Instrument Science Report WFPC2 98-03
WFPC2 Long-Term Photometric Stability
S. Baggett and S. Gonzaga
October 21, 1998
ABSTRACT
We examine the WFPC2 stellar photometric monitoring data obtained over the last four
years. The new analysis shows that, for bright targets, (1) the long-term photometric
throughput continues to be quite stable: fluctuations are ~2% or less peak-to-peak over 4
years in filters longwards of and including F336W, (2) the UV throughput has gradually
evolved, that is, the clean count rates (immediately after the decons) have increased over
the 4 years in some filters, e.g., F160BW by ~12% in the PC, and F170W by ~9%, while
decreasing slightly in others, e.g., F255W by ~3% and, (3) the contamination growth rates
have slowed slightly for the UV filters, e.g., ~1%/day to 0.5%/day in F160BW on the PC.
The small zeropoint discontinuity around Feb. 1995 is discussed. These results are being
used to update the WFPC2 time-dependent photometric calibration within SYNPHOT.
1. Introduction
The presence of contaminants within WFPC2 causes a gradual build-up of material on
the cold CCD faceplates of the cameras, resulting in a decrease in the UV throughput.
Regular decontamination procedures (“decons”), which turn the thermo-electric coolers
off, and turn the CCD and heat pipe heaters on, warm the cameras to +20oC and have been
successful at restoring the normal UV throughput (Table 4 in the Appendix lists all decons
to date). To quantify the effect of contamination on WFPC2 photometry, measurements of
a photometric standard star have been taken before and after the decons since on-orbit
installation of the instrument, at a variety of wavelengths. In general, the count rates of the
spectrophotometric standard have remained stable to better than 2% (see Figure 3), even
across the Second Servicing Mission in 1997 (SM97): before and after SM97, the sensitivity was shown to be the same to within 0.7% rms (Biretta et al., ISR 97-09; Whitmore,
1997). Indications of a slight secular increase in throughput had been noted in F160BW
and F170W (Whitmore et al., ISR 96-04). This report summarizes all available monitoring
data, presents the throughput and zeropoint variations found, and provides recommendations for applying a contamination correction to science observations.
1
2. Data
The spectrophotometric standard star observed is grw+70d5824, a DA3 white dwarf
with V=12.77 and B-V=-0.09 (Turnshek et al., 1990). Filters used for photometric monitoring are the standard broadband filters (F160BW, F170W, F218W, F255W, F336W,
F439W, F555W, F675W, and F814W); Table 1 summarizes the details of the calibration
programs, cameras and filters used, and frequency of observations. In general, early in the
mission, monitoring was fairly extensive and frequent, 2 chips twice a month in all filters
and all 4 chips twice a week in F170W. Since the instrument has proven to be relatively
stable from decon to decon, the photometric monitor observations are now made in only in
a single chip, after every decon, and before every other decon. The chip used is changed
every decon so that over the course of a year, all chips have been used as a monitor several
times. F555W PC images are included every month for focus measurements; they are
included here for the throughput analysis as well.
The data used in this report span the time period from May 1,1994 to June 29,1998;
data taken at the warmer -76oC operating temperature (prior to Apr 24,1994) are excluded,
as are the Dec. 1994 measurements taken with clocks=ON. Previous studies of the
grw+70d5824 photometric throughput data have examined data taken through mid-1994
(Holtzman et al. (1995a, 1995b)), through mid-1995 (Baggett et al. (ISR 96-02)), and
through mid-1996 (Whitmore et al. (ISR 96-04)). The present ISR adds to, and updates,
results from these earlier studies.
Table 1: Summary of WFPC2 Photometric Monitoring Proposals
prop
dates
cameras
filters
frequency
5563
5/01/94
to
2/13/95
PC,WF3
F160BW, F170W, F218W,
F255W, F336W, F439W,
F555W, F675W, F814W
once before and once after decon
5629
4/25/94
to
7/26/95
all
F170W
twice a week
6143
3/7/95
to
7/06/95
PC,WF3
F160BW, F170W, F218W,
F255W, F336W, F439W,
F555W, F675W, F814W
once before, once after decon
6184
7/27/95
to
6/24/96
all
F160BW, F170W, F218W,
F255W, F300W, F336W,
F439W, F555W, F675W, F814W
F170W in all 4 chips
F555W, PC
other filters had chips alternated every decon
cycle (= each chip monitored every 4 months)
6902
7/27/96
to
7/25/97
all
F160BW, F170W, F218W,
F255W, F300W, F336W,
F439W, F555W, F675W, F814W
F170W in all 4 chips
F439W, F555W, F814W in PC
other filters had chips alternated every decon
cycle (so each chip monitored every 4 months)
2
Table 1: Summary of WFPC2 Photometric Monitoring Proposals
prop
dates
cameras
filters
frequency
7016
3/30/97 to
4/07/97
all
F170W,F555W
weekly (SMOV2 monitor))
7618
7/22/97
to now
all
F160BW, F170W, F218W,
F255W, F300W, F336W,
F439W, F555W, F675W, F814W
F170W in all 4 chips
F439W, F555W, F814W in PC
other filters had chips alternated every decon
cycle except that pre-decon visits only performed every other decon
3. Analysis
Photometry
The images were calibrated using the WFPC2 pipeline (STSDAS task calwp2) along
with the best reference files available at the time of pipeline processing. All WF2, WF3,
and WF4 data, taken between December 1994 and January 1997, were recalibrated manually in order to correct a bug in calwp2 which affected all non-PC, single-chip readout
images. PC and 4-group data were not affected by the bug. Cosmic rays were identified
visually and, using the IRAF imedit task, affected pixels were replaced with an average
based on the surrounding pixels. Typically, only a few cosmic rays were found in each
image, although some of the longer exposures had up to 10-12 cosmic rays within the area
of interest. If a cosmic ray fell within the PSF or the removal was at all questionable, the
conservative approach of excluding the image was taken. Using the IRAF phot task (in the
noao.digiphot.apphot package), standard aperture photometry was performed on the
cleaned images with a 0.5” radius aperture. Sky was determined from an annulus around
the star: for PC, the annulus had an inner radius of 32 pixels and width of 11 pixels, in the
WF chips, the inner radius was 15 pixels and the width 5 pixels. The resulting count rates
in DN/sec, and their errors, are tabulated as a function of day since decon; the regularlyupdated WWW Photometric Monitoring memo contains the full table of data (Gonzaga et
al., see References for URL).
