<title>In-flight degradation results for the UARS SOLSTICE instrument</title>

Abstract

The Solar Stellar Irradiance Comparison Experiment is a three channel spectrometer designed for measuring the solar UV irradiance from 119 to 420 nm with a spectral resolution of 0.1 to 0.3 nm. The three channels are designated as the G, F, and N channels that cover the 119-190 nm, 170-320 nm, and 280-420 nm regions respectively. The SOLSTICE is aboard the NASA Upper Atmosphere Research Satellite (UARS) which was launched on September 12, 1991. The degradation of the SOLSTICE sensitivity is primarily tracked in-flight by measuring a set of bright, early-type stars with the same optics and detectors and by only changing slit sizes and integration times. While the Sun changes by 1 percent in the near UV and by as much as a factor of 2 in the far UV, early-type main sequence stars are not expected to change by more than 1 percent in the UV for long time periods. The ensemble average of the SOLSTICE stellar observation indicate that these stars are indeed stable to 2 percent or better. Since the launch of UARS, the SOLSTICE sensitivity has decreased by a few percent per year. We attribute the degradation primarily to again effects for the photomultiplier tubes for all three channels and to diffusion between layers in the broadband interference filters for the F and N channels. There also appears degradation for the G channel diffraction grating related to excessive heating of the grating on a few days during the UARS mission. There appears only minor degradation associated with optical contamination, mainly because of the strict use of low-outgassing materials in the SOLSTICE instrument and maintenance of class 10,000 clean rooms and oil-free vacuum systems for all pre-flight testing of the SOLSTICE instrument.