular setting, or both. Similarly, selection of the different filters for direct photography and for separating orders in the spectrographs will require either a command sys- tem or monitor read-out, or both. It may be desirable to incorporate into the cameras a meter which pro- vides the observer-astronaut with information about the amount of unexposed film remaining in them. Shutters on the c_neras should have va_yi_ and selectable speeds w_ch can be chosen by the observer. A continuing series of exposures, as well as single and intermittent exposures, will be desired for some observations. Thus the cameras should be "programmable." C. Comparison of Telescope Designs 1. PERFORMANCE The Schupmann system is superior to the Compact system in terms of its relative freedom from spurious instrumental polarization. The relative merits of other optical aspects should be the subject of detailed optical studies. 2. STRUC_JRE AND GUIDANCE A comparative study was not undertaken in detail. However, we can guess that: (i) The lighter weight (see Section III) of the Compact system is an advantage during launch. (ii) The shorter Compact system may offer some advantage in the launch configuration. However, the Schupmann does fit in the LEM adapter. (iii) The principal moment of inertia of the Compact system is less than for the Schupmann telescope (see Appendix B), which may have some significance in expenditure of fuel for guidance. _9 (iv) The Compact system's rotatable turret, as conceived here_ may be difficult to align and to keep in adjustment. Fur- ther, rotation of the turret is a great source of torque and the resulting angular momentum must be somehow removed from the telescope. The Schupmann telescope's rotatable mirror_ while also a source of torque, will produce much less angular momentum. Of the two instruments, the more valuable from the standpoint of the astronomical experiments to be performed is the Schupmann telescope. 3O I I I I I I I I I I I I I I I I I I I m t I I I I I I I I I I I I I I I | III. ENGINEERING THE TELESCOPES Appendix B of this report is the engineering report by the subcontractor. He has devised structures for both telescopes. These are truss-column, and are to be covered with a two-layer heat shield to maintain an equilibrium thermal environment. The telescope is to be open to vacuum. Estimated dimensions and weights are: (i) (ii) Schupmann System: Compact System: 762 cm x 80 cm x 60 cm; 440 lb. 498 cm x 80 cm x 70 cm; 390 lb. Methods of gimballing the telescope are discussed by the subcontractor, assuming it to be attached to the spacecraft body. 31 IV. CONTROLANDGUIDANCEOFTELESCOPES Appendix B contains the subcontractor's report on control and guidance. He has assumedthe telescope to be mountedin a mannedApollo vehicle in earth orbit. This assumption provides a point of departure from which control and guidance methods and requirements maybe developed. TwoApollo mountings are assessed: (a) one in which the telescope is fixed to the vehicle and the entire vehicle is pointed and guided; and (b) one in which the telescope is mounted to the vehicle on gimbals--the vehicle is pointed as coarse control while the telescope is driven against it for fine pointing and tracking. A third aprroach_use of the telescope as a separately-orbiting but tethered vehicle_is also explored by the subcontractor. A. Distrubance Torques An analysis of the sources of disturbance torques leads to the usual con- clusion that manmotion is of several orders of magnitude more importance than other sources. Table 5_ Appendix B, summarizesthe calculations. B. Pointing and Guidance To utilize fully the resolving power of the telescope, it must be guided with an accuracy of better than 0.3 arc-second. The subcontractor's report states that there is no sensor available to supply error signals for guidance of this accuracy and therefore one must be developed. In assessing guidance requirements it was assumedthat the angular drift rate of Apollo vehicle is 3.6°/hr in all three axes. Table III on page 21 of this report lists exposure times for various operational modesof the tele- scopes. In that table is listed the angular spacecraft drift about any axis during an exposure (column 3). If we assumea field of view of 5 arc-minutes, then the periphery of the field of view at a focal surface will be rotated through the linear distance given in column 4. The linear distance at a focus through which the image is translated_ during an exposure_ due to drift in pitch and yaw is listed in column 5. It is clear that no roll guidance is needed, since the displacement of 32 I I I I I I I I I I I I I I I I I I I II . I I I I I I il i I I I I I I I I I I the field periphery is well below the emulsion resolution at all foci, and only a fraction of the slit widths of both spectrographs. However_ fine guidance in pitch and yaw is needed. Different methods of pointing and guidance are explored by the subcontrac- tor in Appendix B, where both sensors and guidance techniques are evaluated, for the different ways in which the telescope may be mounted (body-fixed_ gim- balled, tethered). It is concluded by the subcontractor that: (i) The body-fixed mounting will be satisfactory if disturbance torques due to main motion are kept below O.1 ft-lb for a duration of 0.7 second. An improvement in the Apollo guid- ance system is required for this, however. (ii) The gimballed mounting is of practical advantage, though there remains sugnificant technical problems in the area of actuation techniques. (iii) The tethered concept is wholly within the state-of-art knowledge (except for fine-pointing sensors, as mentioned above). Although a strict roll guidance is not indicated for the duration of a single exposure, it will be desirable to maintain a given roll orientation with precision, as for example during a time-series of exposures of spicules. The solar vector may be defined by a fine-angle sun-sensor. Roll orientation about this vector requires use of a star-tracking sensor. The precision of roll orientation desired is about i arc-minute, corresponding to a 2-micron displacement of the periphery of the field of view at the f/42 focus° Because the telescope is in an orbit about the earth, the sun's eastward motion among the stars, as seen from it, will not be uniform. For this rea- son, the solar vector should be defined by the sun itself. The use_ for ex- ample_ of three star trackers for orientation would be cumbersome, requiring on-board computers. Also for this reason the single star-tracker which sup- plies roll orientation should be directed nearly normally to the orbit plane. 33 V. TEE ROLE OF MAN The design, intended mode of operation and means of acquiring data are based upon a close relationship of man and instrument. A. Telescope Maintenance Any maintenance or service could be supplied by the observer-astronauts. Attention towards a possible need for realignment of the telescope and spectro- graph optics while in orbit should be paid during further design stages. Doubtless special mountings, fittings, etc., will be mandatory to permit a suc- cessful alignment or realignment effort. Failure of any component will require its replacement, again a function which a man can accomplish if components are suitably designed. Access hatches to the replaceable or alignable components will be needed. If body-mounted (either gimballed or fixed), some or most of the tele- scope could be directly accessible from inside the spacecraft. Still, the work would be in the weightless condition and in a vaculnn. B. Mode of Operation To reduce the work effort on the observer-astronaut, the telescope sys- tems should be programmable under the observer's control. Complete operation from a single console containing at least one TV monitor appears highly de- sirable. The length of an "observing session" should be set in large part by studies of astronauts in present programs. There is no astronomical reason to think of observing sessions lasting longer than an hour or two, except in unusual circumstances (great flares at the limb) when perhaps four or five hours continuing observation would be of great importance. (We have in mind here a recommendation to be made later; a polar retrograde orbit which permits nearly continuous solar observation, or a high orbit of long period.) How- ever_ for a low orbit of low inclincation, periodic respite for the observer is afforded by entering earth's shadow. A TV camera is incorporated in our design which views the slit of the large spectrograph. This camera can be used by the observer in guiding the telescope to place a particular feature on the slit_ or to line up the tele- scope on some event of interest. Therefore hand controls for him are needed. 34 I I I I I I I I I I I I I I I I I I I m • I. I I l I I I I I I I I I l I I I However, fine guidance should proceed automatically. FiLms must be changed by the man. If the telescope is body-mounted, this may be accomplished from within the spacecraft in a vacuum. If the telescope is tethered, extra-vehicular excursions will be required. Access hatches, film cassettes, releases, etc., must be designed with the gloved condition in mind. Air supply quantities will be determined by how often films are to be changed. A special storage location must be provided for both exposed and un- exposed films, to protect them from radiation damage and from loss of moisture. Similar cautions apply to the films while in use in the telescope. A significant fraction of telescope time will be devoted to study of phenomena which are at present not entirely predictable. It may be that the phenomena will not yet be predictable when a space coronoagraph is flown. These phenomena may have to be treated as "targets of opportunity." The man's role in observing them will be of great importance, since by their un- expected nature the telescope could not automatically be set for their observa- tion. However, his success will be only as good as his ability to detect the beginning of the events. How a detection system might be designed and employed is open to consid- eration. Three possibilities are (i) an all-sun H_ monitor, with image vis- ible to the astronaut; (2) a UV sensor set, for example, at the He II 304 line and with an alarm threshold; and (3) a direct link from ground-based solar observations to satellite to warn of events. It is our recommendation, in any event, that a link to the orbiting tele- scope be established to permit ground-based solar observers to talk to the observer-astronaut. By such means observations could be coordinated and dis- cussed, plans elaborated or changed, new programs initiated. 35 VI. SPECIAL PROBLEMS The subcontractor has discussed_ in his report_ some of the basic tech- nical problems associated with a manned orbiting coronagraph. A few problems which may also be mentioned are the following. A. Micrometeoritic Hazard Micrometeoritic particles in the vicinity of the earth have been inten- sively investigated. These pose a threat to the surface of the objective lens. The rate at which scattered light in the instrument increases with time, due to pitting of the objective, should be assessed before the feasi- bility of successful flight operation is finally judged. B. Cleanliness of Environment Sources of local contamination such as venting of crew wastes, condensa- tion of exhaust gases_ etc., must be minimized for successful observation of the low corona. Local contamination must be virtually eliminated if the K- corona is to be observed_ using external occultation. Co External Occultation Provision of an external occulter will extend the range of coronal ob- servation indefinitely outward from the sun. The external occulter shields the objective from direct photospheric radiation, reducing the scattered sun- light and fluorescence in the telescope to very low values, below 10-9 B@. External occultation has been discussed at length by Newkirk and Bohlin. ±I Since each part of the lens is to be shielded by the occulter, there re- mains a portion above the solar limb which is vignetted. If _ is the angular size of the vignetted portion above the limb in radians, _ is the objective llNewkirk, G., Jr._ and David Bohlin; Applied Optics, _, 131, 1963. 36 I I I I I I I I I I I I I I I I I I I |. " I I I I I I I I I I I lens diameter and d is the distance from lens to occulter, then a d - The diameter D of the occulter should be, approximately, D = 0.00931 d + a . Some values for our lO-inch diameter lens are d D 3' arc 291 meters 3.0 meters 6' 147 1.6 i0' 87 1.0 30' 29 0.5 60' 15 0.4 For a fixed-diameter occulter--we here choose 3 m--the vignetting at var- ious distances d is D d 3.0 meters 291 meters 3' arc 3.0 200 101 3.o 150 18' 3.o lOO 35' 3.0 50 87' 3.0 25 i90' I Thus an occulter of fixed diameter at varying distances from the tele- scope may also be employed. It will probably not be feasible dynamically to attach an occulter to the telescope via a boom of the length required_ and therefore, if employed, the occulter should be a separately-orbiting, loosely-tethered (or a free) device. Whether it is possible to deploy such an object to the required position relative to the telescope is a question we have not attempted to answer. 