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Technical Instrument Report WFPC2-97-07

WFPC2 internal monitoring for SM97
(Program 7022)
Max Mutchler and Massimo Stiavelli August 28, 1997

ABSTRACT The goal of SMOV proposal 7022 was to verify that there was no change in basic WFPC2 instrument health and internal calibrations. A variety of internal exposures were obtained in order to provide a monitor of the integrity of the CCD camera electronics in both bays (gain 7 and gain 15), a test for quantum efficiency in the CCDs, and an internal check on the alignment of the WFPC2 optical chain. Analysis of the data from program 7022 shows that the servicing mission caused no significant changes to these parameters.

1. Introduction
This report describes the results of the internal WFPC2 calibration observations (program 7022) which were conducted as part of the Servicing Mission Orbital Verification (SMOV) effort following the 2nd Hubble Space Telescope (HST) servicing mission (SM97) in February 1997. Internal WFPC2 observations were made at a gain of both 7 and 15. sisted of two bias frames at both gains (to verify the electronic integrity estimate noise) and two F555W flat fields at both gains (to verify optical ratios). Two Kelsall spot ("KSPOT") images (one optimized for the PC WFs), and five 1800s dark frames were also taken at gain=15. Each visit conof WFPC2 and to integrity and gain and one for the

This entire set ran weekly, on a non-interference basis. The first visit started shortly after WFPC2 became operational on February 23, 1997, with the CCDs at a temperature of -43C. The remaining visits began after WFPC2 was cooled to -88C on February 27.

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2. Analysis
The goal of this program was to measure any changes in bias and dark frames to ~1.4 electrons per pixel, and to measure any changes in flat fields to ~1% accuracy. The noise was estimated by running the imstat task on the bias frames. Cosmic rays were ignored by setting the upper pixel value cutoff to be ten sigmas above the mean. The table below shows that the noise level was high by a factor of 2 or 3 for the first visit on February 23 (when the CCDs were at a temperature of -43C), and that the noise returned to nominal levels after WFPC2 was cooled to -88C.
Noise on Feb 23 at -43C 15.12e 17.00e 16.82e 18.71e 20.09e 21.14e 17.72e 20.15e Noise on Feb 27 at -88C 5.31e 7.70e 5.30e 8.11e 5.21e 7.94e 5.21e 7.27e Nominal pre-SM97 noise at -88C 5.24e +/- 0.30 7.02e +/- 0.41 5.51e +/- 0.37 7.84e+/- 0.46 5.22e +/- 0.28 6.99e +/- 0.38 5.19e +/- 0.36 8.32e +/- 0.46

CCD

Gain

PC1 PC1 WF2 WF2 WF3 WF3 WF4 WF4

7 15 7 15 7 15 7 15

The gain ratio was verified by comparing the flux measured in flat field exposures through the same shutter with different gains. Shutter B was used for all the flat field exposures. The table below shows that the gain ratios for each chip were nominal after WFPC2 was cooled down to -88C on February 27.
counts/sec at gain=7 58.7 197.2 207.5 144.1 counts/sec at gain=15 28.8 98.0 105.1 73.9 gain ratio at -88C 2.04 2.01 1.97 1.95 Nominal pre-SM97 gain ratio 1.99 +/- 0.02 2.00 +/- 0.02 2.01 +/- 0.02 1.96 +/- 0.02

CCD

PC1 WF2 WF3 WF4

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To check the internal alignment of the WFPC2 optical chain, we compared the positions of the Kelsall spots in pre-SM97 and post-SM97 exposures. We found a small shift on the order of 5-10 milliarcseconds (see Figure 1) which is consistent with the general trend over WFPC2's on-orbit lifetime (see Figure 2).

Figure 1. Shifts of WFPC2 Kelsall spots after 1997 servicing mission. The pre-SM97 coordinates (from February 9) were all normalized to 0,0. The relative location of each spot on February 27 is plotted.

The 2nd servicing mission occurred on February 11-25, 1997. Figure 3 shows that the small shifts measured between February 9 (day 1062) and February 27 (day 1080) -- i.e. before and after the servicing mission -- are within the expected range.

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Figure 2. Average shifts of WFPC2 Kelsall spots from March 15, 1994 (day 0) to April 5, 1997 (day 1117).

Figure 3. Average WFPC2 Kelsall spot shifts around the 2nd HST servicing mission.

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The dark current was estimated by combining calibrated (i.e. bias subtracted) dark exposures with the crrej task to remove cosmic rays, and then measuring the median pixel value for each CCD for each shutter setting. All the dark exposures were 1800s at gain=7 at -88C. Here we use the three dark exposures with shutter B.
Total exp time 5400s 5400s 5400s 5400s Median counts per pixel 3.515 DN 2.304 DN 3.187 DN 2.734 DN Dark current e/sec/pix 0.0046 0.0030 0.0041 0.0035 Nominal pre-SM97 dark current e/sec/pix 0.0058 0.0025 0.0038 0.0038

CCD

PC1 WF2 WF3 WF4

The pre-SM97 and SMOV dark currents are in reasonable agreement. The small difference for PC1 is likely due to the strong dependence of it's dark current on the cosmic ray rate (see WFPC2 Handbook version 4, page 74).

3. Summary
In general, analysis of the data from program 7022 shows that the 2nd Hubble Space Telescope servicing mission of February 1997 caused no significant changes to the basic health and internal calibration of WFPC2.

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