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. Each visit consisted of two bias frames at both gains (to verify the electronic integrity of WFPC2 and to
estimate noise) and two F555W flat fields at both gains (to verify optical integrity and gain
ratios). Two Kelsall spot (“KSPOT”) images (one optimized for the PC and one for the
WFs), and five 1800s dark frames were also taken at gain=15.
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.
CCD
Gain
Noise on
Feb 23
at -43C
Noise on
Feb 27
at -88C
Nominal
pre-SM97 noise
at -88C
PC1
7
15.12e
5.31e
5.24e +/- 0.30
PC1
15
17.00e
7.70e
7.02e +/- 0.41
WF2
7
16.82e
5.30e
5.51e +/- 0.37
WF2
15
18.71e
8.11e
7.84e+/- 0.46
WF3
7
20.09e
5.21e
5.22e +/- 0.28
WF3
15
21.14e
7.94e
6.99e +/- 0.38
WF4
7
17.72e
5.21e
5.19e +/- 0.36
WF4
15
20.15e
7.27e
8.32e +/- 0.46
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.
CCD
counts/sec
at gain=7
counts/sec
at gain=15
gain ratio
at -88C
Nominal
pre-SM97
gain ratio
PC1
58.7
28.8
2.04
1.99 +/- 0.02
WF2
197.2
98.0
2.01
2.00 +/- 0.02
WF3
207.5
105.1
1.97
2.01 +/- 0.02
WF4
144.1
73.9
1.95
1.96 +/- 0.02
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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.
CCD
Total
exp
time
Median
counts
per pixel
Dark
current
e/sec/pix
Nominal
pre-SM97 dark
current
e/sec/pix
PC1
5400s
3.515 DN
0.0046
0.0058
WF2
5400s
2.304 DN
0.0030
0.0025
WF3
5400s
3.187 DN
0.0041
0.0038
WF4
5400s
2.734 DN
0.0035
0.0038
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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