Instrument Science Report WFPC2 2002-07
Updated Contamination Rates
for WFPC2 UV Filters
Matt McMaster and Brad Whitmore
October 24, 2002
ABSTRACT
Photometric monitoring observations of a white dwarf standard (GRW+70D5824) have
been used to update the contamination rates for WFPC2 UV filters. Observations from
April 1994 through May 2002 were used in the analysis. In general, the contamination
rates have declined by roughly 50% for most of the filters during this period. Leastsquares fits have been made to the data to allow observers to remove the effects of contamination in their WFPC2 observations.
Introduction
Contaminants within the WFPC2 instrument gradually build up on the cold CCD faceplate which results in a decrease in the UV throughput. Approximately once per month,
these contaminants are evaporated off of the faceplate by way of a decontamination procedure (decon) that involves warming the camera heads, which restores the UV throughput
to its nominal value. The dates of these decons can be found on the WFPC2 web pages at:
http://www.stsci.edu/instruments/wfpc2/Wfpc2_memos/wfpc2_history.html#Dec
Contamination rates have been presented in two previous WFPC2 ISRs (96-04 and 9803) and are also given in the HST Data Handbook for WFPC2. This study extends the earlier analyses and provides least-squares fits to the yearly contamination rates. The
resulting formulae provide both more accurate and more easily used corrections for the
effects of contamination on WFPC2 UV data.
Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report WFPC2 2002-07
Data
The data used in this study were taken from the F160BW, F170W, F218W, F255W,
F336W, and F439W photometric monitoring observations of the DA3 white dwarf,
GRW+70D5824 and can be found at: http://www.stsci.edu/instruments/wfpc2/
Wfpc2_memos/wfpc2_stdstar_phot3.html. Details on the observations and the measurements are available in WFPC2 ISR 98-03 and on the web page mentioned in the last
sentence. The data cover the time period from April 1994 (after the cool down from -76˚C
to -88˚C) to May 2002.
Analysis and Discussion
The first step in the analysis was determining the contamination rate as a function of the
number of days after decontamination for each year. This was done using the polyfit task
within the cl.utilities package of IRAF. The resulting least-square fits for the period 04/9704/98 and 04/01-04/02 are shown in Figures 1-6 for each of the UV filters studied
(F160BW, F170W, F218W, F255W, F336W, and F439W). As a matter of clarity, only the
97-98 data were compared with the 01-02; for similar plots of earlier data, please see
WFPC2 Instrument Science Reports 96-04 and 98-03. The contamination rates for filters
further into the red are negligible and have not been included. We note, as shown by the
shallower slopes for the more recent data, that the contamination rates have declined with
time.
In Figures 1 and 2, F160BW and F170W respectively, the count rates (DN/sec) in
the PC at or near zero days since decontamination for 2001-2002 are generally higher than
those for 1997-1998. The opposite is true for Figures 3-6 (F218W, F255W, F336W, and
F439W) where the count rates for the PC near zero days since decontamination are lower
by 3 - 7 percent in 1997-1998 as compared to 2001-2002. Both of these phenomena can be
seen more clearly in the photometric monitoring plots in WFPC2 ISR 98-03 and WFPC2
Technical Instrument Report (TIR) 02-041.
Along with contamination, the throughput is also affected by CTE. It is now known
that throughput loss due to CTE has been linearly increasing with time (WFPC2 ISR 0109). Because CTE is a function of position on the chip rather than the filter used, one
would expect to see the count rate (DN/sec) to decrease due to the increase in CTE. While
this seems to be the case for the NUV data (F218W, F255W, F336W, and F439W filters),
it does not explain the increase in the count rates for the FUV data (F160BW and F170W
filters) since an increase in CTE would not produce an increase in the count rate. It is also
known that the NUV filters are less affected by contamination than the FUV filters
1. The abstract for this TIR is available on the web at: http://www.stsci.edu/instruments/wfpc2/
Wfpc2_tir/tir0204.html. For a copy of this report, please send a request to help@stsci.edu.
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Instrument Science Report WFPC2 2002-07
(WFPC2 ISR 96-04), which may be an indication that some of the long-lived contaminants (i.e. those that are not removed by the monthly decons) have outgassed from the PC
over time. Since a decrease in count rates is not seen for F160BW and F170W, it can be
concluded that these filters are more affected by the loss of long-lived contaminants than
they are by the increase in CTE.
