Cryogenic optical performance of the Cassini Composite InfraRed Spectrometer (CIRS) flight telescope

The Cassini Composite InfraRed Spectrometer (CIRS) half-meter diameter beryllium flight telescope's optical performance was tested at the instrument operating temperature of 170 Kelvin. The telescope components were designed at Goddard Space Flight Center (GSFC) but fabricated out-of-house and then assembled, aligned, and tested upon receipt at GSFC. A 24-inch aperture cryogenic test facility utilizing a 1024 x 1024 CCD array was developed at GSFC specifically for this test. The telescope's image quality (measured as encircled energy), boresight stability and focus stability were measured. The gold coated beryllium design exceeded the cold image performance requirement of 80% encircled energy within a 460 micron diameter circle.