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Instrument Science Report WFPC2 98-01

WFPC2 Cycle 6 Calibration Closure Report
S. Baggett, S. Casertano, and WFPC2 group. April 21, 1998

ABSTRACT This report describes in detail the WFPC2 observations used to maintain and improve the quality of WFPC2 calibrations during Cycle 6, and the status of their analysis as of February 1998. Also included are summaries for three Cycle 5 proposals (Flat Field Check, UV Throughput, and Polarization and Ramps) which have been completed since the writing of the Cycle 5 Closure Report .

1. Overview
After more than four years in orbit, the most frequently used WFPC2 observing modes have been calibrated to a level sufficient for the large majority of science programs, and the performance of the camera has generally proved remarkably stable over the long term. Cycle 6 saw the successful continuation of the routine monthly decontamination procedures to remove the buildup of contaminants on the CCD windows. The associated routine monitoring of the basic properties of the camera (flat field, throughput, focus, internal electronics) revealed no major variations except for those that were expected: the throughput decrease in the UV due to contamination, with the nominal throughput fully restored after each decontamination; the monthly growth in the number of hot pixels, with the normal number restored at each decontamination; the slow focus drift due to OTA desorption, with focus restored by movements of the secondary mirror; and the continuing expected decline (due to usage) in the VISFLAT and UVFLAT lamp throughputs. The second servicing mission (SM97), which took place roughly halfway through Cycle 6 in Feb. 1997, entailed additional monitoring and checkouts of the camera which verified that no performance changes occurred as a consequence of SM97. The details of the SM97 programs and their results are presented in ISRs 97-03 (SM97 Calibration Plans, Biretta et al.) and 97-09 (Results of the SM97 Calibration Program, Biretta et al.).

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In addition to the routine and SM97 calibrations, a few special Cycle 6 programs, designed to provide data for use in addressing some of the remaining issues such as precision photometry and astrometry, were planned as well. These special proposals had two general goals: to improve the absolute accuracy (particularly in the popular wide-band filters) and to improve our understanding of the WFPC2 photometric systems compared to other systems. Acquiring these objectives meant achieving a better understanding of CTE and other nonlinearities at low light levels, both with and without an underlying background, and effects on the PSF due to focus position, location on chip, and exposure time. A few of these Cycle 6 programs have not yet been completed due to the shift in scheduling priorities required to maximize the use of NICMOS during its shortened lifetime; once the data are taken and analyzed, new results will be presented in the usual manner (e.g., TIPS meetings, ISRs, Data and Instrument Handbooks) and the results will be summarized in the Cycle 7 Closure report. We also take this opportunity to close out a few remaining Cycle 5 programs that had not been completed at the time of the Cycle 5 Closure Report ISR (97-02, Casertano et al.): the Flat Field Check (proposal 6195), which has yielded some important new results concerning CTE, the UV Throughput (proposal 6186) program, the results of which have been incorporated into the SYNPHOT and photometric zeropoint updates, and the Polarization and Ramps (proposal 6194) proposal, which has resulted in an improved calibration of polarization data as well as new WWW tools to aid in the analysis. As for all ISRs, an electronic version of this document is available via WWW on the WFPC2 instrument pages; the individual proposals in their Phase II format are available on the WWW HST Program Information page maintained by PRESTO (see Bibliography for addresses of sites).

2. Format of this document
Table 1 presents the calibration plan while Table 2 details the current status as of February 1998, including actual orbits used so far (statistics supplied by the PRESTO DBSA group), products, and accuracy achieved. The remainder of this document consists of detailed descriptions of each calibration program in a standard format. For each program, the first page ("Plan") contains the original description of the planned observations, their purpose and expected results; the second page ("Results") includes any modifications to the Plan, details on the execution, actual resources used, results achieved, a timeline of activity, and plans for any future continuation of the program. If the program is not complete, an estimate is given of the resources that will be necessary for completion. All Cycle 6 programs are presented first, followed by the three Cycle 5 proposals that have now been closed out. Finally, a detailed bibliography is provided for many of the WFPC2 documents to date, including WWW items.

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Table 1: WFPC2 Cycle 6 Calibration Plan
Estimated Time (orbits) ID Proposal Title Frequency "External" "Internal" Products Routine Monitoring Programs 6902 6903 6904 6905 6906 6907 6908 6909 6936 6934 6935 6937 6938 6939 6940 6941 6942 6943 Photometric Monitoring Decontamination Darks Internal Monitor Visflat Monitor Intflat Monitor UV Flat Field Monitor Earth Flats UV Throughput and Lyman Photometric Zeropoint Photometric Transformation CTE Calibration PSF Characterization Linear Ramp Filters Polarizers Astrometry Verification Camera Electronics Verification Narrow Band Throughput 2/4 weeks 1/4 weeks weekly weekly 2/4 weeks 1/4 weeks 2/cycle continuous 2/cycle 1 2 1 1 1 1 1 1 1 12 6 12 3 7 6 4 4 1 10 91 649 26 168 195 39 62 18 12 155 Synphot CDBS CDBS CDBS,ISR ISR ISR ISR CDBS Synphot Synphot ISR ISR CDBS CDBS CDBS STSDAS ISR Synphot 1-2% n/a 1 e-/hr 0.8 e
-

Accuracy

Notes

Also focus monitor Used together with darks, internals Also hot pixel lists on WWW Also monitor lamp health

0.3% 0.3% 2-8% 0.3% 3-10% 1-2% 2-5% 1% 10% 3% 3-5% 0.01'' 0.5% 3%

Also LRF, Methane quads Include BD+75D325 Add 2 new standards Also test zeropoint differences between chips, UV vignetting, astrometry Shift target field by a few arcsec Accuracy dominated by systematics

Special Calibration Programs

SNAP

TOTAL TIME (including all executions)


Table 2: WFPC2 Cycle 6 Calibration Closure Summary
Time Used (orbits) ID Proposal Title "External" "Internal" Routine Monitoring Programs 6902 Photometric Monitoring 6903 Decontamination 6904 Darks 6905 Internal Monitor 6906 Visflat Monitor 6907 Intflat Monitor 6908 UV Flat Field Monitor 6909 Earth Flats 6936 UV Throughput and Lyman 11 25 92 131 37 36 25 8 187 Synphot CDBS CDBS CDBS, ISR ISR ISR TIPS CDBS Synphot, ISR ~2-10% 1-2% n/a 0.05 e/hr 0.5 e ~0.5% ~0.5% 2-8% done done; decontamination date list kept updated on WWW done; hotpixel lists maintained on WWW; accuracy given is for typical superdark done; accuracy listed is for typical superbias in progress in progress done updates to flat fields in progress in progress; see results for 6186 Products Accuracy Status of analysis, notes

Special Calibration Programs 6934 Photometric Zeropoint 6935 Photometric Transformation 6937 CTE Calibration 6938 PSF Characterization 6939 Linear Ramp Filters 6940 Polarizers 6941 Astrometry Verification 6942 Camera Electronics Verification 6943 Narrow Band Throughput TOTAL CYCLE 6 TIME 4 1 8 86 516 Closeout of Remaining Cycle 5 Programs 6186 UV Throughput 6194 Polarization and Ramps 6195 Flat Field Check 6 24 2 76 Synphot, ISR ~1%, 5%, 10% done; accuracies dependent upon observing mode ISR ISR 1.5%; 8% 1.5%, 0.6% done; accuracies for polarizers and ramps, respectively done; accuracies for single chip & chip-to-chip 6 13 2 Synphot ISR ISR CDBS CDBS,ISR ISR STSDAS,ISR ISR,TIR Synphot,ISR 0.5% ~5-10% ~8% ~1-2% ~2-5% 1% analysis underway; preliminary results reported at Calib.Workshop in progress. done; results to be included with Cycle 7 (7630) results data to be taken in summer 1998 see results for 6194 on hold; see section on 6194 analysis in progress done nearly complete


Figure 1: Results of the photometric monitoring for WF3 and PC1 from Feb. 1994 through Nov. 1997, illustrating, particularly in the UV, the restoration to "normal" throughput after each decon (from WWW Memo, Gonzaga et al.). Data for the other cameras show similar trends. Starting with Cycle 5 (July 1995), images were taken in a different camera each month (except for F170W), resulting in the sparser data points - and halving the required number of orbits. See WWW Memo for a table of all data and a copy of the most recent figure.