Corrections to Measured Count Rates
Before performing the throughput analysis, the measured count rates were corrected
for CTE and aperture size. In addition, the error bars for count rates in the UV were
increased to account for the noisier UV flatfields. These corrections are described in more
detail below.
A correction for the long-versus-short effect1 was considered as well, since the exposure times for some observing modes were changed over time by up to a factor of four.
However, in the end, such a correction was not applied: the total counts in the aperture
3
were at least 3070 DN and ranged up to 21000 DN. The measured median long-vs-short
error after CTE correction in the range 300 to 10000 DN is less than 0.5%; that is, the
expected corrections ranged from only 0.5% (1000 DN) to 0.05% (10,000 DN) (Casertano
& Mutchler, ISR98-02; Casertano, 1997). This correction was deemed too small to be
important.
In the past, a 4% linear ramp has been applied in order to correct for the CTE losses in
the standard star photometry (Holtzman, 1995a; Baggett et al., ISR97-10). This study
takes advantage of the new recommendations for Y- and X- CTE corrections (Whitmore
and Heyer, ISR 97-08), which correct for CTE loss based on the counts from the star, its
position in X and Y on the chip, and the background counts. Specifically, we use equations
1, 2d, and 3d, from ISR 97-08, along with average measured background count rates for
each observing mode, to compute and apply the CTE correction2. A time-dependent CTE
correction was not required (Whitmore, 1998), due to the high number of counts in each
observation.
For the aperture correction, we maintain consistency with prior SYNPHOT updates:
we adopt a flat 0.1 magnitude as the correction for 0.5” radius aperture to infinity, regardless of observing mode (Baggett et al., ISR 97-10). That is, the CTE-corrected count rates
at 0.5” radius were multiplied by a factor of 10(0.10/2.5).
Finally, since the UV flatfields are noisier than the flatfields in the optical, an additional error term was included in the UV count rate errors . The additional error amounted
to 5.2% and 1.8% for F160BW (PC and WF, respectively), 0.4% for F170W (all chips),
and 1.0% and 0.6% for F218W (PC and WF, respectively; Biretta, 1998); the error rates
for other filters are as tabulated in the WWW memo (Gonzaga et al., see References for
URL).
Throughput Decline Rates
The gradual build-up of contaminants on the CCD face plates causes the UV throughput to decline in the weeks following each decon. Since a hint of a time-dependence in the
UV decline rates had been noticed in earlier data (Whitmore et al., ISR 96-04), we began
by dividing the count rate data into four epochs. The first three groups are each one year in
length and the last one is somewhat longer: Apr 94 to Apr 95, Apr 95 to Apr 96, Apr 96 to
Apr 97, and finally Apr 97 to June 98. Within each of these epochs, the corrected count
rates for each mode were plotted as a function of day since decon and fitted via linear
regression; this provided the UV throughput decline rates and clean count rates (at the
1. The long-versus-short effect is a position-independent non-linearity in the camera that is primarily
a function of total counts: targets are unexpectedly faint in short exposures compared to long exposures.
See WFPC2 WWW pages (URL in References) for the most current information on the effect.
2. The 4% ramp was tried as well; the throughput decline rates were not significantly different from
those derived from data corrected with the ISR 97-08 CTE corrections.
4
intercept). Table 2 lists the percent decline rate per day as a function of epoch, filter, and
chip. Figure 4 in the Appendix illustrates the fits for PC, last epoch (Apr 97-Jul 98).
As illustrated in Table 2, the decline rates in the UV have generally slowed over time.
The largest change is seen in F160BW for the PC: from ~0.9% decline/per to 0.6%/day.
The other filters and cameras show less dramatic changes over time. This may indicate
that some of the contaminants are slowly dissipating from WFPC2.
