Instrument Science Report 99-01:
Internal Flat Field Monitoring II. Stability of the Lamps, Flat Fields, and Gain Ratios
C. O'Dea, M. Mutchler, and M. S. Wiggs
April 27, 1999
We have compared the on-orbit behavior of the VISFLAT and INTFLAT lamps and the "flat fields" that they produce over the period from early 1994 to early 1998. We find the following: the VISFLAT lamps continue to degrade in intensity at the expected rate. The loss is less in the blue due to the Welch-Allyn Bulb becoming hotter. One (or more) INTFLAT lamps have increased output slightly (the net change is ~1.3% in 4 years). On timescales of ~6-35 s, the output of the Carley bulbs is relatively constant. The relative gain ratios of the electronics are stable in WF2 and WF4, though there is a steady 0.3% drift in the WF3 gain ratio, and a jump in PC gain ratio at SM2 of 0.7%. The pixel-to-pixel response of the chips is very stable. There are variations in the flat fields (donuts and blemishes) not due to the chips but due to dust on optical surfaces either moving or appearing. There are changes in the large-scale structure of the INTFLAT fields with time probably related to varying lamp brightness. The effects of contamination (location of "worms") are stable with time. Overall, the results from the VISFLATs and INTFLATs are in good agreement. If the VISFLAT lamps become unusable, the INTFLAT lamps would provide useful diagnostics of all properties except possibly for the stability of the flat fields on large scales which is affected by the variations in the Carley bulbs.