WFPC2 Instrument Handbook for Cycle 10

Short-term Time Dependence of UV Response

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The UV throughput of the WFPC2 degrades in a predictable way after each monthly decontamination. The photometric calibration given in System Throughput is applicable at the start of each cycle, and measurements taken at other times must be corrected to account for the change in sensitivity since the last decontamination. In addition, a long-term change in sensitivity is present for the F160BW and F170W filter observations on the PC, and may be present to a lesser degree at other wavelengths.

Figure 6.11 shows the photometric monitoring data for the standard star GRW+70D5824 (a white dwarf classified DA3; B-V = -0.09) in the WF3 and PC1 for the set of filters which are routinely monitored. Only data after April 24, 1994, when the CCD operating temperatures were lowered from -76°C to -88°C, are shown. Figure 6.11 shows that the effect of contamination on the F675W and F814W filter observations is essentially negligible. However, at UV wavelengths contamination effects are readily apparent; the upper envelope of points indicate measurements made shortly after a decontamination, while the lower envelope are data taken shortly prior to a decontamination. Contamination effects are largest for the F160BW filter where they cause a 30% - 40% modulation in throughput. Table 6.10

Table 6.10: Change in WFPC2 Throughput Over 30 Days¹.

Filter	PC1		WF2		WF3		WF4
F160BW	-0.263 ± 0.030				-0.393 ± 0.051
F170W	-0.160 ± 0.011		-0.284 ± 0.005		-0.285 ± 0.006		-0.232 ± 0.006
F218W	-0.138 ± 0.009				-0.255 ± 0.010
F255W	-0.070 ± 0.007				-0.143 ± 0.009
F336W	-0.016 ± 0.008				-0.057 ± 0.011
	(-0.038 ± 0.018)		(-0.043 ± 0.010)		(-0.046 ± 0.008)		(-0.047 ± 0.007)
F439W	-0.002 ± 0.007				-0.021 ± 0.010
	(0.002 ± 0.014)		(-0.022 ± 0.007)		(-0.023 ± 0.009)		(-0.023 ± 0.007)
F555W	-0.014 ± 0.006				-0.016 ± 0.008
	(0.007 ± 0.013)		(-0.007 ± 0.007)		(-0.009 ± 0.009)		(-0.008 ± 0.008)
F675W	-0.001 ± 0.006				-0.001 ± 0.006
	(-0.020 ± 0.020)		(0.001 ± 0.011)		(0.002 ± 0.011)		(0.004 ± 0.011)
F814W	0.007 ± 0.007				0.003 ± 0.008
	(0.013 ± 0.019)		(-0.002 ± 0.009)		(-0.000 ± 0.009)		(-0.002 ± 0.010)

1 Values in parentheses are from the Cen observations.

shows the monthly decline in throughput based on this data. The values in parentheses are based on similar observations of the globular cluster Cen (NGC 5139; mean B-V ~ 0.7 mag). In general, the values derived from the Cen data are in good agreement with the values derived from GRW+70D5824 data.

A slight difference between the throughput declines for GRW+70D5824 and Cen might be expected due to differences in spectral shape, especially for filters like F336W which have a substantial red leak. However, even in the case of F336W the effect should be less than 0.01 mag based on SYNPHOT simulations.

Figure 6.12 and Figure 6.13 show the throughput decline in all four chips as a function of days since the last decontamination for the F170W filter. The contamination rate is remarkably constant during each decontamination cycle, and can be accurately modeled by a simple linear decline following the decontaminations, which appear to return the throughput to roughly the nominal value each month. While the contamination rates are similar for the three WF chips, the values for the PC are significantly lower.

In addition to the monthly changes in throughput there is evidence for a long-term variation in the F170W data on the PC, where the throughput has increased at the rate of approximately 3.3% ± 0.2% per year. This is evident in Figure 6.11, but is much clearer in the top panel of Figure 6.12 where lines are fitted separately to the epoch ~1994 (dotted line) and ~1998 data (solid line). The effect is most evident in Figure 6.14 where only data taken 4 days or less after a decontamination are shown. The F160BW filter shows an even stronger trend but with larger uncertainties (i.e., an increase of 9.0% ± 1.7% per year). The WF chips do not show this effect, nor do the observations on the PC at longer wavelengths. One possible explanation of the throughput increase is that WFPC2 was flown with some initial contaminant on the PC1 optics which is slowly evaporating on-orbit. The pre-launch thermal vacuum test gave evidence of elevated contamination in PC1, which is consistent with this hypothesis.

A second long-term effect is also apparent in Figure 6.12 and Figure 6.13. In all four CCDs the line fitted to the later data show a shallower slope, which indicates a slower throughput decline. The decline rate is reduced by 19% (PC) to 30% (WF4) over the four-year interval between the dotted and solid lines in each panel. This is likely caused by contamination slowly escaping the camera.

ISRs WFPC2 96-4 and WFPC2 98-3 describe detailed results of this monitoring (available from our WWW site). Observers are advised to consult the STScI WFPC2 WWW page for the latest information at the following address:

http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_top.html

Figure 6.11: Photometric Monitoring Data for WFPC2.
 
Figure 6.12: Post-decontamination Throughput for F170W Filter in PC and WF2.
 
Figure 6.13: Post-decontamination Throughput for F170W Filter in WF3 and WF4.
 
Figure 6.14: Change in Throughput vs. Time.^1
 
1 Only data taken 4 days or less after a decontamination are shown. Data taken 0 to 60 days after service missions are also excluded. The fit is to data prior to MJD 51100.