SMOV3B WFPC2 Photometry Check

Technical Instrument Report WFPC2-02-04

SMOV3B WFPC2 Photometry Check

Brad Whitmore and Inge HeyerMay 28, 2002

ABSTRACT

A check of the photometric throughput of the WFPC2 was performed March 31, 2002(program ID: 8953). The standard star GRW+70d5824 was observed with a selection offilters and the standard star was centered in each of the four CCDs. The data indicate thatany changes in the photometric throughputs due to SM3B are less than 1% in most of thevisible wavelength filters, and less than a few percent in the UV filters. The distributionshows a mean value of 0.34 +/- 0.26 sigma (where sigma is defined separately for each fil-ter-chip combination) for all the observations, essentially consistent with an unchangedphotometric throughput. This corresponds to about 0.4% on an absolute scale. Hence, theresponse of the WFPC2 was essentially unchanged by the servicing mission. We also findthat the long-term throughput decline is consistent with the expected CTE loss.

1. Introduction

The goal of the relative photometry check (proposal 8953) was to verify that the pho-tometric accuracy remained unchanged at the 1-2% level after the latest servicing missionSM3b. Our regular photometric standard star, GRW+70D5824, was observed in a varietyof filters (F160BW, F170W, F185W, F218W, F255W, F300W, F336W, F439W, F555W,F675W, and F814W), with the star centered in each of the 4 CCDs (one orbit per CCD).This is the same photometry check performed after the previous servicing missions, whichshowed no decline in the photometric performance.

HST was released by the shuttle crew at 5:04 AM EST, March 9, 2002., and WFPC2was in protect Decon for about 11 days 6 hours, from March 12-23, 2002, duringSMOV3B. The observations were obtained on March 31, 2002, (MJD 52364) about 0.3days after WFPC2 was in a 9-hour special Decon on March30, 2002.

2. Observations

Contaminants collect on the cold CCD windows and reduce the UV throughput of theWFPC2. A warm up decontamination (Decon) procedure is performed monthly to evapo-

1

rate contaminants from the CCD windows which is followed by a cool down. Following aDecon on March 30, 2002 (MJD=52363.7516), observations were taken for the purpose ofdetermining if any chance in the photometric performance had occurred as a result ofSM3B.

Observations of standard star GRW+70d5824 (mv = 12.7, B-V = -0.09) for the pho-tometry check were obtained on March 31, 2002 (program ID: 8953) which wasapproximately 0.3 days following the Decon on March 30, 2002. The star was positionedin the center of a camera during four 1-orbit visits, one visit per camera. The respectivesingle camera images were read out and sent to the ground for analysis. Observations wereobtained with filters F160BW, F170W, F185W, F218W, F255W, F300W, F336W, F439W,F555W, F675W, and F814W. Table 1 lists the observations.

Table 1. 8953 Photometry Monitor Observations.

3. Calibration and Data Reduction

The OPUS pipeline calibrated data were used for the analysis. No other calibrationsteps were performed.

Photometry was performed using the APPHOT task phot with the star positions auto-

matically identified for each camera as input1. For the PC1 frames, a photometry apertureradius of 11 pixels was used with the sky fitting region parameters set to annulus=32 pix-

filter PC1PC1

exptime(sec.)

WF2 WF3 WF4WF

exptime(sec.)

F160BW u8bd0101r 200.0 u8bd0201r u8bd0301r u8bd0401r 100.0

F170W u8bd0102r 40.0 u8bd0202r u8bd0302r u8bd0402r 40.0








F675W u8bd010ar 8.0 u8bd020ar u8bd030ar u8bd040ar 4.0

F814W u8bd010br 14.0 u8bd020br u8bd030br u8bd040br 7.0

2

els and dannulus=11 pixels. For the WF frames, a photometry aperture radius of 5 pixelswas used with the sky fitting region parameters set to annulus=15 pixels and dannulus=5pixels. The sky fitting algorithm was set to “ofilter.” The centering algorithm was set to“centroid”. The photometry values are listed in Tables 2-5. For Tables 2-5, the table head-ers are:

filter - WFPC2 filter used.mjd - modified Julian Date (Julian Date - 2400000.5) for the observation.ct_rate - countrate (DN/s) for the respective aperture.

Table 2. PC1 photometry.

Table 3. WF2 photometry.

1. The scripts used for this work can be found at the following locations:/data/denali11/smov3b/script_smov_8953 (photometry)/data/nerys5/script_library/scripts_smov2002/script_smov_8953_autoplot_*.cl (creating Figures 1-4)/data/denali11/smov3b/script_sigma_diagram (creating Figure 5)data/nerys5/script_library/scripts_smov2002/script_smov_cte_comp_pc1.cl (creating Figure 6)

filter mjd ct_rate

F160BW 52364.22 88.301

F170W 52364.23 170.478

F185W 52364.23 92.641

F218W 52364.24 128.388

F255W 52364.24 150.319

F300W 52364.24 941.872

F336W 52364.24 740.793

F439W 52364.24 886.434

F555W 52364.25 3724.889

F675W 52364.25 2063.758

F814W 52364.25 1355.537

filter mjd ct_rate

F160BW 52364.29 76.366

F170W 52364.29 192.428

F185W 52364.30 102.416

F218W 52364.30 139.858

F255W 52364.30 159.448

3



F300W 52364.30 955.157

F336W 52364.31 756.947

F439W 52364.31 883.425

F555W 52364.31 3726.059

F675W 52364.31 2098.310

F814W 52364.31 1377.432

filter mjd ct_rate

F160BW 52364.36 66.767

F170W 52364.36 152.701

F185W 52364.36 82.069

F218W 52364.37 127.220

F255W 52364.37 157.696

F300W 52364.37 959.739

F336W 52364.37 753.756

F439W 52364.38 859.329

F555W 52364.38 3681.950

F675W 52364.38 2036.548

F814W 52364.38 1338.025

filter mjd ct_rate

F160BW 52364.09 67.101

F170W 52364.09 167.927

F185W 52364.10 94.994

F218W 52364.10 138.287

F255W 52364.10 157.344

F300W 52364.10 984.035

F336W 52364.11 750.429

F439W 52364.11 865.116

F555W 52364.11 3735.275

filter mjd ct_rate

4

4. The Results

The 8953 calibration program executed approximately 0.34 - 0.63 days following theDecon on March 30, 2002.