Table 2: Throughput Decline Rates for GRW+70D5824
PC
WF2
WF3
WF4
filter
yeara
Nb
% decline
per day
error
N
% decline
per day
error
N
% decline
per day
error
N
% decline
per day
error
160
4/94-4/95
23
-0.885
0.104
--
--
--
23
-1.277
0.038
--
--
--
160
4/95-4/96
12
-0.840
0.137
5
-1.281
0.089
9
-1.227
0.057
3
-0.930
0.102
160
4/96-4/97
6
-0.690
0.159
4
-1.303
0.094
6
-1.142
0.069
5
-1.079
0.067
160
4/97-6/98
9
-0.613
0.138
6
-1.202
0.077
6
-1.093
0.065
7
-0.893
0.067
170
4/94-4/95
75
-0.564
0.009
51
-0.949
0.011
75
-0.988
0.009
51
-0.801
0.012
170
4/95-4/96
39
-0.516
0.012
33
-0.901
0.012
39
-0.956
0.011
33
-0.736
0.013
170
4/96-4/97
32
-0.509
0.012
29
-0.901
0.011
29
-0.943
0.012
29
-0.752
0.012
170
4/97-6/98
28
-0.452
0.013
28
-0.804
0.011
28
-0.772
0.012
28
-0.643
0.012
218
4/94-4/95
23
-0.478
0.028
--
--
--
23
-0.846
0.019
--
--
--
218
4/95-4/96
12
-0.433
0.036
6
-0.757
0.039
10
-0.787
0.026
4
-0.704
0.047
218
4/96-4/97
6
-0.442
0.044
--
--
--
6
-0.845
0.033
--
--
--
218
4/97-6/98
7
-0.398
0.046
--
--
--
3
-0.633
0.047
--
--
--
255
4/94-4/95
23
-0.236
0.008
--
--
--
23
-0.471
0.007
--
--
--
255
4/95-4/96
12
-0.235
0.007
5
-0.445
0.013
10
-0.490
0.007
4
-0.385
0.013
255
4/96-4/97
6
-0.215
0.008
4
-0.398
0.017
5
-0.333
0.013
6
-0.298
0.011
255
4/97-6/98
9
-0.183
0.007
6
-0.311
0.014
6
-0.327
0.012
7
-0.234
0.012
300
4/95-4/96
--
--
--
5
-0.260
0.015
--
--
--
--
--
--
336
4/94-4/95
23
-0.031
0.007
--
--
--
23
-0.198
0.007
--
--
--
336
4/95-4/96
12
-0.108
0.008
5
-0.188
0.013
10
-0.208
0.009
4
-0.226
0.014
336
4/96-4/97
6
-0.068
0.009
4
-0.286
0.014
6
-0.285
0.010
6
-0.153
0.010
336
4/97-6/98
9
-0.062
0.008
6
-0.177
0.012
6
-0.181
0.010
7
-0.053
0.010
439
4/94-4/95
23
-0.008
0.008
--
--
--
23
-0.078
0.008
--
--
--
439
4/95-4/96
12
-0.037
0.007
5
-0.121
0.013
9
-0.080
0.009
4
-0.127
0.014
5
Table 2: Throughput Decline Rates for GRW+70D5824
PC
WF2
WF3
WF4
filter
yeara
Nb
% decline
per day
error
N
% decline
per day
error
N
% decline
per day
error
N
% decline
per day
error
439
4/96-4/97
11
-0.037
0.007
4
-0.146
0.017
6
-0.083
0.011
6
-0.031
0.011
439
4/97-6/98
19
-0.026
0.005
5
-0.147
0.017
6
-0.094
0.012
5
-0.014
0.014
555
4/94-4/95
23
-0.018
0.007
--
--
--
23
-0.067
0.007
--
--
--
555
4/95-4/96
24
-0.039
0.005
6
-0.050
0.013
10
-0.051
0.009
4
0.016
0.016
555
4/96-4/97
32
-0.028
0.004
14
-0.055
0.011
16
-0.012
0.009
16
0.023
0.009
555
4/97-6/98
33
-0.010
0.004
6
-0.048
0.012
6
-0.031
0.011
7
-0.020
0.011
675
4/94-4/95
23
-0.009
0.008
--
--
--
23
-0.009
0.008
--
--
--
675
4/95-4/96
12
-0.012
0.006
6
-0.009
0.013
10
0.006
0.009
4
-0.020
0.016
675
4/96-4/97
6
-0.034
0.007
--
--
--
0
0.000
0.000
--
--
--
675
4/97-6/98
8
0.024
0.006
--
--
--
--
--
--
3
0.013
0.016
814
4/94-4/95
23
0.041
0.007
--
--
--
23
0.005
0.008
--
--
--
814
4/95-4/96
12
-0.014
0.006
6
-0.001
0.012
10
-0.038
0.009
4
0.045
0.015
814
4/96-4/97
13
-0.005
0.005
4
-0.035
0.015
6
-0.005
0.011
6
0.012
0.010
814
4/97-6/98
30
-0.000
0.003
6
-0.087
0.012
6
0.014
0.010
7
-0.043
0.010
a. Epochs boundaries are at Apr. 25, 1994 (MJD=49467), Apr. 25, 1995 (MJD=49832), Apr. 25, 1996
(MJD=50198), Apr. 25, 1997 (MJD=50563), and Jun. 29, 1998 (MJD=50993).
b. N is the number of observations.
Post-Decon (clean) Throughput
The linear fits also reveal that there have been changes in the “clean” count rates, that
is, countrates immediately after the decons. The changes in intercept for PC are plotted in
Figure 1, as count rates vs year. The “percent change” at the bottom of each plot refers to
the percent difference between the maximum and minimum intercept values for each filter.
For completeness, the computed values for all cameras are tabulated in Table 5 in the
Appendix. The PC experienced the most dramatic change over the four years: ~12%
improvement in F160BW and ~9% in F170W. The F218W and F255W clean count rates
showed changes of 2-3%, while other filters showed changes of less than 2%. WF3
improved by ~5% in F160BW but experienced a 4.5% decrease in F170W and F218W.
These results are not completely understood; certainly the PC changes -- the higher
clean count rates and the improving throughput decline rates (see previous section) -could be interpreted as the slow outgassing of a long-lived contaminant. However, there
must be additional factors at work since not all clean count rates appear to be improving.
For example, the PC, F255W clean count rates appear to be decreasing over the same
6
interval that the F160BW and F170W clean count rates are increasing (see Figure 1) while
in WF3, the clean count rates improve in F160BW (by ~5%) and decrease in F170W and
F218W (both by ~5%). These differences may be due to changes in the scattering and
absorbing properties of the contaminant mix, which in turn are due to the composition of
the contaminants changing over time (e.g., certain types of contaminants outgassing while
other contaminants remain and begin to dominate the properties of the mix).
Figure 1: Intercept values (clean count rates) for PC as a function of year.
Corrections for SYNPHOT zeropoints
Another way to view the intercept changes is presented in Figure 2. Here, the corrected
count rates (including contamination correction) are plotted as a function of Modified
Julian Date (MJD). Each plot represents a camera+filter combination, noted in the title of
each plot; not all filters are available for all cameras. For each plot, all count rates were
normalized to the average of the data taken between MJD 49750-50508, marked on the
plot with a horizontal bar. This was done because the average of the contamination-corrected data in this time interval were used to derive the most recent SYNPHOT zeropoint
values (i.e., PHOTFLAM; Baggett et al., ISR 97-10).