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Proposal ID 6902: WFPC2 Cycle 6: Photometric Monitor

Plan
Purpose: Monthly external check of instrumental stability. Description: The standard star grw+70d5829 is observed before and after each decontamination (thus twice in a four-week period) using three different strategies: (1) F170W in all four chips to monitor contamination in the far UV; (2) F439W, F555W, F814W on the PC to monitor focus; (3) F160W, F185W, F218W, F255W, F300W, F336W, F439W, F555W, F675W, F814W in a different chip each month. Some filters may be cut because of lack of time (F185W cut first, then F300W, then F675W, then F218W). Based on Cycle 5 program 6184; added focus monitoring in F439W, F814W at the expense of some UV filters. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: 26 pointed orbits. Special Requirements: Needs to be scheduled shortly before and after each decontamination (up to 5 days). Accuracy: Overall discrepancies between the results of this test need to be measured to better than 2% and are expected to be less than 1% rms. The point of the test is to measure this variation. Products: Instrument Handbook, reports at monthly TIPS meetings, WWW (sensitivity trends); updates in UV sensitivity variation used in SYNPHOT.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 25 orbits. Accuracy Achieved: The standard star (grw+70d5824) countrates have remained stable to better than 1% (see Figure 1), even across the Second Servicing Mission (SM97): before and after the SM97, the sensitivity was shown to be the same to within 0.7% rms (Biretta et al., ISR 97-09 and Whitmore, 1997, Calibration Workshop). A slight secular increase in throughput - at the 1% level - has been observed in some filter/chip combinations (ISR in progress). Products: The data from this monitoring proposal are used in a variety of studies. The results have been presented in the Instrument and HST Data Handbooks, reports at TIPS meeting and 1997 Calibration Workshop, focus monitoring, WWW memo updates, and SYNPHOT update. Included here are the results from three studies that have this data. - The photometric monitoring results in WF3 and PC1 are presented in Figure 1 (Gonzaga et al.) and illustrate the improvement in throughput after each monthly decon. - The drift in focus is also monitored with data from this proposal; Figure 2 shows the drift in focus since Jan. 1994 (Suchkov & Casertano, ISR 97-01). The magnitude of the routine focus updates (1-2 per year) are determined from the results of these observations. Details of the focus monitoring are kept updated on WWW (see Reference section). - The impact of focus drift, both long and short term, on aperture photometry has been evaluated (Suchkov & Casertano, ISR 97-01). The effect was found to be small for both WF and PC when large aperture radii are used (~5 pixels or larger). However, the magnitude corrections for defocusing become important for smaller aperture radii: short-term variations (orbital timescales) can vary by a few percent and up to 10% for aperture radii of 1 pixel. On a longer timescale, the PC photometric zeropoint has drifted: the spectrophotometric standard star looks brighter by ~0.05 mag in Jan. 1997 than it did it April 1994 (preliminary results show the same brightening in the WF chips). An example of this longterm drift is shown in Figure 3; more details are provided in ISR 97-01 (Suchkov & Casertano). - The data have also been used in determining the necessary SYNPHOT table update (summary of proposal 6186 - this ISR - contains details). Timeline: Observations were taken regularly every month as planned. Continuation Plan: Cycle 7 photometric monitor will be essentially the same as this proposal after each decon; given the instrument's stability, the pre-decon measurements will only be taken before every other decon, thereby saving ~12 orbits of observing time.

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Proposal ID 6903: WFPC2 Cycle 6: Decontamination

Plan
Purpose: UV blocking contaminants are removed by warming the CCDs. Description: The decontamination itself is implemented via the DECON mode, in which the TECs (thermo-electric coolers) are turned off and the CCD and heat pipe heaters are turned on to warm the detectors and window surfaces (cold junctions warmed to ~+22C). Keeping WFPC2 warm for ~6 hours has been shown in previous cycles to be sufficient to remove the contaminants and anneal many hot pixels; continuation of 6-hour decons is anticipated for Cycle 6. The internal observations taken before and after each decontamination consist of: 4 biases (2 at each gain setting), 4 INTFLATs (2 at each gain setting), 2 K-spots (both at gain 15, one short and one long exposure, optimized for PC and WF), and finally, 5 darks (gain 7, clocks off). To minimize time-dependent effects, each set of internals will be grouped within 2 days and performed no more than 1 day before the decon and no later that 12 hours after the decon. To protect against residual images in the darks (which results in the irretrievable loss of the critical pre-decon hot pixel status), the darks will be executed as a non-interruptible sequence at least 30 minutes after any WFPC2 activity. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: None pointed, 168 internal. Special Requirements: Requires scheduling at 4 week intervals. Prevents WFPC2 from being used for several hours, although other instruments can be used most of that time. Dark frames taken before decontaminations need to be protected from possible residual images from overexposed sources. Accuracy: This proposal is mainly designed to maintain the health of the instrument. Biases, darks, and other internals taken with this proposal are used in generating appropriate reference files (see Proposals 6904 and 6905). Products: Those obtained from use of darks, biases and other internals (see Proposals 6904 and 6905).

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 92 occultation periods. Accuracy Achieved: N/A; objective is to insure that the contaminants are periodically removed from the CCD windows. Products: Data was used in generating dark reference files for the pipeline (Wiggs et al.), hot pixel lists for the WWW (Wiggs & Casertano), and regular updates to WWW History File on Web (Baggett & Wiggs). The table below lists all decontamination procedures done to date; the Photometric Monitoring proposal (6902) section contains a figure illustrating the restoration to nominal throughput after each decon. Timeline: N/A Continuation Plan: Decons for Cycle 7 are anticipated to be clones of the Cycle 6 decons.

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Figure 2: Focus History. Data points are focus measurements (PC, F555W) obtained via phase retrieval; focus adjustments are marked with a vertical line (from Suchkov & Casertano, ISR 97-01; plot kept updated on WWW, URL is included in Reference section).

Figure 3: Long term magnitude variations in F555W Casertano, ISR 97-01). Aperture radius is given along Thick lines are linear regression fits to the magnitudes focus adjustment dates; left and right panels mark PC

due to focus drift (from Suchkov & the right hand side of each data set. in each aperture. Vertical lines mark and WF, respectively.

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Table 3. 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, maintained by Baggett and Wiggs).
date 1994 Feb 22 11:37 Mar 24 11:08 Apr 24 00:49 May 23 15:00 Jun 13 11:02 Jul 10 11:40 Jul 28 07:12 Aug 27 09:46 Sep 25 00:46 Oct 21 00:41 Nov 19 17:29 Dec 18 06:00 49405.4840 49435.4639 49466.0340 49495.6250 49516.4597 49543.4861 49561.3000 49591.4069 49620.0319 49646.0285 49675.7285 49704.2500 1995 13 Jan 16:14 12 Feb 01:54 11 Mar 14:30 8 Apr 10:29 7 May 01:13 2 Jun 18:30 27 Jun 20:00 30 Jul 08:50 27 Aug 05:43 49730.6764 49760.0792 49787.6042 49815.4368 49844.0507 49870.7708 49895.8333 49928.3681 49956.2382 6 6 6 6 6 6 6 6 6 07 Jan 23:41 09 Feb 00:00 23 Feb 19:08 6 6 6d 5.5e 12 12 12 12 12 12 6 6 11 Jan 23:24 11 Feb 00:30 10 Mar 00:21 02 Apr 00:16 04 May 17:09 28 May 06:16 22 Jun 22:15 28 Jul 13:34 23 Aug 10:10 18 Sep 16:25 18 Oct 07:46 12 Nov 09:40 15 Dec 00:00 19 Dec 12:33 MJD ta date 22 Sep 03:40 17 Oct 09:43 15 Nov 08:53 14 Dec 07:03 MJD 49982.1528 50007.4053 50036.3706 50065.2929 1996 50093.9750 50124.0208 50152.0147 50175.0111 50207.7146 50231.2614 50256.9277 50292.5653 50318.4242 50344.6840 50374.3236 50399.4031 50432.0417 50436.5229 1997 50455.9875 50488.0006 50502.7978 6 6 6f 6 6 6 6 6 6 6 6 6 6 6 6 8 6 Jan 08 00:03 Feb 01 19:15 Mar 06 09:18 ta 6 6 6 6 date 27 Feb 06:31 04 Mar 10:16 21 Mar 03:35 05 Apr 08:50 25 Apr 23:00 15 May 20:18 07 Jun 13:06 24 Jun 11:04 24 Jul 18:42 20 Aug 02:17 17 Sep 17:24 13 Oct 18:00 14 Nov 05:19 10 Dec 09:40 MJD 50506.2721 50511.4278 50528.1494 50543.3681 50563.9583 50583.8460 50606.5461 50623.4612 50653.7795 50680.0952 50708.7256 50734.7506 50766.2217 50792.4027 1998 50821.0025 50821.0025 50878.3877 6 6 6 ta 6b 6c 6 6 6 6 6 6 6 6 6 6 6 6

a. b. c. d. e. f.

t is length of time chips are kept warm. Post-SM97 decontamination procedure: CCD's to -88° C, TEC on 07:10:48. Post-SM97 decontamination procedure: CCD's to -88° C, TEC on 10:55:46 Operating temperature set colder (-88°C). Realtime contingency decon used. Post-SM97 decontamination procedure: CCD's to -43° C, TEC on 19:40:46.

