Optimized reflective wide-angle Michelson phase-stepping interferometer.

A fully achromatic wide-angle interferometer that uses cat's-eye retroreflectors has been developed for optical Doppler imaging in the ultraviolet region. Here the solar flux is much weaker than in the visible, so the solar background control system ordinarily used in spacecraft optical instruments becomes unnecessary, which results in a much smaller instrument and a broader viewing geometry. The theory and the test results associated with the development of this instrument are described. Minimizing the aberrations involved in cat's-eye geometry resulted in optimum cat's-eye and interferometer designs. The results of tests conducted in the visible and the near-UV regions demonstrated experimentally that the field of view of the interferometer is 5 degrees and that the residual wave distortions are approximately 1/8 wave with visibilities sufficient for high-resolution Doppler imaging.