Technical Instrument Report WFPC2-02-04
SMOV3B WFPC2 Photometry Check
Brad Whitmore and Inge Heyer
May 28, 2002
ABSTRACT
A check of the photometric throughput of the WFPC2 was performed March 31, 2002
(program ID: 8953). The standard star GRW+70d5824 was observed with a selection of
filters and the standard star was centered in each of the four CCDs. The data indicate that
any changes in the photometric throughputs due to SM3B are less than 1% in most of the
visible wavelength filters, and less than a few percent in the UV filters. The distribution
shows a mean value of 0.34 +/- 0.26 sigma (where sigma is defined separately for each filter-chip combination) for all the observations, essentially consistent with an unchanged
photometric throughput. This corresponds to about 0.4% on an absolute scale. Hence, the
response of the WFPC2 was essentially unchanged by the servicing mission. We also find
that the long-term throughput decline is consistent with the expected CTE loss.
1. Introduction
The goal of the relative photometry check (proposal 8953) was to verify that the photometric accuracy remained unchanged at the 1-2% level after the latest servicing mission
SM3b. Our regular photometric standard star, GRW+70D5824, was observed in a variety
of filters (F160BW, F170W, F185W, F218W, F255W, F300W, F336W, F439W, F555W,
F675W, and F814W), with the star centered in each of the 4 CCDs (one orbit per CCD).
This is the same photometry check performed after the previous servicing missions, which
showed no decline in the photometric performance.
HST was released by the shuttle crew at 5:04 AM EST, March 9, 2002., and WFPC2
was in protect Decon for about 11 days 6 hours, from March 12-23, 2002, during
SMOV3B. The observations were obtained on March 31, 2002, (MJD 52364) about 0.3
days after WFPC2 was in a 9-hour special Decon on March30, 2002.
2. Observations
Contaminants collect on the cold CCD windows and reduce the UV throughput of the
WFPC2. A warm up decontamination (Decon) procedure is performed monthly to evapo-
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rate contaminants from the CCD windows which is followed by a cool down. Following a
Decon on March 30, 2002 (MJD=52363.7516), observations were taken for the purpose of
determining if any chance in the photometric performance had occurred as a result of
SM3B.
Observations of standard star GRW+70d5824 (mv = 12.7, B-V = -0.09) for the photometry check were obtained on March 31, 2002 (program ID: 8953) which was
approximately 0.3 days following the Decon on March 30, 2002. The star was positioned
in the center of a camera during four 1-orbit visits, one visit per camera. The respective
single camera images were read out and sent to the ground for analysis. Observations were
obtained with filters F160BW, F170W, F185W, F218W, F255W, F300W, F336W, F439W,
F555W, F675W, and F814W. Table 1 lists the observations.
Table 1. 8953 Photometry Monitor Observations.
filter
PC1
PC1
exptime
(sec.)
WF2
WF3
WF4
WF
exptime
(sec.)
F160BW
u8bd0101r
200.0
u8bd0201r
u8bd0301r
u8bd0401r
100.0
F170W
u8bd0102r
40.0
u8bd0202r
u8bd0302r
u8bd0402r
40.0
F185W
u8bd0103r
100.0
u8bd0203r
u8bd0303r
u8bd0403r
100.0
F218W
u8bd0104r
40.0
u8bd0204r
u8bd0304r
u8bd0404r
40.0
F255W
u8bd0105r
80.0
u8bd0205r
u8bd0305r
u8bd0405r
40.0
F300W
u8bd0106r
10.0
u8bd0206r
u8bd0306r
u8bd0406r
10.0
F336W
u8bd0107r
14.0
u8bd0207r
u8bd0307r
u8bd0407r
12.0
F439W
u8bd0108r
14.0
u8bd0208r
u8bd0308r
u8bd0408r
8.0
F555W
u8bd0109r
3.5
u8bd0209r
u8bd0309r
u8bd0409r
2.3
F675W
u8bd010ar
8.0
u8bd020ar
u8bd030ar
u8bd040ar
4.0
F814W
u8bd010br
14.0
u8bd020br
u8bd030br
u8bd040br
7.0
3. Calibration and Data Reduction
The OPUS pipeline calibrated data were used for the analysis. No other calibration
steps were performed.