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Proposal ID 6904: WFPC2 Cycle 6: Darks

Plan
Purpose: Measure dark current on individual pixels and identify hot pixels at frequent intervals. Description: Every week, five 1800s exposures are taken with the shutter closed. The length of the exposures is chosen to fit nicely within an occultation period. The weekly frequency is required because of the high formation rate of new hot pixels (several tens per CCD per day). Five darks a week are required for cosmic ray rejection, to counterbalance losses due to residual images, and to improve the noise of individual measurements. Even with these measures, some weeks no usable darks will be available because of residual images. Normally this results only in a longer-than-usual gap in the hot pixel lists, but in a decontamination week, information on pixels that became hot and then annealed would be lost irretrievably. For this reason, pre-decon darks are to be executed NON-INT and at least 30 minutes after any WFPC2 activity (see Proposal 6903). Normal darks do not need to be protected in this fashion. Fraction of GO/GTO Programs Supported: 90% Resources: Observation: 195 internal orbits (occultation periods). Special Requirements: None. (Darks associated with decontaminations (proposal 6903) are protected from residual images due to saturated sources). Accuracy: The required accuracy for darks is about 1 e-/hour (single-pixel rms) for the vast majority of science applications. The expected accuracy in a typical superdark is 0.05 e-/hour for normal pixels. The need for regular dark frames is driven by systematic effects, such as dark glow (a spatially and temporally variable component of dark signal) and hot pixels, which cause errors that may exceed these limits significantly. Products: Weekly dark frames delivered to CDBS and monthly tables of hot pixels on the Web.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 131 occultation periods. Accuracy Achieved: Typical superdark accuracy is ~0.05 e-/hour. Products: Dark reference files are regularly delivered to CDBS; WWW hot pixel lists are maintained, in STSDAS format for use with the STSDAS task `warmpix', and in ASCII format for use outside of IRAF (Wiggs et al.). An excerpt from one of the hot pixel lists is shown in the table below. Timeline: Cycle 6 dark frames were taken weekly and new calibration files delivered generally within two weeks (primarily dependent on archive turnaround); the warm pixel lists are made available ~monthly, typically within 1 month of nominal date (Wiggs et al.). Continuation Plan: The 5 dark frames per week continue in Cycle 7. In addition, daily 1000 sec darks (up to 3 frames per day) are taken on a non-interference, low-priority basis as a service to the GO community, to help with hot pixel rejection. There is no plan to use the supplemental darks for the standard pipeline products. Table 4. Excerpt from a hot pixel list (decon_971013_971114.tab); dark current is given in DN/sec at 7 electrons/DN. The hot pixel tables typically span the time interval between decons and contain hot pixel information derived from the weekly dark files taken during that period; the table section below is from the period Oct 13,1997 to Nov 14,1997. The STSDAS warmpix task uses the locations and darkcounts to flag and/or fix hotpixels in an image; pixels with darkcounts set to "-100" before and after the observation are considered to be fine and are not changed by warmpix in the image or in the data quality file. For more details on warmpix, see the online help (type "help warmpix" at the IRAF prompt).
chip 1 1 1 1 1 1 1 1 1 X (pixels) 83 83 83 83 83 83 83 83 83 Y (pixels) 82 82 82 82 82 82 82 82 82 date (MJD) 50734.000000 50734.994603 50735.427242 50741.070992 50748.146686 50755.043908 50762.178631 50765.342520 50766.000000 darkcount(DN/sec) -100.0000 0.00311 0.00329 0.00363 0.00331 0.00326 0.00302 0.00397 -100.0000

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Proposal ID 6905: WFPC2 Cycle 6: Internal Monitor

Plan
Purpose: Verification of short-term instrument stability for both gain settings. Description: The internal observations will consist of 8 biases (4 at each gain) and 4 intflats (2 at each gain). The entire set should be run once per week, except for decon weeks), on a non-interference basis. This proposal is similar to the Cycle 5 Internal Monitor (6250), except that the K-spot images have been removed (these are being taken with the Decon Proposal). The execution frequency during Cycle 6 has also been reduced, from twice a week to once a week, although the total number of biases has been increased to continue to provide an adequate number of frames for the generation of pipeline reference files. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: 39 internal orbits (occultation periods). Special Requirements: None. Accuracy: Approximately 120 bias frames will be used for each pipeline reference file; accuracy is required to be better than 1.5 e-/pixel, and is expected to be 0.8 e-/pixel. Products: Superbiases delivered ~every year to CDBS; TIPS reports on possible buildup of contaminants on the CCD windows (worms) as well as gain ratio stability, based on INTFLATs. An ISR will be issued if significant changes occur.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 37 occultation periods. Accuracy Achieved: Sets of 120 bias frames are used to generate the pipeline reference files: the accuracy is ~0.5 electrons/pixel. Other bias-related results are detailed in the ISR (O'Dea et al., ISR 97-04), and can be summarized as follows. · · Superbiases: These are similar to previous superbiases; see table below for statistics. Bias eral, stant even Overscan: The analysis also included a study of the overscan properties. In genthe overscans were found to be well-behaved and, to a good approximation, conas a function of row number, confirming that subtraction of a single value for and odd columns, the BIASEVEN and BIASODD keywords, is appropriate.

·

Bias Jumps: these jumps, as detected in the .x0h by calwp2, were found to occur a significant fraction of the time in both gain settings (63% and 45% for gain 7 and 15, respectively). PC tends to dominate the statistics in gain 7, while WF4 dominates in gain 15. The average bias jump magnitudes were 0.127 (gain 7) and 0.147 (gain 15). There is no evidence for any long-term changes in frequency of jumps, their mean or median amplitude, or dispersion in amplitudes (O'Dea et al., TIR 97-11).

Products: Results presented at TIPS; updated bias reference files delivered to CDBS (Gonzaga et al.); ISR 97-04 (O'Dea et al.) and TIR 97-11 (O'Dea et al.) completed. Timeline: Continuation Plan: Maintain current policies. Table 5. Statistics of the two most recent sets of superbiases (h* were generated from biases dating May 95 - July 96 while i* were generated from Aug 96 - Nov 97 biases).
Chip 1 2 3 4 1 2 3 4 h1612404u (gain 15) CDBS File h161240au (gain 7) Mean 0.346 0.312 0.301 0.330 0.176 0.153 0.146 0.170 RMS 0.382 0.217 0.654 0.183 0.105 0.134 0.295 0.100 Max 287.8 91.5 494.2 59.5 53.4 69.2 223.6 19.7 Min -0.057 -0.197 -0.147 -0.165 -0.103 -0.232 -0.117 -0.295 i2h1025iu (gain 15) CDBS File i2i1201qu (gain 7) Mean 0.344 0.314 0.297 0.319 0.176 0.160 0.149 0.169 RMS 0.221 0.303 0.323 0.297 0.114 0.150 0.155 0.142 Max 73.941 133.497 146.916 159.772 42.423 61.924 73.203 58.485 Min 0.004 -0.121 -0.032 -0.083 -0.072 -0.077 -0.109 -0.169

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Proposal ID 6906: WFPC2 Cycle 6: Visflat Monitor

Plan
Purpose: Monitor the stability of the camera and filter responses via the VISFLAT channel. Description: Twice a month, internal flat fields (VISFLATs) will be obtained using the visible cal-channel lamp with the photometric filter set plus one of the linear ramp filters. The images will be used to monitor WFPC2's flat field response as well as to build a database of high S/N flat fields, which will provide information on the pixel-to-pixel response in the cameras and any possible long-term, contamination-induced changes. The linear ramp filter (FR533N) exposures, one at each gain, taken after decon will provide a monitor of the performance of the ADC. Histograms generated from the ramp filter flats will be used to trace the ADC transfer curve. ON HOLD: In addition to the monitor observations, an initial filter sweep is done to obtain VISFLATs in all visible filters; these will be compared to the Cycle 5 filter-sweep data to verify that none of the filters are developing any problems and to provide a check of the cal-channel's long-term stability. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: 62 internal orbits (occultation periods). Special Requirements: Uses the VISFLAT calibration channel, whose Welch Allyn bulb is apparently wearing out (see ISR 96-01; a back-up exists for the Welch Allyn bulb). The Cycle 6 proposal has been redesigned to limit the number of ON/OFF cycles placed on this channel to a level believed safe over 10-15 years. The sweep part of the proposal, which puts the heaviest usage on the lamp, is on hold, pending verification of the lamp health from the short monthly executions. The INTFLAT Monitor (Proposal 6907) can obtain similar information if necessary. Accuracy: The VISFLAT response is stable to about 0.3%, both in overall level (lamp degradation aside) and in spatial variations. The point of this proposal is to verify this stability on a regular basis and to monitor the lamp degradation. Products: ISR, TIPS reports.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 36 occultation periods. Accuracy Achieved: Products: Analysis is still in progress and is part of the intflat study (proposal #6907, O'Dea et al.). Final results will be presented, as usual, at TIPS meetings, STAN, WWW, Instrument and Data Handbook updates, and Instrument Science Report. Timeline: Continuation Plan: Maintain current policy: minimize use of VISFLAT lamp to conserve remaining lifetime.

