The degradation was found by measuring the flux of the star through the finding aperture (VCLRV_T). Two separate targets, VID998 and BD+75D325, were used as photometric standards. The accuracy of the flux calibrations was at the level of photon counting statistics or better for the test. The VIS tube has decreased by about a factor of 3.2 from April 1991 through August 1993, but the final calibrations between October and December of 1993 showed a return to levels typical of the beginning of calibrations in April 1991. This level held through several calibrations including the last taken on the final day of HSP operations. Figure 42.1 shows the VIS flux from observations of VID998 and BD+75D325 over this period. The other detectors showed no change in flux measurements over time.
The UV1, UV2, and POL detector sensitivities were all constant within 1% over the same time period.
Figure 42.1: VIS Degradation