The development and analysis of cryogenic optical systems for the rapid infrared imager/spectrometer

The Observational Cosmology Laboratory at NASA’s Goddard Space Flight Center (GSFC), in collaboration with the University of Maryland, is building the Rapid Infrared Imager/Spectrometer (RIMAS) for the new 4.3 meter Discovery Channel Telescope (DCT). The instrument is designed to observe gamma-ray burst (GRB) afterglows following their initial detection by the Swift satellite. RIMAS will operate in the near infrared (0.9 – 2.4 microns) with all of its optics cooled to ~60 K. The primary optical design includes a collimator lens assembly, a dichroic dividing the wavelength coverage into the “YJ band” and “HK band” optical arms, and camera lens assemblies for each arm. Additionally, filters and dispersive elements are attached to wheels positioned prior to each arm’s camera, allowing the instrument to quickly change from its imaging modes to spectroscopic modes. Optics have also been designed to image the sky surrounding spectroscopic slits to help observers pass light from target sources through these slits. Because the optical systems are entirely cryogenic, it was necessary to account for changing refractive indices and model the effects of thermal contraction. One result of this work is a lens mount design that keeps lenses centered on the optical axis as the system is cooled. Efforts to design, tolerance and assemble these cryogenic optical systems are presented.