Figure 1: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F160BW. Note that for the PC, the count rates are higher near 0 Days Since Decontamination for the more recent data.
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Instrument Science Report WFPC2 2002-07
Figure 2: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F170W. The count rates at 0 Days Since Decontamination for the PC are generally
higher for the more recent data.
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Instrument Science Report WFPC2 2002-07
Figure 3: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F218W. There were not enough data points to determine fits for both epochs for WF2
and for 1997-1998 for WF4.
5
Instrument Science Report WFPC2 2002-07
Figure 4: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F255W. Note that the count rates at 0 Days Since Decontamination in all chips for the
2001-2002 data are lower than that for 1997-1998, due to the increase in CTE.
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Instrument Science Report WFPC2 2002-07
Figure 5: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F336W.
7
Instrument Science Report WFPC2 2002-07
Figure 6: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)
for F439W. There were not enough data points to determine fits for 2001-2002 for WF2
and WF3. For WF4, observational scatter make it appear as if the contamination rate for
the 2001-2002 data has increased
As shown in Table 1 below, the contamination rates in the UV have generally slowed
over time. The largest changes are seen in the PC for F160BW, where the contamination
rate changes from ~0.9%/day to ~0.5%/day, and the WF chips in F170W where there is
about a 50% change between 1994-1995 and 2001-2002. The remaining filters and cameras show similar though less dramatic changes. This decline may be an indication that
short-lived contaminants (i.e. those that are removed by monthly decons) may be slowly
dissipating from the instrument, especially in the PC (WFPC2 ISR 96-04).
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Instrument Science Report WFPC2 2002-07
Table 1: Yearly Contamination 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
--
---
---
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
4/97-4/98
8
0.606
0.129
6
1.224
0.072
5
1.116
0.059
5
1.044
0.150
160
4/98-4/99
4
0.633
0.081
6
0.929
0.027
6
0.998
0.065
5
0.797
0.041
160
4/99-4/00
--
---
---
3
0.865
0.044
--
---
---
--
---
---
160
4/00-4/01
5
0.640
0.206
5
0.925
0.164
3
0.818
0.146
4
0.745
0.050
160
4/01-4/02
6
0.460
0.236
3
0.684
0.071
7
0.621
0.107
3
0.626
0.082
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
4/97-4/98
28
0.446
0.034
28
0.831
0.028
28
0.795
0.036
28
0.664
0.028
170
4/98-4/99
21
0.428
0.020
21
0.713
0.027
21
0.738
0.016
21
0.572
0.026
170
4/99-4/00
18
0.461
0.076
18
0.749
0.083
18
0.746
0.086
18
0.607
0.073
170
4/00-4/01
18
0.411
0.023
18
0.638
0.033
18
0.657
0.032
17
0.541
0.025
170
4/01-4/02
17
0.330
0.021
18
0.486
0.024
16
0.516
0.019
16
0.373
0.030
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
4/97-4/98
6
0.418
0.084
--
--
--
4
0.638
0.073
--
--
--
218
4/98-4/99
4
0.413
0.018
--
--
--
4
0.535
0.036
--
--
--
218
4/99-4/00
7
0.369
0.133
--
--
--
5
0.687
0.130
--
--
--
218
4/00-4/01
5
0.374
0.060
4
0.556
0.070
3
0.528
0.052
5
0.382
0.085
218
4/01-4/02
3
0.318
0.084
--
--
--
5
0.491
0.061
3
0.362
0.030
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
4/97-4/98
8
0.192
0.044
6
0.331
0.062
5
0.309
0.131
5
0.286
0.043
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Instrument Science Report WFPC2 2002-07
Table 1: Yearly Contamination 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
255
4/98-4/99
4
0.165
0.057
6
0.285
0.041
6
0.318
0.086
5
0.221
0.026
255
4/99-4/00
6
0.185
0.093
3
0.351
0.013
5
0.485
0.099
--
--
--
255
4/00-4/01
5
0.122
0.059
5
0.291
0.052
3
0.389
0.205
5
0.213
0.044
255
4/01-4/02
4
0.143
0.012
4
0.244
0.043
5
0.196
0.059
3
0.188
0.075
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
4/97-4/98
8
0.063
0.015
6
0.207
0.082
5
0.144
0.073
5
0.132
0.085
336
4/98-4/99
4
0.129
0.041
6
0.164
0.059
6
0.117
0.055
5
0.055
0.072
336
4/99-4/00
7
0.051
0.055
3
0.157
0.073
5
0.146
0.031
--
--
--
