A wide-field relay optics system for the Caltech Submillimeter Observatory

Relay optics designed for use with imaging arrays at the Caltech Submillimeter Observatory are described. An off-axis ellipsoidal mirror with foci displaced from the conjugate planes is used to achieve a Strehl ratio >0.88 over a 2′ square field of view (corresponding to a far field of 15 × 15 diffraction beams) at a wavelength of 350µm. The mirror also provides an aperture-stop image just prior to focus which allows compact entry into cryogenic camera dewars. Using a compensating ellipsoid in subsequent camera optics, the Strehl ratio can be improved to >0.95 at all points across the field, even for off-axis chop angles of the telescope’s secondary mirror as large as 2′.