updated contamination rates for wfpc2 uv filters · 2002. 10. 24. · photometric monitoring...

Instrument Science Report WFPC2 2002-07

Updated Contamination Ratesfor WFPC2 UV Filters

Matt McMaster and Brad WhitmoreOctober 24, 2002

ABSTRACT

Photometric monitoring observations of a white dwarf standard (GRW+70D5824) havebeen used to update the contamination rates for WFPC2 UV filters. Observations fromApril 1994 through May 2002 were used in the analysis. In general, the contaminationrates have declined by roughly 50% for most of the filters during this period. Least-squares fits have been made to the data to allow observers to remove the effects of contam-ination in their WFPC2 observations.

Introduction

Contaminants within the WFPC2 instrument gradually build up on the cold CCD face-plate 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 proce-dure (decon) that involves warming the camera heads, which restores the UV throughputto 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 98-03) and are also given in the HST Data Handbook for WFPC2. This study extends the ear-lier analyses and provides least-squares fits to the yearly contamination rates. Theresulting formulae provide both more accurate and more easily used corrections for theeffects of contamination on WFPC2 UV data.

Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.


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 measure-ments are available in WFPC2 ISR 98-03 and on the web page mentioned in the lastsentence. The data cover the time period from April 1994 (after the cool down from -76˚Cto -88˚C) to May 2002.

Analysis and Discussion

The first step in the analysis was determining the contamination rate as a function of thenumber of days after decontamination for each year. This was done using the polyfit taskwithin the cl.utilities package of IRAF. The resulting least-square fits for the period 04/97-04/98 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 the97-98 data were compared with the 01-02; for similar plots of earlier data, please seeWFPC2 Instrument Science Reports 96-04 and 98-03. The contamination rates for filtersfurther into the red are negligible and have not been included. We note, as shown by theshallower slopes for the more recent data, that the contamination rates have declined withtime.

In Figures 1 and 2, F160BW and F170W respectively, the count rates (DN/sec) inthe PC at or near zero days since decontamination for 2001-2002 are generally higher thanthose for 1997-1998. The opposite is true for Figures 3-6 (F218W, F255W, F336W, andF439W) where the count rates for the PC near zero days since decontamination are lowerby 3 - 7 percent in 1997-1998 as compared to 2001-2002. Both of these phenomena can beseen 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 knownthat throughput loss due to CTE has been linearly increasing with time (WFPC2 ISR 01-09). Because CTE is a function of position on the chip rather than the filter used, onewould expect to see the count rate (DN/sec) to decrease due to the increase in CTE. Whilethis 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 F170Wfilters) since an increase in CTE would not produce an increase in the count rate. It is alsoknown 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 [email protected].

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(WFPC2 ISR 96-04), which may be an indication that some of the long-lived contami-nants (i.e. those that are not removed by the monthly decons) have outgassed from the PCover time. Since a decrease in count rates is not seen for F160BW and F170W, it can beconcluded that these filters are more affected by the loss of long-lived contaminants thanthey 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 Decontam-ination for the more recent data.

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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 generallyhigher for the more recent data.

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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 WF2and for 1997-1998 for WF4.

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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 the2001-2002 data are lower than that for 1997-1998, due to the increase in CTE.

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Figure 5: Contamination rates for 1997-1998 (open circles) and 2001-2002 (filled circles)for F336W.

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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 WF2and WF3. For WF4, observational scatter make it appear as if the contamination rate forthe 2001-2002 data has increased

As shown in Table 1 below, the contamination rates in the UV have generally slowedover time. The largest changes are seen in the PC for F160BW, where the contaminationrate changes from ~0.9%/day to ~0.5%/day, and the WF chips in F170W where there isabout a 50% change between 1994-1995 and 2001-2002. The remaining filters and cam-eras show similar though less dramatic changes. This decline may be an indication thatshort-lived contaminants (i.e. those that are removed by monthly decons) may be slowlydissipating from the instrument, especially in the PC (WFPC2 ISR 96-04).