37 However, assuming it to have been deployed_ we attempted to determine its usefulness, once in position it must be permitted to drift, since use of gas jets for steering during an exposure will result in objectionable light-scat- tering to the detriment of the observation. The rate of drift of the occulter_ relative to the telescope-sun line, was computed by integrating the equations of motion for both the telescope and occulter. We assumed that the occulter was on the telescope-sun line at time t = 0_ and that at t = 0 both objects were traveling parallel and at identical speeds. is the angle_ measured at the telescope, between the lines telescope-sun _nd te!escope-occulter. _ is initially zero, and increases as the occulter drifts off the telescope-sun line. Its rate of drift may depend upon its initial distance from the telescope _ upon the semimajor axis of the telescope orbit and upon the orientation of the initial telescope-occulter vector relative to the telescope's intial veloc- ity vector. Two circular telescope orbits were investigated: one a 330 km high or- bit, the other a synchronous 24-hour orbit. For both of these, two occulter distances _ were investigated: p = 30 m and 0 = 300 m. For each of these four combinations, seven initial configurations of telescope-occulter vector and telescope velocity vector were assessed. The selected initial configura- tions are displayed in Figure 8. Calculations made for initial configurations in the single octant used by us may be approximately generalized for initial configurations in the other octants, because of the spatial symmetry involved. The integrations span a time interval of 5 minutes, starting from t = O. INITIAL CONFIGURATIONS INVESTIGATED Semimajor Axis of Telescope Orbit 0ccnlter Distance p (meters) Yo_ @o Degrees 6,700 30 42,240 300 O, 90 (A) 45, 90 (B) 90, 90 (C) 0, 45 (D) _5, 45 (_) 90, 45 (F) --, 90 (G) Results of the integrations are displayed in Figures 9 through 15. Each figure refers to a particular initial configuration (_o, @o) and contans data referring to both orbits and both inital occulter distances. We deduce that, 38 I I I I I I I I I I I I I I I I I I I I • I e l l l I N i 39 0 0 0 .r.I ,-I o r-I o '_ O i11 i11 r-t 111 I1_ O •,-40 4-_ 4_ O d_ O X \Q. c_ .--I i i I ! ! sa_,nu!w- :)je '® 4o ,---,I 0 0 o .p 0 h. -,-I .p c_ ,-I 0 0 4o 0 4o ,r-I 4o 0 o 4_ % 0 I I I I I I I I I I I I I I I I I I I I. I I I I I I I I I I I I i I I I I I I I , • _ _ _ _ _ _ _ se_,nu!uJ- :}Je '¢ 43. o L,- O :3 O t- O L_ r-- U T- e i I I A I I I O rJ i-I O ¢- _ E E o o o 0 0 0 ID ,,,.4 .i--I 0 S ril r,,. II 0 CD(J o II Qo I I p-I N .r4 (_ P_ 0 0 0 4o °_ _ 0 r-I ,,_ i:= o 0 o o . 0 E o o m -r-I 0 I I I I I I I I I I I I I I I I I I I II I I I I I I I I I I I I I I I II I I i I I I I / e I 43 s_ G_ r- E G_ E °_ -M O m _o 0 >d _ 0 0 r-I 0 '_ 0 0 4-_ _ 0 8I c: o I.=. r'-, Cr, _o Q.. I ¢- t.- E c_ I i s_nu!w -:)Je '(1) I 1 44 o v') o t- o f- t- I I i i u'_ 0 .M o o ,-1 0 _ 0 r-t .,-I ¢=. •4-, o 0 o o _ 0 E ,-4 0 _ 4_ -0 .,--t 0 0 4-_ 4-_ % 0 -M ,el I I I I I i I I I I I I I I I I I I I I I. I I I I I I I I I I l I I Q.. II 0 CD l.&. @ 8_ II I I spu00os- :)Je ' ® I 4_ o I o -! I,- E o_ t,,-,- Pl 0 rj m i.-I 4-) 0 4a _-r2 _ 0 4-_ .,.4 r--t 0 0 0 0 0 r-t 0 _ 4_ 4.) ._ ,,r't _ 0 0 0 _° 1--.,I _D I I I _o I I se)nu!w_3j e ,® 46 r"" o o t- o e- e-," ..@ e- °_ e- i I J e- l= Q) _= °_ I,-,- o 0 0 r_ _q 40 0 4° _o,r-I 4-_ -,-4 _ 0 0 r--t O',M 4_ 0 0 4-_ 4-_ 0 rl,, I I I I I I I I I I I I I I I I I I I II I I I I I I I I I I I I I I I for a 5-minute interval: (i) The radial rate of occulter drift, relative to telescope, is such that the curve p(t)/p o is independent of Po, for any circular telescope orbit. (ii) The angular rate of drift, d_/dt, is such that the curve _(t) is independent of Po, for any circular telescope orbit. Hence each figure contains a single curve p(t)I@o and _(t) for each telescope orbit, each curve referring to both values of @o- (iii) The rate of drift of the occulter is acceptably small for all initial configurations in the synchronous or- bit, but is too large in the low orbit for most initial configurations. (iv) The rate of occulter drift is acceptably small for both orbits when the occulter is placed on a telescope-sun line which is perpendicular to the plane of the tele- scope orbit (configuration G, Figure 15). We therefore conclude that, after deployment, an external, freely-drift- ing occulter can be of some use over a reasonable time period. The required conditions are (a) a telescope orbit of large major axis o__r(b) a telescope orbit plane perpendicular to the solar direction. It should be here emphasized that the astronomical importance of the telescopes investigated by us will be considerable, even if external occulta- tion is not provided. The last two rows of Table III, page 21, list the exposure times which can be expected if occultation can be provided. 47 Vll. RESUME A. Optical Design There are no developmental requirements on the optical design. Achieve- ment of diffraction-limited resolution is practical. The major difficulty will be in selection of a lens of suitable scattering and fluorescing properties. A crucial question is the rate of increase of scattered light due to mi- cromeoritic pitting of the objective lens. B. Mechanical Design The subcontractor's report indicates feasibility and suggests areas for further design studies. The important areas are in thermal design and in adoption of a gimballing system. C. Control and Guidance It will be necessary to develop an error sensor for fine-guidance of the telescope. There remain significant technical problems in guiding a telescope fixed to a manned spacecraft. Some of these can be avoided through operating the telescope in a tethered condition. D. Platform A platform with Apollo characteristics of mass_ moments of inertia and angular drift rate will serve satisfactorily as a mount for a gimballed tele- scope. A similar platform_ but with finer pointing capability and much slower angular drift rate could serve for a body-fixed telescope. In both cases the presence of man will perturb the pointing control drastically. E. Orbit A lengthened duty cycle can be attained if the telescope can be placed into a high orbit or a polar retrograde orbit without injury to its observers. 48 I I I I I I I I I I I I I I I I I I !m I I I I I I I I I I I I i I i I i_i APPENDIX A CALCULATION OF EXPOSURE TIMES • i CALCULATION OF EXPOSURE TIMES Let the specific intensity of radiation from an observable solar feature be I = f l(k)d ergs cm -2 sec -I ster -I. The 10-inch coronagraph lens sub- tends, at the sun 4_ _(12.7 cm)2 = 7.20 x 10 -25 steradians 4_(1.5 x 1013 cm) 2 Hence from unit area on the sun, the telescope receives I x 7.20 x 10 -25 ergs sec -I (per square centimeter) . sun. A unit area on the photographic emulsion corresponds to an area _ on the At the f/42 focus, i cm corresponds to Ffl arc-sec 104 microns_ x _15 km A42 : L\45 microns x i cm Y arc-sec x 10 5 _)I 2 = 2.33 x 10 20 cm 2 At the f/3.6 focus, I cm 2 corresponds to microns_ cmarc-sec x 104 -- x .15 x 107 A3.6 = mlcrons cm / arc-secjj = 3.2 x 1022 cm 2 Hence an area of i cm on the emulsion at a focus receives energy at a rate A i x I x 7.20 x 10 -25 ergs sec -I (per square centimeter) 5O I I I I I I I I I I I I I I I I I I II • I I I I I I I I I I I I I I I I I The quantity of energy received at 1 cm 2 in a focal surface is, in photo- metric units 0.680meter-candle-seconds ergs cm -2 x A i x I x 7.20 x 10 -25 ergs sec -I cm -2 = A x I x 4.9 x lO -25 meter-candles - Losses by absorption in the telescope, in filters and due to dispersion into various grating orders may be estimated. If the total light loss is expressed as a decimal fraction L, then the quantity of energy received at 1 cm2 of a focal surface is (i - L) x A x I x 4.9 x 10 -25 meter-candles We assume the use of a fast, fine-grain emulsion for which the required exposure is O.1 meter-candle-second. Hence the exposure time at a focal sur- face is O.1 T = seconds (l-L) x A i x I x 4.9 x 10 -25 In the large spectrograph, the third order dispersion at Ha is 1.5 A1/mm. A 25_-width slit projects to 24_ at the spectrum focal plane. Thus Projected slit width = 0.035 Spicule energy in H_ is 2.4 x 10 4 ergs cm -2 sec -I ster -I over a band width of 3.4 _ = 97 projected slit widths. Since a 30_ transmitting filter is retained to eliminate overlapping orders, and assuming a grating efficiency in the third order of 30_, the exposure time for the spicule Ha line in the large spectrograph is 97 x exposure time at f/42 direct focus 0.30 = 71 seconds 51 H r_) o'] O H D] E_ F_ cQ O Q. DI. -.... ° r_ O I I I- c_ q Od OJ O r-I O R _o i1)1 OI ol O ©1 r--II ¢ Ell % ® m O _ _1 r-t r_ °H 0,1 • r_ R Oa'_ o "1 O _- 0.1 ) i N 0d I1 [ Od O t_ ,-t O O O _ Ctl i1) II b0 O_ ,-1 4-_ 0_ • O (U OJ _ ',,.00 -H r_) ,-4 O,J OJ O r-I OJ O q_ o • I_ ._1 oJ 6 L_ ,-4 6 0 t-- 0 0J ¢) ,-I II b0 Q. r-_ 4-_ cd o 00 ',.o o .H r_ ,--t 0d 0 ,-I 0 _d P_ _-i u o o Od o P-_ __) cd 0 LC_ 6 0 L_ 0 II ,-_ 0 <1 cO • O II M o_ O o.J 0 ,-j N L_ d Od % 92 Od Oa O 6 O u"N •-P 0 0 II o ._ cO II IM o_ oa oJ o ,-t o.1 I i I I I I i I I I I I I I I I I I I Im I I I I I I I I I I I I I I I I APPENDIX B SUBCONTRACTOR' S REPORT (Evaluation by the Bendix Systems Division of the Bendix Corporation) !1 ¸¸ mid ! ! ! ! ! ! ! ! ! | ! ! ! ! ! ! t3ttml, 11_, CORONAGRAPH EVALUATION STUDY FINAL REPORT December 1965 to March 1966 BSR IZ75 March 31, 1966 Prepared for: The University of Michigan Ann Arbor, Michigan under Subcontract No. 3 of Contract No. NASr-54(09) Authors: R. Blythe H. I. McDevitt E. D. Shoua T. T. Trexler BENDIX SYSTEMS DIVISION of THE BENDIX CORPORATION Ann Arbor, Michigan L I I I I I I I :I I I L I° 2. . i i i iii iii!ili_I iilill'i/iii i I_ i i _ _ _ _i: i I i i I ! J CONTENTS INTRODUCTION I-I OPTICAL EVALUATION Z-i Z. 1 LINEAR DESIGN 2-1 Z.l.1 2.1.3 3.1.3 2.1.4 2.1.5 2.1.6 Objective Lens Occulter Movement Occulter Positioning Mechanism ManginCorrector and Relay Lens Czerny- Turner Spectrograph Bowen Spectrograph -.:__::_2_:_::_.::F_Ster__.0bji__ Live Lens 2. 2 FOLDED DESIGN 2. 3 EXPOSURE CALCULATION 2.4 OPTICAL EVALUATION CONCLUSIONS Z-I 2-4 2--4 2-6 2-6 2-8 Z-8 2-'1 t 2-11 2-t3 2-15 CONCI_.PTUAL DIESIGN AND ANALYSIS OF THE C ORONAGRAPH SUPER-STRUCTURE 3. 1 MAIN FEATURES OF THE SUPER STRUCTURE "_ 3. Z CHARACTERISTICS OF THE CORONAGRAPH 3. Z. 1 Unfolded Coronagraph Design -3. _-. 2 Fotded-Coronagraph Design 3:-3 OBJECTIVE LENS HOOD AND COVER 3.4 TEMPERATURE CONTROL 3-1 3-1 3-3 3-3 3- 8 i_ :-_._ 3-9 3- 9 ....: 3.5 LOCATION OF GIMBALLING SUPPORT ON SUPER- STR UC TUR E 3.6 CAMERA LOCATIONS OF SUPER-:STRUCTURE . .-, IZY 3-12 .... -q'7_ 3- 14 ............._" ! m • I I ! I I I ! ;B I l !1 | t -2_: 2 . CONTENTS (CONT.) _. 7 I,OCATION OF CORONAGRAPH ON THE APOLLO SPACE CRAFT 3, 8 ASTRONA.UT SUPPORT CORONAGRAPH CONTROL ANALYSIS 4: t PROBLEM DEFINITION 3-14 .- 3-16 4-1 4-1 ,! 4.1.3 4.1.4 4.1 5 4.16 4.1.7 4._:c_Ro<_c_P_ 4. 3 FINE ANGLE ERROR SENSOR 4. 4 ROLL STABILIZATION M_ssion Defir_ition A.ttit_Jde Controi and Stabilization Operational Mod_:s Guror.agrapi_ Mounting Modes Mls sio_. Analysis Disturbance Torques Prime Vehic!e Characteristics, Physical and Gontrot Instrumentatibn State of the Art 4-1 4-3 4-4 4-7 4-12 4-23 4-24 4-30 4-34 4. 4. I Body F,xed Apoilo S/C 4-34 4. 4. Z Gimbailed, Apollo S/C; Bod_ Fixed, Tethered Satellite 4- 34 i. 5 CANDIDATE GONTROL CONGEPT PERFORMANGE 4-35 ANALYSIS 4. 5. I Body Mounted, Apolto S/G 4-'3-5 4. 5. 2 G:mballed Apollo S/C 4-42 4. 5. 3 Tethered Satellite 4-47 COST AND DEVELOPMENT SGHEDUL3_S 5-1 _. t OPTICAL DF_.StGN 5- 1 5. 2 STRUCTURAL FABRICATION 5-5 5. 3 CONTROL DESIGN ANALYSIS 5-6 5.4 COST SUMMARY 5-7 .... _ -a ..... L m u ! I I I I I I l IL LUS T RA TIONS Figure Titl____ee 1 Optical Arrangement - Unfolded Design 2 Optical Arrangement - Folded Design 3 Spicule Observation Geometry 4 Unfolded Coronagraph Design 5 Unfotded Coronagraph Design 6 Folded Coronagraph Design 7 Location of Center of Gravity - Unfolded Design 8 Location of Center of Gravity - Folded Design 9 Pitch and Yaw Gimballing 10 Roll, Pitch, and Yaw Gimballing 11 ...... l_ll Pitch _ YawlGimballin.g 13 14 15 16 17 18 lq 20 I1 zl Z3 Z4 Z5 z6 27 28 Z9 30 31 3Z Circular Orbit Characteristics Tracki_ A ngl_ Commands Maximum Pointing Angie Amplitude Sotav Occultation Ratio Relative Drift Average Gravitational Disturbance Torques Typical Magnetic Field Disturbance Torques Solar Pressure Disturbance Torques 90 Percentile Man Disturbance Torques, Trans[atory Man Motion Disturbance Torques, Rotational Man Motion Disturbance Torques, Coronagraph Reaction Torque CSM/LEM Adapter Fine Track Mode-Body Fixed Apollo S/C Fine Track Mode-GimbaIled Apollo SIC Fine Track Mode-Body Fixed Tethered Satellite Ana!y_ic at.Control Loop_ • Body Fixed Apollo Limit Cvcle Rate Feedback Contro[ Error Rate Control page 2-2 2-12 2-14 3-2 3-4 3-5 3-3 3-8 3-12 1 3-13 3--15 4-8 4-9 i4-I0 4-11 4-13 4-15 4-16 4-17 4--18 4-19 4-20 4-22 .... 4-23 .... 4-27 4-28 4-29 " 4- 36 ..... ,;.;":-,.F-_;:.?_ - -22i 4-37 4-38 • _, .i i.)