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Proposal ID 6907: WFPC2 Cycle 6: Intflat Monitor

Plan
Purpose: Provide backup database of INTFLATS if VISFLAT channel fails. Description: This proposal consists of two parts: (1) an INTFLAT filter sweep and (2) a series of exposures to test the linearity of the camera. (1) The sweep is a complete set of internal flats cycling through both shutter blades and both gains. The signal-to-noise ratio per pixel is estimated to be similar to the VISFLATs (0.6%) but the spatial and wavelength variations in the illumination pattern are much larger. However, the INTFLATs will provide a baseline comparison of INTFLAT vs. VISFLAT, in the event of a cal-channel system failure, and track temporal variations in the flat fields at the 1% level. In addition, these images will provide a good measurement (better than 1%) of the stability of the gain ratios. (2) The linearity test portion is aimed at obtaining a series of INTFLATs with both gains and both shutters. Since the INTFLATs have significant spatial structure, any nonlinearity would appear as a non-uniform ratio of INTFLATs with different exposure times. Exposures are also taken with gain 7, shutter B, and clocks=YES to test these modes. Fraction of GO/GTO Programs Supported: Backup. Resources: Observation: 18 internal orbits (occultation periods). Special Requirements: None. Accuracy: The signal-to-noise ratio per pixel is similar to that obtained in the VISFLAT program (0.6%) but there are much larger spatial and wavelength variations in the illumination pattern. As a result, this dataset will not form any part of the pipeline calibration. This baseline is necessary in case the VISFLAT channel fails and there are temporal variations in the camera flat fields at the 1% level. Products: TIPS report, ISR if any significant variations are observed.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 25 occultation periods. Accuracy Achieved: Preliminary results were presented at TIPS (see figure below): the lamp output has remained relatively stable and exposures repeatable since the start of the WFPC2 mission. The jump early on (MJD~49500) is attributed to the change to cooler CCD operating temperature in April 1994; since then, the lamps have slowly increased their output by ~1%. The increase is attributed to the lamps because, 1) this sort of behavior is typical for Carley-type bulbs and, 2) observations of the photometric standard stars exhibit no such trend. Products: Analysis is still in progress; final results will be presented in the standard fashion: TIPS, STAN, WWW, Instrument and Data Handbook updates, and ISR. Timeline: Continuation Plan: Maintain current policy.
62 INTFLATS MEANC300/EXPTIME PC1 F555W Gain 7 Blade A March 1994 to Feb 1998 60

58

56

Julian Day

Figure 4: Countrates for F555W INTFLAT exposures, from early 1994 until early 1998 (O'Dea et al.).

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Proposal ID 6908: WFPC2 Cycle 6: UV Flat Field Monitor

Plan
Purpose: Monitor the stability of UV flat field. Description: UV flat fields will be obtained with the cal-channel's ultraviolet lamp (UVFLAT) using the UV filters F122M, F170W, F160BW, F185W, and F336W. The UV flats will be used to monitor UV flat field stability and the stability of the Woods filter (F160BW) by using F170W as the control. The F336W ratio of VISFLAT (Cycle 6 proposal 6906) to UVFLAT will provide a diagnostic of the UV flat field degradation and tie the UVFLAT and VISFLAT flat field patterns together. Two supplemental dark frames must be obtained immediately after each use of the lamp, in order to check for possible afterimages. Fraction of GO/GTO Programs Supported: 10% Resources: Observation: 12 internal orbits (occultation periods). Special Requirements: Uses the limited life UV lamp. In order to prevent excessive degradation of the lamp, the SU (scheduling unit) duration for each UVFLAT visit should be kept the same as the durations used during Cycle 5 (proposal 6191); the lamp should not remain on for periods of time longer than those used in Cycle 5. To be executed once before and once after the refurbishment mission, shortly after a decontamination. Accuracy: About 2-8% pixel-to-pixel expected (depending on filter). Products: New UV flat fields if changes are detected.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 8 internal orbits. Accuracy Achieved: 2% on lamp level, 5-10% on pixel-to-pixel sensitivity variation, depending on filter. Products: Results presented at TIPS meeting; the figure below illustrates the UV lamp decline since 1994. Timeline: Observations executed in January and April 1997. Continuation Plan: Maintain current policies.
F170W, UVFLATS 1.1
mean

F336W, UVFLATS 1.1
mean

1.05 1 .95 1.1

PC

1.05 1 .95 1.1

PC

mean

1 .95 1.1

mean

1.05

WF2

1.05 1 .95 1.1

WF2

mean

1 .95 1.1

mean

1.05

WF3

1.05 1 .95 1.1

WF3

mean

1 .95 49500 49750 50000 MJD 50250 50500

mean

1.05

WF4

1.05 1 .95 49500 49750 50000 MJD

WF4
50250 50500

Figure 5: Variation in UV flats over time in two filters for the four cameras. The countrates were averaged over the central 300x300 pixels and normalized to the October 1994 set of UV flats (MJD of 49500 is May 28,1994, MJD of 50500 is Feb 21,1997). The lamp degradation in the far UV is apparent from the F170W data (Baggett).

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Proposal ID 6909: WFPC2 Cycle 6: Earth Flats

Plan
Purpose: Monitor flat field stability. Description: As in Cycle 5 program 6187, sets of 200 Earth-streak flats are taken to construct high quality narrow-band flat fields with the filters F160BW, F375N, F502N, F656N and F953N. Of these 200 perhaps 50 will be at a suitable exposure level for destreaking. The resulting Earth superflats map the OTA illumination pattern and will be combined with SLTV data (and calibration channel data in case of variation) for the WFPC2 filter set to generate a set of superflats capable of removing both the OTA illumination and pixel-topixel variations in flat field. The Cycle 4 plan is being largely repeated except: (1) UV filters are dropped because measurement is generally only of red leak. (2) F160BW is retained in order to check for developing pinholes. (3) Crossed filters used as neutral densities are eliminated (illumination pattern is wrong). (4) Observations with the Methane Quad filters will be taken to define its vignetting pattern. (5) Observations with crossed narrow-band and ramp filters will be taken to verify the relative stability of their wavelength. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: 155 non-science-impacting orbits (during occultation periods). Special Requirements: None. Accuracy: The single-pixel noise expected in the flat field is 0.3%. Products: New flat fields to CDBS if changes detected.

22


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 187 occultation periods. Accuracy Achieved: Flat fields currently in CDBS are based primarily upon Cycle 5 Earth flats; errors are generally < 0.5% rms. Earth flats from this Cycle 6 proposal are now being used to investigate possible improvements to the CDBS flat fields. Products: Analysis is still in progress (Biretta & Wiggs); final results will be presented at TIPS meetings, STAN, WWW, Instrument and Data Handbook updates, and ISR. Timeline: CDBS file updates are estimated for later in 1998. Continuation Plan: Maintain current policies.

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Proposal ID 6934: WFPC2 Cycle 6: Photometric Zeropoint

Plan
Purpose: Verify synthetic zeropoint of WFPC2 filters. Description: Standard stars are observed through all filters longward of F336W (inclusive) with the limit of one orbit per target. Targets include: the spectrophotometric standard grw+70d5824 in PC and WF3; the solar-type standards P041-C and P330-E selected by the NICMOS team (P177-D was observed in Cycle 5); and the Landolt standard fields SA98 and Rubin 149, containing 5 and 7 useful targets, respectively. Observations of grw+70d5824 will be directly comparable to the corresponding observations for cycles 4 and 5 (programs 5572 and 6179) and will verify the stability of the filters as well as improve the accuracy of the calibration. The other standards are observed to provide cross-instrument calibration and to increase the range of colors used for photometric verification. This proposal will help all observers who want to do quantitative photometry at the 2% level. Fraction of GO/GTO Programs Supported: 70% Resources: Observation: 6 pointed orbits. Special Requirements: Specific orientations will be required for the two fields of standards in order to fit the maximum number of stars. All observations should be executed within a week after decontamination. Accuracy: 2% required, 1% expected for our main spectrophotometric standard grw+70d5824. Expected accuracy for the other standards is between 2% and 5%, depending on spectral type and filter; most of the error derives from limited knowledge of the transformations between ground-based and WFPC2 photometric systems. Products: TIPS report, SYNPHOT updates if necessary, ISR.

