The Solar UV Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 115 nm to 410 nm daily since October 1991. The primary difficulty in maintaining the calibration of these long-term measurements is the correct accounting of degradation in the instrument's responsivity. Accordingly, SUSIM was equipped with redundant optical elements and four stable deuterium lamps, any one of which can replace the sun in the optical path. Periodic calibration of the responsivity of the daily used optical paths is accomplished by comparison of the solar signals from these paths with the solar signals from redundant optical paths containing less-frequently used elements which, in turn, are calibrated using the lamps. Measurements of optical responsivity changes during the first 6.3 years of the SUSIM UARS mission are presented in this paper. The degradation is found to be almost entirely dependent on UV exposure. Consequently, most of the measured degradation takes place in optical elements which precede dispersion. Of these, degradation of transmission elements is strong and has a wavelength dependence similar to that of the lens on the calibration lamps on which contaminant layers apparently also form. The time dependence of the degradation also appears to follow a particular functional form for most wavelengths. Degradation in reflective elements is more moderate and, for some wavelengths, has been observed to reverse, i.e. to increase in responsivity. As an example, the standard channel responsivity at Lyman-(alpha) has increased from its minimum by a factor of more than three.
[1]
G. E. Brueckner,et al.
The solar ultraviolet spectral irradiance monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS)
,
1993
.
[2]
W. Hunter,et al.
Laboratory experiments to study surface contamination and degradation of optical coatings and materials in simulated space environments.
,
1970,
Applied optics.
[3]
D. K. Prinz,et al.
Cleaning of Contaminated UV Optics by Low Energy O
,
1991
.
[4]
D. K. Prinz,et al.
Maintaining calibration during the long-term space flight of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)
,
1996,
Optics & Photonics.
[5]
D. K. Prinz,et al.
Solar ultraviolet spectral-irradiance observations from the SUSIM-UARS experiment
,
1995
.
[6]
Dianne K. Prinz,et al.
On-orbit performance of deuterium calibration lamps during four years of SUSIM operations on the UARS
,
1996,
Optics & Photonics.
[7]
R C Preston,et al.
Magnesium fluoride windowed deuterium lamps as radiance transfer standards between 115 and 370 nm
,
1980
.
[8]
J F Osantowski,et al.
Auger spectroscopic examination of MgF(2)-coated Al mirrors before and after uv irradiation.
,
1977,
Applied optics.
[9]
D. K. Prinz,et al.
Validation of the UARS solar ultraviolet irradiances: Comparison with the ATLAS 1 and 2 measurements
,
1996
.