System Throughput and SNR / Exposure Time Estimation

6.9 Time Dependence of UV Response

The UV
throughput of the WFPC2 degrades in a predictable way after each monthly decontamination. The photometric calibration given in Section 6.1 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. The dotted vertical lines mark the dates of the decontaminations. The solid vertical lines labeled "-88" mark the date the operating temperature was changed from -76 degrees C to -88 degrees C (April 24, 1994). The following information is for the period following the cooldown date. Figure 6.11 shows that the effect of contamination on the F675W and F814W filter observations is essentially negligible, but it rises toward the UV until it reaches values of 30% - 40% per month for the F160BW filter. Table 6.10

Table 6.10: Change in WFPC2 Throughput Over 30 Days.

shows the monthly decline in throughput based on this data. The values in parentheses are based on similar observations of the globular cluster
w Cen (NGC 5139; mean B-V ~ 0.7 mag). In general the values derived from the w 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 w 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 shows 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 in throughput 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 much lower.

In addition to the monthly changes in throughput there is now evidence for a long-term variation in the F170W data on the PC, where the throughput has increased at the rate of 4.8% +/- 0.3% per year. This is evident in Figure 6.11, but is much clearer in Figure 6.12 where the data before and after Jan. 1, 1995 are compared, and in Figure 6.13 where the effect of the monthly decontamination is removed. 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.

ISR 96-4 will 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:

Figure 6.11: Photometric Monitoring Data for WFPC2.

Figure 6.12: Throughput for the F170W Filter Following Decontaminations.

Figure 6.13: Change in Throughput vs. Time.

Figure 6.11: - Photometric Monitoring Data for WFPC2.
Figure 6.12: - Throughput for the F170W Filter Following Decontaminations.
Figure 6.13: - Change in Throughput vs. Time.