24


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 6 pointed orbits. Accuracy Achieved: The data have been used to verify the current SYNPHOT tables (Casertano et al.): the SYNPHOT predictions for the photometric filter set are within 1-2% of the observations (see Table 6 below), slightly higher for F555W (consistent with the absolute flux scale from the FOS measurements), and ~3% level for the other filters. Small adjustments to the SYNPHOT tables to further improve the predictions will be made later this year (Baggett et al.). Products: Progress reports have been made at the TIPS meeting and 1997 Calibration workshop (Casertano, 1997). Timeline: Observations completed Nov 1996, May, August, and December 1997. Continuation Plan: Continuing monitoring with grw+70D5824 standard only. Table 6. Ratio of SYNPHOT predictions to observed countrates (from Casertano, 1997).
filter HZ-44 P041-C P177-D P330-E

photometric filter set F336W F439W F555W F675W F814W 0.997 0.992 0.994 0.993 0.000 1.009 1.007 1.031 1.006 1.019 other filters F380W F410M F450W F467M F547M F606W F622W F702W F785LP 1.000 1.018 1.008 0.992 0.987 0.973 0.000 1.005 0.000 1.042 0.996 1.027 1.032 1.049 1.032 1.030 1.022 1.044 1.045 1.003 1.022 1.034 1.045 1.026 0.000 1.008 1.031 1.024 0.986 1.014 1.023 1.061 1.011 0.000 1.004 1.011 1.022 1.006 1.030 1.005 1.018 0.983 0.989 1.017 1.006 1.012

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Proposal ID 6935: WFPC2 Cycle 6: Photometric Transformation

Plan
Purpose: (1) Update photometric transformations to Johnson-Cousins system and Stromgren system; (2) Determine spatial dependence of contamination; (3) Check the astrometric solution using M67; (4) Spot check of gain = 7 vs. gain = 15 ratios; (5) Spot check short vs. long exposure zeropoints; (6) Determine cross-chip zeropoint differences in UV for point-source photometry; (7) Determine the vignetting function in rotated filters. Description: Three photometric standard star fields in Cen (Galactic globular cluster), M67 (old open cluster), and NGC 2100 (young LMC globular cluster) are observed before and after a decontamination. Four different filter sets are used: (1) The five filters generally used to match the Johnson-Cousins system (F336W, F439W, F555W, F675W, F814W). (2) The wide-band equivalents for the Johnson-Cousins system (F300W, F380W,F450W, F606W, F702W). (3) The Stromgren equivalents (F336W, F410M, F467M, F547M). (4) Two filters farther toward the UV (F255W, F170W), so that contamination over the full field of view can be measured. F255W is not used for the reddest cluster ( Cen). F170W is only used for the bluest cluster (NGC 2100). For the brighter clusters (M67 and NGC 2100) long and short exposures are taken in the UBVRI equivalents both to extend the dynamic range and to check for differences in photometric zeropoints. A spot check to compare gain=7 and gain=15 is also included for Cen and M67, and an observation of Cen with a 60o change in orientation - which moves individual stars across chips - is used to test the cross-chip zero-point calibration. Finally, NGC 2100 is observed in F160BW in both normal and rotated (N15) position in order to determine the vignetting function of the latter. Fraction of GO/GTO Programs Supported: 40% Resources: Observation: 12 pointed orbits. Special Requirements: The first visit for each target must be taken within 3 days after a decontamination. The second visit, including only the UV filters, must be taken more than 25 days after the first visit, but before the next decontamination. Accuracy: The photometric transformations should be accurate to 2-5%. The stability of these transformations will be measured to the 1% level. The astrometry should be good to 0.1" (absolute) and 0.05" (relative). Products: ISR, Instrument Handbook, and part of a planned paper on 1% photometry.

26


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 13 pointed orbits. Accuracy Achieved: Products: Analysis is still in progress (Whitmore et al.); final results will be presented at TIPS meetings, STAN, WWW, Instrument and Data Handbook updates, and ISR. Timeline: Data taken Sep/Oct 96 and Jun/Oct 97. Continuation Plan: Repeat a shorter version (merging of 6934 and 6935) in Cycle 7, in order to monitor any time-variable effects in the zeropoint, spatial dependence of zeropoint, or contamination. May also include a new globular cluster target to aid in photometric transformations.

27


Proposal ID 6936: WFPC2 Cycle 6: UV Throughput and Lyman Verification

Plan
Purpose: Verify throughput for all UV filters, including Lyman test to monitor possible contamination on pick-off mirror. Description: Spectrophotometric standards are observed shortly before and after a DECON through all the UV filters in each chip and through F160BW crossed with F130LP, F185LP, and F165LP to determine the wavelength dependence of the throughput across the bandpass (for color terms). This proposal is based on the Cycle 5 UV Throughput proposal (6186) but includes also the standard BD+75d325 used in Cycle 4 (proposal 5778) to establish the Lyman throughput calibration. Some of the observations will be repeated after the servicing mission to verify the stability of the camera. Fraction of GO/GTO Programs Supported: 10% + check for contamination on pick-off mirror. Resources: Observation: 12 pointed orbits. Special Requirements: Timing requirements with respect to decontaminations. Accuracy: The UV throughput will be measured to better than 3%. Accuracy in Lyman throughput is expected to be between 5 and 10%, because of the residual uncertainty of the red leak correction after observations with crossed F122M and F130LP. Products: TIPS, SYNPHOT update if necessary, ISR.

28


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 11 pointed orbits. Accuracy Achieved: Analysis is currently in progress, in conjunction with the solar analog standard star data from proposal 6934 (Baggett et al.). Products: Results will be reported in the standard fashion, via TIPS meetings, STANs, WWW memos, and Instrument and Data Handbooks updates. Any necessary SYNPHOT updates will be implemented later in 1998. Timeline: Data taken in spring and fall of 1997. Continuation Plan: UV throughput check will be cut back to just one iteration, midway through the next cycle.

29


Proposal ID 6937: WFPC2 Cycle 6: CTE Calibration

Plan
Purpose: (1) Test whether the exposure-time dependence of the photometric calibration is due to CTE (long vs. short exposure problem); (2) refine flux and background-level dependent aperture corrections. Description: The globular cluster NGC 2419 will be observed through F555W with a combination of exposure times (between 5 and 1400 s) and preflash levels (0 to 500 e-). Analysis of cycle 5 CTE calibration data (proposal 6192) suggests that magnitude errors due to charge transfer effects are greatly reduced (if not entirely eliminated) at background levels of 160 e- or greater. CTE effects have been proposed as the solution to reported differences in the magnitudes of stars measured on frames with short exposures vs. long exposures. If CTE is the cause, then the differences should disappear with preflash. We will reobserve the cluster NGC 2419 with and without preflash to test this hypothesis. This data set will also provide a large number of stars with which to refine existing measurements of the effects of CTE on the wings of the PSF. A range of preflash levels will be explored at the 60 sec exposure time. Fraction of GO/GTO Programs Supported: 30% Resources: Observation: 2 pointed orbits. Special Requirements: Observations should be made at the same position and roll angle as the previous NGC 2419 "long" exposures (proposal GO 5481). Accuracy: The reported short vs. long effect is ~0.05 mag. We wish to reduce this to less than 0.01 mag. Products: ISR; if appropriate, a special task to correct the CTE effect will be generated.

30


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 2 pointed orbits. Accuracy Achieved: The long vs. short anomaly, or nonlinearity, is characterized by shorter exposures, with less total signal, providing lower countrates than long exposures with more total signal (see Figures 6 and 7 on following page). For more details about previous studies, see the WFPC2 WWW Clearinghouse page and the CTE WWW memo. The magnitude discrepancies are ~0 for well-exposed stars (>2000 DN in short exposure) and >0.10 mag for faint sources (~100 DN). The total signal in the short exposure appears to determine the discrepancy, i.e., nonlinearity is determined by total amount of signal, not the exposure length, and the discrepancy increases with increasing disparity of exposure times. An empirical correction (adding ~2-2.5 electrons per pixel in the target aperture), although unphysical, appears to remove the effect (see WWW memo). Despite the different: it position on Workshop) · · superficial similarity with the CTE problem, the long vs. short nonlinearity is is not affected by the background level and there is no apparent dependence on the chip. Analysis of the data from this proposal (Casertano, 1997, Calibration showed that:

there have been no longterm changes in the long vs. short nonlinearity behavior (the problem exists at the same level as it did back in 1994), and preflashing at 200 e-/pixel has no measurable effect (see Figure 7 on next page).

Products: Preliminary results were presented at TIPS and at 1997 Calibration Workshop (Casertano). The analysis is still in progress (Casertano et al.); final results will be presented in the standard fashion, via TIPS meetings, STAN, WWW, Instrument and Data Handbook updates, and ISR. Timeline: Continuation Plan: More data is planned for Cycle 7 (proposal 7630) in order to allow a more detailed characterization of the anomaly.

31


Figure 6: Long vs. Short Effect. Plotted is the difference in magnitude between the short and long exposures (60 and 1400 seconds), without preflash, as a function of the long exposure magnitude (Casertano & Mutchler).

Figure 7: Same as Figure 6, but data has been preflashed at ~250e- level.