•_ l m • I I I 1 I I I ii i i t I F.! _ u r e _3 34 35 Table 2 3 4 5 6 7 8 ill US T RATIONS Title (CONT.) OpTical Desigv Schedule Goronagraph Super-Structure Prototype Schedule Goronagraph Gontrol System Detail Design Schedule 5-5 5-6 TA BLE Title Pnysical Constants Design Phvsicai Consta:,ts Mounting Modes Man Mode i Di sl,urbar_,:'e Torque Summary Fine Track Attitude Semsors Actuation Techniques G o7 onag ratph (2..ot:trol G one epts of Goronagraph Unfolded of Coronagraph Folded Design 3-6 3-7 4-5 4-21 4-21 4-24 4-25 4-26 I m I I I I I I I SECTION 1 INTRODUCTION The Bendix Systems Div, slon under contract with The University of Michigan and the technical direction of the Department of Astronomy, Chairman Prof. Mohler, and Dr. Teske, performed a preliminary analysis _._f the feasibihtv of coronagraph application for solar observations from _n orbiting space vehicle. The analysis and preliminary conceptual design effort consisted of three tasks I. Evaluation of proposed optical designs .... i ....i t $ ....Evilua_i on: of _andida/_ ¢¢mtrol techniques. The general cor_clusion derived from this preliminary work is that :t :s entirety' feasible and within the current state-of-the art of all techno- logical and physical areas to fle_tgn, fabr_.cate, laun_:h, deploy, and per- f_rrn meaningful observations v,'ith an orbiting coronagraph. -_5"7 ..... i n I I I i t I I I I I I I r. SECTION Z OP TICA L EVA LUA T ION Two designs for a coronagraph have been considered and evaluated. The first design (linear) consists of optical components similar to the Rush- Schnable 1 design except that most angles of incidence are near normal to ease the problem of polarization analysis. The second design uses folded optics for the purpose of conserving space but consequently places a heavy burden on the polarization analysis, Z. I LINEAR DESIGN Z. 1. 1 Objective Lens -i Refer-ringt_:_ig_e l_0__he first element to be considered is the , 10 inch objective lens (a). The limiting resolution of the system to this point is given by tt_e:Fr, aunhofer d_ffraction of the circutar aperture (lens A), therefore the diameter (a) of the image spot between the first mimina is 1. 22fk a - d (t) where f = nom,_nal focal length of lens d : diameter of lens k = wavelength corresponding to f For a focal length for_ (A) of 457 ca(at 5890 A °) (A) is approximately 13 microns. This figure is next compared with a figure obtained using the spherical aber- ration-obtained for the best shape factor for spherical surfaces to see whether or not aspheric figuring is necessary. The shape factor (gD) for minimum spherical aberration is just s!ightly different from that for zero coma at the in_:ex]nD) corresponding tO 5890 A ° for BSC-Z glass. For zero coma Z : ..... =_;:: .A - ...... 7_i_-Z 1 j. RUS_and G" Schnable, Applied Optics, Dec. t964, p. 1347. _/:-;>211;?-2-2 Jenkins and White, Fundamentals of Optics ................._ ................._ ........;....... : .... ,-::_2;;_........._Y; . .2;i!=?22:: II I I i I I I I_l I l-i I II 0 .p, I I . . i-. ° ! :> r I I I I where Zf p =_-I= -IS G qD -- W p {Z) I I I I I I I G = 3(Zn + 1 ) 4n 3(n+l) W= 4n (n - ] ) For nD= 1.517; G = 1.99, W = 2.41, andqD= .83. The radii of curvature for this lens would be Z58 cm. and Z780 cm. With the more convex surface toward the sun. The change in image dis- tance for this lens is obtained from 3 ZI 1 h , [n_2 Z n(n-l) hn - I q 3 z n ] n-i + 4(n _ I) pq * {3n _ 2_ _n - I) p (3) /_ where fo r S ' is the image distance for a ray entering the lens n at a distance h from the optical axis S' is the image distance for paraxial rays h= 12.7 cm n = I. 517 II I I I I I I !-i f = S _ = 457 cm From this calculation S h' = 456.64 which indicates that peripheral rays cross the axis approximately 0. 36 cm. in front of the nominal focal plane. At the nominal focal plane, the lateral spherical aberration corres- ponding to this Sh' would be 202 x 10 -2 cm which is approximately fifteen times the diffraction limited spot size. Some aspherizing is therefore necessary to obtain diffraction limited resolution in the system. Z. 1.Z Occulter Movement The chromatic aberration determines the on-axis movement of the occutter (b}. Using the index of refraction at 0.33_t to be 1.540 for DSG-2, the image distance for 0.33_ is 436± 2cm. The image distance fork = l_t (n= l:5t_is46Z-_2 cm::::The occutter therefore should have an: axial move- ment of_pproximateI 7 30cm for the wavelength region 3300 A ° ' 10,000 A °. The occulter diameter (do) should be somewhat greater than the largest image of the sun which would occur at the lowest index (n= 1.510). d sd =m f 0 S Ill S where d s S S = diameter of sun = distance to sun I f. = 46Z cm; image distance at 1,in At tg. d o would be 4.3t cm. therefore the occulter should be perhaps 4.4 cm in diameter. Z. 1.3 Occulter Positioning Mechanism Inasmu.¢h as the occutter will be completely movable within the area of the. field stop (field lens}, it would he desirable to have the occulter II I I I I I I I I I I I considerably larger than d o (eg. 5 cm) to ease the requirements on the positioning mechanism. For this configuration, the requirements on the positioning and holding servo can be loose at all times except when observ- ing close to the limb. In this case the holding servo should be maintained by a tight loop. A suggested design for the occulter and its associated mechanism would have the occulter mounted at ttae vertex of a tetrahedral frame. Spider elements (thin perpendicular to the optical axis and thick along the optical axis) would connect the vertex with the four points of the base. The base would be the moving part of a 3 axis lathe .type slide mechanism _similar to the Kulicke and Soffa m_crometer positioning unit) with motor drive applied to the lead screws. For an observation position at a considerabIe distance from the sun's limb (> 1/10 r s from the timb), the occulter can automatically be placed over the sun's image. The occulter is moved to a given Z o position outside of the desired wavelength region (Z1; - Z 33_t ) above which is an insensiti_ve-IR detector in a semi-circular arc. T'he occulter is then in- ,_,_,-,_d-i_ x _d ,e_:,_n_d in v {or vice versa_ where three index steps are necessary to cause a 5 em occutter to cover the entire transfer lens diameter ..... The detector output (amplitude as a function of the scanned direction} is fed to processing c_rcuits which determine the mean y position .y + Y_/ t ,.\ , ) - ) and drive the occulter to that position. The occulter is then scanned in x and the above process repeated to find the proper x position. The occulter is thus set in its x and y positions automatically and driven by the control c.enter (astronaut) to its desired Z posit_on corresponding to a particular wavelength region. The lateral movement of theoccutter should correspond to the diameter of the field stop which in this case ts the field lens (C). The occutter should idealty have a lateral movement of approximately 13 cm. This wo,atd correspond to a field of two solar diameter on either side of the _tmts-limb.. I. I I I I I I I I I I l ,I I I ?-. 1.4 Mangin Corrector and Relay Lens The field lens (C} images the primary objective (A) onto the stop in front of the Mangin Mirror (D}. The distance from the field lens (C) to the stop in this design wii1 be about 273.5 cm. The purpose of the stop is to eliminate the light diffracted from the edge of (A), therefore the size (diam.) of the stop would be slightly less than the size of the image of (A) at (D). For a focal length of(C) equal to 172 cm, the size of the image of A at D is 15 cm so that a 13 cm diameter stop in front of D is more than enough to eliminate several hundred fringes. The change in diameter corresponding to the first fringe is about 10 -3 cm. The Mangin Mirror should be slightly larger than the stop (approx, 14 cm) and would have spherical surfaces with the back surface aluminized except for a small spot in the center {back vertex) to allow the image due to the second surface reflection within the primary lens (A) to pass. The second surface reflection is imaged approximately 64, 5 cm from lens (A) and is approximately 0. 6 cm in diameter. Using this as the object for the field lens gives an image at the Mangin mirror of approximately 0, 45 cm in diameter. The spot should therefore be slightly larger than this diameter. The Mangin Mirror is canted slightly off axis to allow the light to just miss the field lens on the return. If the Mangin has a focal length of 140 crn, it wilt image the occulter and associated focal plane in the vicinity of the field tens and slightly below it. A 2. 5 ° tilt of the Mangin gives sufficient clearance to form a 9 cm image below (C). The Mangm _s figured to correct for the chromatic aberration introduced by (A) and focuses all wavelengths at approximately the same position. The image- plane is used as the object plane for the relay lens (E), Lens (F) will also be figured to correct for the astigmatism produced by the Mangin. 2-.I. 5 Czerny-Turner Spectrograph For a field of view of 5 arc-minutes the slit height of the Czerny Turner should be about 1. 3 cm. The slit jaws are conceived to be totally reflecting (either glass or metal) and at an angle of 45 ° to the optical axis. The light which doesn't go through the silt is then reflected into a TV camera which serves as an alignment tool as well as a real time data record. There will be a monitor in the control capsule which the astro- naut will view to align the coronagraph on a particular solar region. For J • _A n • ! ! ! ! ! ! ! ! ! ! ! ! ! t ! times other than alignment, the camera will be switched to a video tape mode which will provide the experiment with a real time data record. When the Bower. spectrograph is in use, light reflected from its jaws is fed into tile same TV camera for the _azne purpose. The collimating mirror (H), which directs the tight to the grating (G) is situated at the focat distance (302 cm) from the slit. The pertinent angles, dispersion orders etc, are obtained from the grating equation: d(sin i 4 sin 8) =mk (5) where m = order \ = wavelength i = angle of incidence 8 = diffraction angle d = grating line spacing and its derivative d cos O = mdk/d@ The linear dispersion is dR dl d cos @ mf (6) (7) where f = i/e Referring to Figure 1, the angle HGH' is about 4. 5 ° which means that i +4.5 ° = 8. Putting this value into the grating equation for a 600 l/ram grating and . 5 x 10-4 crn wavelength gives a I st order diffraction angle O of tl ° and incidence angle of 6.5 ° . For gnd order O is about 20 ° and i is about 15. 5 °. The 1st order dispersion is obtained from Equation (7) and is about 5.65 A°lmm. L For 1st order use, the blaze angle _ should be 9° where --- - -- --27-- . :._ ....: ---==: _2 ...... ...... . :. _ . -[ .......... . -- ___i>_}re:{_:_": .............._ .........: =_/::!e'_!5_--_--':: ':-.................... _:_ :)._==-:-<7---_c?:-_:-?:?/::._:.':isUi72:::_-::it)72::.: -z_:7: ............>:_e-.:_q_ iI I I I I I I I I I I I i+0 _3- 2 (8) The camera crJirror H (focal length = 280 cm) focuses the spectrum on the exit camera Ps which is discussed in a later section. 2, 1.6 Bowen Spectrograph The folding mirror (F) directs the light either into the Czerny- Turner Spectrograph (SHGH'Ps) , which has just been discussed, or the Bowen spectrograph (G'KG'LPa) through a telephoto lens (QR) or into the camera Pd for direct observation. A 9 cm diameter mirror is used to avoid vignetting in any direction. Lens (Q) is placed at its focal length (-218 cm) from the relay tens (E) and essentially collimates the light from E. R is then placed such that the slit opening of the Bowen is at the focal length of R (30 cm). The diameter of these optics is chosen to be com- patible with the speed of the Bowen which is chosen to be F/3.6 (entrance). The Bowen slit is mounted into the back side of the grating G' (600 I/mm) and has totally reflecting jaws. The light reflected from the jaws is directed into the TV Camera as previously discussed. Mounted in front of the grating is a corrector plate to correct for residual spherical aber- ration. The angle of incidence corresponds to the size of the mirror K and is app_ximately 37 ° in order to fill the grating, Using Equation (5), the diffraction angle 0 for Znd order, 5000 A o is about 0 ° from the grating normal. From Equation (7), the Znd order dispersion is approximate! 7 30 A°/mm for a focal length for L of 30 cm. The camera mirror (L) moves about the grating such that other wavelength and orders may be sampled. The appropriate blaze angle _would be 18.5 ° for Znd order use. The camera Pa is attached to the same frame and moves with L in order to photograph the spectrum. The camera Pf is mounted on the same frame as the grating (G') and can be rotated into the place of the grating for photographic observa- tion of the distant solar regions. 