336
4/00-4/01
5
0.052
0.071
4
0.146
0.052
3
0.104
0.071
5
0.039
0.050
336
4/01-4/02
4
0.022
0.022
4
0.093
0.052
5
0.114
0.024
3
0.099
0.039
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
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
4/97-4/98
17
0.036
0.023
5
0.169
0.042
6
0.061
0.077
5
0.002
0.041
439
4/98-4/99
14
0.025
0.017
5
0.064
0.033
6
0.109
0.016
3
0.079
0.021
439
4/99-4/00
8
0.007
0.025
3
0.213
0.093
3
0.139
0.034
-
--
--
439
4/00-4/01
5
0.047
0.024
3
0.128
0.045
--
--
--
3
0.087
0.004
439
4/01-4/02
4
0.002
0.049
--
--
--
--
--
--
3
0.050
0.071
a. Epoch boundaries are the mean MJD for a given year (from April - March) and vary from filter to filter and chip to
chip. The first three entries for a filter were taken from Table 2 in ISR 98-03 and the boundaries occur at Apr. 25,
1994 (MJD=49467), Apr. 25, 1995 (MJD=49832), and Apr. 25, 1996 (MJD=50198).
b. N is the number of observations.
The second part of the analysis consisted of plotting the yearly contamination rates as
a function of time and making a least-squares fit to the data. These fits can be used as correction formulae, and are made possible due to the longer temporal baseline. The
subsequent smoothing and averaging of the data provide an improvement of our past technique of listing the yearly rates, where occasional increases were inferred due to
observational scatter. Figures 7-12 show the linear fits to the data in Table 1.
10
Instrument Science Report WFPC2 2002-07
Figure 7: Fits to the contamination rates for F160BW. The dashed lines represent Servicing Mission 2 (MJD=50490.03) and Servicing Mission 3A (MJD=51531.16). Note the
similarity in position of the last two points for all the chips (one just above the line, the
second one below it); this is probably an effect after Servicing Mission 3A (see Figure 8
for more details). The lines were fit to the points above despite the third points in PC1 and
WF3, the first point in WF4 and the second and fifth points in WF2 straddling Servicing
Missions 2 and 3A.
11
Instrument Science Report WFPC2 2002-07
Figure 8: Fits to the contamination rates for F170W. Note the step-like structure in the
plots; this is probably an indication of an effect after Servicing Missions 2 and 3A,
MJD=50490.03 and 51531.16 respectively (dashed lines in the plots). Only the solid
points were used in determining the fits, including the third and sixth points which span
the two servicing missions mentioned; note that the asterisks (*) were not used in determining the fits. The asterisks to the left of the dashed lines are the contamination rates
before the servicing missions (from several months to a few days before), and those to the
right are the contamination rates after the servicing missions (from a few days to a few
months). Note that all of the asterisks to the right of the dashed lines are higher than those
to the left, indicating an increase in the contamination rate just after a servicing mission
and would explain the step-like structure seen in the plots. Note also that while the contamination rates immediately after a servicing mission can be quite high (almost twice as
high after SM3A), they return to the general trend soon afterward.
12
Instrument Science Report WFPC2 2002-07
Figure 9: Fits to the contamination rates for F218W. The dashed line in the WF2 plot is an
average of the fits for WF3 and WF4 and is the slope is the recommended value to use.
13
Instrument Science Report WFPC2 2002-07
Figure 10: Fits to the contamination rates for F255W.
14
Instrument Science Report WFPC2 2002-07
Figure 11: Fits to the contamination rates for F336W.
15
Instrument Science Report WFPC2 2002-07
Figure 12: Fits to the contamination rates for F439W. Note that due to observational scatter, it appears that the rates for WF2 and WF3 are increasing. Since this is not thought to
be true, a mean rate (dotted line) is also provided for each of the chips; we recommend
using this mean rate.
Notice that the last two points in the F160BW plots (Figure 7) are similar; the first
point is above the fitted line while the second is below it. Similarly for the F170W plots
(Figure 8); the last two points seem to drop more steeply than a linear fit would imply.
