AN INTERFEROMETRIC APPROACH TO SUPPRESSION OF SCATTERED RADIANT ENERGY

The concept of using an interferometric process to reduce scattered light in optical systems has been examined from a physical optics viewpoint. An experiment is described which illustrates the basic concepts of a particular scheme addressing the important case of a telescope viewing a distant point object whose image is degraded by scattering from dust on the primary lens. A basic conceptual scheme is described, utilizing a modified Twyman-Green interferometer, which introduces a π phase shift and a focal power difference between the arms. The interferometer is inserted into the optical train of an elementary telescope to null the scattered energy while preserving the image intensity associated with the distant point object, thereby enhancing the signal-to-noise ratio. The effect of this interferometer on the image of a distant point object is examined in detail. Computer techniques were used to solve Lommel function based series representations of the diffraction integral, and generate the intensity distributions which are presented as a series of iso-intensity contour maps. The results indicate that a distant point object could be reimaged successfully. Further analytic evaluations reveal specific operating conditions for the interferometer that optimize the peak intensity at the focal plane. A wavefront model was used to derive the radially dependent intensity distribution at the focal plane from the single on-axis scattering point. To evaluate the effectiveness of the scatter-nulling interferometric technique, an analytic expression for the gain in signal-to-noise ratio was developed, utilizing elementary models of a reimaging system with and without the inclusion of a nulling interferometer. Gains up to a factor of 10⁸ were observed with the assumption of a reasonable set of system parameters and for the limited set of conditions employed. Results imply that the interferometric approach could be successful and effective under restricted conditions. Imagery from the experimental arrangement is presented showing a projection of the concentrated distant source image along with the broad null field from the scattering source