WF/PC Instrument Report- WF/PC 91-04:
WF/PC Photometric Calibration during Jan-May 1991
-Keith Horne, Lisa E. Walter, and Christine E. Ritchie
May, 1991
Summary excerpted from the report:
Observations of 3 spectrophotometric standard stars taken between 31 Jan. 1991 and 23 May 1991 are used to investigate the sensitivity of the WF and PC cameras relative to the pre-launch baseline predictions. The analysis considers all spectrophotometric standard stars observed between the UV flood on 26-28 Dec. 1990 and the latest observation on 23 May 1991, which occurred after the deep safing event of 02 May 1991. The main results are summarized in Figures 3a and 3b, which show the ratio of observed to predicted count rates as a function of wavelength in the WF and PC camers respectively. Figures 4a and 4b compare the baseline and corrected quantum efficiency curves for WF2 and PC6 respectively.
At optical wavelengths the measurements indicate stable sensitivites in WF2 and PC6 to a few percent in F555W, and F785LP and 5-10 percent in F336W. In the UV the sensitivity is not stable.
Comparison of the observed and predicted counts indicates that the ground-based sensitivity calibrations differ significantly from on-orbit performance. The sensitivity in F785LP is down by 35-40 percent in SF2 and PC6,7, confirming the result of our earlier analysis of Omega Cen data. At F555W, WF2 is up by 10 percent while PC6 and 7 are down by 10-20 percent. At F439W, WF2 is up by 20 percent, PC6 is down by 10-15, and PC7 is down by 35 percent. At F336W, WF2 is up 70-90 percent while PC6 is down 45-50 percent. F284W is up by 45-60 percent in WF2, and the other UV filters are down by 20-97 percent and unstable as noted above.
The issue of flat field normalization in relation to photometric calibrations is briefly discussed. A reccomendation is made that flat fields be normalized to the median value in a 200 by 200 pixel box centered on the prime chip (WF2 or PC6) rather than to a mean over all 4 chips.
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