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6.4 Differences between lab RSRF and in-orbit RSRF

The differences between the RSRF obtained in the lab and in orbit are twofold:

(1) The overall shape agrees mostly within the current uncertainties of the models and the observations. The only place where the discrepancies are so big that a correction of the calibration makes sense at this stage, is at the short-wavelength edges of bands 1a and 2a  . The relative responsivities as measured in the lab appeared up to 60% higher than in orbit. This can be explained as a leakage  problem since the temperature of the calibration source was only 300K.

(2) In the lab only band 3  showed fringes , while in orbit bands 1 and 2 also show fringes. The filters in the detector block have been identified as the cause of these fringes, with the thickness of the filters matching the Fabry-Pérot gap calculated from the observed fringe frequency. The width of the fringes is close to the resolution  limit of the instrument. This explains why the effect was not observed in the lab: the external black body source was extended and thus the resolution of the measurements was not high enough to show the fringes.

K. Leech with contributions from
the SWS Instrument Dedicated Team (SIDT)
and the SWS Instrument Support Team (SIST)