Discontinuity near February 1995
The changes in the clean count rates are clearly visible, along with an obvious discontinuity around Feb. 1995 (MJD~49770, marked on the plot with a vertical line), a feature
first noted by Whitmore & Heyer (ISR97-08). The cause for the discontinuity is not completely clear; as noted in ISR97-08, it may be due to a change in exposure times during
7
that time, although as discussed earlier, the corrections for the long-vs-short effect are too
small to account for the offset seen in most of the clean count rates. Another possibility is
that the discontinuity is due to changing positions on the chip or distance from chip center:
before Feb. 1995, the star ranged from 200 to 600 pixels in X and in Y on the chip (from
~50 to ~100 pixels or more in radius from the center) while after Feb. 1995, the position
remained fairly stable near the center of each chip. However, we note that for many camera and filter combinations, the longterm changes after the discontinuity have been such
that the clean count rates currently are nearly equivalent to the clean count rates before the
discontinuity. For example, in F218W on PC, the contamination-corrected normalized
countrate was ~0.95 around May 1994, was near ~1.02 at the time of the discontinuity in
Feb. 1995, and now in Oct. 1998, is back down near 0.95 (see Figure 2). These post-discontinuity longterm changes cannot be due to either exposure time or positional changes,
since those characteristics have remained constant since Feb. 1995.
Quantifying the Longterm Changes
To allow researchers some flexibility in the type of corrections they may wish to apply,
two types of fits were performed to the data. First, linear fits were determined for data in
the time periods before and after Feb. 1995, and are tabulated in Table 3 as well as
included in the plots in Figure 2. To use the linear fits, simply compute the estimated shift
from the slopes and intercepts listed in Table 3, that is, the correction C = Slope(from
Table 3) * MJD (science observation) + intercept.
In addition, we provide plain averages of the normalized, clean count rates, both
before and after the SYNPHOT epoch3 (MJD 49750-50508); these count rates were normalized to the average of the clean count rates from the SYNPHOT epoch. To apply this
correction (instead of the linear fits), determine the science data’s epoch from the image
header and apply the average shift listed in Table 3. The SYNPHOT contamination tables
incorporate these average offsets. For data taken before and after the SYNPHOT update
epoch (prior to MJD 49750 and after MJD 50508, respectively), SYNPHOT includes both
a monthly decline correction (Table 2) and a longterm correction (averages from Table 3).
For data taken during the SYNPHOT update epoch (MJD 49750-50508), SYNPHOT will
compute only a decline correction based on the number of days since a decon (longterm
correction is set to 1 during this time period.
The “SYNPHOT epoch” here is defined as the time range of the data that went
into generating the most recent zeropoints in SYNPHOT (i.e., PHOTFLAMs; see ISR
97-10).
3.
8
Figure 2: Contamination-corrected normalized count rates as a function of MJD.
9
Table 3: Long-Term Corrections to Zeropoints
MJD < 49772 (Feb 24,1995)
MJD > 494772
MJD < 49750
MJD > 50508
cam
filt
intercept
slope
intercept
slope
average
average
PC
160
-14.67 +/- 3.24
3.14e-04 +/- 6.53e-05
-2.50 +/- 0.61
7.00e-05 +/- 1.21e-05
0.911 +-1.087e-03
1.044+-1.578e-03
PC
170
-7.29 +/- 0.45
1.66e-04 +/- 9.07e-06
-2.23 +/- 0.08
6.46e-05 +/- 1.66e-06
0.948 +-2.831e-04
1.037+-3.365e-04
PC
218
-3.32 +/- 1.17
8.65e-05 +/- 2.35e-05
3.53 +/- 0.22
-5.05e-05 +/- 4.40e-06
0.966 +-6.443e-04
0.975+-2.107e-04
PC
255
0.17 +/- 0.65
1.68e-05 +/- 1.31e-05
2.71 +/- 0.09
-3.42e-05 +/- 1.78e-06
1.00 3 +-3.867e-04
0.979+-1.584e-04
PC
336
-0.18 +/- 0.27
2.32e-05 +/- 5.50e-06
1.87 +/- 0.04
-1.73e-05 +/- 8.49e-07
0.972 +-7.089e-05
0.989+-3.450e-05
PC
439
-0.59 +/- 0.28
3.16e-05 +/- 5.70e-06
1.31 +/- 0.03
-6.20e-06 +/- 5.98e-07
0.974 +-8.577e-05
0.997+-8.727e-05
PC
555
-0.28 +/- 0.13
2.55e-05 +/- 2.57e-06
1.75 +/- 0.01
-1.50e-05 +/- 2.20e-07
0.987 +-6.310e-05
0.990+-5.706e-05
PC
675
-0.09 +/- 0.19
2.14e-05 +/- 3.78e-06
1.49 +/- 0.02
-9.70e-06 +/- 4.83e-07
0.975 +-3.603e-05
0.992+-6.172e-06
PC
814
0.19 +/- 0.24
1.57e-05 +/- 4.80e-06
0.71 +/- 0.02
5.73e-06 +/- 3.66e-07
0.968 +-6.847e-05
1.002+-4.731e-05
W2
160
---
---
0.27 +/- 0.64
