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Technical Instrument Report WFPC2-97-08

SMOV Flat Field Check (prop-ID 7019)
J. Biretta and M. Wiggs January 20, 1998

ABSTRACT We compare WFPC2 Earthflats taken before and the field-of-view shows no change (<<1%) in flat and occur within a few pixels of the CCD corners utable to long-term changes in the camera / OTA

after the 1997 service mission. Most of field calibration. The largest changes and reach ~1.5%; these are likely attribgeometry, rather than SM97.

1. Introduction
As part of our post-servicing check-out of WFPC2, we have observed a series of F502N Earthflats to test the flat field stability. The goal of these observations is to test for any unexpected obscuration or contamination in the OTA or WFPC2 due to HST servicing. The flats are also capable of revealing changes in the OTA / WFPC2 geometry, as well as any QE changes localized to one CCD camera or to a small region of the field-of-view. While internal flats can provide some of this information, the Earthflats are unique in providing an end-to-end test of the OTA+WFPC2 system. Detailed discussion of Earthflats and WFPC2 flat fields can be found elsewhere (Biretta 1995).

2. Observations and Analyses
The Earthflat observations made for SMOV were identical to those obtained for the routine Cycle 4 to Cycle 6 calibration programs. They are 1.2 second exposures of the bright Earth made with gain 15 in filter F502N. Since we are interested primarily in changes to the flat field, we have also selected a set of Cycle 6 Eathflats to compare with those taken during SMOV. For the pre-SMOV observations we examined 48 Earthflats in F502N taken between May 1996 and Feb 1997 as part of proposal 6909. From these we discarded ones with mean counts on WF3 below 1000 DN, ones with less than 500000 good pixels on any CCD (usually due to saturation), and ones with streaks exceeding about 1% peak-to-peak amplitude. In order to evaluate this last criteria, the raw Earthflats were multiplied by the current F502N flat field reference file, and then examined with IMSTAT and displayed.

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The remaining seven images, u3ek1b03t, u3ek1l04t, u3ek1v03t, u3ek2603t, u3ek2604t, u3ek6603t, and u3ek9604t, were combined with the task STREAKFLAT to produce an averaged, de-streaked pre-SMOV flat. Similar criteria were applied to the 108 Earthflats taken after the service mission as part of program 7019. This resulted in five images being selected, u3sc0502r, u3sc0703r, u3sc0803r, u3sc1102r, and u3sc1503r, which were combined with STREAKFLAT to produce an SMOV flat. We then divided the SMOV flat by the pre-SMOV image, and normalized so that the center 400 x 400 pixels of WF3 had a mean of unity. The resulting SMOV / pre-SMOV ratio image shown in Figures 1 and 2.

3. Results
As Figures 1 and 2 show, the pre- and post-servicing flats are essentially identical. The most significant changes are seen in the corners of WF2 and WF4 near the pyramid apex, where departures from unity reach about 1.5% within about 10 pixels of the pyramid apex. Similar effects are seen in the corners of WF3. These effects in the CCD corners are most likely due to small changes in the camera vignetting, which in turn result from small changes in the geometry of WFPC2 and the OTA. Other evidence of small on-going geometric changes from K-spot images is described by Mutchler and Stiavelli (1997). There are also faint traces of the "worm" features on WF2 at the 0.2% peak-to-peak level, which are related to contamination on the CCD windows. This is to be expected, since we have not restricted our selection of Earthflats with regards to time delay after decontaminations. The pixel-to-pixel fluctuations (over central 400 x 400 pixels) in the ratio image are typically 0.4% RMS for the WFC CCDs, and 0.8% RMS for PC1, which is entirely consistent with photon statistical noise. After smoothing with a 10-pixel FWHM Gaussian function, the fluctuations decrease to <0.1% RMS. No change in the chip-to-chip sensitivity is seen. The average ratio of post-SMOV / pre-SMOV counts over the central 400 x 400 pixels of each CCD is 0.9995 for PC1, 0.9998 for WF2, 1.0000 for WF3 (by definition), and 1.0003 for WF4.

4. References
Biretta, "WFPC2 Flat Field Calibration," in Calibrating HST: Post Servicing Mission, eds. A. Koratkar and C. Leitherer, STScI, p. 257, 1995. Mutchler and Stiavelli, "WFPC2 Internal Monitoring for SM97," Technical Instrument Report WFPC2-97-xx, in press.

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Figure 1: Ratio of SMOV / pre-SMOV flats taken in F502N. The display greyscale ranges from 0.98 (black) to 1.02 (white). The CCDs are indicated by the key at the upper left. For display purposes the original 1600 x 1600 pixel image has been smoothed using a 5x5 pixel box average.

PC1 WF2

WF4 WF3

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Figure 2: Same as Figure 1, except that the display greyscale ranges from 0.995 (black) to 1.005 (white).

PC1 WF2

WF4 WF3

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