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Filt error

16 ---

16 ---

16 0.067

16 0.150

16 0.041

16 ---

16 0.050

16 0.082

17 0.012

17 0.013

17 0.012

17 0.028

17 0.026

17 0.073

17 0.025

17 0.030

21 --

21 0.047

21 --

21 --

21 --

21 --

21 0.085

21 0.030

25 --

25 0.013

25 0.011

25 0.043

Table 1: Yearly Contamination Rates for GRW+70D5824

PC WF2 WF3 WF4

er Yeara Nb % declineper day

error N% declineper day



0 4/94-4/95 23 0.885 0.104 -- --- --- 23 1.277 0.038 -- ---

0 4/95-4/96 12 0.840 0.137 5 1.281 0.089 9 1.227 0.057 -- ---

0 4/96-4/97 6 0.690 0.159 4 1.303 0.094 6 1.142 0.069 5 1.079

0 4/97-4/98 8 0.606 0.129 6 1.224 0.072 5 1.116 0.059 5 1.044

0 4/98-4/99 4 0.633 0.081 6 0.929 0.027 6 0.998 0.065 5 0.797

0 4/99-4/00 -- --- --- 3 0.865 0.044 -- --- --- -- ---

0 4/00-4/01 5 0.640 0.206 5 0.925 0.164 3 0.818 0.146 4 0.745

0 4/01-4/02 6 0.460 0.236 3 0.684 0.071 7 0.621 0.107 3 0.626

0 4/94-4/95 75 0.564 0.009 51 0.949 0.011 75 0.988 0.009 51 0.801

0 4/95-4/96 39 0.516 0.012 33 0.901 0.012 39 0.956 0.011 33 0.736

0 4/96-4/97 32 0.509 0.012 29 0.901 0.011 29 0.943 0.012 29 0.752

0 4/97-4/98 28 0.446 0.034 28 0.831 0.028 28 0.795 0.036 28 0.664

0 4/98-4/99 21 0.428 0.020 21 0.713 0.027 21 0.738 0.016 21 0.572

0 4/99-4/00 18 0.461 0.076 18 0.749 0.083 18 0.746 0.086 18 0.607

0 4/00-4/01 18 0.411 0.023 18 0.638 0.033 18 0.657 0.032 17 0.541

0 4/01-4/02 17 0.330 0.021 18 0.486 0.024 16 0.516 0.019 16 0.373

8 4/94-4/95 23 0.478 0.028 -- -- -- 23 0.846 0.019 -- --

8 4/95-4/96 12 0.433 0.036 6 0.757 0.039 10 0.787 0.026 4 0.704

8 4/96-4/97 6 0.442 0.044 -- -- -- 6 0.845 0.033 -- --

8 4/97-4/98 6 0.418 0.084 -- -- -- 4 0.638 0.073 -- --

8 4/98-4/99 4 0.413 0.018 -- -- -- 4 0.535 0.036 -- --

8 4/99-4/00 7 0.369 0.133 -- -- -- 5 0.687 0.130 -- --

8 4/00-4/01 5 0.374 0.060 4 0.556 0.070 3 0.528 0.052 5 0.382

8 4/01-4/02 3 0.318 0.084 -- -- -- 5 0.491 0.061 3 0.362

5 4/94-4/95 23 0.236 0.008 -- -- -- 23 0.471 0.007 -- --

5 4/95-4/96 12 0.235 0.007 5 0.445 0.013 10 0.490 0.007 4 0.385

5 4/96-4/97 6 0.215 0.008 4 0.398 0.017 5 0.333 0.013 6 0.298

5 4/97-4/98 8 0.192 0.044 6 0.331 0.062 5 0.309 0.131 5 0.286

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25 0.026

25 --

25 0.044

25 0.075

33 --

33 0.014

33 0.010

33 0.085

33 0.072

33 --

33 0.050

33 0.039

43 --

43 0.014

43 0.011

43 0.041

43 0.021

43 --

43 0.004

43 0.071

a p to25,

b

Filt error

The second part of the analysis consisted of plotting the yearly contamination rates asa function of time and making a least-squares fit to the data. These fits can be used as cor-rection formulae, and are made possible due to the longer temporal baseline. Thesubsequent smoothing and averaging of the data provide an improvement of our past tech-nique of listing the yearly rates, where occasional increases were inferred due toobservational scatter. Figures 7-12 show the linear fits to the data in Table 1.