Figure 1 through Figure 4 show the evolution of the photometric measurements from1995 until the present, for nine filters and all four chips. The star represents the post -SM3b data point. The data has been normalized to a mean of 1.0 based on the historicaldataset, and fitted from MJD 51000 to the present, with the exception of F675W for PC1and WF2, which were fitted from MJD 50200 and 50700, respectively, due to the paucityof recent data. The 1-sigma scatter (based on the empirical scatter) of the baseline mea-surements are shown by dashed lines.

The observed long-term trend is a decline of the throughput, to varying degrees in thedifferent filters. The post-SM3b data point seems to follow the most recent trend. Theoverall data for several UV filters (most notably F255W) seem to show a curious sharpdownward trend around MJD 51000, and we are currently investigating possible causes.

Figure 5 shows the statistical distribution of the post-SM3b photometry data pointsaround the predicted values based on the fits shown in Figure 1 through Figure 4, and nor-malized by the 1-sigma error bars shown by the dotted lines. We find that the distributionhas a mean value of 0.34 +/- 0.26 sigma for all the observations, hence there is no obviouschange in the throughput before and after SM3b. This compares to a value of -0.17 +/-0.14 for SM2 (Biretta et al. 1997). The mean value in the PC appears to be slightly higher,with a value of 1.31 +/- 0.56 sigma. However, even this is only about a 2-sigma result, andis probably due to low number statistics. We note that in SM2 a similar 2-sigma result wasfound for the PC, but in the opposite sense (i.e. a decline instead of an increase). The factthat the width of the distributions in Figure 5 are larger than 1 sigma, suggests that thereare other sources of uncertainty besides the historical random uncertainties, such as largefluctuations in focus or breathing.

Figure 6 shows the observed throughput decline compared with that predicted fromDolphin’s CTE equations (2001) for F336W, F555W, and F814W. For all three filters thethroughput decline is consistent with the expected CTE loss, within the uncertainties.

F675W 52364.11 2075.017

F814W 52364.11 1363.641

filter mjd ct_rate

5

Figure 1: Photometric Throughput for PC1 in 9 filters. The star represents the post-SM3bdata point. The data has been normalized to a mean of 1.0 based on the baseline observa-tions, and fitted from MJD 51000 to the present, with the exception of F675W, which wasfitted from MJD 50200 due to the paucity of recent data. The 1-sigma scatter (based on theempirical scatter) of the baseline measurements are shown by dashed lines.

6

Figure 2: Same as Figure 1 for WF2. F675W was fitted from MJD 50700 due to the pau-city of recent data.

7

Figure 3: Same as Figure 1 for WF3.

8

Figure 4: Same as Figure 1 for WF4.

9

Figure 5: Statistical distribution of the post-SM3b photometry data points (number ofdata points vs. sigma).

10

Figure 6: Actual throughput (solid lines) vs. expected decline from Dolphin’s CTE equa-tions (dashed lines) for F336W, F555W, and F814W for the PC1 chip. This shows that thelong-term trends are due to CTE loss, within the measurement uncertainty. The disconti-nuity for the early data points for F814W is due to a change in the exposure times.

11

5. Conclusions and Recommendations

The WFPC2 calibration program 8953 was executed post-SM3b to check that the rela-tive photometric throughput had not changed. Observations of the standard starGRW+70D582 were obtained with each camera and with the normal selection of monitorfilters (11 filters). Each camera performed as expected, and WFPC2 was unaffected by theSM3b Servicing Mission. The distribution shows a mean value of 0.34 +/- 0.26 sigma forall the observations, consistent with an unchanged photometric throughput. Hence, theresponse of the WFPC2 was essentially unchanged by the servicing mission. We also findthat the long-term throughput decline is consistent with the expected CTE loss.

6. References

Baggett, S. and Gonzaga, S. 1998, WFPC2 Long-Term Photometric Stability, WFPC2-ISR-98-03. (http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9803.html)

Biretta J., Heyer I., Baggett S., et al. 1997, Results of the WFPC2 Post-Servicing Mis-sion-2 Calibration Program, WFPC2-ISR-97-09. (http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9709.html)

Biretta J., McMaster M., Baggett S., and Gonzaga S. 1997, Summary of WFPC2SM97 Plans, WFPC2-ISR-97-03. (http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9703.html)

Casertano, S., et al., editors, The 1997 HST Calibration Workshop Proceedings,STScI, 1998, p.318. (http://www.stsci.edu/stsci/meetings/cal97/proceedings.html)

Dolphin, A. E. 2001 (April 2), private communication, http://www.noao.edu/staff/dol-phin/wfpc2_calib/

Gonzaga, S., Ritchie, C., Baggett, S., Casertano, S., Whitmore, B., and Mutchler, M.2000, “Standard Star Monitoring Memo”, (http://www.stsci.edu/instruments/wfpc2/Wfpc2_memos/wfpc2_stdstar_phot3.html)

12

Documents

SMOV3B WFPC2 Photometry Check