1.44e-05 +/- 1.27e-05
---
1.002+-2.589e-04
W2
170
1.96 +/- 0.51
-1.91e-05 +/- 1.02e-05
2.40 +/- 0.08
-2.80e-05 +/- 1.63e-06
1.018+-4.202e-04
0.988+-3.723e-04
W2
255
---
---
2.55 +/- 0.17
-3.10e-05 +/- 3.29e-06
---
0.980+-1.350e-04
W2
336
---
---
2.26 +/- 0.07
-2.52e-05 +/- 1.43e-06
---
0.980+-2.521e-04
W2
439
---
---
1.53 +/- 0.07
-1.06e-05 +/- 1.44e-06
---
0.992+-3.715e-05
W2
555
---
---
1.34 +/- 0.03
-6.73e-06 +/- 6.43e-07
---
0.994+-9.891e-05
W2
814
---
---
0.32 +/- 0.05
1.35e-05 +/- 1.06e-06
---
1.005+-1.227e-04
W3
160
3.66 +/- 2.25
-5.40e-05 +/- 4.53e-05
-0.74 +/- 0.43
3.48e-05 +/- 8.60e-06
0.980+-2.830e-04
1.026+-2.551e-04
W3
170
7.69 +/- 0.47
-1.35e-04 +/- 9.54e-06
3.19 +/- 0.08
-4.38e-05 +/- 1.66e-06
1.018+-3.217e-04
0.977+-2.232e-04
W3
218
---
---
3.32 +/- 0.26
-4.63e-05 +/- 5.27e-06
---
0.980+-5.552e-04
W3
255
1.23 +/- 0.60
-4.53e-06 +/- 1.21e-05
2.47 +/- 0.10
-2.93e-05 +/- 2.03e-06
1.002+-1.450e-04
0.984+-4.288e-04
W3
336
1.61 +/- 0.26
-1.28e-05 +/- 5.23e-06
1.38 +/- 0.05
-7.56e-06 +/- 8.98e-07
0.980+-1.686e-04
0.994+-2.105e-04
W3
439
0.10 +/- 0.27
1.78e-05 +/- 5.43e-06
1.27 +/- 0.04
-5.35e-06 +/- 8.86e-07
0.978+-1.186e-04
0.994+-1.470e-04
W3
555
0.13 +/- 0.12
1.73e-05 +/- 2.44e-06
1.96 +/- 0.02
-1.93e-05 +/- 3.86e-07
0.987+-8.351e-05
0.983+-1.018e-04
W3
675
2.41 +/- 0.18
-2.85e-05 +/- 3.62e-06
2.48 +/- 0.15
-2.96e-05 +/- 3.01e-06
0.994+-1.071e-04
---
W3
814
2.95 +/- 0.22
-3.97e-05 +/- 4.51e-06
1.09 +/- 0.03
-1.82e-06 +/- 6.93e-07
0.979+-1.829e-04
0.994+-1.933e-05
W4
160
---
---
-2.92 +/- 0.61
7.79e-05 +/- 1.20e-05
---
1.040+-8.507e-04
W4
170
4.23 +/- 0.54
-6.44e-05 +/- 1.09e-05
2.28 +/- 0.08
-2.56e-05 +/- 1.51e-06
1.033+-2.250e-04
0.985+-3.399e-04
W4
255
---
---
1.50 +/- 0.13
-9.87e-06 +/- 2.59e-06
---
0.996+-1.085e-04
W4
336
---
---
3.09 +/- 0.06
-4.16e-05 +/- 1.20e-06
---
0.986+-3.099e-04
10
Table 3: Long-Term Corrections to Zeropoints
MJD < 49772 (Feb 24,1995)
MJD > 494772
MJD < 49750
MJD > 50508
cam
filt
intercept
slope
intercept
slope
average
average
W4
439
---
---
2.12 +/- 0.07
-2.23e-05 +/- 1.33e-06
---
0.994+-2.590e-05
W4
555
---
---
0.67 +/- 0.03
6.60e-06 +/- 5.44e-07
---
1.000+-1.273e-04
W4
814
---
---
0.11 +/- 0.04
1.77e-05 +/- 8.79e-07
---
1.011+-1.544e-05
4. Correcting for Contamination
The calibration pipeline does not automatically correct for contamination (e.g., the
PHOTFLAM value in the data headers assume no contamination). The correction must be
applied manually using the results in Tables 2 through 4 or using SYNPHOT. Alternatively, the stellar monitor results, posted and updated regularly on WWW, can be used
directly rather than relying on the current linear model used to populate the SYNPHOT
contamination tables.
In SYNPHOT tasks, the “obsmode” field defines the observation mode of the image,
similar to the group parameter PHOTMODE in the science data headers. The time-dependent photometric calibration can be included in the “obsmode” field via the “cont#”
keyword, followed by the Modified Julian Date of the exposure. The implementation of
the contamination correction in SYNPHOT has been previously described (Baggett et al.,
ISR 96-02); those details are not repeated here, but for convenience, we include examples
of using SYNPHOT to compute the contamination correction. MJD is provided in the
header keyword EXPSTART or the STSDAS task ‘epoch’ can convert the calendar date to
MJD:
epoch "aug 27,1998:00:00:00" qual="" dmy="mdy" printout=mjd,date
27 Aug 1998 00:00:00.00000
MJD 51052.000000000
THU
For example, for WFPC2, camera 1, without the correction for contamination, the
SYNPHOT command and output are:
calcphot “wfpc2,1,f336w,a2d7,cal” spect=crcalspec$grw_70d5824_005.tab
form=counts
Mode = band(wfpc2,1,f336w,a2d7,cal)
Pivot
Equiv Gaussian
Wavelength
FWHM
3359.492
481.5496 band(wfpc2,1,f336w,a2d7,cal)
Spectrum: crcalspec$grw_70d5824_005.tab
VZERO
(COUNTS s^-1 hstarea^-1)
0.
1733.664
11
The grw+70d5824 spectrum came from the cdbs/calspec directories that are part of the
installation of STSDAS/IRAF and the SYNPHOT tables. The spectra are also available on
the WWW4. The same computation, including the contamination correction shows:
calcphot "wfpc2,1,f336w,a2d7,cal,cont#51052.0" spect=crcalspec$grw_70d5824_005.tab form=counts
Mode = band(wfpc2,1,f336w,a2d7,cal,cont#51052.0)
Pivot
Equiv Gaussian
Wavelength
FWHM
3359.155
481.683 band(wfpc2,1,f336w,a2d7,cal,cont#51052.0)
Spectrum: crcalspec$grw_70d5824_005.tab
VZERO
(COUNTS s^-1 hstarea^-1)
0.
