Study of the image quality and stray light in the critical design phase of the Compact Echelle Spectrograph for Aeronomical Research (CESAR)

The success of the high resolution nightglow studies conducted with the Keck telescopes on Mauna Kea and the Very Large Telescopes in Chile led to the design of the Compact Echelle Spectrograph for Aeronomical Research (CESAR). This is an echelle spectrograph with grating post-dispersion that will be dedicated to nightglow studies at high spectral resolution (R ~ 20000) between 300-1000 nm, and that will be easily deployable at different sites. The development of CESAR is conducted by SRI International, and INO is involved in the optical design and integration of the spectrograph camera, whose all-spherical form is based on the camera of the HIRES spectrograph at the Keck I telescope. The detailed optical design is used to calculate the position of the spectral elements on the detector, predict their image quality, and estimate the level of stray light. This paper presents the methodology used in these analyses.