2.1.7 Cameras With the exception of the optical guiding TV camera, there are four recording cameras called for in the design oI the solar coronagraph. Both cine cameras and TV recording systems were explored; the latter because of the high astronaut oxygen expenditure used in loading and |. " I I I I I I I I I I I I I unloading the film (cine) cameras. The TV recording system was dis- carded in favor of the film system because of the lack of availability of a high resolution (ZS0 line pairs/ram) vidicon or orthicon, as well as the weight and environmental problems inherent in a TV system. The features deemed desirable for all of the cine cameras are as follows: NASA certified spaceworthiness, due to the outgassing problems basic to normal camera lubricating =y_'-"'_............. al rubber gaskets, many plastics, such as teflon, and normal camera film substances Touch-release fitm magazine loading and unloading because of lack of astronaut manual dexterity due to necessary use of relatively inflexible vacuum g!oves, and because experience in Gemini flights has shown high oxygen expenditure by the astronaut in loading and unloading the film magazines in space o Automatic film threading variable shutter speeds, film advance, etc for the reasons enumerated in (1) and {Z} immediately above . Large film capacity with up to I000 ft of film, using normai thick based film, with greater capacity using thin based films. The differences between the cameras lies primarily in their uses, which manifests themselves (a) in different film format and orientation and (b) in the amount of film advance per frame. 5. Some method of sealing the camera, perhaps with a plane glass or plastic window over the format aperture, which will be necessary be- cause there are to be no optics directly mounted on the cameras. Turning now to the availability of cameras, it is noted that there are presently available, as off the shelf hardware, only 16 mm and 70 mm cameras, fulfilling requirements one through four listed under desirable feature, above.. It is apparent that 35 mm and 100 mm cameras may be avaiMbte in the near future as they are in the design stage now. To the bestof ore" knowledge, no 8 mm spaceworth camera ispresently available or is under consideration, however, our inquiry may have generated some interest in future 8 mm space cameras. ...... T Z2_ Q• i I | I ! I I I I I I I I Fulfilling condition five, above, appears to be a minor problem, and it is assumed at this point that this problem can be met and solved without undue concern. The unofficially quoted (by telephone) prices of these NASA cer- tified cameras range from a low, for stripped down cameras, lacking features (Z) and/or (3), or parts thereof, of $40,000 for the 16 mm cameras •+,_'_,y v_v t.*a. t.*ae ,,..' zaJL._a caT,_Jeras tu a I..-"--I- ........ UU, UUU 1Ttm and $1ZS, 000 for the 70 mm cameras which completely satisfy conditions one through four, with only minor adjustments to be made to the existing cameras. Graduations in price would of course appear as more features are added or major revisions are called for. The following discussion analyzes the requirements of the individual cameras designated in Figures 1 and 2. The most desirable format size for the camera P would be about s 5 cmx 1:5 cm except when a reference spectra is needed. + In this case the format size should be somewhat larger in the shorter dimension. The 5 cm dimension w0uld at[ow a 100 A ° look at a dispersion of 2 AO/mm, Ideatly, this Camera should be a 70 mm camera (for the 5 cm dimension) with a variable fractional frame advance and a variable shutter speed from 1 to t000 milliseconds. Two certified spaceworthy versions of this size camera are presently- available with slight modifications. Camera Pd is used for direct photographic observation and would r:eed a I. 3 x !. 3 cm. format for a 5' field of view. This is very close to the format slze on presently available certified 16 mm cameras which are capable of meeting the above requirements. For a 5 rain field of view, the slit height of the Bowen spectrograph would be about 1. 5 ram. To see the entire visible region at a nominal dis- persion (40 A°/mm) would require about 8 cm of film. The film format should therefore be 8 cmx 1. 5 ram. A 100 mm camera would be ideal but it appears that none is presently available which meets all requirements although companies are planning to produce such a camera. An alternative would be an 8 mm camera mounted perpendicular to the preferred way with an 8 em (i0 frame) aperture and 10 frames advance. This camera is also not presently available, and not likely to be in the near future. The sug- gested way is than to use the 70 mm camera with some loss of spectrat bandwidth. The other alternative would be to look into the state of 100 mrn cameras presently under development " -'J:71z---"-_I .............. _ ..... 2 ........ • " -- i 22 2 _ 7- l o I i I i I I H Camera Pf, which swings into position in place of the Bowen grating (GI), should ideally be an 8 mm camera which would, give about a Z0 rain field of view. Inasmuch as a spaceworthy camera is not available in 8 mm size, either a development program should be undertaken to develop such a camera, or the 16 mm camera could be used at the expense of using more film. The 16 mm camera would give about a 40 minute field of view. 1. 8 Faster Objective Lens In order to shorten the entire structure, a shorter focal length abjectlve lens (A) would be necessary. A brief analysis on an F/15 381 crn focal length lens was carried out. The diffraction limited spot size between ]st minlma for this lens would be about 9_ at k = 5890 A °. The lateral spherical aberration at the nominal focal length would be about Z91_ which means that more aspherizing is needed as compared with the F/18 lens. The chromatic aberration for this lens would be about the same as for the F/18 lens, I I I I it is concluded that it would be feasible to shorten the focal length by- about 76 cm and consequently shorten the entire structure {assuming a shorter focal length field lens) by about lOT0 but the additional figuring re- q_ired on the objective lens (A) might make it undesirable. 2. 2 FOLDED DESIGN The-folded design, shown In Figure Z, was conceived to compact and shorten the length d_menslon o_ the corona graph. The major dis- advantage of thls design is that the two rlght angle reflections (at C and D) i_Iroduce additional polarization into the light incident on E in such a way that it is difficult to correct for the introduced polarization. The optics of the folded design are very similar to those of the linear design consequently the discussion will be minimal. : I The objective lens A (focal length --- 457 cm. ) focuses the solar image on the occulter and its associated focal plane. The spherical and chromatic aberration of lens A, size of the occulter, its movement and positioning mechanism are all the same in the linear design. Lens (C) image s A onto the stop in front of (E). For a focal length of C of the 54 ca, the distance between C and [he stop should be about 62 Ca. The righ£ angle prism and the folding mirror D may be placed almost any-avhere within that distance ..- but the preferable positioning would be for C to be attached to the right ............. ......... " ..... " ........................ L_ ........... :-. -=_4+_4-7+ ...................................... _- - ==.==== It _L ! ! II | t • ° "_"b I: l i_ • :illi__il -i.i_--:i,--.-%-- _ _ "_...........: ....:.................F:",.- _.::,-............. ............................... I I I I I I angle prism and D to be placed such that AD approximately equals AC. Lens E images B and its focal plane onto the slit S. For a focal length of 45 cm for (El the distance KS equals 124 cm and D should be about 20 cm from C. The proposed design has BCDE rotating about ABC into three positions to direct the light into the Czerny Turner, the Modified Bowen and the camera Pd directly. For a i5 minute fieid of view the slit height on the Czerny Turner should be about 3,3 cm, Camera Pd should therefore be a 70 mm camera. The Czerny Turner spectrograph itself has the same grating and mirrors as in the linear design, consequently the dispersion is the same. Camera Ps would therefore require a 5 cm (for a 100 A ° bandwidth) x 5 cm {slight height) format and should also be a 70 mm camera with the film axis oriented in either direction with respect to the spectrum axis. I I I I I I I The third step around the axis brings the image through (FJ and (R_ and focuses on the slit mounted in the backside of the Bowen grating G'. Mounted on the same structure as the grating is camera Pf which swings into the ptace of the gratmg when direct observation is desired, The corn7 ponents of the Bowen are the same as in theJinear design, consequently the minimum format size is 8 cm (for 3200 A bandwidth) x 3. 3 mm (slit height). A t00 mm camera would be best suited for this configuration with a second choice being an 8 mm camera with the film axis parallel to the spectrum. Z. 3 EXPOSURE CALCULATION The intensity of a small spicule in the Hc_ light for a bandwidth of about 3.4 A ° is approximately Z x 104 erg cm -2 sec -1 ster -1. (4) tn order to obtain the energy density on the film, we shall assume (without loss oi exactness) a 1 cm 2 film area being exposed. Then the equivalent area of spicu!e intensity at the sun can be found from the following geometry of (4)R- B. Dunn, Photometry of the Solar Chromosphere p. 17-9-130 Tabie 14 AFCRL 65-398, June 1965, Environmental Research Papers No. t09. !I • I I I I I I I I I I I I I LI ;I 1 9.3x 107mi _ cm 2 film 457 cm Figure 3 Spicule Observation Geometry f9.3x 107 03 2 , x 5. 28 x I x 30 cm ", 2 1020 A = ( 2 ) x 1 cm = 10.6x _' 4. 57 x I0 cm cm The energy density emitted is I 2.4 x 104 erg 1020 2 . 1024 ergE = x 10.6x cm = 21 Zx 2 sec ster cm ster sec (9} The energy collected by a 10" diameter lens at the earth is _o 24 .¢_r )2 oe 21.2x 10 cm 4 "25"4 cm 50 erg E" = E'_ -- x 24 2 -- secf2 sec ster oe 21 6 x 1 0 cm (10) 2 This is incident upon 1 cm of film, so the energy density upon film, if this were at the first focal plane (at the occulter position}, is 1 0 4 2. E=--E' _ 50 erg watt sec 500 lumens xl meter candtex cm a 2 x 7 x watt _ Z cm sec 10 erg lumen/re" m (11) = 2.5 meter candles. If the exposure required is about 1/30 meter candle second {5) t " 1/30 m. c. s. 25 m.c. = 1t750 second for no filter or other losses. (12) (5)For fast spectroscopic o£ Tri-X speed Iitm exposed to a density of 1. 1 I I I I I Ii I I I I I I I :% Since an H a filter has about 10% transmission and the system beyond magnifies the images about twice, the exposure time would, for the camera Pd be increased about 40 times additionally, hence t = 40 x 1/750 sec : t120 sec (13) For a grating efficiency of 60%, and a slit width of 10_, the exposure ....... 1-"..... g'=l.',, _e,.o.,_ ,,uuuL I/Z second. During this time, at a drift rate of 3 arc sec/sec, the sun's image will move approximately 0. Z slit widths. Thus for the spectrescopic work, and especially for faint spicules, it is advisable to have a drift rate certainly not greater than 3 arc seconds per second of time, and preferably less. Z. 4 CONCLUSIONS Each of the designs--linear and folded, have advantages and dis- advantages. The major disadvantages of the straight through design are the limited field of view and the increased length. The advantages are that it does permit polarization measurements, it has a simpler beamshifting mechanism to select which instrument is to be used, and it can fit into the LEM adapter. The disadvantages of the folded telescope are that polarization measure- ments, which are a very important part of the scientific experiments, can- not feasibly be done, the design does not permit a simple chromatic cor- rection such as is done with the Mangin mirror, and it requires more com- plicated and heavy mechanism to accomplish the task of switching the beam from instrument to instrument. The advantages are that it will fit into the smaller command module, and it may have a somewhat greater undis- torted field of view. It is concluded, since the scientific value of the straight through design is so much greater, that this design be used. I I I I I I I I I I I I I I t l o SECTION 3 CONCEPTUAL DESIGN AND ANALYSIS OF THE CORONAGRAPH SUPER- STR UCT UR E Depending on the relative positions of the optical elements, as dic- tated by the requirements and nature of the experiments to be performed, two conceptual designs of a super-structure are presented. The aim of such a s,aper-structure is to house the optical elements, and to ensure that their relative positions are fixed and constant at all times; taking into account the following two factors: l. The integrity of the super-structure should be such that it can resist any external load imposed on it here on earth or at launch with negligible resulting deformations. This could be achieved by elesig_aing a super-structure such that the maximum applied load be well within the elastic limit of the material used in design. _f the elastic limit of the material used varies with temperature, then the design should be for the minimum ex- pected lil_.it The material used in design should be vlsco-elastic in nature and capable of rapidly damping any free or forced vibrations of small magnitudes (similar to those expected to result from astronaut's movement, motors, servo-mechanisms, voice, etc) that will be imposed on the coronagraph during its operation. 