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Proposal ID 6938: WFPC2 Cycle 6: PSF Characterization

Plan
Purpose: Provide a subsampled PSF over the full field to allow PSF fitting photometry, test PSF subtraction as well as dithering techniques (cf. effects of the OTA breathing and CCD gain). Description: Measure PSF over full field in photometric filters in order to update the TIM and TINYTIM models and to allow accurate empirical PSFs to be derived for PSF fitting photometry. These observations will also be useful in order to test PSF subtraction and dithering techniques at various locations on the CCD chips. With one orbit per photometric filter, a spatial scan is performed over a 4x4 grid on the CCD. The step size is 0.125 arcseconds; this gives a critically sampled PSF over most of the visible range. This program uses the same specially chosen field in Cen as the Cycle 5 proposal 6193, but with a few arcsec shift in order to better map the PSF variation. The standard `photometric' filters are used. Two additional orbits are used to explore the effects of OTA breathing and CCD gain on dithering and PSF subtraction techniques. Data volume will be a problem, so special tape recorder management will be required. The proposal also allows a check for subpixel phase effects on the integrated photometry. Fraction of GO/GTO Programs Supported: 15% Resources: Observation: 7 pointed orbits. Special Requirements: Needs same pointing and orientation as Cycle 5 observations for proposal 6193, thus should be scheduled within a similar time frame. Accuracy: Provides measurement of pixel phase effect on photometry (sub-pixel QE variations exist). The chosen field will have tens of well exposed stars in each chip. Each star will be measured 16 times per filter at different pixel phase. The proposal therefore provides, in principle, a high signal-to-noise, critically sampled PSF. This would leave PSF fitting photometrists in a much better position than now, where pixel undersampling clearly limits the results. The result will be largely limited by breathing variations in focus. It is difficult to predict the PSF accuracy that will result. If breathing is less than 5 microns peak to peak, the resulting PSFs should be good to about 10% in each pixel. Breathing effects will be investigated (1 additional orbit) as well as the gain dependence (1 additional orbit). PSF fitting results using this calibration would of course be much more accurate. In addition, the test gives a direct measurement of sub-pixel phase effects on photometry, which should be measured to better than 1%. Products: PSF library (CDBS); ISR -----------------------------------------------------------------------------------------------------------Results Current status: Not yet executed; in the Long Range Plan, observations are set for June/ July 1998.

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Proposal ID 6939: WFPC2 Cycle 6: Linear Ramp Filters

Plan
Purpose: Verify wavelength and throughput calibration for Linear Ramp Filters at selected wavelengths. Description: The throughput calibration is obtained by observing the spectrophotometric standard grw+70D5824 at several filter rotations and wavelengths. This completes the program carried out in Cycle 5, in which some wavelength ranges and rotations could not be covered. The stability of the wavelength calibration is verified by observing an extended line source (the Orion nebula) and checking the position of the bright spots corresponding to the bright nebular emission. An additional verification is obtained through crossed narrow-band and linear ramp filters (see Proposal 6909). Fraction of GO/GTO Programs Supported: 7% Resources: Observation: 6 pointed orbits. Special Requirements: None. Accuracy: Throughput accuracy should be verified to ~3%. Wavelength variations of more than 0.2% (0.6-1.4 nm), or about 15% of the filter width, should be detectable; a factor of 2 better can be expected. Products: Update SYNPHOT throughput tables if necessary.

34


Results (based on data from Cycle 5 Proposal 6194)
Modifications: Proposal 6939 was initially a placeholder, with proposal development pending results from Cycle 5 Polarizer and Ramps Proposal (6194). Because of the delay in executing 6194, the Cycle 6 version (6939) has now been withdrawn, in favor of a Cycle 7 ramps proposal. Execution: Observations near 7400е and those between 8500 and 8600е were lost due to an error in the proposal. Resources Used: Observation: 8 pointed orbits for LRF portion of 6194. Accuracy Achieved: A recent analysis of the ramp data taken as part of Proposal 6194 showed that, on average, there is 1% agreement between the ground-based calibration and new on-orbit data, but there also ~8% RMS scatter between individual data points which is not completely understood (O'Dea & McMaster). Products: Work is still in progress; results will be reported as usual, at the TIPS meetings, in STANs, on WWW, in Instrument & Data Handbooks and possibly a SYNPHOT update. Timeline: Ramp portion of 6194 executed April-June 1997. Continuation Plan: While the 8% accuracy reported could be considered "acceptable" for ground-based narrow band imaging, the expectation is that ~3% accuracy should be possible for WFPC2; additional ramp filter observations are being planned for Cycle 7.

Figure 8: Countrates of ramp observations compared to SYNPHOT predictions (O'Dea & McMaster).

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Proposal ID 6940: WFPC2 Cycle 6: Polarizers

Plan
Purpose: Verify stability of polarization calibration. Description: The goal of this proposal is to check for any changes in the polarization calibration since Cycle 5. Observations are made in F555W+POLQ of both polarized and unpolarized stars, in addition to VISFLATs. Data are taken in all four quads of the polarizer, as well as in three rotated positions of the POLQ. Fraction of GO/GTO Programs Supported: 5% Resources: Observation: 4 pointed orbits. Special Requirements: Requires specific orientations. Accuracy: The error in the polarization calibration is dominated by systematic instrumental effects. The Cycle 5 observations will be taken shortly and will allow a better assessment of the systematics. The practical limit for useful polarization observations is probably around 3%; sources with smaller polarization probably cannot yield reliable polarization measurements. Products: Update SYNPHOT throughput tables if necessary.

36


Results (from Cycle 5 Proposal 6194)
Modifications: Cycle 6 Proposal 6940 is a placeholder pending completion of the analysis of 6194. Here, we present results from the Cycle 5 Polarization Proposal 6194. The scope of the polarization section of 6194 was expanded (to include multiple combinations of filter, aperture, and telescope roll) after Cycle 4 observations showed that significant instrument polarization was present. Execution: Observations from visit 2 (target G191B2B) failed due to error in proposal. Resources Used: Observation: 16 pointed orbits, 76 occultation periods (24 VISFLATs and 52 Earth flats). Accuracy Achieved: A model of the polarization effects of the WFPC2 optics has been developed, including effects of the pick-off mirror as well as the polarizer filters. Countrates predicted by this model agree with on-orbit data to ~1.5% RMS (Biretta & McMaster, ISR 97-11). Products: ISR 97-11 and WWW tools for use in calibrating GO polarization data (Biretta & McMaster). Timeline: Last 6194 polarizer visit executed in December 1997; other portions of proposal were completed Nov/Oct 1996, and Jan, Apr-Aug, and October 1997. Continuation Plan: The present model and tools should be sufficient for most GOs. Areas for future work include the following (details in ISR 97-11, Biretta & McMaster). - Obtain additional flat fields in more filters using VISFLAT lamp (Earth flats are too bright); however, the degradation of the VISFLAT lamp (see results from proposal 6906) has limited its use, so these flats are competing with other VISFLAT calibrations. - Obtain additional on-orbit data to verify the model in more filters and apertures; this may reduce calibration errors to < 1%; these are in the queue for early 1998 (proposal 6194). - Study possible distortion effects due to polarized filters when using large apertures (>20") and possible spurious reflections against the filters (effects estimated ~1-2%). - Since polarization studies rely on measuring small differences between observations made at different chip locations, CTE is likely to have substantial effect on polarization observations and may be the largest remaining error source. - The existing WWW tools would benefit from additional testing using a larger variety of targets as well as improvements in the least squares solution (to allow >4 apertures). - A long-term goal is to develop an advanced STSDAS tool to handle the image alignments, distortion corrections, and computation of the Stokes vector components.

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Proposal ID 6941: WFPC2 Cycle 6: Astrometry Verification

Plan
Purpose: Verify accuracy and stability of geometric transformation and relative astrometry solution. Description: A very rich star field in Cen will be observed in five different positions with relative shifts of 40'' in each coordinate. Positions of more than 2000 stars per chip will be compared between pointings using the three different astrometric solutions provided by Gilmozzi, Holtzman, and Trauger, in order to verify and refine their accuracy. (Differences of up to 1 PC pixel exist in some regions of the field of view.) A very densely populated field is chosen in order to achieve better coverage of the field of view, even if at the expense of the accuracy of individual position measurements. Observations will be carried out in three filters, F555W, F300W, and F814W, to provide a verification and/or correction of the wavelength dependence of the solution. The F555W observation is repeated with smaller shifts of 15'' to ensure a better coverage of the PC. K-spot observations will be taken at the same time, in order to assess the relation between K-spot position and astrometry. Fraction of GO/GTO Programs Supported: 20% - also supports WFPC2-assisted target acquisitions for other instruments. Resources: Observation: 4 pointed orbits. Special Requirements: None. Accuracy: Expected better than 0.01''; required better than 0.05'' (full field of view). Products: Improvements in the STSDAS task metric and in the aperture reference file if required; ISR.

38


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 4 pointed orbits. Accuracy Achieved: Products: The analysis is currently in progress (Fruchter et al.); preliminary results show that over the longterm, there have been small motions (~0.1") between the chips. Final results will be reported via TIPS meeting, Instrument and Data Handbooks, STAN, and WWW memo. Timeline: Observations taken June 1997. Continuation Plan: A subset of the program will be repeated twice during Cycle 7, as a monitor of the relative positions of the chips.