There also seems to be a step-like pattern to the F170W plots. The first three points line up
at nearly a constant value, then the fourth, in which the value drops by nearly 20% in
WF3, through the sixth line up, and then the final two. These phenomena seem to be due
to an effect from Servicing Mission 3A (the dashed lines in the F160BW and F170W
plots). There seems to be a similar effect around Servicing Mission 2.
The asterisks (*) in Figure 8 show that for a few months after a servicing mission, the
contamination rate is higher than just before or for the periods in between the missions.
Note, however, that the rate returns to the general trend within a few months of the
increase in the contamination rate. A similar process appears to be happening with the
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Instrument Science Report WFPC2 2002-07
F160BW data, but there were not enough points to determine the contamination rates
before or after the servicing missions. Despite the increase in the contamination rates
shortly after a servicing mission, we feel that a simple linear fit appears to represent the
data quite well.
An exception to this is the F439W (Figure 12) data, where due to observational scatter,
the contamination rate appears to be increasing over time for the WF2 and WF3 chips.
Since this is not thought to be happening, as evidenced by the decline in the rates in the
more sensitive FUV data, a mean value was determined for the four chips and represents
our recommended value. Table 2 gives the resulting fits to the contamination rate data.
Table 2: Fits to Yearly Contamination Rates
PC
WF2
WF3
WF4
Filter
Consta
Slope
error
Const
Slope
error
Const
Slope
error
Const
Slope
error
F160BW
0.857
-1.348E-4
3.042E-5
1.499
-2.985E-4
4.821E-5
1.344
-2.401E-4
2.210E-5
1.285
-2.443E-4
4.474E-5
F170W
0.562
-7.440E-5
1.219E-5
1.004
-1.659E-4
2.144E-5
1.038
-1.755E-4
1.934E-5
0.835
-1.457E-4
2.106E-5
F218Wb
0.478
-5.185E-5
6.497E-6
0.829
-1.519E-4
---
0.863
-1.387E-4
3.091E-5
0.795
-1.651E-4
1.673E-5
F255W
0.246
-4.240E-5
6.803E-6
0.462
-8.050E-5
1.926E-5
0.459
-6.093E-5
3.958E-5
0.387
-7.822E-5
1.750E-5
F336W
0.081
-1.084E-5
1.569E-5
0.276
-6.245E-5
2.212E-5
0.234
-5.005E-5
1.862E-5
0.214
-6.210E-5
2.597E-5
F439Wcd
0.029
-3.275E-6
(0.025)
7.541E-5
(0.017)
0.128
+9.022E-6
(0.140)
3.912E-5
(0.050)
0.062
+2.874E-5
(0.092)
1.281E-5
(0.028)
0.069
-4.115E-6
(0.063)
2.647E-5
(0.044)
a. The value of the percentage throughput loss at MJD = 49500 (May 28, 1994).
b. The values for WF2 are an average of those for WF3 and WF4 and are recommended when determining the contamination correction for this filter/chip combination.
c. A plus (+) sign indicates where the rate appears to be increasing, though this is probably due to observational scatter rather than a
larger number of contaminants falling on the chips.
d. The numbers in parentheses are the mean values of the yearly contamination rates (in percent throughput loss per days since decontamination) and should be used when correcting for contamination effects in this filter.
The equation below demonstrates how the contamination correction can be made.
COUNT S OBS
COUN TS CORR =  ------------------------------------------------------------------------------------------------------------------------------------------------------ (1)
 1.0 – ( ( Slope × ( MJD – 49500 ) + Cons tan t ) × ( DSD ⁄ 100 ) )
where: COUNTSOBS is the count rate, in DN/second, measured in the image; Slope is
the percentage throughput loss per day per MJD; MJD is the Modified Julian Date; Con-
17
Instrument Science Report WFPC2 2002-07
stant is the value of the throughput loss per day at MJD=49500 (May 28, 1994); and DSD
is the number of Days Since Decontamination.
