Technical Instrument Report WFPC2 97-01
Results of the WFPC-2 SMOV Relative
Photometry Check (prop-ID 7020)
B. Whitmore, S. Gonzaga, I. Heyer
December 1, 2008
ABSTRACT
The photometric throughput of the WFPC2 was essentially unaffected by the 1997 servicing mission. The mean change in throughput is observed/predicted = 0.9984 +/- 0.0023.
The largest deviation is a decline of 0.4 +/- 0.2% in the PC1. Even this should be considered tentative until new observations become available during the next few months, since
it represents only a 2-sigma deviation.
1. Introduction
The second HST Servicing Mission (SM97) concluded February 18, 1997. During this
mission STIS, NICMOS, and various other components were installed. While WFPC2
was not serviced, it is still important to ensure that neither the astronaut activities nor contamination from any other source, such as the installment and subsequent powering up of
new instruments, altered the performance of WFPC2. Herein we describe the procedure
and results for the WFPC2 relative photometry check (proposal 7020).
The goal of the relative photometry check was to verify that the photometric accuracy
remained unchanged at the 1-2% level. 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) on March 5, 1997 (MJD
50512), with the star centered in each of the 4 CCDs (one orbit per CCD). Each orbit
started with an observation with the F555W filter in the PC1, in order to monitor any drifts
in the throughput. This program was run one day after the “protect decon” which executed
13 days after HST release from the shuttle. WFPC2 was at its normal operating temperature of -88C. The results were compared to observations of the same object taken prior to
the servicing mission.
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2. Procedure
Photometric count rates were measured independently by using two methods, using
pipeline-calibrated data. Method #1 used IRAF scripts developed by Christine Ritchie for
routine photometric monitoring of WFPC2 over the past several years. Method #2 used
more automated IRAF scripts developed by Whitmore and Heyer (script_smov_7020 to
be found at /data/rainier1/smov). Both cases used an aperture radius of 0.5 arcsecs (5 pixels in the WF chips; 11 pixels in the PC1 chip) for the targets, and an annulus from radius
30 to 35 pixels for determining the sky values.
The agreement between both methods was very good (method #1 / method #2 =
0.9993 +/- 0.0002). The values from method #1 were used for the comparative analysis,
since this method was also used to establish the baseline measurements. The baseline measurements have been corrected for contamination using the contamination rates in ISR
WFPC2 96-04.
Figures 1a -1i show the baseline observations over roughly the past 2 years (since
MJD = 49750, i.e. Feb. 2, 1995) and the March 5, 1997, observations from proposal 7020
for the full set of filters and chips. The measurements have been normalized to a mean of
1.000 based on the baseline observations. The 1-sigma scatter (based on the empirical
scatter) of the baseline measurements are shown by dashed lines. In cases with less than
10 baseline measurements (i.e. WF2 and WF4 for all but F170W), the scatter from WF3
has been adopted. In cases where no baseline measurements were available the value on
WF3 was used.
The observations were taken on March 5, 1997 (MJD 50512.22 to 50512.44), roughly
one day after a decontamination (MJD 50511.43). One observation with the F255W filter
in the WF3 chip was excluded from the analysis because the star’s central pixel was saturated. A contamination rate of 1.7 times the pre-SMOV values has been adopted, based on
measurements from proposal 7016 (see forthcoming TIR). The contamination correction
is very small in all cases (maximum of 1.3% for F160BW on WF4), since the observations
were taken within 1 day of the decontamination. The observed scatter in the F555W observations on the PC1 taken at the start of each orbit is less than the predicted scatter, hence
no correction for a “drift” from orbit to orbit was made.
Figures 1a-1i show that in all cases the observed count rates from March 5, 1997,
are in good agreement with pre-SMOV data, showing that WFPC2 throughput was
essentially unaffected by the 1997 servicing mission. A flat mean has been adopted in
all cases, except for F160BW and F170W on the PC1 chip, where a slow increase in the
throughput as a function of time has been observed previously (see ISR WFPC2 96-4). For
these two cases a linear fit was adopted, using baseline data, to predict the value for March
5, 1997.
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Figures 2a and 2b show the observed versus predicted values for all cases with more
than 10 baseline measurements (N=22 in the resulting sample) both in terms of the ratio of
observed to predicted values, and as the 1-sigma scatter (shown as dashed lines in Figures
1a - 1i). Overall, the agreement is excellent, with only a very small (1 sigma) hint that the
throughput has decreased by about 0.16% (i.e. observed/predicted = 0.9984 +/- 0.0023).