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Proposal ID 6942: WFPC2 Cycle 6: Camera Electronics Verification

Plan
Purpose: Verify several aspects of the WFPC2 camera electronics: linearity, gain ratios, effect of CLOCKS, and effect of CTE on extended sources. Description: A very extended non-uniform target (the giant elliptical galaxy NGC 4472) will be observed with different exposure times and electronic configurations. The linearity test is carried out by taking exposures of NGC 4472, centered in WFALL, with multiple exposure times. Since the galaxy is non-uniform, the ratio of these exposures is directly related to the camera linearity. The exposures will be taken with GAIN=7. However, one exposure will also be taken with GAIN=15. Additional exposures will be taken with CLOCKS=YES and with a preflash. These observations complement those of the internal calibration proposal 6907. The two major advantages of these observations compared to the 6907 ones are the possibility of studying the effect of preflash (since the light distribution of the preflash is different from that of NGC 4472) and the possibility of measuring an absolute response curve, since NGC 4472, unlike the INTFLAT lamp, does not have temporal variations in luminosity. NGC 4472 has been chosen as target galaxy because it is large enough to produce significant signal in all chips and bright enough to allow us to explore the highest counts without excessive integration times. Fraction of GO/GTO Programs Supported: 100% Resources: Observation: 1 pointed orbit. Special Requirements: None. Accuracy: expected: 0.5% for linearity and CTE, 0.1-0.2% for gain ratios and CLOCKS. Required: less than 1% on each item. Products: ISR, TIPS reports.

40


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 1 pointed orbit. Accuracy Achieved: ISR (Stiavelli & Mutchler, ISR 97-04) details the results achieved with the observations from this proposal. In summary: · Linearity: For countrates exceeding 0.5DN/sec, the linearity is better than 0.5% (see Figure 9 below). Due to low S/N, results at the lowest countrates are less conclusive: the formal errors of ~1% assume that nearby pixels are uncorrelated, which is probably incorrect (due to pixel response function and/or transfer inefficiencies). Clocks on: although there is no trend with countrate, there is a 0.3% shift when comparing clocks on to off (see Figure 10 below). The exposure shortening effect due to the shutter (see WFPC2 Handbook Section 2.6), is not large enough (at 0.125%) to account for the shift. There may be some residual, as yet unknown, effect which is shortening the clocks ON exposures. Preflash: preflashed images do not show any effect as a function of countrate. There is, however, a uniform shift of 13 electrons per pixel (possibly due to higher than normal sky background, e.g., from Earth limb?).

·

·

Products: TIPS presentations and ISR 97-07 (Stiavelli & Mutchler). Timeline: Observations executed in February 1997. Continuation Plan: None for Cycle 7.

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Proposal ID 6943: WFPC2 Cycle 6: Throughput Verification for Narrow Band Filters

Plan
Purpose: Direct verification of throughput of narrow band filters through observations of emission line objects. Description: The current throughput calibration of narrow-band filters is based on filter profiles from data obtained before launch and on observations of continuum sources. This program will verify the accuracy of the calibration, and indirectly the stability of the filters, by observing compact planetary nebulae with well-established line fluxes. The targets, chosen for consistency across instruments, are the FOS standard NGC 6833 and the six planetary nebulae selected for the NICMOS calibration of narrow-band filters. In addition, two PN with existing Cycle 4 observations will be included to verify the stability of the narrow-band filters. The observations can be executed in SNAPSHOT mode since they will be short and none is specifically required. Fraction of GO/GTO Programs Supported: 20% Resources: Observation: 10 SNAP orbits. Special Requirements: None. Accuracy: Expected 2%, required 3%. Products: Update SYNPHOT tables if required; ISR.

42


Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 9 SNAPs. Accuracy Achieved: Work is still in progress(Casertano & Gonzaga). Preliminary results have shown that there is no measurable time dependence to the narrowband throughput and that the derived WFPC2 narrowband fluxes are in good agreement with ground-based observations. In more detail: · Second-epoch observations of NGC 6543 were compared with first-epoch observations obtained in 1994. The narrow-band fluxes did not vary measurably. The measurements have an uncertainty of about 5%, dominated by actual changes in the nebular structure - NGC 6543 turned out to be closer than previously thought, and we detected a measurable expansion in the nebula. The narrow-band flux measured for three planetary nebulae, M4-16, K3-15, and NGC 6833, was found in agreement with available ground-based measurements, generally within 5-10%. Ground-based measurements have a catalog uncertainty of 25%. A detailed analysis of the differences will be carried out after the last observation of this program is executed.

·

Products: In progress. Timeline: Of the 10 SNAPs, 9 were completed by Dec 1997. Visit 2 (target K3-69) is tentatively scheduled for next year, Feb 1999. Continuation Plan: None for Cycle 7.

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Figure 9: WFPC2 Linearity (taken from Figure 1 in ISR 97-07, Stiavelli & Mutchler). Open squares are 350 sec vs. 50 sec data, filled triangles are 200 sec vs. 50 sec data, and stars correspond to 350 sec vs. 100 sec data. Lowest bin is < 3 electrons/sec.

Figure 10: Effect of CLOCKS=YES (from ISR 97-07, Stiavelli & Mutchler). The data point at a countrate of ~130 electrons/sec corresponds to pixels in the WF3 saturated star.

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filter new/ old F160BW F170W F185W F218W F255W F300W F336W F380W F439W F450W F547M F555W F569W F606W F622W F675W F702W F791W F814W 1.113 1.044 1.074 1.051 1.071 1.035 0.980 1.008 0.984 1.008 0.996 0.998 0.995 1.010 0.994 0.998 1.001 1.009 1.002 new PHOT FLAM

PC1 Vega ZP 14.985 16.335 16.025 16.557 17.019 19.406 19.429 20.939 20.884 21.987 21.662 22.545 22.241 22.887 22.363 22.042 22.428 21.498 21.639

WF2 new PHOT FLAM 4.563e-15 1.398e-15 1.872e-15 9.887e-16 5.414e-16 5.891e-17 5.445e-17 2.508e-17 2.895e-17 8.856e-18 7.502e-18 3.396e-18 4.040e-18 1.842e-18 2.700e-18 2.797e-18 1.809e-18 2.883e-18 2.458e-18 Vega ZP 15.126 16.454 16.132 16.646 17.082 19.451 19.462 20.959 20.903 22.007 21.689 22.571 22.269 22.919 22.397 22.080 22.466 21.529 21.665 new/ old 1.168 1.072 1.095 1.058 1.063 1.019 0.961 0.987 0.965 0.992 0.993 0.995 0.995 1.013 1.000 1.007 1.008 1.003 0.988

WF3 new PHOT FLAM 5.418e-15 1.578e-15 2.083e-15 1.069e-15 5.640e-16 5.985e-17 5.451e-17 2.481e-17 2.860e-17 8.797e-18 7.595e-18 3.439e-18 4.108e-18 1.888e-18 2.778e-18 2.898e-18 1.867e-18 2.913e-18 2.449e-18 Vega ZP 14.946 16.313 16.014 16.558 17.037 19.433 19.460 20.972 20.916 22.016 21.676 22.561 22.253 22.896 22.368 22.042 22.431 21.512 21.659

WF4 new PHOT FLAM 5.133e-15 1.531e-15 2.036e-15 1.059e-15 5.681e-16 6.097e-17 5.590e-17 2.558e-17 2.951e-17 9.053e-18 7.747e-18 3.507e-18 4.181e-18 1.914e-18 2.811e-18 2.919e-18 1.883e-18 2.956e-18 2.498e-18 Vega ZP 15.002 16.350 16.040 16.570 17.029 19.413 19.433 20.938 20.882 21.984 21.654 22.538 22.233 22.880 22.354 22.034 22.422 21.498 21.641

conv

5.212e-15 1.551e-15 2.063e-15 1.071e-15 5.736e-16 6.137e-17 5.613e-17 2.558e-17 2.945e-17 9.022e-18 7.691e-18 3.483e-18 4.150e-18 1.900e-18 2.789e-18 2.899e-18 1.872e-18 2.960e-18 2.508e-18

0.378 0.412 0.411 0.232 0.015 -0.024 -0.098 0.559 0.657 0.475 -0.023 -0.000 -0.114 -0.316 -0.424 -0.703 -0.791 -1.224 -1.263

Table 7. PHOTFLAMs and zeropoints for the more popular observing modes; the above values should be applied to counts referenced to a nominal "infinite" aperture, defined by an aperture correction of 0.10 mag with respect to 0.5" radius standard aperture. The full table is available on WWW Documentation page as well as in ISR 97-10 (Baggett et al.). PHOTFLAM values for gain=15 can be obtained by multiplying by the gain ratio: 1.987, 2.003, 2.006, and 1.955 for PC, WF2, WF3, and WF4, respectively (from Holtzman et al. 1995). Zeropoints are given in the Vega system; the required conversion factor used for transforming ST magnitudes1 to Vega magnitudes is provided in the `conv' column. The size of the SYNPHOT changes are given for PC1 and WF3 in the new/old columns.
1. ST magnitudes are defined as -2.5*log10(PHOTFLAM)-21.1 and result in constant magnitudes for spectra having constant flux per unit wavelength. See STSDAS Synphot User's Guide for more details.