3. 1 MAIN FEATURES OF THE SUPER-STRUCTURE Two conceptual designs have been defined to house the optical elements: ]. A reinforced spiral column fixed between two or more end and in_err/nediate p!ates (see drawing Figure 4). The plates could be a honeycomb or sandwich type which offers a high flexural stiffness to weight ratio; while the spiral could be made of hollow tubing ! I " Q _ _ii ] i _'_r -q. 0 ! t I • I I I I ! I . A statically determinate truss-column fixed between two or more end or intermediate sandwich plates (see drawing Figure 5 for the unfolded design and Figure 6 for the folded design). Because of the accuracy of alignment involved, the dimensions of the different members of the coronagraph should be within ± 0.02"_- 3.2 CHARACTERISTICS OF THE CORONAGRAPH The following characteristic features of the unfolded and folded coronagraph designs are summarized hereunder (see Tables 1 and 2). B. Z. 1 Unfolded Coronagraph Design ! -I ! Overall Dimensions - 762 cm long x 80 cm wide x 60 cm breadth. Weight - Approximate total weight including optical elements and cameras = 440 lb. Location of Center of Gravity - The center of gravity- has been evaluated using the center of the mirror D as a reference point (see Table 1 and Figures 1 and 7). I Z X Z [ Mirror D I • Center of Gravity Figure 7 Location of Center of Gravity - Unfolded Design !ri. _ 21 ! ii -:! i\1:\ i u. E '4:) , C._ 7 I *1 I-,r fl l-4 Z ..J r_ _.i_Z i ! f4 ;> Z :Ilpl" 7 c.L, C 0 "-0 L 3-42 I Figure 5(bl LEM Adapter - Garmagraph Comparison 3 -4b II I I I \ \ \ I !i t 1 I i I I I ! 1 I I i i i I I i I i I i i o,_l _tn_ _+ _..o 7 g Y _o OF" /"l/P,_o_¢ C. _. Coo_i F,1 Polished Metals - body tends to be hot 3-10 ;i, I. I I I I I I I I I I I I I I I At approximately the earth's radius from the sun, surfaces with _/E = 0. 2 tend to equilibrate (adiabatic plate) near 230°K; surfaces with e/e= 1 equilibrate near 313°K; surfaces with _/E = 3.5 tend *_ eq ....... te nea_ 500°K. Uniformity of temperature distribution depends on the substrate Materials which may be useful for thermal control of the coronagraph'._ .......... super-structure may be _,_-_-,m .........y g,_ v_,_,,_,.,a as to exterior use or interior use The exterior materials can probably be limited to: a. Inorganic white paint, a/ £ = O. Z b. Second surface Vycor - Silver mirrors, c. Inorganic Black paint, ale = 1" o, 0.06 E- 0.81 - 0.075 do eo Aluminum flake pigmented paint, ¢t/¢ : 1 Evaporated g01d surfaces, erIE : 10 The interior surfaces can be limited almost entirely to: a. Inorganic Black paint b. Evaporated gold surfaces If the inner shield operates under cold conditions during part of the mission, electrical healing elements will be mounted to the inner shield to raise its temperature to the equilibrium level of approximately room temperature (75°F). Thermostat controlled heaters could maintain this temperature to within ± 5°F. The objective lens cover wilt be kept closed except during specific exposure periods to avoid major changes in the thermal balance Within the structure in order to ensure that no warping of the super-structure takes place due to an uneven thermal stress distribution, !!it¸ 3-11 ! ! ! ! i 3.5 LOCATION OF GIMBALLING SUPPORT ON SUPER-STRUCTURE The best theoretical position to place the gimballing mechanism and support the coronagraph to a platform is at its Center of Gravity, or as I close to it as possible. However, since the coronagraph will have three axes of stabilization (roll, pitch and yaw), plus constant pointing towards i the sun; then two possibilities exist. I. If the roll of the spacecraft is controlled to within ±3°/hr {which i is required to attain the accuracy of the line image on the photo-graphic film) then only two xes of stabilization {pitch and yaw) will be required for the coronagraph. This will be attained by gimballed supports from the platform to the super-structure I placed in a plane containing the center of gravity of the corona- graph as shown below. See Figure 9. ! I / Platform ! iimballed Supports I for Pitch Stabilization I Gimballed Supportsfor Yaw Stabilization Figure 9 Pitch and Yaw Gimballing o If the roll of the spacecraft cannot be controlled to within ± 30/hr. then stabilization for the three axes of the coronagraph will be required. Since the roll stabilization desired is a pure rotation of the coronagraph about the optic axes AD, then two alternatives exist ! ! 3-12 I I I I I I I I I I I l I I a. The gimballing system could be attached to the back plate at the center of Mirror D (as shown in Figure 10). This has the disadvantage of cantilevering the super-structure which makes it very susceptible to vibrations Center of Objective Lens A Optical Axis Center of Mirror D !_ Roll GirnbalI ?! Platform Gimbal Do Figure 10 Roll, Pitch and Yaw Gimballing The gimballing systems and supports for pitch and yaw could be placed on a plane containing the Center of Gravity of the system, however it has to be supported on an eccentric roller support which will permit a roll stabilization about the optic axis AD, (see Figure 11). This arrangement has the disadvantage of requiring more space and equipment 3-13 Gimballed Supports for Yaw Stabilization Center of Optical Axis Center of Gravity Gimballed Supports for Pitch Stabilization Roller Supports for Roll Stabilization I I 3.6 Figure 11 Roll, Pitch and Yaw Gimballing CAMERA LOCATIONS ON SUPER-STRUCTURE I I It is highly desirable to have all the film - cameras located at or near the super-structure so that they can be easily accessible to the astronauts. However, if such an arrangement cannot be fulfilled, then openings shall be provided on the super-structure through which the Apollo crew can change or recover experiment films. I I I 3.7 LOCATION OF CORONAGRAPH ON THE APOLLO SPACECRAFT From the overall dimensions of the super-structures of both the folded (Figure 5) and the unfolded (Figure 3) designs, the only feasible position which will allow enough space to accommodate the coronagraph will be the LEM adapter (see Figure 12). I i!i!| 3-14 I I I I I I I I I 336 '_ i17" t r'- i- I I i 260" ,., LEM Adaptor Coronagraph Unfolded De sign) Scale: I" = 75" Figure 12 Possible Location of Coronagraph 3-15 Im • I I I I I I I I I I I I I I I I 3.8 ASTRONAUT SUPPORT A. Restrict their movements to an absolute minimum during the experiments since such movements will be the biggest source of vibrations. B. Restrict any liquid waste disposal during each experiment. C. Change or recover experiment films, 3-16 mI. I I I I I I I I I I I I I I I SECTION 4 CORONAGRAPH CONTROL ANALYSIS Based upon astronomical objectives of coronagraph appiication, various coronagraph control techniques have been evaluated. The objec- tives have been to specify the prime space vehicle constraints, and to recommend candidate control concepts which satisfy the coronagraph oper- ational requirements. The results are, by necessity, conceptual in nature and reflect design parameters which candidate control concepts must meet 4. 1 PROBLEM DEFINITION In order that the control analysis may achieve the overall objectives of assessing the feasibility of an orbiting coronagraph, the general problem is constrained. The guide Iines and necessary assumptions are outlined below. The control requirements are defined and outlined and are based upon operational modes of the orbiting coronagraph. The operational modes, or the manner in which the coronagraph is used, are considered on the basis of achieving desirable astronomical objectives. 4. 1 1 Mission Defimtxon 4_ i. 1. 1 Orbit 1. Circular e = 0.0 Altitude 200 n. mi. to synchronous 19,368 n. mi 500 n. mi. (OSO, OAO) 3. Occultations - minimu.m 4. inclination 33 ° (OSO), 32 ° (OAO) 5. Flight Duration 1 to 1 1/2 years I I. I I I I I I I I I I I I I I I I 4. I. I. 2 Vehicle I. Prime: Manned, Apollo, CSM/LEM Lab. 2. Secondary: deployed satellite tethered to prime vehicle. 4, I. 1. 3" Astronomical Operational Modes I. Inner Corona Observation a. Direct solar line pointing b. Duration of data taking: 5 rain to 2 hrs. c. Duty Cycle: I/Orbital Revolution to Continuous d. . e. Accuracy: 0. 3 arc sec Minimum Resolving Power Drift Accuracy: 1.22k = = 0. 35 arc sec D I0 in.D k = 0 35_ Exposure Time 0. 1 Outer Corona Observation a. Off Axis Solar Pointing: b° C. d. e. Duration of Data Tanking: Duty Cycle: Same sec, 3 deg/hr. Continual(0. 3 sec)to 5 Solar Radii {1. 3 deg}, roll axis control to 5 deg/hr. Same Accuracy: 2%, Camera FOV (5 arc mini = 6 arc sec Drift Accuracy: 3 deg/hr 4-2 I I I I I I I I I I I I I I I I I i' 4. 1. 1.4 Operational Control Modes 1. Fine Solar Axis Tracking 2. Fine Off Axis Solar Pointing 3. Fine Off Axis Solar Pointing: Roll Control 4. 1.2 Attitude Control and Stabilization Operational Modes The specific nature and components of an attitude control and stabilization system for an orbiting vehicle with an on-board pointing requirement are greatly affected by the technique of mounting the pointing instrument to the spacecraft: Gimballed mountings as opposed to body fixed methods alter the final form of the system. However, irrespective of the mounting technique, there are a class of several operational man- euvers of **" _,,e spacecraft/pointing instrument which are required of the vehicle control system. The general operational maneuvers are: I. Initial Orientation and Stabilization Prior to a mission mode operation such as solar observation, the spacecraft must acquire a gross reference and eliminate tumbling. Whether the orientation and stabilization is initiated following launch or simply after free flight, such as might occur during solar occultation, the control and instrumentation mech- anization is the same. 2. Observable Acquisition Once the gross initial reference has been acquired, the pointing instrument must be slewed to acquire the desired reference observable. The similarities between the observable acquisi- tion and initial orientation mode are great but dependent upon the basic technique of attitude reference. The observable ac- quisition mode requires pointing command of the pointing instrument relative to the gross reference. The pointing com- mands are dictated by the fechniqhe of search pattern and de- I_endent upon the field of view0f the acquisitibn sensor and errors in the initial reference. 4-3 m • I I I I I I I I I I I I I I I | . Coarse Tracking : Acquisition of the selected observable enables coarse tracking to be initiated. If the pointing instrument is g!mballed, tracking is maintained by torquing the iaastrument in response to the coarse tracker error signal, relative to the gimbal_tvehicle mounts or effectively tt_e basic attitude reference Of the vehicle. In the r_th_r instance when the pointing instrument is,body fixed or when gimbal angular freedom is restricted, coarse track is maintained by torquing the spacecraft to eliminate the sensed error signal. However, the problem is one of tracking and not pointing. . Fine Tracking When the steady-state limit cycle .of the coarse track mode is achieved, the fine tracking mode is initiated. Fine tracking operational modes are identical to coarse tracking technique, but depending upon overall system requirements, fine actuating techniques and sophisticated error sensing is needed. Roll stabilization about the observable LOS is necessary. 5 Fine Pointing - Off Axis Tracking Off axis tracking with the accuracy of the fine tracking mode requires torquing the pQinting axis relative to a highly accurate previously established reference attitude. Roll stabilization is provided by orthogonal celestial lines of reference. 4. 1, 3 Coronagraph Mounting Modes There are basically two alternate techniques for mounting the coronagraph to the vehicle structure: t. Body Fixed Z. Gimballed The feasibility of the body fixed technique depends upon structural rigidity and the ability of the spacecraft control system to null out disturbance torques due to man motion and the physical environment with the major constraint of high inertia and required fine tracking. 