However, there is some evidence for a slightly larger effect on the PC1 alone, at about the
2-sigma level (i.e. N=12, observed/predicted = 0.9958 +/- 0.019 for the PC1, N=8,
observed/predicted = 1.0010 +/- 0.015 for the WF3). Including all the data (i.e. chips WF2
with two baseline observations and WF4 with five baseline observations) yields a value of
observed/predicted = 0.9968 +/- 0.0026.
Table 1 shows the numbers from the analysis (from the file /data/rainier1/smov/
final_7020_shireen_brad.dat).
It is interesting to note that the observed value of the scatter is less than the predicted
scatter (i.e. the width in Figure 2a is 0.64, less than 1.0). This indicates that there is more
structure in the long-term baseline data than just random Gaussian noise. Examples can be
seen in the F170W WF3 and F218W WF3 baseline observations (Figures 1b and 1c),
where the throughput during days 49750-50000 appears to be about 3% higher than the
more recent observations.
Table 1: Proposal 7020 WFPC2 Photometry Check Results
Pre - SM97 Baseline
File
Filter
Chip
N
Post - SM97
predicted (DN/sec)
sigma(DN/sec)
Mar.5 (DN/sec)
U3SG0101R
F555W
1
61
3744.80
41.36
3708.52
2R
F170W
1
64
164.24
1.75
163.80
3R
F160BW
1
27
84.22
3.73
84.09
5R
F218W
1
27
140.16
2.63
138.78
6R
F255W
1
27
160.75
2.02
158.26
7R
F336W
1
27
773.33
9.98
773.24
8R
F439W
1
36
894.33
9.14
901.99
9R
F675W
1
27
2103.52
17.24
2093.15
AR
F814W
1
43
1359.81
15.25
1358.94
U3SG0201R
F555W
1
61
3744.80
41.36
3701.69
2R
F160BW
2
2
79.16
2.24
76.22
3R
F170W
2
12
192.37
3.78
191.99
4R
F218W
2
1
141.67
3.47
145.90
3
Post - SM97
Pre - SM97 Baseline
File
Filter
Chip
N
predicted (DN/sec)
sigma(DN/sec)
Mar.5 (DN/sec)
5R
F255W
2
2
169.13
3.27
164.78
7R
F336W
2
2
789.06
13.02
784.19
8R
F439W
2
2
905.09
7.57
909.61
9R
F555W
2
2
3811.86
34.08
3787.27
AR
F675W
2
0
INDEF
18.23
2153.77
BR
F814W
2
2
1393.16
18.24
1388.03
U3SG0301M
F555W
1
61
3744.80
41.36
3735.94
2R
F160BW
3
23
69.21
2.24
71.37
3R
F170W
3
61
160.30
2.88
158.58
4R
F218W
3
24
135.97
3.47
134.26
6R
F336W
3
24
800.19
13.02
790.90
7R
F439W
3
23
893.60
7.57
887.34
8R
F555W
3
24
3818.50
34.09
3822.54
9R
F675W
3
18
2087.88
18.23
2093.15
AR
F814W
3
24
1359.79
18.25
1380.38
U3SG0401R
F555W
1
61
3744.80
41.36
3746.28
2R
F160BW
4
5
72.80
2.24
70.45
3R
F170W
4
13
169.84
3.57
171.48
4R
F218W
4
0
INDEF
3.47
144.04
5R
F255W
4
5
168.69
3.27
167.44
6R
F336W
4
5
786.98
13.02
817.72
7R
F439W
4
5
892.54
7.57
889.03
8R
F555W
4
5
3829.37
34.08
3756.38
9R
F675W
4
0
INDEF
18.23
2097.01
AR
F814W
4
5
1379.88
18.25
1366.47
3. Summary
The throughput of WFPC2 was essentially unaffected by the 1997 servicing mission.
The mean change in throughput is observed/predicted = 0.9984 +/- 0.0023. There is a
slight (2-sigma) tendency for the PC1 chip to be 0.4% lower following the servicing mission. This result should be considered tentative until new observations become available
during the next few months.
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Figure 1a: F160BW
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Figure 1b: F170W
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Figure 1c: F218W
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Figure 1d: F255W
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Figure 1e: F336W
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Figure 1f: F439W
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Figure 1g: F555W
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Figure 1h: F675W
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Figure 1i: F814W
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Figure 2a: Observed vs Predicted Values
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Figure 2b: Distribution in terms of Sigma
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