1686.405
The obsmode must be enclosed in double quotes to prevent SYNPHOT from assuming
that the characters to the right of ‘#’ are part of a comment. The correction in this example,
~3%, is not very large since MJD=51052 is only ~5.5 days after a decon procedure; the
majority of the change is due to the zeropoint offset. The SYNPHOT value is an approximate correction only, due to the interpolation in time and wavelength that is performed.
Instead of using calcphot, the bandpar or calcband tasks may be used with the same
updated OBSMODE keyword. These two tasks will both compute the photometric keywords including PHOTFLAM (equivalent to URESP); the calcband task also provides a
synthetic passband table5 of wavelength versus throughput. For example, to compute
updated photometric keywords, including a contamination correction, for a WFPC2, PC1
observation using F336W and gain 7:
bandpar obsmode=”wfpc2,1,f336w,a2d7,cal,cont#51052.0” photlist=all
refwav=indef wavetab=””
# OBSMODE
URESP6 PIVWV
BANDW
wfpc2,1,f336w,a2d7,cal,cont#51052.0 5.7726E-17
3359.2
# OBSMODE
FWHM
TPEAK
AVGWV
wfpc2,1,f336w,a2d7,cal,cont#51052.0
481.68
0.0048011
# OBSMODE
QTLAM
EQUVW
RECTW
wfpc2,1,f336w,a2d7,cal,cont#51052.0
6.7412E-4
2.2582
# OBSMODE
EMFLX
REFWAVE
TLAMBDA
wfpc2,1,f336w,a2d7,cal,cont#51052.0 2.9388E-14
3368.5
204.55
3368.5
470.35
0.0044357
or
4. Standard spectra on WWW can be found at:
http://www.stsci.edu/ftp/instrument_news/Observatory/cdbs/astronomical_catalogs_alt.html
(see also Synphot User’s Guide, Appendix B)
5. The calcband output table is a standard STSDAS binary table; the wavelength and throughput data
can be plotted (e.g., using tasks sgraph or plband), printed to an ascii table (tprint); the photometric keywords can be accessed by dumping the table (tdump task ) to view the table header keywords.
6. The uresp parameter corresponds to the PHOTFLAM header keyword.
12
calcband obsmode=”wfpc2,2,f336w,a2d7,cal,cont#51052.0” output=mybandpass
wavetab=””
Calcband computes an STSDAS table of wavelength and throughput; the photometry
parameters are stored as table header keywords, which can be viewed, e.g., via tprint:
tprint mybandpass row=0 prparam+ showhdr+
# Table mybandpass.tab Tue 10:05:08 29-Sep-98
#K GRFTABLE mtab$i9l1044hm.tmg
#K CMPTABLE mtab$i9l1044im.tmc
#K EXPR wfpc2,2,f336w,a2d7,cal,c
#K APERAREA 45238.93
#K ZEROPT -21.1
#K URESP 5.444830E-17
#K PIVWV 3358.973
#K BANDW 204.723
#K TPEAK 0.005087134
#K EQUVW 2.394235
#K RECTW 470.6451
#K EMFLX 2.767610E-14
# WAVELENGTH
THROUGHPUT
#
angstroms
In these examples, bandpar and calcband were allowed to generate their own default
wavelength set to cover the passband wavelength range (as set by wavetab), which is sufficient for most filters. For narrow band filters, however, smaller wavelength steps are
required: the STSDAS task genwave can be used to compute a custom wavelength table
for use in the bandpar (calcband) wavetab parameter.
5. Summary and Recommendations
Based upon the more than 4 years of photometric data available for the bright spectrophotometric standard grw+70d5824, we have found that the long-term photometric
throughput continues to be stable to better than ~2% in the visible wavelength filters. In
addition, the contamination growth rates have slowed slightly for the UV filters, and the
throughput after decons has generally improved in the far UV. These results have been
summarized in Tables 2, 3, and 4, and are being used to update the SYNPHOT WFPC2
time-dependent photometric calibration.
To correct UV data for the variable decline rates and zeropoint shifts:
1) Use the decline rates from Table 2 to correct for contamination based on the number
of days since a decon procedure for the science exposure (see Table 4 for decon dates),
and consider including a small zeropoint offset using either the fits or the epoch averages
from Table 3. In general, filters redward of F336W do not require a correction based on
day since decon but may require a small zeropoint correction.
13
2) Or, use the updated SYNPHOT contamination tables, which contain both monthly
contamination effects as well as the longterm zeropoint changes (i.e., run calcphot, calcband, or bandpar). Note, however, that SYNPHOT linearly interpolates in both
wavelength and date to obtain the final contamination estimate; the only wavelengths currently in the SYNPHOT contamination tables are the central wavelengths of the
monitoring filters.
3) Or, use the WWW table of detailed monitoring results to obtain data close in time
and wavelength to the science observation and derive an independent model of the contamination effects.
6. References
Baggett, S., “WFPC2 SYNPHOT Tables,” May 1997 (http://www.stsci.edu/ftp/
instrument_news/WFPC2/Wfpc2_phot/wfpc2_synphot.html).
Baggett, S., and Whitmore, B., “WFPC2 History File” (http://www.stsci.edu/ftp/
instrument_news/WFPC2/Wfpc2_memos/wfpc2_history.html).
Baggett, S., Casertano, S., Gonzaga, S., and Ritchie, C., “WFPC2 Synphot Table
Update,” ISR 97-10, Oct 1997.
Baggett, S., Sparks, W., Ritchie, C., and MacKenty, J., “Contamination Correction in
SYNPHOT for WFPC2 and WF/PC-1,”, ISR 96-02, Feb 1996.
Biretta, J., private communication.
Biretta, J., Heyer, I., and Baggett, S., “Results of the WFPC2 Post-Servicing Mission2 Calibration Program,” ISR 97-09, Sep 1997.