4-4 fa. I I II i i I I i i I i II II I I I Elimination of large disturbance torques due to man motion is possible in a tethered body fixed deployed open loop configuration, but disturbance torques are transmitted through the flexible tether. Also translational thrusting capabilities are necessary to periodically eliminate relative vehicle drifting due to orbital separation. Gimballing the telescope to the spacecraft, which has relatively high inertia as compared to the telescope, depends upon the maintenance of fine track with sufficient servo response of the massive gimballed tele- scope inertia. Since the problem is one of fine tracking, gimbal alignment errors are not important. However, off-axis satellite pointing accuracies greater than 13 arc sec represent a significant technical problem. The coronagraph operation requires that high tracking accuracies be maintained. Since the experiment requires on-board manned support, and since the basic class of vehicle is of the Apollo class, various modes of control defined above do not require evaluation. For instance, the assumption can be made that the initial orientation and stabilization con- trol mode is performed by the existing on-board attitude control system. Also since acquisition can be manually aided, regardless of structural implementation, the study concentrates on the requirements of the coarse track, fine track, and point while fine track modes. The options on mounting the coronagraph to the vehicle are sum- marized in Table 3, together with required operational modes. TABLE 3 MOUNTING MODES Operational Control Modes Solar Axis Tracking Off Solar Axis Pointing "(Command R011 Mode) Vehicle Type Manned Apollo Tethered Satellite Mounting T e chnique s Body Fixed Gimballed 4-5 II I :| I I I I I I I I I I I The body fixed system, when in the tracking mode, requires the '_ehicle actuation system to cancel disturbance torques. The signal for corrective torquing is received from the tracking error sensor which indicates an off null correction is required- The principal difficulty in this configuration is, slewing the mass of the entire vehicle and telescope with the required speed of response so tha2 the null detector error signal remains within the allowable pointing angle limit cycle amplitude region. The off-axis pointing mode complicates the body fixed control mode. To point at a prescribed angular orientation from the sun line is an effective open loop maneuver. Therefore an attitude reference is required to provide angular orientation about the sun line of sight. The angular orientation reference can be accomplished through inertial techniques, celestial orientation sightings of one star to fix the locus of the pointing, and two stars to fix the line of the commanded LOS, or a single star tracking instrument providing off axis pointing capabilities. The pointing and tracking commands are implemented by the vehicle actu- ation system and in the presence of large disturbing torques, high inertia. The gimbal mounted technique has an advantage for solar tracking when the moments of inertia of the vehicle are high with respect to the coronagraph inertia. The gimbal degree of freedom requirements depend upon the accuracy of the vehicle attitude control system. During tracking, the vehicle attitude control system should remain in a fixed inertial system so the gimbat motion required would be small perturbation compensation. Again, ratios of coronagraph to spacecraft inertia are important due to reaction torque coupling. The gimbal technique is advantageous during off axis pointing since a point command may be given through girnbal angular orientation. But machining, gimbal alignment, and pointing angle transducers represent a problem due to relatively large errors which induce unsatisfactory pointing errors on this open loop maneuver. Therefore the requirement again is for an off axis solar tracker. The problems which arise for the prime manned vehicle are identi- cal to those of the tethered satellite. However, disturbance torques are minimized. 4-6 ! i 4 I I ! i I ! I I I I I I i i ! I 4. 1.4 Mission Analysis 4. 1.4. 1 Circular Orbit Characteristics Figure 13 shows typical circular orbit characteristics. Plotted against altitude h c are the variables spacecraft tangential velocity Vc0 angular velocity _, and orbital period T. It is important to note that low altitude orbits have orbital periods from 1. 5 to Z. i hours which implies a relatively high frequency of solar occultation. 4. 1.4.2 Commanded Pointing Angle Inputs Regardless of telescope mounting techniques, the fine track mode requires alignment of the coronagraph boresight axis to the solar line of sight LOS in the presence of spacecraft S/C orbital motion and solar motion on the celestial sphere. Figure 14, shows azimuth and elevation tracking angle commands as a function of time for parametric values of S/C orbit (inclination i c, solar LOS, orbit line of nodes angle _c, altitude hc). The tracking commands are measured relative to inertial alignment upon fine track acquisition. The major tracking component is seen to be due to solar drift with sinusoidal variations imposed upon the solar drift component. Note also, although the tracking angles are extremely small, the magnitude is twice the order of magnitude of the pointing accuracies. Figure 15 shows typical orbital component pointing angle amplitudes. 4. 1.4. 30cculation Conditions The total available time for data gathering is a direct function of ratio of orbital time spent in occultation to orbital period. Figure 16 shows the proportion of time spent in the earth eclipse in the form of a normalization factor, o-/&_ max. where er = S/C angular subtense from earth/sun LOS Ay = maximum o- 4-7 I I I I I I I I I I I I I I I I I C_ Ii JO 0 0 0 0 £OT xaq/T w-u- A 0 i 0 °r.a 0 re_ 0 'CD CM 0 '0 ,=,=# 4-8 r_ r-* 0 .,=_ m • i I I ! I I I I I I I I I I I I E E 0 0 .1.4 u_ k,z_ 0 0 "4 0 _U E 0 0 t; o_,_ o© o 0 0 0 0 E 0 0 glmm mmm m i n u in m m _m am imm m <21 o o r_ v _'_'@m NIl i lm m ..-, E c_ o L_ E c) o I! L) C 0 o o II ¢o _ II It 0 0 0 0 0 0 _s _- _i_u' V _u._._& 0 0 N "-4 o ! -!_ ,,2 0 0 4-9 ,C c_ e-' U 24 < e- 2_ L I • ! I Z6 Orbital Component Fixed Sun 25- Pointing Angle. Rate. I z4 m i o _-22- I '_ 21 o ! I I ! i i 200 400 600 800 1000 ! ! Maximum Pointing Angle Variation Plane of Ecliptic ! ! -Altitude-n. mi ! ! 16 0 i i 1 I I 200 400 600 800 1000 h - .altitude - n. mi C Figure ! 5 Maximum Pointing Angle Amplitude J 4-I0 II I I 0 0 0o o_..o o_ _ "_',.o "\ ll' Oo ill o,:o o°'-" __ ' / , i! ! I I I I I II / _!q=o DDO 0 0 0 ,,==II o c:3 0c °_,=i E L o ,D "0 :C, o 0 0 o mle_ 6 4-II '"T C. © mI I I I I I I I I I I I I I I I Whenever ¢< 8.8 arc sec and the spacecraft is on the earth's far side, relative to the sun, occultation occurs. Figure 16 shows the proportion of orbital time with respect to orbital period spent in the eclipsed condition.. Nominal values are 30 to 40%. Certainly for low altitude orbits, the efficiency of the data gathering duty cycle and continuous tracking are severely limited. 4. I- 4.4 Drifting Between Two Bodies in Rendezvous The technique of utilizing a deployed satellite as an external occulter for the purpose of shading the objective lens in the off axis pointing mode requires alignment accuracies typical of the 0.3 arc sec requirement. Two major problem areas in the use of the external occulter are station keeping and propulsion requirements to eliminate orbital drift, Figure 17 shows the orbital drift conditions between two bodies in rendezvous. The normalized drift ratio 5/_ is effectively the perturbation in the occulter satellite relative to the spacecraft celestial observable LOS, or in the event a tethered satellite is considered, the relative drift between the prime ve- hicle and the deployed vehicle. The drift rate at proposed low altitudes are intolerably, large. At synchronous altitudes, conditions of drift rates appear favorable. 4. 1 5 Disturbance Torques The principal disturbance torques considered are earth gravity gradient, earth magnetic field, solar pressure, rotational and translatory man motion, and coronagraph reaction torques. The magnitude of these disturbance torques must be cancelled by the vehicle reaction control system within the dynamical response time required to maintain fine solar tracking. The following data indicates maximum or average magnitude levels and does not reflect transient responses. 4. 1. 6. 1 Gravity Gradient given by The average gravity gradient disturbance torque magnitude is T = 3/4¢0 g{I - I ) sin Z0 ave : x y (14) 4-12 I m I I I I l I I I I I I I II ! ! II I. 24 I. 1. 1. S/A 1.0_ 1, 1,0 0 h = C n. mi ...,---- ......G= I, 0° o=0. I° 4 6 Figure 17 Two Body Short Term Orbit Stability -- = Drift Ratio A A = Initial Separation, ft in orbital plane along observable LOS 6 = Drift, ft Ang_Jar Misalignment = o ,_(t)=[ l, _][s','. 31aeg 8 10 t - rain 19,368 n. ml Synchronous L IZ 14 16 Relative Drift 4-13 18 I. I I I I I I I I I I I I I I I I _i_!ii_ii• i where _0 = S/C inertial angular velocity I = S/C longitudinal (roll) moment of inertia X I = SIC lateral (pitch) moment of inertial Y e = inclination of S/C longitudinal axis to the orbital plane Figure 18 shows the maximum magnitude gravity gradient torque. 4. 1. 5. 2 Earth Magnetic Field The maximum value of the magnetic field induced torque as a function of altitude is shown in Figure 19. The parametric conditions are normalized coil data on-board the S/C. 4. 1. 5. 3 Solar Pressure Disturbance Torques Figure P-0 shows maximum average solar pressure disturbance torques as a function of cg-cp moment arm and effective resultant area. 4. 1. 5.4 Internal Disturbance Principal internal disturbance torques arise from man motion and coronagraph motion inertial reaction torques. Generally internal disturb- ance torques not only act as forcing function to the vehicle attitude equations of motion, but also effect the coefficients of the differential equations des- scribing the vehicle attitude motion. However, this analysis is constrained to simplification and considers only the forcing function aspect. The linear time variable parameter approach is not taken. where For translatorg motion relative to the vehicle c. g., T = -m(1 - _1 (ro + r (t)) x a (t) m = element mass (151 M = system mass 4-14 4 I i I • I I 2 3 I x o_ ! I -_ ! ! I , 1 I I I o ,.,-I i i l I i I I ,--I OT xq'[ - _._ -£- I I I I I I ,--,4 o 0 0 6 i:I 4-15 c@ 0 ,,-4 0 < _F m m. " m m m 14_ _====== 10 d_nne -cm ! IZ ! m _ 10 , _ Condition s :m _ s[.. r rq_ey_,_p t_rn _ on. ootg 6 E .m...O. O01 oz-in l ! [-- -_ _o°-_ m o i i I m i i I IZOO 400 600 800 lO00 ZZ00 1400 1600 i i i Figure 19 Altitude - h c Typical Magnetic Field Disturbance Torques 4-16 4-17 0 N I I I I I I l I I I I l I I I I I r = initial position vector o r(t) = position vector perturbation a{t} = element acceleration For rotational motion relative to the vehicle c. g. , wh e r e x _x Iy_D -" "-"T = I t_ D + (_xI } (16}x x element longitudinal moment of inertia I Y '_D = element diametral moment of inertia = element angular velocity w = S/C inertial angular velocity The man model is based upon the 90 percentile man. Figure 21 shows the geometric composition. Table 4 shows the physical character, istics. Figures 22 and 23 show typical maximum disturbance torques due to translational and rotational motion respectively. Torso Head f r = 0.42ft H = 2.62ft 7- H A_ Arm/Leg Figure 21 90 Percentile Man 4-18 I i __,2,,_ors_ 1 .... I Lo 0 iO. 0 Lig I ° l!i i ,.o o. / _i,.ol 1.-_fl , _.o I _o._,oi_o;;:,o.,o_ o., , , I /_f o._1_.,, l>s 0. I O.Z 0.3 0.4 0.5 O. Ol 0. I 0.2. 0.3 0,4 0.5 2 Element Acceleration - ft/sec Figure 22 Disturbance Torques, Translatory Man Motion 4-19 l. I I X x -m 6.0 301 I _ __/'_OOn. m_gz.o ...... ic _ ...... m C 00 . . '0 O. 0.2 0.3 0.4 0 I 0.2 0 3 ,; Rotational Rate - Rad/sec m ,-o I m 3O ! m -_ 2o) ! I ° I0 o m _D ! o I2 Arm 8 h c = 200 j _'" 5oo n. mi 0 0.1 0.2 0.3 0.4 0 0 Rotational Rate - Rad Head _,_ h c= 200 JJt 0.1 0.2 0,3 0.4 sec m m Figure 23 Disturbance Torques, Rotational Man Motion 4-20 I I I I TABLE 4 MAN MODEL I I I I I I I I I I I I I Element Head Torso Leg Arm .Volume x slug-ft 2 O. 026 1.04 O. 018 O. 013 I Y Z slug-ft O. 024 1.15 O. 053 O. 040 Mass Slug s O. 227 5.72 0.0881 O. 0881 Total Mass 0.3 2.9 "0. 8 "0.6 0.3 Total Mass 6.0 slugs = 19Z _ The inertial reaction disturbanc'e torques arising from coronagraph angular velocity in the fine track mod.e are shown in Figure 24. 4. 1. 5. 5 Disturbance Torque Summary A summary of the disturbance torques acting on the coronagraph space platform is shown in Table 5. TABLE 5 DISTURBANCE TORQUE SUMMARY Disturbance Torque Gravity Magnetic Field Solar Pressure Man Motion Coronagraph Reaction Magnitude - Slug- ft Vehicle I Manned Apollo Tethered Satellite O. 002 -5 10 10 -5 0. i -7 5x I0 O. 0002. -6 10 -7 10 -8 5x I0 4-21 i • I I L0 -5 .... Fine Track Mode I I | 10 I , ! I I I I I I I I I0 0 E-, n 10"8 10 lill A: S/C Limit Cycle B: SIC Inertial Angular Velocity I I I II 1 I I III 10 100 1000 Coronagraph Moment of Inertia ly - Slug - ft 2 I illl 10 4 Figure 24 Disturbance Torques, Coronagraph Reaction Torque 4-22 ! ! i From an analysis of the disturbance torque curves and the summary i table, the conclusion is made that man motion exceeds the perturbation in- duced by other disturbances. Hence man-motion should be restricted to a high degree. i 4. 1. 6 Prime Vehicle Characteristics; Physical and Control I The following data may be considered ordy as nominal operating point data for the Apollo CSM and LEM Adapter vehicle. The dimensions (inches) and weights of the S/C are shown in Figure 25, and are estimated I to provide data inputs to the control anal_ sis. 32 ___ .__._ 13 5__ i 1Z 1 -----_ _--' / _ 336 ._Z Weights - lb Full Fuel DryI I I i I CM 9000 3000 6000 SM 52000 32000 20000 CG Location: Within 5 in. diameter of longitudinal axis. Figure 25 CSM/LEM Adapter Calculated (through geometrical approximations) moment of inertias of the CSM are approxlmately X I = Y Z 15. 2 x 103 slug ft 2 20. 1 x 103 slug ft 2 21. 2 x 103 slug ft 2 Attitude control sensor dead bands p, y = ±0. I° r = ±0.5 ° i 4-Z3 I I I I I i I I I I I I I I I I I 4. 