Photometry was performed using the APPHOT task phot with the star positions automatically identified for each camera as input1. For the PC1 frames, a photometry aperture
radius of 11 pixels was used with the sky fitting region parameters set to annulus=32 pix-
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els and dannulus=11 pixels. For the WF frames, a photometry aperture radius of 5 pixels
was used with the sky fitting region parameters set to annulus=15 pixels and dannulus=5
pixels. The sky fitting algorithm was set to “ofilter.” The centering algorithm was set to
“centroid”. The photometry values are listed in Tables 2-5. For Tables 2-5, the table headers are:
filter - WFPC2 filter used.
mjd - modified Julian Date (Julian Date - 2400000.5) for the observation.
ct_rate - countrate (DN/s) for the respective aperture.
Table 2. PC1 photometry.
filter
mjd
ct_rate
F160BW
52364.22
88.301
F170W
52364.23
170.478
F185W
52364.23
92.641
F218W
52364.24
128.388
F255W
52364.24
150.319
F300W
52364.24
941.872
F336W
52364.24
740.793
F439W
52364.24
886.434
F555W
52364.25
3724.889
F675W
52364.25
2063.758
F814W
52364.25
1355.537
filter
mjd
ct_rate
F160BW
52364.29
76.366
F170W
52364.29
192.428
F185W
52364.30
102.416
F218W
52364.30
139.858
F255W
52364.30
159.448
Table 3. WF2 photometry.
1. The scripts used for this work can be found at the following locations:
/data/denali11/smov3b/script_smov_8953 (photometry)
/data/nerys5/script_library/scripts_smov2002/script_smov_8953_autoplot_*.cl (creating Figures 1-4)
/data/denali11/smov3b/script_sigma_diagram (creating Figure 5)
data/nerys5/script_library/scripts_smov2002/script_smov_cte_comp_pc1.cl (creating Figure 6)
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filter
mjd
ct_rate
F300W
52364.30
955.157
F336W
52364.31
756.947
F439W
52364.31
883.425
F555W
52364.31
3726.059
F675W
52364.31
2098.310
F814W
52364.31
1377.432
filter
mjd
ct_rate
F160BW
52364.36
66.767
F170W
52364.36
152.701
F185W
52364.36
82.069
F218W
52364.37
127.220
F255W
52364.37
157.696
F300W
52364.37
959.739
F336W
52364.37
753.756
F439W
52364.38
859.329
F555W
52364.38
3681.950
F675W
52364.38
2036.548
F814W
52364.38
1338.025
filter
mjd
ct_rate
F160BW
52364.09
67.101
F170W
52364.09
167.927
F185W
52364.10
94.994
F218W
52364.10
138.287
F255W
52364.10
157.344
F300W
52364.10
984.035
F336W
52364.11
750.429
F439W
52364.11
865.116
F555W
52364.11
3735.275
Table 4. WF3 photometry.
Table 5. WF4 photometry.
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filter
mjd
ct_rate
F675W
52364.11
2075.017
F814W
52364.11
1363.641
4. The Results
The 8953 calibration program executed approximately 0.34 - 0.63 days following the
Decon on March 30, 2002.
Figure 1 through Figure 4 show the evolution of the photometric measurements from
1995 until the present, for nine filters and all four chips. The star represents the post SM3b data point. The data has been normalized to a mean of 1.0 based on the historical
dataset, and fitted from MJD 51000 to the present, with the exception of F675W for PC1
and WF2, which were fitted from MJD 50200 and 50700, respectively, due to the paucity
of recent data. The 1-sigma scatter (based on the empirical scatter) of the baseline measurements are shown by dashed lines.
The observed long-term trend is a decline of the throughput, to varying degrees in the
different filters. The post-SM3b data point seems to follow the most recent trend. The
overall data for several UV filters (most notably F255W) seem to show a curious sharp
downward trend around MJD 51000, and we are currently investigating possible causes.
