Optical and opto-mechanical design of spectrometers for PRIMA

The PRobe Infrared Mission for Astrophysics (PRIMA) will be a multi-mode, cryogenic, far-infrared space-based observatory consisting of a 1.8 meter collecting telescope followed by multiple spectrometer and imager instruments. There is a Fourier Transform Module (FTM) that is optionally inserted between the telescope and Far-Infrared Enhanced Survey Spectrometer (FIRESS), which consists of an objective group that receives the collimated beam from the scan mirror and creates a focus at the slit for all spectrometers. A total of four spectrometers, working in different spectral bands, disperse the light and create a focus at FPAs. Most commonly, imaging spectrometers work at 1:1 magnification between slit and FPA, but in PRIMA the magnification of the four spectrometers ranges from 0.6x to 1.8x. The non-unity magnification led to a configuration trade for the spectrometer optics, and required the use of low-order Zernike aspheres (freeform surfaces) to achieve the requirements on wavefront error, keystone error, and smile. This paper describes the requirements, configuration trade and development, preliminary optical design, and performance analysis of FIRESS.