1.7 Instrumentation State of the Art (SOA) As = r,_lude tr_......................._fining _nr_ag_aph e_ntrnl rnncepts, a SOA as- sessment is made examining current instrumentation technologies. Ta- ble 6 is a matrix of attitude sensor capabilities, and Table 7 is a presen- tation of SOA actuation techniques. TA BLE 6 i Com pone nt Sun Tracker IR Earth Tracker OAO Star Tracker Star Tracker AOT Telescope Displacement Gyros Rate Gyros FINE TRACK ATTITUDE SENSORS 3 o- SOA Accuracy ¢J 1 arc sec360 arc sec 0. I arc sec Z arc sec 10 arc sec 0. 001°/hr 0. O05°/sec 4. Z CONTROL CONCEPTS The matrix of concepts for coronagraph control is shown in Table 8. The deplo_nent modes considered are the Apollo body fixed and gimballed modes, and body fixed tethered satellite. The high accuracy null detector in each concept is a 0. 1 arc sec accuracy sun tracker with off-axis point- ing capabilities to 2 ° . The sensor pickoffs determine azimuth and eleva- tion relative to a defined plane. The plane definition is provided by an orthogonally mounted star tracker (or trackers, dependent upon stellar density of celestial sphere locus). Single axis flow diagrams for each of the preliminary concepts are shown in Figures 26 to Z8. As an aid to occulter control, the off axis adapter of the sun tracker could provide reference data for occulter positioning. ;. 4-24 lB. ! l l I I I I I ! I I I I I I I I l'-- < T 1 I ! I:: O_ Q i.._l U I_l! _r M _u ° 1_ g iF il I J t Ot -_ z C [-., X 0 0 0 0 0 0 0 0 0 _g 0 < I 0 0 ,,D ! 0 0 U C m C 0 X = 0 II 0 I_1 ,, E _0°_._,_ 6 il In II n_ _c; c_d 0 0 0 0 0 0 aO0_ C) 0 X xO 0 00 00 ooo 0 0 o6 0 0 0 0 0 0 v,a ._a 0 C_ 0 0 A A A v I 0 _ _--_ 0 0 0 C) 0 0 0 un 0 aO aO < 0 < < _J 0 < 0 < 0 < 0 a_ E _ o _ _-_. 0 0 .2 0 0 0 0 • 0 0 0 6 0 0 0 6 0 .2 0 0 0 0 0 0 0 U ..2 = _ Et,) ._ o _0._. I o 0 o ii (I,} m -_ _ o._° _ _0 0 0 o o < 0 4-25 I I I I I I I I I I I I l I I I TABLE 8 CORONAGRAPH CONTROL CONCEPTS Deployment Mode i • ..... _-'_ _ IAt_ __ ZVIu unL _L*/_ ,v_ ud _ Stabilized Reference r-roll p-pitch (elev) y-yaw (am) Acquisition (and Coarse Track) P rinc ipal A ctuation T e c hnique s Coarse Actuator Fine Actuator Fine Null Sensor Position Rate Roll Reference Off Axis Reference Apollo CSM/LEM Lab Body Fixed Gim_ba!led Inertial-r Solar LOS-p, y Wide Angle Sun Sensor (Man- ua11y Aided) Reaction Jets (BipropeIlant) Celestial- r Solar LOS-p, y Wide Angle Sun Sensor (Man- ua11y Aided) Reaction Jets (Bipropellant) Reaction Jets Microrockets Sun Tracker (Az, Elev) Rate Gyro Displacement Gyro Sun Tracker Reaction Jets Servo Motor (Hydraulic, DC Sun Tracker (A z, Elev) Signal Pro- cessing Star Tracker Sun Tracker Fethered Satellite Body Fixed CelestiaI- r Solar LOS-p, y Wide Angle Sun Sensor (Man- ually Aided) Mic rorockets (Monopropellant) (Momentum Dump- ing) Coarse Inertia Wheels Fine Inertia Wheels Sun Tracker (Az, Elev) Signal Pro- ce s sing Star Tracker Sun Tracker I 4 -26 ! o ! ! I oT Eli ! ! ! ! _J C | _ ! 0 cz>_J !ili¸ i! L) cO 0 ,,-.t 0 .< "0 _J P, 0 t "0 0 o,,,_ o1,,4 4-27 I • ' il u i o_ I ._L u I ._ 0 0 _ I._ i al 12t _I ! ° I _o I Y 4) U •_ 0 ! U 0 0 0 I 0 t) ._,,4 r_ t.I 4-28 I • I. I I l I I I I I l I I I l I I oI . _,i_1 I/ Iv *_,,d & I ID *el E < ] u .__; ID ..-,4 u o,..1 _ m _¢=ol _ ... PI _ _ t._! ,%)[...., x :>., 0 4-Z9 t ,J u 1- *,-4 I I I I I. I I I I I I I I I I I I I 4. 3 FINE ANGLE ERROR SENSOR The fine angle error sensor listed in Table 8 is the source of the error signal whic_ is used to actuate the coronagraph control system in the fine track mode. The error sensor (also depicted in Figures 26, 27, andZ8, ) performs three functions in the conceptual design. The tracker: i. Indicates solar LOS to a 3 _ null accuracy of crG = 0. I arc sec i. Generates elevation and azimuth (pitch, yaw) error signals o Provides off axis tracking capabilities out to 5 solar radii (I. 50). The error sensor, during the Fine Solar Track Mode must be parallel to the telescope optical boresight. In the Fine Track Off Axis Pointing Mode the relative angle between the error sensor and the coronagraph bore- sight axis must be maintained at a prescribed azimuth and elevation. The two techniques for off axis pointing are: Orientation of solar LOS by precisely driving two prisms mounted internally to the fine angle error sensor. The sensor is body fixed to the coronagraph structure Driving the error sensor boresight axis, which is mounted in a pitch and yaw gimbal configuration, relative to the coronagraph structure. Both techniques are open loop pointing procedures. The 3or null errors between the error sensor boresight axis and the coronagraph op- ticai axis are: In the former case (1.), [ Z 2 2 ] 1/2= _ + _ + _ (17) e a p a 4-30 I I I I I I I I I I I I I I I I I i where a = 3¢ pointing error e _r = 3tr instrument null error cr = 30- prism orientation error P 0- = 3_ structural alignment error a Typical state-of-the-art values are: ff = I arc sec d_ cr = 1 arc sec P = O. l arc sec a ° .o- = 1.4 arc sec e In the latter case, [¢ 2 2 2 2 icr = +_ +_ +0- (18) _" ¢_ g a t where then g (y -- t gimbat alignment error ~ 10 arc sec pointing angle transducer error _ 2 arc sec = 10. 2 arc sec. e Thus the conceptual error sensor design will be boresighted to the telescope optical axis with internal off axis pointing capabilities. The fine track position error sensor which is utilized in each of the three modes of coronagraph deployment is a high accuracy wide angle sun sensor. The sensor has the two axis capability for generating off null sig- nals from the solar LOS in both azimuth and elevation (yaw and pitch). Ad- ditionally, the wide angle field of view (FOV), in conjunction with the two 4-31 I I I I I I I I I I ! ! precisely driven psisms allow off axis tracking to any commanded azimuth and elevation within the initial LOS/FOV. The sun tracker in all instances of c_perational modes is mounted body fixed, parallel to the telescope bore- sight axis. The FOV required to point off axis to 5 solar radii from the sun LOS is approximately I. 5°, which is the FOV required of the instrument. The Eclipse-Pioneer Division of the Bendix Corporation designed and developed a fine sun sensor with an off axis adapter which provides the op- erational off axis performance capability desired. The sun sensor was de- slgned for space applications and has a quoted null accuracy of 1 arc sec 30-. However, the 30- null accuracy required for coronagraph operation is 0. 3 arc sec. Generally, the state-of-the-art null accuracy for field opera- tional sun sensors is 5 arc sec 3_. In a tightly controlled laboratory environ- naent, the representative null accuracies which can be achieved in off-the- shelf hardware is about 1 arc sec 3_. Thus, the requirement for the null sensor is not met by state-of-the-art hardware and the equipment must be developed for the coronagraph experiment utilization. Application of other sensors also fail. TV systems for the null sensor are restricted also since the accuracy requirement is not met. For ex- ample: a 1. 5 ° FOV, Z000 line system has a 30- null accuracy of 2.7 arc sec. To provide the specified accuracy and FOV constraints, the following parameters are typical of the optical systena of the prescribed error sensor: 2. r. -l 1 0- = tan (19) a f -1 ¢r = Z tan I. ZZ k < -- i rf rf q- (z0) (21) 4'32 I i I I I I I I l I I I I I I I I I I where Now then so if = 3_ instrument accuracy of rf = colIector radius r. = image radius I of = FOV k = wavelength f = focal length 0- Of Of = O. I arc sec = 1. 5 deg f = 7. 65 rf -4 r = 0. I86 x 10 1 -3 rf > 0. 43 x 10 rf rf = 1 in. f = 7. 65 in. 4-33 l l I I I I I i I I I I l l I I I I 4. 4 ROLL STABILIZATION Roll stabilization requirements arise due to excessive spacecraft limit cycle drift rates, or the scientific experiment astronomical ob- jectives. In the former case, vehicle or coronagraph drift rates greater than 3 deg/hr exceed the control requirement specified by film exposure time while in the near corona fine track mode. To satisfy the experiment requirement, the coronagraph optical axis is rolled so that the grating spectrograph slits are properly oriented to the solar or coronal feature under observation. Thus the two specifications arise, maintenance of roll rates < 3 deg/hr, (5 deg/hr off axis) and roll orientation. 4.4. I Body Fixed, Apollo S/C The minimum limit cycle conditions to be expected of the Apollo class of vehicles is nominally a 3.6 deg/hr drift rate, in three axes, and null accuracies of ±0. 1° in pi/;ch and roll and ±0. 3 ° in yaw. However, if the reaction control system RGS thrusters are supplemented with fine actuation monopropellant microrockets {a typical dynamic response ex- ample is shown in Section 4. 5. 1) the roll stabilization requirement can be met with state-of-the-art inertial technology. 4. 4.2 Gimballed, Apollo S/C; Body Fixed, Tethered Satellite Roll stabilization of the coronagraph optical axis, and the celestial reference to define a specified roll orientation can be provided by star tracker references. Two degree of freedom, pitch and yaw (azimuth and elevation) gimballed star trackers mounted orthogonally to the telescope structures will supply the third inertial reference. Typical state-of-the-art star tracker accuracies are listed in Table 6 and represent performance in ideal environments. However, star trackers such as OAO back up trackers are highly suited for the coronagraph application. Representative tracker accuracies are: ty = 20 arc sec = 13 arc sec g = 2_ arc sec t Or = l arc sec a .'.ff = 24 arc sec. e 4-34 l I I I I I I l I I I I I I I I I I The OAO trackers FOV is I °.* Since the earth subtends approxi- mately 120 ° to 140 ° at the low earth orbits, three star trackers required to ensure the orthogonal reference. Upon initial acquisition and commencing of celestial tracking, roll commands'must be generated. The rod loop is typical of the loop in Fig- ure 27 with the star tracker and computer coordinate transformations re- placing the fine error detector. The coordinate transformation required are star tracker space (p,y) to star tracker mounting space, to coronagraph body fixed space to the vehicle space. Typical dynamic limit cycles are shown in Figure 30 and 31 of Section 4. 5. 4. 5 CANDIDATE CONTROL CONCEPT PERFORMANCE ANALYSIS The analytical control block diagrams for each of the candidate con- trol modes are shown in Figures Z9 (a), (b) and (c). Figure 29 (a) is the single axis bang bang control loop of the body mounted Apollo SIC con- figuration. Figure Z9 (b) is the gimballed mounting mode of the S/C con- figuration, and Figure 29 (c) is the fine reaction wheel control loop of the tethered satellite. The control loops in Figure 29 are the simplified analytical representation of the conceptual flow diagrams in Figures 26, 27 and Z8, respectively. 4. 5. I Body Mounted, Apollo S/C 4. 5. I. I Analysis The following parameters are assumed to be representative of the Apollo S/C configurations as shown in Figure 29 (a). 2 -_- = 3 deg/sec (100 lb thrust) (Z2) _r Z --_ = O. 15 deg/sec (5 Ib thrust) (23) 2 --_ = 0.03 deg/sec (1 Ib thrust) (24) Gimbal capability is ± 60 °. 4-35 i. I .0. j I i¸ - I ! ! [_r--! _ i/_ i® 4-36 4.J x 0 m T. tM N I • i I I I I I I I I I I I I I I I I C J_ Jt i_ al ® \ {J p_ -X o II _=_ o o n_ -.='4 o o o_=_ 17 • 4-37 il -, I • I I i I I I I I II II i I I I I GimbaUed, Apollo S/C Fine Track Mode -8 -4 -2 * -4 E x 10 deg/sec i,i!8 6 1- | ! I I k -6 Figure 31 Rate Feedback Control t 4 E x 10 -4 Deg 4-38 it . I I I I I I I I I l I I I i I fined: Then from the control h_op, the system state variables are de- x z = _ pointing rate, deg/sec (25) x 1 : _ pointing angle, deg (26) and the equations of motion are: o x 1 = x Z (27) where T Do (3r xz = --i-+ sgn [-xl - a xz] 7- (zs) T D = S/C disturbance torque, ft-lb a = rate feedback gain, sec ............... 1 ........... _-_,,_J..C. t_ril_c.ip_Xluoment of ine rtia, slug/ft z = S/C torque level; ft-lb £ = 3_ deadband, deg. The phase plane plot of the system state variables is shown in Figure 30 for: ff I a -4 3x10 = 1 = 20 x 103 (0.01 lb thrust) (zg) T D = 1, 0. 1 for t = 0.46 sec, 0.7, 1.0, 3.0 4-39 i o ! I I ! I I I I ! I i I I l I I i| ¸ ....... %:/ .... For satisfactory system operation during the data taking or fine mode, the locus of the phase plane trajectory must be bounded by: = x z < 3 0 deg/hr (30) < 0. 3 arc sec (31) ¢ = x I __ This region is indicated in Figure 30 by the dashed lines near the origin. Three trajectories are shown in Figure 30 I. Acquisition Phase: An example is given of switching from the wide angle sun sensor to the fine sun sensor and the coarse reaction control system to the fine jet system. This is the transition from the Coarse Track Mode to the Fine Track Mode. For the given set of initial conditions, the pointing axis is driven to the specified limit cycle region at the origin; total time is 4 sec 2. Fine Track Mode: The system is initially at the origin and a pulse disturbance torque of I ft-lb acts upon the space- craft. Fine Track is broken if the duration of the pulse exceeds 60 millisec 3_ Fine Track Mode: The system, initially at the origin, is acted upon by a disturbance torque of 0. 