For example, a star observed on March 9, 2000 through the F170W filter on the PC1
chip has a count rate of 300 DN/sec. The MJD for this observation is 51612.2 which is
given in the header parameter, EXPSTART. The last decontamination before this date
occurred on February 25, 2000 (MJD = 51599.4) which means DSD would be 12.8
(51612.2 - 51599.4). Note that the dates of decontamination can be found on the WFPC2
Decontamination Date web page, located at: http://www.stsci.edu/instruments/wfpc2/
Wfpc2_memos/wfpc2_decon_dates.html. Table 2 shows that the slope is -7.440E-05 percent throughput loss per DSD per MJD, and the constant is 0.562. Therefore:
COUNTSCORR = 300/(1.0 - (-7.440E-05 * (51612.2-49500) + 0.562) * (12.8/100)) DN/s
COUNTSCORR = 316.40 DN/s
For an object observed on the F439W filter, the correction formula would be:
COUNTS OBS
COUNT S CORR = ---------------------------------------------------------------( 1 – ( Mean × DSD ⁄ 100 ) )
(2)
where: COUNTSOBS is the count rate, in DN/second, measured in the image; Mean is
the average value of the yearly contamination rate in percentage throughput loss per days
since decontamination (the number in parentheses in Table 2); and DSD is the number of
Days Since Decontamination.
For example, the same object in the first example is observed on the same day through
the F439W filter on WF3. The mean value of the contamination rate is 0.092 which is
multiplied by the number of days since the last decontamination, in this case 12.8. Note
that MJD, Slope, and Constant are not needed here. If the star had a count rate of 350 DN/
sec in this filter, we’d have:
COUNTSCORR = 350/(1.0 − (0.092 ∗ (12.8/100)) DN/s
COUNTSCORR = 354.17 DN/s
Red Leaks
Some of these UV filters have substantial red leaks which have not been accounted for
in the photometric monitoring data. In fact, red leaks can account for a significant percentage in the overall count rate for an observation in the FUV filters (see Fig. 1 in WFPC2
TIR 02-05) and strictly speaking, the corrections presented here are valid only for an
18
Instrument Science Report WFPC2 2002-07
object with the same spectral distribution as GRW+70D5824 (a white dwarf). The red leak
is minimal for this case since the star is a white dwarf (i.e. very blue DA3 star). Table 3.13
in the WFPC2 Instrument Handbook indicates that roughly 6% of the light comes from the
red leak. We caution users about using the contamination rates presented in this report for
very red sources. SYNPHOT can be used to determine more realistic corrections for these
objects.
Conclusions
Photometric monitoring observations from April 1994 to May 2002 have shown a
decrease in the rate of contamination in the UV filters, with F160BW and F170W showing
the steepest drop off.
While it appears that the data show a correlation with the Servicing Missions, especially in the F170W filter, a least squares fit seems to be adequate. These fits, listed in
Table 2, along with the equations given above, can be used to correct for the effects of contamination in WFPC2 UV photometric data.
Recommendations
The corrections given here supersede those given in previous ISRs and when using
SYNPHOT, the Space Telescope Science Institute’s SYNthetic PHOTometry package,
when accounting for contamination in the obsmode parameter (e.g. wfpc2, 2, a2d7,
f170w, cont#MJD). If resources permit, the SYNPHOT tables will be updated in the future
with the appropriate corrections.
References
Baggett, S., and Gonzaga, S. 1998, WFPC2 Long-Term Photometric Stability, WFPC2
Instrument Science Report 98-03
Biretta J., Lubin L., et al. 2002, WFPC2 Instrument Handbook, Version 7.0 (Baltimore: STScI)
Heyer, I. 2001, The WFPC2 Photometric CTE Monitor, WFPC2 Instrument Science
Report 01-09
Lubin, L.M., Whitmore, B., Koekemoer, A.M., and Heyer, I. 2002, SMOV3b WFPC2
Lyman-α Throughput Check, WFPC2 Technical Instrument Report 02-051
1. The abstracts for TIRs are available on the web at: http://www.stsci.edu/instruments/wfpc2/
wfpc2_tir_list.htmll. For a copy of a report, please send a request to help@stsci.edu.
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Instrument Science Report WFPC2 2002-07
Whitmore, B., and Heyer, I. 2002, SMOV3b WFPC2 Photometry Check, WFPC2
Technical Instrument Report 02-041
Whitmore, B., Heyer, I., and Baggett, S. 1996, Effects of Contamination on WFPC2
Photometry, WFPC2 Instrument Science Report 96-04
WFPC2 Decontamination Date Web Page, http://www.stsci.edu/instruments/wfpc2/
Wfpc2_memos/wfpc2_decon_dates.html
WFPC2 Standard Star Monitoring Memo, http://www.stsci.edu/instruments/wfpc2/
Wfpc2_memos/wfpc2_stdstar_phot3.html
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