ULTRAVIOLET IMAGER FOR THE INTERNATIONAL SOLAR-TERRESTRIAL PHYSICS MISSION

Imaging of the earth's auroral regions in the ultraviolet provides information on a global scale on the energy flux and characteristics of precipitating particles and on the composition of the atmosphere in which the energy is deposited. We report the design of an imager with 0.6-mrad angular resolution over an 8-deg field of view sampled with 39,500 pixels, yielding global auroral coherent imaging from above 6 R(E)(Earth radii). High-performance filters provide spectrally pure measurements of four key far-UV (FUV) features, with 5 x 10(-5) out-of-band rejection. Together with a solar blind intensified CCD detector, a net rejection of 10(-9) of all out-of-band emissions is achieved. The optical design comprises a three-mirror f/3 system that yields a noise equivalent sensitivity of 10 rayleighs (R) for a 37-s frame rate. The intrascene and interscene dynamic ranges are 1000 and 10(5), respectively. The optical surface microroughness is less than 2 nm, providing exceptionally low light scattering characteristics, allowing simultaneous observations of very weak and bright emissions. The imager should provide about two orders of magnitude improvement in performance over previous designs.