The optical design of PRIMAger, the hyperspectral and polarimetric imager of the PRIMA space mission

PRIMAger is an infrared imager onboard the PRobe far-Infrared Mission for Astrophysics (PRIMA), an infrared observatory for the next decade, currently in Phase A, with a 1.8 m telescope actively cooled to 4.5K. PRIMAger will provide observers with a coverage of mid-infrared to far-infrared wavelengths from 24 to 264μm and will offer two imaging modes: the hyperspectral mode from 24 to 84μm wavelength range with a spectral resolution R ≥ 8, and the polarimetric mode in four broadbands from 80 to 264μm. The current paper presents the baseline optical design of PRIMAger. This design, based on off-axis freeform aluminum mirrors, permits to meet the critical requirements in terms of field of view, focal ratio, image quality, telecentricity and pupil stability. A detailed description of the principle and the analysis of the optical performances and tolerances are studied. Other important aspects such as mirror manufacturing feasibility, straylight minimization and self-polarization behavior will be discussed. The alignment strategy and the verification plan will also be presented.