In-flight degradation results for the UARS SOLSTICE instrument

The SOLar STellar Irradiance Comparison Experiment (SOLSTICE) is a three-channel spectrometer designed for measuring the solar ultraviolet (UV) irradiance from 119 nm to 420 nm with a spectral resolution of 0.1 nm to 0.3 nm. The SOLSTICE is aboard the NASA Upper Atmosphere Research Satellite (UARS), which was launched on 12 September 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 changing only slit sizes and integration times. While the Sun changes by 1% 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% in the UV for long time periods. The ensemble average of the SOLSTICE stellar observations indicates that these stars are indeed stable to 2% or better. Since the launch of the UARS, the SOLSTICE sensitivity has decreased by a few percent per year. We attribute the degradation primarily to ageing of the photomultiplier tubes for all three channels and to diffusion of layers in the broadband interference filters for the F and N channels. 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 10000 clean rooms and oil-free vacuum systems for all pre-flight testing of the instrument.