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.
Intensity measurements of the two lines of the sodium nightglow, at 589.0 and 589.6 nm, show that their ratio is not constant. It is generally assumed that this ratio is fixed, with a value of I(589.0)/ I(589.6) = 2.0. In measurements made at a variety of sites, most often utilizing the echelle spectrographs at large telescopes, it is demonstrated that the ratio typically varies within the 1.3-1.8 range, with 1.6 being a common value. Because the nightglow emission is relatively strong, individual measurements are quite precise. Both the intensity ratio and the summed intensity fluctuate with a semi- annual oscillation. Laboratory spectra of the ratio show a similar variability of values, and the current hypothesis is that the ratio reflects the [O(<sup>3</sup>P)]/[O<sub>2</sub>] ratio of the environment, in the laboratory or in the mesosphere.