IFIRS : an imaging Fourier Transform Spectrometer for NGST.

To accomplish the scientific objectives of NGST, the observatory must be equipped with instruments suitable for panchromatic observations across the 1-15 mu m spectral region on the faintest detectable objects. A wide-field imaging spectrometer that is efficient in the confusion limit, which may occur in deep field images, will maximize the scientific return and opportunities for serendipity from NGST. An imaging Fourier transform spectrometer (IFTS) supports these requirements in a low-cost, efficient instrument package that functions as an electronically programmable infrared filter with both imaging and spectroscopic capability. The conceptual design of the Integral Field Infrared Spectrograph (IFIRS) is an imaging FTS configured as a 4-port Michelson interferometer. The added ports are obtained by the use of cube-corner retroreflectors. A 4-port design delivers complementary symmetric and antisymmetric interferograms to the primary and secondary focal plane assemblies (FPAs). In this design, the object field of the complementary input is also imaged and superimposed on each image of the primary input. In operation, when observing the sky in the primary input, the secondary input would be fed with a cold blackbody having negligible radiance. The final interferogram is constructed from the difference between the two outputs (which is therefore also immune to common mode noise) while the normalized ratio of the difference to the sum of the two outputs serves to compensate for temporal variations in the object radiance, and may reveal systematic variations due to telescope throughput or detector drifts. The interferometer aperture, field angle, beam waist control, beamsplitter/beamcombiner co-planar alignment, maximum optical frequency and maximum resolution have been traded at a conceptual level of detail. These tradeoffs suggest that a 12 cm beam splitter diameter is sufficient to accept the throughput of an 8 m primary over a 5.'3 x 5.'3 square field of view