Bushouse, H., “STSDAS Synphot User’s Guide,” Version 1.3.3, March 1995. (http://
ra.stsci.edu/STSDAS.html under Documentation).
Casertano, S., “WFPC2 Photometric Calibration,” HST Calibration Workshop, Sep
1997, pg 327 (http://www.stsci.edu/stsci/meetings/cal97/proceedings.html).
Casertano, S., and Ferguson, H., “Status Report on CTE and Photometric Differences
Between Long and Short Exposure Times,” (http://www.stsci.edu/ftp/instrument_news/
WFPC2/Wfpc2_cte/ctetop.html).
Casertano, S., and Mutchler, M., “Long vs. Short Exposure Anomaly in WFPC2
Images,” ISR 98-02, Sep 1998.
Gonzaga, S., et al., “WFPC 2 Photometric Monitoring of Standard Star” (http://
www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_memos/wfpc2_stdstar_phot3.html).
14
Holtzman, J., Hester, J., Casertano, S., Trauger, J., Ballester, G., Burrows, C., Clarke,
J., Crisp, D., Gallegher, J., Griffiths, R., Hoessel, J., Mould, J., Scowen, P., Stapelfeldt, K.,
Watson, A., and Westphal, J., “The Performance and Calibration of the WFPC2,” PASP
107,156 (1995a).
Holtzman, J., Burrows, C., Casertano, S., Hester, J., Trauger, J., Watson, A., Worthey,
G., “The Photometric Performance and Calibration of WFPC2” PASP, 107, 1065(1995b).
Turnshek, D.A., Bohlin, R.C., Williamson, R.L., Lupie, O.L., and Koornneef, J., “An
Atlas of HST Photometric, Spectrophotometric, and Polarimetric Calibration Objects”, AJ
99, 1243.
Whitmore, B., “WFPC2 Status and Overview,” HST Calibration Workshop, Sep 1997,
pg 317 (http://www.stsci.edu/stsci/meetings/cal97/proceedings.html).
Whitmore, B., “Time Dependence of the Charge Transfer Efficiency on the WFPC2,”
July 1998 (http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_advisory.html).
Whitmore, B., Heyer, I., and Baggett, S., “Effects of Contamination on WFPC2 Photometry,” ISR 96-04, June 1996.
General WWW References
WFPC2 -- http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_top.html
WFPC2 ISRs -- http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_bib.html
WWW memos -- http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_doc.html
Spectral Atlases -- http://www.stsci.edu/ftp/instrument_news/Observatory/cdbs/
astronomical_catalogs_alt.html
Papercopies may be obtained via request to help@stsci.edu.
7. Acknowledgements
We thank John Biretta and Stefano Casertano for useful discussions, John Biretta for
comments on this ISR, and Mike Wiggs for assistance in keeping the monitoring data upto-date on the WWW.
15
8. Appendix
Figure 3: Photometric Monitoring Results in PC1 and WF3, from Feb. 4/94 - 4/95
through Aug. 1998 (from WWW Memo, Gonzaga et al.). Data for the other cameras show
similar trends. For UV filters, the data oscillate between high, post-decon (clean) values
and low, pre-decon (contaminated) values.
16
Table 4. Summary of WFPC2 Decontamination Dates and Parameters (from the WFPC2
History Memo on the WWW (http://www.stsci.edu/ftp/instrument_news/WFPC2/
Wfpc2_memos/wfpc2_history.html).
date
MJD
ta
1994
date
MJD
ta
date
MJD
ta
date
MJD
ta
2 Jun 18:30
49870.7708
6
18 Oct 07:46
50374.3236
6
13 Oct 18:00
50734.7506
6
Feb 22 11:37
49405.4840
6
27 Jun 20:00
49895.8333
6
12 Nov 09:40
50399.4031
6
14 Nov 05:19
50766.2217
6
Mar 24 11:08
49435.4639
6
30 Jul 08:50
49928.3681
6
15 Dec 00:00
50432.0417
8
10 Dec 09:40
50792.4027
6
Apr 24 00:49
49466.0340
6b
27 Aug 05:43
49956.2382
6
19 Dec 12:33
50436.5229
6
May 23 15:00
49495.6250
5.5
22 Sep 03:40
49982.1528
6
Jun 13 11:02
49516.4597
12
17 Oct 09:43
50007.4053
6
07 Jan 23:41
50455.9875
Jul 10 11:40
49543.4861
12
15 Nov 08:53
50036.3706
6
09 Feb 00:00
Jul 28 07:12
49561.3000
12
14 Dec 07:03
50065.2929
6
Aug 27 09:46
49591.4069
12
Sep 25 00:46
49620.0319
12
11 Jan 23:24
50093.9750
Oct 21 00:41
49646.0285
12
11 Feb 00:30
Nov 19 17:29
49675.7285
6
Dec 18 06:00
49704.2500
6
1995
1998
Jan 08 00:03
50821.0025
6
6
Feb 01 19:15
50845.8021
6
50488.0006
6
Mar 06 09:18
50878.3877
6
23 Feb 19:08
50502.7978
6
Mar 31 12:54
50903.5376
6
27 Feb 06:31
50506.2721
6
May 02 12:26
50935.5186
6
6
04 Mar 10:16
50511.4278
6
Jun 07 21:01
50971.8757
24
50124.0208
6
21 Mar 03:35
50528.1494
6
Jun 09 23:59
50973.9993
24
10 Mar 00:21
50152.0147
6
05 Apr 08:50
50543.3681
6
Jun 12 08:01
50975.3340
24
02 Apr 00:16
50175.0111
6
25 Apr 23:00
50563.9583
6
Jun 25 06:59
50989.2910
24
04 May 17:09
50207.7146
6
15 May 20:18
50583.8460
6
Jun 28 14:06
50992.5881
6
1997
1996
13 Jan 16:14
49730.6764
6
28 May 06:16
50231.2614
6
07 Jun 13:06
50606.5461
6
Jul 22 18:56
51016.7889
6
12 Feb 01:54
49760.0792
6
22 Jun 22:15
50256.9277
6
24 Jun 11:04
50623.4612
6
Aug 21 12:23
51046.5161
6
11 Mar 14:30
49787.6042
6
28 Jul 13:34
50292.5653
6
24 Jul 18:42
50653.7795
6
Sep 15 02:18
51071.0963
6
8 Apr 10:29
49815.4368
6
23 Aug 10:10
50318.4242
6
20 Aug 02:17
50680.0952
6
Oct 14 02:39
51100.1104
6
7 May 01:13
49844.0507
6
18 Sep 16:25
50344.6840
6
17 Sep 17:24
50708.7256
6
a. ‘t’ is length of time (in hours) that the chips were kept warm.
