SOLSTICE Version 11 (January 2012):
The MUV degradation correction (180-310 nm) has been significantly improved. In this version, the weekly alignment measurements have been used at eight wavelengths to measure the degradation of the optics as a function of field-of-view (FOV) angle. In previous versions, only four wavelengths were used. The algorithm for calculating this FOV correction was revised and produces a much better statistical uncertainty in each week's measurement. The MUV degradation correction uses the stellar calibration data, the new FOV correction, plus the calibration transfer from the redundant SOLSTICE B instrument. An error in the version 10 degradation correction in the A/B transfer is now believed to have produced the unusually large variation at some wavelengths in the MUV data. The FOV correction in the 180-190 nm range is still preliminary. The correction factor for this wavelength range is currently extrapolated based on longer wavelength trends in the FOV measurement, but analysis of the alignment data from the FUV channel in the wavelength overlap region will reduce the uncertainty of this correction for the next data version. The FUV degradation correction extends the stellar calibration observations to the current epoch but no changes in the FUV algorithm have been implemented version 11. The overall uncertainty in the SOLSTICE degradation correction is still meeting the 0.5% per year requirement.
SOLSTICE Version 10 (September 2009):Degradation Correction:
The SOLSTICE A MUV channel observations have been corrected for degradation by cross-calibration to SOLSTICE B. Only SOLSTICE B makes stellar calibration observations, due to the anomalous behavior of SOLSTICE A's entrance aperture mechanism. Weekly A/B comparison experiments are used to correct any drift between the two instruments. Corrections for differences in field of view between solar and stellar modes at four wavelengths have been included. Future versions will include an additional four wavelengths. The uncertainty in the long-term degradation is currently about 2% (1-sigma).
SOLSTICE Version 9 (March 2007):
The following updates and modifications have been made to the SOLSTICE calibration data and processing code. Improvements have allowed the extension of the wavelength range to 310 nm bridging the gap to the SIM spectral data.
- Slit Anomaly: In January of 2006, an anomaly became apparent which was contributing to a 1 to 3% (depending on wavelength) increase in subsequent irradiance data for the SOLSTICE A instrument, which provides the MUV data. Analysis showed that the anomaly is due to a slit misalignment which directs the incident light to a different region of the detector which has a higher sensitivity due to less degradation. A new instrument misalignment calibration was applied with a map of the sensitivities in the instrument's field of view derived from an in-flight experiment performed shortly after the anomaly. A small discontinuity can still be seen at some wavelengths at the time of the anomaly. More detailed analysis remains to be done.
- Instrument Misalignment: Improved analysis of the cruciform alignment experiments (haystacks) have yielded a better set of data for quantifying the relative misalignment of the SOLSTICE instruments and the SORCE spacecraft. Instead of the constant misalignment use in previous data versions, a time dependent model is now applied. A simple step function was used to account for the apparent difference in pointing before and after the slit anomaly. The effect on the data is as described above for the slit anomaly. More analysis remains to be done of the haystack measurements and the time dependent behavior of the instrument misalignment.
- Field of View Maps: The pre-flight data from SURF was reprocessed to generate a new map of the sensitivities in the field of view of the SOLSTICE instruments as a function of wavelength. These maps, with the target coordinates, were applied to the FUV data. New MUV field of view maps were derived from an in-flight experiment performed shortly after the slit anomaly. The impact of the new FUV maps was minimal. However, the new MUV maps brought the irradiance down from 0.5 to 1% before the slit anomaly and 2 to 4% after the anomaly. Note that the map, with the new instrument misalignment calibration, introduces a larger variation with wavelength after the slit anomaly. Further analysis of the field of view maps remains to be done.
- Responsivity: The responsivity of the SOLSTICE A instrument, the source of the MUV data, is based on pre-flight data collected at SURF. The responsivity of the SOLSTICE B instrument, the source of the FUV data, is based on an in-flight AB comparison experiment conducted on mission day 69. The new pre-flight field of view maps were used to recalibrate the SOLSTICE B FUV responsivity. This is one of the prominent difference in the FUV irradiance data, introducing changes up to plus or minus 1% depending on wavelength.
- Temperature Gain: An in-flight experiment was performed on mission day 862 to assess the effects of temperature change. The FUV data proved to be too noisy, so only an improved model of the pre-flight data was use introducing minimal differences. Models of the new MUV temperature gain corrections were applied and yielded some improvement in the data, however, more analysis remains to be done.
- Degradation: The FUV detector degradation derived from stellar measurements has adequately corrected the FUV irradiance in previous data versions. However, the stellar data have failed to completely account for the MUV degradation due to different properties of the solar and stellar images on the detector's filed of view. A new field of view component of the degradation was added based on an analysis of the change in count rate during cruciform scans. The result is an increase of the MUV irradiance by up to 1% per year depending on wavelength.