Shearing interferometer as an interferometric filter for the reduction of scattered light

The background radiation problem encountered in astronomical observations is reviewed, and it is observed that a considerable reduction of the background radiation can be achieved by using interferometric techniques. A longitudinal radial shearing interferometer is described which meets the requirements of this application. The formal development of its properties as well as the results of preliminary tests are presented. It is shown that for a small region of the telescope field, this interferometer can be expected to reduce the background radiation originating in the telescope by a factor of 50 to 500, to reduce the background radiation produced by Mie type scattering external to the telescope to a level below that produced by Rayleigh scattering, and to reduce the background radiation from Rayleigh scattering by a factor of two. These properties can be extended to broadband spectral operation and can be achieved without degrading the astrometric qualities of the telescope. The size of the interferometer is quite small making it easy to add to an existing telescope.