Figure 5 shows the statistical distribution of the post-SM3b photometry data points
around the predicted values based on the fits shown in Figure 1 through Figure 4, and normalized by the 1-sigma error bars shown by the dotted lines. We find that the distribution
has a mean value of 0.34 +/- 0.26 sigma for all the observations, hence there is no obvious
change in the throughput before and after SM3b. This compares to a value of -0.17 +/0.14 for SM2 (Biretta et al. 1997). The mean value in the PC appears to be slightly higher,
with a value of 1.31 +/- 0.56 sigma. However, even this is only about a 2-sigma result, and
is probably due to low number statistics. We note that in SM2 a similar 2-sigma result was
found for the PC, but in the opposite sense (i.e. a decline instead of an increase). The fact
that the width of the distributions in Figure 5 are larger than 1 sigma, suggests that there
are other sources of uncertainty besides the historical random uncertainties, such as large
fluctuations in focus or breathing.
Figure 6 shows the observed throughput decline compared with that predicted from
Dolphin’s CTE equations (2001) for F336W, F555W, and F814W. For all three filters the
throughput decline is consistent with the expected CTE loss, within the uncertainties.
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Figure 1: Photometric Throughput for PC1 in 9 filters. The star represents the post-SM3b
data point. The data has been normalized to a mean of 1.0 based on the baseline observations, and fitted from MJD 51000 to the present, with the exception of F675W, which was
fitted from MJD 50200 due to the paucity of recent data. The 1-sigma scatter (based on the
empirical scatter) of the baseline measurements are shown by dashed lines.
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Figure 2: Same as Figure 1 for WF2. F675W was fitted from MJD 50700 due to the paucity of recent data.
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Figure 3: Same as Figure 1 for WF3.
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Figure 4: Same as Figure 1 for WF4.
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Figure 5: Statistical distribution of the post-SM3b photometry data points (number of
data points vs. sigma).
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Figure 6: Actual throughput (solid lines) vs. expected decline from Dolphin’s CTE equations (dashed lines) for F336W, F555W, and F814W for the PC1 chip. This shows that the
long-term trends are due to CTE loss, within the measurement uncertainty. The discontinuity for the early data points for F814W is due to a change in the exposure times.
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5. Conclusions and Recommendations
The WFPC2 calibration program 8953 was executed post-SM3b to check that the relative photometric throughput had not changed. Observations of the standard star
GRW+70D582 were obtained with each camera and with the normal selection of monitor
filters (11 filters). Each camera performed as expected, and WFPC2 was unaffected by the
SM3b Servicing Mission. The distribution shows a mean value of 0.34 +/- 0.26 sigma for
all the observations, consistent with an unchanged photometric throughput. Hence, the
response of the WFPC2 was essentially unchanged by the servicing mission. We also find
that the long-term throughput decline is consistent with the expected CTE loss.
6. References
Baggett, S. and Gonzaga, S. 1998, WFPC2 Long-Term Photometric Stability, WFPC2ISR-98-03. (http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9803.html)
Biretta J., Heyer I., Baggett S., et al. 1997, Results of the WFPC2 Post-Servicing Mission-2 Calibration Program, WFPC2-ISR-97-09. (http://www.stsci.edu/instruments/wfpc2/
Wfpc2_isr/wfpc2_isr9709.html)
Biretta J., McMaster M., Baggett S., and Gonzaga S. 1997, Summary of WFPC2
SM97 Plans, WFPC2-ISR-97-03. (http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/
wfpc2_isr9703.html)
Casertano, S., et al., editors, The 1997 HST Calibration Workshop Proceedings,
STScI, 1998, p.318. (http://www.stsci.edu/stsci/meetings/cal97/proceedings.html)
Dolphin, A. E. 2001 (April 2), private communication, http://www.noao.edu/staff/dolphin/wfpc2_calib/
Gonzaga, S., Ritchie, C., Baggett, S., Casertano, S., Whitmore, B., and Mutchler, M.
2000, “Standard Star Monitoring Memo”, (http://www.stsci.edu/instruments/wfpc2/
Wfpc2_memos/wfpc2_stdstar_phot3.html)
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