5 4/98-4/99 4 0.165 0.057 6 0.285 0.041 6 0.318 0.086 5 0.221

5 4/99-4/00 6 0.185 0.093 3 0.351 0.013 5 0.485 0.099 -- --

5 4/00-4/01 5 0.122 0.059 5 0.291 0.052 3 0.389 0.205 5 0.213

5 4/01-4/02 4 0.143 0.012 4 0.244 0.043 5 0.196 0.059 3 0.188

6 4/94-4/95 23 0.031 0.007 -- -- -- 23 0.198 0.007 -- --

6 4/95-4/96 12 0.108 0.008 5 0.188 0.013 10 0.208 0.009 4 0.226

6 4/96-4/97 6 0.068 0.009 4 0.286 0.014 6 0.285 0.010 6 0.153

6 4/97-4/98 8 0.063 0.015 6 0.207 0.082 5 0.144 0.073 5 0.132

6 4/98-4/99 4 0.129 0.041 6 0.164 0.059 6 0.117 0.055 5 0.055

6 4/99-4/00 7 0.051 0.055 3 0.157 0.073 5 0.146 0.031 -- --

6 4/00-4/01 5 0.052 0.071 4 0.146 0.052 3 0.104 0.071 5 0.039

6 4/01-4/02 4 0.022 0.022 4 0.093 0.052 5 0.114 0.024 3 0.099

9 4/94-4/95 23 0.008 0.008 -- -- -- 23 -0.078 0.008 -- --

9 4/95-4/96 12 0.037 0.007 5 0.121 0.013 9 0.080 0.009 4 0.127

9 4/96-4/97 11 0.037 0.007 4 0.146 0.017 6 0.083 0.011 6 0.031

9 4/97-4/98 17 0.036 0.023 5 0.169 0.042 6 0.061 0.077 5 0.002

9 4/98-4/99 14 0.025 0.017 5 0.064 0.033 6 0.109 0.016 3 0.079

9 4/99-4/00 8 0.007 0.025 3 0.213 0.093 3 0.139 0.034 - --

9 4/00-4/01 5 0.047 0.024 3 0.128 0.045 -- -- -- 3 0.087

9 4/01-4/02 4 0.002 0.049 -- -- -- -- -- -- 3 0.050

. Epoch boundaries are the mean MJD for a given year (from April - March) and vary from filter to filter and chichip. The first three entries for a filter were taken from Table 2 in ISR 98-03 and the boundaries occur at Apr. 1994 (MJD=49467), Apr. 25, 1995 (MJD=49832), and Apr. 25, 1996 (MJD=50198).

. N is the number of observations.

Table 1: Yearly Contamination Rates for GRW+70D5824

PC WF2 WF3 WF4

er Yeara Nb % declineper day




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Figure 7: Fits to the contamination rates for F160BW. The dashed lines represent Servic-ing Mission 2 (MJD=50490.03) and Servicing Mission 3A (MJD=51531.16). Note thesimilarity in position of the last two points for all the chips (one just above the line, thesecond one below it); this is probably an effect after Servicing Mission 3A (see Figure 8for more details). The lines were fit to the points above despite the third points in PC1 andWF3, the first point in WF4 and the second and fifth points in WF2 straddling ServicingMissions 2 and 3A.

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Figure 8: Fits to the contamination rates for F170W. Note the step-like structure in theplots; 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 solidpoints were used in determining the fits, including the third and sixth points which spanthe two servicing missions mentioned; note that the asterisks (*) were not used in deter-mining the fits. The asterisks to the left of the dashed lines are the contamination ratesbefore the servicing missions (from several months to a few days before), and those to theright are the contamination rates after the servicing missions (from a few days to a fewmonths). Note that all of the asterisks to the right of the dashed lines are higher than thoseto the left, indicating an increase in the contamination rate just after a servicing missionand would explain the step-like structure seen in the plots. Note also that while the con-tamination rates immediately after a servicing mission can be quite high (almost twice ashigh after SM3A), they return to the general trend soon afterward.

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Figure 9: Fits to the contamination rates for F218W. The dashed line in the WF2 plot is anaverage of the fits for WF3 and WF4 and is the slope is the recommended value to use.

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Figure 10: Fits to the contamination rates for F255W.

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Figure 11: Fits to the contamination rates for F336W.

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Figure 12: Fits to the contamination rates for F439W. Note that due to observational scat-ter, it appears that the rates for WF2 and WF3 are increasing. Since this is not thought tobe true, a mean rate (dotted line) is also provided for each of the chips; we recommendusing this mean rate.