b. Operating temperature set colder (-88˚C).
17
Figure 4: Normalized count rates as a function of day since decon, for PC, Apr 97 to Jul
98 (see Table 2 for all fit results). Count rates have been corrected for CTE, aperture, and
flatfield noise (as described on page 4).
18
Table 5: Computed Clean Count Rates
PC
WF2
WF3
WF4
filt
year
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
160
4/94-4/95
23
84.5
1.6
-
-
-
23
76.2
0.5
-
-
-
160
4/95-4/96
12
90.2
2.5
5
92.5
1.6
9
78.0
0.9
3
76.6
1.8
160
4/96-4/97
6
92.7
3.2
4
92.8
1.6
6
78.4
1.1
5
80.7
1.1
160
4/97-6/98
9
94.9
2.2
6
93.0
1.3
6
79.8
1.0
7
82.2
1.0
170
4/94-4/95
75
177.4
0.3
51
221.6
0.4
75
188.1
0.3
51
198.8
0.4
170
4/95-4/96
39
184.0
0.4
33
216.1
0.5
39
185.0
0.4
33
191.5
0.5
170
4/96-4/97
32
189.6
0.4
29
215.2
0.4
29
180.7
0.4
29
192.4
0.4
170
4/97-6/98
28
193.5
0.4
28
213.1
0.4
28
179.7
0.4
28
188.8
0.4
218
4/94-4/95
23
196.9
1.0
-
-
-
23
161.8
0.5
-
-
-
218
4/95-4/96
12
200.2
1.4
6
169.3
1.1
10
159.7
0.8
4
172.9
1.6
218
4/96-4/97
6
195.2
1.8
-
-
-
6
157.3
1.0
-
-
-
218
4/97-6/98
7
195.2
1.3
-
-
-
3
154.8
1.3
-
-
-
255
4/94-4/95
23
194.7
0.3
-
-
-
23
200.2
0.2
-
-
-
255
4/95-4/96
12
193.6
0.3
5
197.5
0.5
10
201.7
0.3
4
198.3
0.5
255
4/96-4/97
6
190.3
0.3
4
194.2
0.6
5
195.1
0.6
6
193.4
0.4
255
4/97-6/98
9
189.1
0.2
6
192.2
0.4
6
196.9
0.4
7
194.6
0.4
300
4/95-4/96
-
-
-
5
1179.0
2.4
-
-
-
-
-
-
336
4/94-4/95
23
907.5
1.1
-
-
-
23
919.4
1.1
-
-
-
336
4/95-4/96
12
926.4
1.4
5
925.2
2.2
10
929.0
1.5
4
937.1
2.6
336
4/96-4/97
6
920.2
1.7
4
925.2
2.2
6
937.2
1.8
6
906.2
1.7
336
4/97-6/98
9
913.7
1.1
6
903.4
1.7
6
923.1
1.5
7
905.3
1.6
439
4/94-4/95
23
1043.4
1.4
-
-
-
23
1018.9
1.3
-
-
-
439
4/95-4/96
12
1056.9
1.5
5
1061.4
2.4
9
1027.8
1.7
4
1046.5
2.8
439
4/96-4/97
11
1056.4
1.4
4
1058.7
2.9
6
1038.2
2.1
6
1023.2
2.1
439
4/97-6/98
19
1052.1
0.9
5
1050.5
2.4
6
1024.8
2.0
5
1026.6
3.0
555
4/94-4/95
23
4427.5
5.4
-
-
-
23
4342.5
5.2
-
-
-
555
4/95-4/96
24
4428.2
4.0
6
4392.7
9.3
10
4373.9
7.4
4
4338.5
13.1
555
4/96-4/97
32
4412.1
3.3
14
4355.5
5.7
16
4319.7
5.1
16
4315.5
5.0
19
Table 5: Computed Clean Count Rates
PC
WF2
WF3
WF4
filt
year
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
N
count rate
(DN/s)
error
555
4/97-6/98
33
4388.6
2.8
6
4375.7
8.8
6
4326.0
7.5
7
4355.8
7.8
675
4/94-4/95
23
2470.4
3.2
-
-
-
23
2404.3
3.3
-
-
-
675
4/95-4/96
12
2489.8
3.0
6
2463.5
5.3
10
2418.3
4.2
4
2460.0
7.5
675
4/96-4/97
6
2495.2
3.5
-
-
-
-
-
-
-
-
-
675
4/97-6/98
8
2471.3
2.5
-
-
-
-
-
-
3
2418.6
8.9
814
4/94-4/95
23
1559.7
2.0
-
-
-
23
1549.1
2.1
-
-
-
814
4/95-4/96
12
1588.8
1.8
6
1610.2
3.3
10
1581.5
2.6
4
1573.9
4.5
814
4/96-4/97
13
1599.1
1.4
4
1613.9
3.9
6
1591.1
3.1
6
1583.1
2.9
814
4/97-6/98
30
1593.5
0.8
6
1621.7
3.1
6
1566.9
2.6
7
1597.0
2.7
20