SOLAR SPECTROGRAPH DESIGN CONCEPT FOR OCCULTATION AND SOLAR FLUX MEASUREMENTS IN THE SOLAR-SYSTEM

Future space missions, large or small, that address planetary atmosphe res should have the capability to observe occultations of the Sun by t he atmosphere and to monitor the solar EUV and FUV flux. To be a viabl e candidate, the solar spectrograph must not compete with the primary experiments for mass and power. We have designed a compact solar spect rograph that covers the spectral range from approximately 200 to 3200 angstrom. It has no moving parts and records the entire spectrum withi n its range in a single exposure. The solar spectrograph is based on a classical Rowland grazing incidence design. Because of the intensity of the solar flux, small apertures offer adequate throughput. The spec trum is dispersed along the Rowland circle by three gratings and recor ded in three side-by-side segments on a single detector, a CCD. The st rong astigmatism in the grazing incidence design is canceled in the fo reoptics. Control of stray light is a particular concern for a UV sola r spectrograph, because of the extraordinarily large variation in the solar flux across the desired wavelength range. Techniques to minimize the effects of scattering include (1) suppressing it with appropriate coatings and optical components, (2) choosing a detector layout that permits measuring and subtracting the effects of scattered light from the spectrum, and (3) employing a photocathode that discriminates agai nst long wavelengths. We estimate that a flight-ready unit would have a mass of about 1 kg and fit in a volume 20 x 9 x 7 cm.