Optical and opto-mechanical design of the PRIMA telescope

The PRIMA (Probe Far-Infrared Mission for Astrophysics) system will be a 1.8 meter, multi-mode, cryogenic, far-infrared space-based observatory consisting of a collecting afocal telescope followed by multiple spectrometer and imager instruments. The afocal telescope collects light from targets across a total field of view that is selectable by means of two different internal scan mirrors, one each for the spectrometers and the imagers. The primary and secondary mirrors are common to both instrument groups, and working with an angular field offset between instruments. The optics after the secondary and up to the scan mirrors are separate. The telescope provides two separate collimated and demagnified beams emerging from the scan mirrors to the spectrometer and imager instrument groups. Design-driving factors include the need to demagnify the entrance beam to fit a heritage scan mirror, and the need for an accessible field stop located clear of any mirror. This paper describes the requirements, configuration trade and development, preliminary optical design, and performance analysis of the telescope.