Notice that the last two points in the F160BW plots (Figure 7) are similar; the firstpoint 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 upat nearly a constant value, then the fourth, in which the value drops by nearly 20% inWF3, through the sixth line up, and then the final two. These phenomena seem to be dueto an effect from Servicing Mission 3A (the dashed lines in the F160BW and F170Wplots). 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, thecontamination 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 theincrease in the contamination rate. A similar process appears to be happening with the

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4

Filter Co

a. The valu

error

F160BW 0. -4 4.474E-5

F170W 0. -4 2.106E-5

F218Wb

b. The valu n correc-tion for

0. -4 1.673E-5

F255W 0. -5 1.750E-5

F336W 0. -5 2.597E-5

F439Wcd

c. A plus ( er than alarger nu

d. The num nce decon-taminati

0. -6)

2.647E-5(0.044)

F160BW data, but there were not enough points to determine the contamination ratesbefore or after the servicing missions. Despite the increase in the contamination ratesshortly after a servicing mission, we feel that a simple linear fit appears to represent thedata 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 themore sensitive FUV data, a mean value was determined for the four chips and representsour recommended value. Table 2 gives the resulting fits to the contamination rate data.

The equation below demonstrates how the contamination correction can be made.

(1)

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-

Table 2: Fits to Yearly Contamination Rates

PC WF2 WF3 WF

nsta

e of the percentage throughput loss at MJD = 49500 (May 28, 1994).

Slope error Const Slope error Const Slope error Const Slope

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

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

es for WF2 are an average of those for WF3 and WF4 and are recommended when determining the contaminatiothis filter/chip combination.

478 -5.185E-5 6.497E-6 0.829 -1.519E-4 --- 0.863 -1.387E-4 3.091E-5 0.795 -1.651E

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

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

+) sign indicates where the rate appears to be increasing, though this is probably due to observational scatter rathmber of contaminants falling on the chips.bers in parentheses are the mean values of the yearly contamination rates (in percent throughput loss per days si

on) and should be used when correcting for contamination effects in this filter.

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(0.063

COUNTSCORR

COUNT SOBS

1.0 Slope MJD 49500–( ) Cons ttan+×( ) DSD 100⁄( )×( )–------------------------------------------------------------------------------------------------------------------------------------------------------

=

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stant is the value of the throughput loss per day at MJD=49500 (May 28, 1994); and DSDis the number of Days Since Decontamination.

For example, a star observed on March 9, 2000 through the F170W filter on the PC1chip has a count rate of 300 DN/sec. The MJD for this observation is 51612.2 which isgiven in the header parameter, EXPSTART. The last decontamination before this dateoccurred 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 WFPC2Decontamination 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 per-cent 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:

(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 dayssince decontamination (the number in parentheses in Table 2); and DSD is the number ofDays Since Decontamination.

For example, the same object in the first example is observed on the same day throughthe F439W filter on WF3. The mean value of the contamination rate is 0.092 which ismultiplied by the number of days since the last decontamination, in this case 12.8. Notethat 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 = 354.17DN/s

Red LeaksSome 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 percent-age in the overall count rate for an observation in the FUV filters (see Fig. 1 in WFPC2TIR 02-05) and strictly speaking, the corrections presented here are valid only for an

COUNT SCORR

COUNTSOBS

1 Mean DSD 100⁄×( )–( )-----------------------------------------------------------------=

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object with the same spectral distribution as GRW+70D5824 (a white dwarf). The red leakis minimal for this case since the star is a white dwarf (i.e. very blue DA3 star). Table 3.13in the WFPC2 Instrument Handbook indicates that roughly 6% of the light comes from thered leak. We caution users about using the contamination rates presented in this report forvery red sources. SYNPHOT can be used to determine more realistic corrections for theseobjects.

Conclusions

Photometric monitoring observations from April 1994 to May 2002 have shown adecrease in the rate of contamination in the UV filters, with F160BW and F170W showingthe steepest drop off.

While it appears that the data show a correlation with the Servicing Missions, espe-cially in the F170W filter, a least squares fit seems to be adequate. These fits, listed inTable 2, along with the equations given above, can be used to correct for the effects of con-tamination in WFPC2 UV photometric data.

Recommendations

The corrections given here supersede those given in previous ISRs and when usingSYNPHOT, 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 futurewith the appropriate corrections.

References

Baggett, S., and Gonzaga, S. 1998, WFPC2 Long-Term Photometric Stability, WFPC2Instrument Science Report 98-03

Biretta J., Lubin L., et al. 2002, WFPC2 Instrument Handbook, Version 7.0 (Balti-more: STScI)

Heyer, I. 2001, The WFPC2 Photometric CTE Monitor, WFPC2 Instrument ScienceReport 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 [email protected].

19


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 WFPC2Photometry, 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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updated contamination rates for wfpc2 uv filters · 2002. 10. 24. · photometric monitoring...

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