1 ft-lb. Fine Track is maintained up to a pulse duration limit of 700 millisec, The conditions and response trajectories for off axis pointing modes are identical to the given trajectories with the exception that the origin of the phase plane is translated to a given x I = 9c. 4 -40 I. l l I ! I I I i l I I ! I t i The propellant system fuel requirements can be calculated for the fine track limit cycle, The total impulse is Z I x2 I T = 2r x I t, lb-sec (32) where r = moment arm = 6 ft t = duration of Fine Track Mode sec = [mission duration] [ 1 - proportion of occultation-proportion of acquisition] = 1 year [ 1 - 0.40 - 0. I0] = 15 x 106sec then ....... IT .... - 52. 5 X 103 lb-sec. Forarnonopropellant, I = 100, the total fuel weight required is W_ = 525 lb. s t For a bipropellant system, quired is Wf = 195 lb. I = 270, S the total fuel weight re- 4. 5. I. k Conclusions The body fixed Apollo S/C mode is a satisfactory technique of coronagraph mounting if the reaction control system is supplemented with fine reaction control jets (0.01 lb thrust) and disturbance torques due to man motion are restrained to 0. 1 ft-lb for a duration of 0.7 sec. For weight minimization a bipropellant system appears practical aIthough the time constant is 1.9 millisec as compared to a 0.6 millisec monopropellant system. 4-4 1 . 1 l l I I I I I I I I I I I i I 4. 5. Z Gimballed, Apollo S/C 4. 5. 2. 1 Analysis The control loop in Figure 29 {b) represents the single axis point- ing angle control loop of the gimballed, Apollo S/C deployment with the telescope in the fine track operational mode. The control technique may be accomplished either through dc motor or hydrauhc actuation techniques. Since both the dc motor and hydraulic actuation techniques have the same simplified models, the loop parameters may be interpreted.as characteris- tics required of each technique, The control loop in Figure 29 (b) utilizes error rate control as opposed to the rate feedback control depicted in the associated conceptual block diagram in Figure 27. The nomenclature of Figure ?-9 (b) are de- fined as: T D I 9sc ¢c 'e K o = disturbance torque acting on spacecraft ft-lb = spacecraft S]C moment of inertia, slug-ft z = inertial angular perturbation of S/C, deg = coronagraph commanded pointing angle deg = pointing error, deg = loop velocity constant, rad/sec "r = actuator time constant, sec = pointilag angle "r includes the reflected load inertias if torque magnification {gears) tech- niques are utilized. The design requirements which must be satisfied to achieve the desired control response which permits scientific experimentation are the following: 4 -42 I I I I I I I I I I I I I I I I I >: v:he re Null error: < O. 35 arc sec 2 2 b2]E =[o- +o _ +O" L_ a 1/2 <0.3 (34) 0- b = backlash, null resolution of torque magnification device. Maxin_um spacecraft acceleration: ¢SC -2 g = 5. 2 x I0 rad/sec [ I00 Ib thrust] -3 2 = Z. 6 x 10 rad/sec [ 5 lb thrust] -4 2 = 0. 5 x 10 rad/sec [man motion, 1 ft-lb] Maximum spacecraft velocity: o. ¢SC = 1.74 x I0 5 rad/sec -4 = 1.74 x l0 rad/sec -4 = 0. 5 x 10 rad/sec [ 3. 6 deg/hr limit cycle] [ 36 deg/hr limit cycle] [man motion 1 ft-lb, 1 sec] Coronagraph moments of inertia: ( slug-ft Z ) Unfolded Folded About cg At End I = 33.7 41.6 x About cg 11.9 I = 346. 1 1972 Y I = 345. 8 197 1 z 221 214 4 -43 I. I I I I I I I I I I I I I I I I Linear s_:st_-m analysis: £=0. The system natural frequency is given as: ;K- 0 n T The dimensionless damping ratio: I +aK I o = -- o The closed loop transfer function: ¢c TS z K (I + _s) o + s (I +K a) +K O O The error transfer function: (35) (36) (37) s(T + 1) e S o Z TS +(I +aK )s +K o o (38) The selection of error rate control (Figure 29 (b)) over rate feed- back (Figure 27) was based on a comparison of linear system responses. The two te(hniques of control result tn identical values of damping and sys- tem natural frequency, however, error rate control has a faster response and smaller steady state error due to a ramp input. The main control func- tion input will be a ramp input as is evidenced in Figure 14. With error rate control, the steady state pointing error due to a ramp input of magnitude A is A = -- (39) 'e K ' o 4-44 I I i I I I i I I i I I I II I I I for rate feedback, the pointing error is / I +aK \ 0 _e = ( )" _, K o Since a, K o are always greater than zero, the former error is smaller. Lead circuit techniques at low frequency levels will eliminate expected large amplitude noise due to signal diflerentiation. The design parameters are: _= 0.7 maxm_un_ drift _0n = a [ maximum allowable error ] • F ¢SC ] =3/zi JL E (41) = 18. 0 rad/sec T = 0.055 sec. system The minimum actuator time constant can be shown to be T z_ system = (4Z) E Where ,%is the ramp input due to solar sidereal motion. Then T = 0. 542 sec K = O a =2_ 2 T = 175 rad/sec (43) n 2r_ -1 n sec = 0.138 _ (44) K rad O 4-45 m w I I I I I I I I I I I I I I I I I As an example, these _ontrol loop parameters are reflected into component design values where the actuation technique is considered to be adc Ward Leonard position control system. Again, the actuation technique could be hydraulic. Let total armature resistance R be approximately 20 ohms a R = 7-0 ohm s a R = r + r (45) a ag am = 8+12. Let the motor velocity constant be K ~ 2 volt sec/rad r¢l (46) then 2 I = 400 slug-ft, C for maximum acceleration transfer through a gear train I c I = _ = motor armature inertia rn 2 n = 100 Z I = O. 04 slug-it (47} The state variables of the system are pointing error and pointing error rate. x2 - 6e (48) = ¢Xl 'e (49) 4 -46 I i • I I I I ! I I I I I I I I I I I i 7¸¸.¸,¸ _--- =_ = - : '/ :# The equations of motion are: x,, = X Z K a 3 K K ¢ k2 = _xz [I +OL__ j o o -- _SC- --_x I +-- sgn(xl)-T _ SC T (50) - _(51) _her_ T-- oo for x I _< ¢. Figure 31 is a plot of the coronagraph pointing angle limit cycle where the control loop contains rate feedback. Figure 32 shows the limit cycle with the control loop employing error rate control. Curve 1 is the acquisition response and curve 2 the system response to spacecraft rate. 4. 5. 2. 2 Conclusions The analytical results indicate the conceptual design of the gim- balled mounting mode can meet the required pointing limit cycle require- ments in the presence of expected disturbance torque s upon the space ve- hicle. The constraint limit cycle of 3.6 deg/hr must be imposed upon the spa( ecraft. S_gnlficant technical difficulties such as linear actuation at low sig- na!s must be sv, lved for both hydraulic: and dc motor actuation techniques. Additionally if conventional gearing is applied for torque magnification the backlash or null resolution must be limited to less than 0. 1 arc sec. Thus dire(t drive offers significant advantages, and hydraulic acquation appears favorable. 4. 5. 3 Tethered SateIlite The response of the tethered satellite col_figuration is given by the analysis of the control loop block diagram in Figures 27 and Z9 (c). The loop degenerates into the response characteristics of the gimbalIed Apollo S/C configuration. Thus, the loop analysis is not derived, but rather the physical chara(teristics of the control components are defined. 4-47 e8 I I I I I I I I I I I i I I I I xlO 6 I 2 I t I t -4 deg/sec 4 Figure 32 Error Rate Control E x 10 -4 deg 4--48 I , ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! II 4. 5. 3. I Analysis For control of the coronagraph spacecraft; assume a maximum principal moment of inertia of the spacecraft I c to be I max : 500 slug It z. C This figure is calculated about the vehicle cg and represents about a 20% increase in the moment of inertia of the coronagraph, Momentum Storage = (Disturbance Torque) (Duration) + (Spacecraft Angular Momentum) = I. 3 ft-lb-sec (52) Required Maximum Torque = 0, 2 ft-lb = 34. 8 in. oz atc Drive Motor Synchronous Speed ca = 1200 rpm. s The speed torque curve is required to be linear throughout the design torque limit range. Since the specified torque limits are: = 58. 4 < T < 58.4 in. oz m and since the output torque is zero at synchronous speeds, the torque speed curve is defined for operational limits on the reaction wheel speed. Define the required linear range to extend + 1000 rpm, then the torque speed curve is given as: 38.4 T = _ + T (53) 1000 max where T = 46.0 in. oz; the stall torque. max The power required at stall is T p = s max = 83.2 watts (54) s 663 4-49 i I i I I I I I I I I I I I I l I The inertia required is I = . OiZ4 siug-ft z. m The weight of this unit would be approximately 8. Z5 pounds plus 4 pounds motor weight for a total of 1Z. Z5 pounds. The wheel diameter wOUAQ be inches. 4. 5. 3. g Conclusions Based on empirical data derived from existing reaction wheel sys- tems the feasibility of fabricating a reaction wheel control system for a tethered coronagraph satellite is wholly within the state-of-the-art. The fine reaction wheel control system, damped by magnetometers, would be momentum dumped by microrocket rotational thrusters. The dumping would be performed during occulation and at which time translational thrusters would properly position the tethered satellite with respect to the prime space vehicle. 4-50 m o I I I I I I I I I I I I I I I i I=1 •SECTION 5 COST AND DEVELOPMENT SCHEDULES 5. 1 OPTICAL DESIGN Figure 33 shows the optical design schedule to fabricate the selected (coronagraph design). The total effort for detail design, fabrication, and check out is scheduled as 26 ram. The costing of the optical and mechanical elements is shown as follows. CORONAG tLAPH COSTING LINEAR DESIGN Objective (a} Achromat (c) Margin (d} Relay (e) Figured Folding Mirror {f) Spherical Mirror (h) Spherical Mirror (h') Grating (g) (t960) Slit Lens (q) Lens (r) Grating (g') Slit Mi r ro r (k) Mirror (1) Co rrector Plate $ 3,000 800 80O I, 400 I00 20O Z00 2,500 50O I00 i00 Z, 500 l, 000 200 500 500 $14, 400 5-I I I I I I I I I I I I I I I I I I m 0 ! 0 0 v u C ..-'4 °_..* 0 0 C 0 0 '4 o ! 0 4_ C 0 CL E 0 u_ 8_ u 0 0 C ° o_ C _ 0 C C 0 • 0 o o _'_ 0 0 0 0 o| | I l ! "_ ""4 ,--, _v _ _ >,_"_" .0_ _ _ _ " • (0 C'l_ E u_ 5-3 I • I I I I I I I I I I I I I I I L_ J: r, 0 _E o E rJ L_ t-- 0 0 o o 0 0 _-.-_ ..Q O_ o T ! 0 _J 0 0 r" r_ Ou l d 0 o m 0 c- O TI I I L E) 4 J_ _J _J 1) Q •_ _ .., °I° [ u .n o z'_ u 5-Z. i I B i I I I I i I I I I I I I p OCCULTER MECHANISM Occu!ter Occulter Holder Occulter XYZ mechanism including motors electronics Occulter Position (Auto} IR detector 3 Position Folding Mirror (F} and Mechanism Grating Mechanism (G) Mechanism to Move L & Pa Mechanism to interchange G' &Pf $ l, 000 5O0 5,000 2,000 2,000 5O0 700 500 l, 500 Z, 000 $ IS.voo Came ras $300,000 TOTAL $330,000 In reality, since these prices are estimates, a 20% factor at least should be applied, so that the cost of ...._._ optics would be _,_-"the order of $400,000 to $500,000. 5.4) I I I I I I I I I I I I I I I I I I 5.2 STRUCTURAL FABRICATION 1-u .... ,. --u^; .,- a .... '^- ph•_u_,_Lc the coronagra structure is shown in Figure 34. The material and component costs are: FIGURE 34 Coronagraph Super Structure Prototype Schedule (Linear) Task Total Detail Design • _ 6 Fabrication . _ 16 Preliminary ' I Alignment I Z 3 4 5 6 7 (months) 25 mm Materials: $4,000 Coronagraph Super Structure Prototype Schedule (Folded) Task Total Detail Design • I 6 Fabrication i 16 Preliminary l Alignment 1 2 3 4 5 6 7 (months) Z5 mm Materials: $3,000 5-5 II II I l II I I II I I II I I III I I I I 5. 3 CONTROL DESIGN ANALYSIS SCHEDULE Since the specific nature of the coronagraph control system is - -__OAa_&AA " gent upon the coronagraph mounting mode and operational mode which de- pends directly upon the U.S. space program evolution and objectives,. specific and detailed cost schedules cannot be forecasted.. Howeve r, independent of the will not eventual experimental orbiting mis sion is the requirement for a detailed analysis study, componentdesign study, and total control concept design study. The schedule for the control sys- tem development task is shown in Figure 35, and includes development tasks associated with both telescope designs. FIGURE 35 Coronagraph Control System Detail Design Schedule Analog Computer Analysis • Sensor Design Actuator Design Final Concept Formulation (including electronics) 9 Total Period of Effort:. Total Engineering ram: Total Support ram: Total mm 1 2 6 months II 3 ¸ 14 ! I i . I 3 4 5 6 manmonths 5.6 ! I I I I i I I il t I I I I I I II 5.4 COST SUMMARY Total Optical Design Costs Folded Linear Labor (Z9.5 m/m) 36,875 (Z6 m/m)3Z, 500 Burden 44, Z50 39,000 Materials 400,000 400,000 G&A 73, Z75 71, 809 TOTAL _==_Jj-*, 400 $543,309 Total Structural Fabrication Costs Labor {Z5 m/m) 31,Z50 (Z5 m/m)31,g50 Burden 37, 500 31,_uu Materials 3,000 4,000 G&A 10, 9Z7 I l, 080 TOTAL $8Z, 677 $83,830 Total Control Analysis Cost Labor (14 m/m} Burden G&A 17, 500 Z 1,000 5,864 $44, 364 TOTAL COST $681,441 TOTAL (14 m/m}17,500 Zl, 000 5,864 $44, 364 $671,503 5-7

Orbiting coronagraph : final report

Abstract

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Orbiting coronagraph : final report

Abstract

Extracted data

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