WFPC2 Technical Instrument Report 00-04:
WFPC2 Internal Monitoring
-O'Dea, Heyer, and Baggett
We report on the stability of WFPC2 with special attention to any potential changes due to SM3A as determined from internal calibration observations. Over the course of the period Nov. 1999 to Jan. 2000, there were no significant changes in the gain 7 or 15 read noise.
The data from the dark images are consistent with an increase of a few percent (ranging from 2-8% depending on chip) in the dark current between October 1999 and January 2000. The data show that the pre and post SM3A points lie along the line of steadily increasing dark current found by Baggett etal. Thus, the post-SM3A results show only the expected gradual increase in dark current and there appears to be no effect of SM3A itself on the dark current.
Ratios of VISFLATs taken Dec. 1998 and Jan. 2000 show that there are no significant changes in either the flat fields or the VISFLAT illumination pattern due to SM3A. The ratio of gain 7 and gain 15 VISFLATs does show a small increase of order 0.5 percent between Dec. 1998 and Jan. 2000. This may correspond to a change in the gain ratios or to the ratios of VISFLAT lamp brightness over 60 and 120 second intervals.
We note that the general trends in the gain 7 and gain 15 ratios of observed count-rates are similar in the VISFLATs and INTFLATS up through SM2, after which the two sets of data seem to diverge. However, the VISFLAT data are more poorly sampled after SM2. Since the VISFLATs and INTFLATs are independent systems, the trends suggest that from 1994 to 1997 there were small drifts and jumps in the gain of gain 7, however, after that the variations in the ratios are dominated by variations in the lamps.
There have been small (few tenths of a percent) changes in the INTFLAT lamp brightness in gain 7 associated with both servicing missions (but not in gain 15 except for the PC1 at SM2). We see changes in the illumination pattern of the INTFLATs associated with the changes in intensity. Ratios of INTFLATS taken in mid-99 and Jan. 2000 show small amplitude (0.1 - 1%) large scale variations which are chip and wavelength dependent. These variations are somewhat larger than those seen in a similar time interval from Jan. 99 to mid-99. We suggest that the four Carley bulbs are not varying in the same manner and that this induces the observed changes in the illumination pattern of the INTFLATS. The variation in the lamp brightness may be associated with changes in temperature.
Over the period Nov. 3, 1999 to Jan. 11, 2000 the Kelsall spots shifted by on average 10 milliarcsec. These shifts are consistent with the known time evolution of the K-spot positions.
In summary, overall the WFPC2 appears to be very stable exhibiting only the minor changes expected due to known low level variability. The only variations attributable to SM3A are small changes in INTFLAT lamp brightness.