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Proposal ID 6186: WFPC2 Cycle 5: UV Throughput

Plan
Purpose: Update SYNPHOT database for UV throughput. Description: Grw+70d5824 is observed shortly before and after a decontamination (decon) through all the UV filters in each chip and through F160BW crossed with F130LP, F185LP and F165LP (where applicable) to determine the wavelength dependence of the throughput across the bandpass (hence color terms). This program is designed to better characterize the spectral response curve in the UV, and the spectral shape introduced by the contamination. Fraction of GO/GTO Programs Supported: 20% Resources: Observation: 6 pointed orbits. Special Requirements: Must be phased with decon cycle; one execution just before, one just after decon. Accuracy: Overall discrepancies between the updated synthetic photometric products and the results of this test should be 1-2% rms. This does not mean that the UV throughput will be known to this accuracy primarily because of uncertainties in the flux calibration of the standard used (5%), uncertainties in the UV flat fields (maybe 3% near the chip center), and time dependent contamination corrections (3% error), and uncertainties in the CTE correction (2%). The derived UV absolute photometric accuracy at the center of the chips should therefore be about 10%. Products: After pipeline processing, each image will be reduced by aperture photometry. The throughput curves and their normalizations can be updated by trial and error. Expected to run 8/95.

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Results
Modifications: Some pre-decon exposures with crossed filters dropped to save orbits. Execution: Nominal. Resources Used: Observation: 9 pointed orbits. Accuracy Achieved: All UV filters shortward of F439W were updated, as well as some of the more frequently-used broadband filters redward of F439W. With a few exceptions, changes to non-UV filter modes were relatively minor: generally ~1-2% or less (changes in F785LP, F850LP, and F1042M were higher: ~1-4%, ~1-6%, and ~2-15% respectively, depending on the chip.). The UV filters required somewhat larger changes to bring SYNPHOT into agreement with the observations, ranging from ~2% (e.g., F255W and F300W) to ~5% (F170W, F218W) to > 10% (F160BW, F375N). Full details of the update, including a table of the new zeropoints (PHOTFLAMs), are in ISR 97-10 (Baggett et al.). Products: Updated SYNPHOT tables were installed May 1997; ASCII versions are on WWW. Results were reported via TIPS, STAN, 1997 Calibration Workshop and ISR. Timeline: Data and analysis are complete. Continuation Plan: Repeat subset in Cycle 7.

Figure 11: Observed grw+70D5824 countrates compared to SYNPHOT predictions (from ISR 97-10, Baggett et al.).

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Proposal ID 6195: WFPC2 Cycle 5: Flat Field Check

Plan
Purpose: Check quality of flat fields and estimate errors. Description: The crowded Cen field is positioned with a bright star at the center of each CCD in turn. 40 sec images are taken through each of the photometric filters (F336W, F439W, F555W, F675W, F814W) and as many supplementary filters (from F450W, F606W, F702W and F547M) as can fit in the allotted time. If data volume is a problem, single chip readout is acceptable, but should be avoided as much as possible. Based on cycle 4 programs 5659 and 5646. Fraction of GO/GTO Programs Supported: 60% Resources: Observation: 2 pointed orbits. Special Requirements: Assumes that the CTE proposal has been run and gives satisfactory results; if CTE calibration fails, this proposal may need to be run with preflash. Accuracy: Overall discrepancies between the synthetic photometric products and the results of this test should be 1-2% rms. Part of the point of the test is to measure this accuracy. Products: After pipeline processing, each image will be reduced by aperture photometry to measure the RMS errors in the flat fields. The RMS error will be determined by the additional noise in the independent measurements over the expected variance of less than 1% from photon statistics. The single bright star at the center of each chip independently estimates the chip-to-chip normalization error.

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Results
Modifications: None. Execution: Nominal. Resources Used: Observation: 4 pointed orbits. Accuracy Achieved: Conclusions from the data analysis (Whitmore & Heyer, ISR 97-08) can be summarized as: · · · · The flat fields are uniform to at least 1.5% across the chips (0.6% chip to chip), likely better as remaining uncertainties may be due to other sources (e.g., variable PSF). A 2% scatter exists in the normalization of the different chips, understood as differences in the gain ratios and aperture corrections. Applying the new corrections for CTE, aperture corrections, and gain ratios reduces the RMS scatter for a random observation from ~4-7% to 2-3%. The CTE results from Holtzman et al. (1995) were confirmed with this new dataset: - CTE loss is proportionally larger for fainter point sources. - CTE loss is larger for images with low background. - The 4% linear ramp provides a reasonable correction in many cases, although not all. Analysis of this dataset also led to new results concerning the CTE problem: - CTE loss is the same on all three WF chips and appears to be the same on PC. - The CTE loss ranges from 2% (for bright stars on a bright background) to ~15% (faint stars on a faint background). The filter dependence of the CTE loss is shown in the figure below. - The dependence on background has been quanitified (linear in log-log space). - There appears to be weak CTE loss in X-axis (~1-5% level). - Differences in CR-SPLIT exposure pairs can be understood as first exposure acting as a `preflash' for the second exposure. Formulae for correcting the effects of the CTE problem have been developed (see Section 2.7 of ISR 97-08, Whitmore & Heyer); these depend upon X and Y position, the background, the star brightness, and aperture size.

·

·

Products: Results reported in TIPS, STAN and ISR 97-08 (Whitmore & Heyer). Timeline: Observations executed in June 1996. Continuation Plan:

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Figure 12: Values of CTE loss in units of % loss over 800 pixels vs. the brightness of the star in DN (figure taken from ISR 97-08, Whitmore & Heyer); top plot is for 2-pixel radius aperture, bottom is for 5-pixel radius aperture. A small horizontal offset has been made as a function of the wavelength in order to reduce overlaps, and to show the dependence on wavelength.

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3. References
For more details on specific results, please refer to the documents listed below. For paper copies of any documents listed here, please contact help@stsci.edu.

WWW
Advisories Documentation Software Tools User Support Frequently Asked Questions Frequently Asked Questions:
Astrometry WFPC2 Handbook Calibration Reports Instrument Science Reports Calibration Monitoring Memos S.T.A.N. WFPC2 Photometry SYNPHOT WFPC2 PSF Documentation Other WFPC2 Documents PSF Library Search Tool Exposure Time Calculator Linear Ramp Filter Calculator Polarizer Calibration Tools WFPC2 Clearinghouse Calibration Dithering Exposure Times STScI Visits WFPC2 Documentation WFPC2 Group Image Displays Miscellaneous Narrow Band Photometry Photometry Proposal Preparation PSFs Other FAQ Lists

Documentation:

Software Tools:

User Support:

Figure 13: Overview and site layout of WFPC2 WWW pages (Wiggs). This map is accessible via WFPC2 homepage at the WFPC2 home page: http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_top.html. All topics listed on the roadmap are also direct links to the corresponding pages.

General Documents The WFPC2 Instrument Handbook (http://www.stsci.edu/ftp/instrument_news/ WFPC2/wfpc2_bib.html) The HST Data Handbook (http://www.stsci.edu/ftp/documents/html/data-handbook.html)

The 1997 HST Calibration Workshop (http://www.stsci.edu/stsci/meetings/
cal97/)

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STAN, the Space Telescope Analysis Newsletter (http://www.stsci.edu/ftp/
instrument_news/WFPC2/wfpc2_stan.html)

The list of Frequently Asked Questions (http://www.stsci.edu/ftp/ instrument_news/WFPC2/wfpc2_top_faq.html) Proposals in Phase II format, page maintained by PRESTO, at
http://www.stsci.edu/public/propinfo.html

Some of the Memos and Reports on WWW WFPC2 Clearinghouse, Whitmore et al., (http://www.stsci.edu/ftp/
instrument_news/WFPC2/Wfpc2_clear/wfpc2_clrhs.html)

WFPC2 History file, Whitmore, Baggett & Wiggs, updated regularly (http://
www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_memos/ wfpc2_history.html)

Standard Star and UV Monitoring Memo, Gonzaga, et. al., October 1997 (http://
www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_memos/ wfpc2_stdstar_phot3.html)

WFPC2 Reference Files Currently in the Calibration Database, Baggett and Wiggs, updated regularly (http://www.stsci.edu/ftp/instrument_news/WFPC2/ Wfpc2_memos/wfpc2_reffiles.html) WFPC2 IDT Reference Files (http://www.stsci.edu/ftp/instrument_news/WFPC2/ Wfpc2_memos/wfpc2_idtrefs.html) WFPC2 Hot Pixel Lists for Use with Warmpix (http://www.stsci.edu/ftp/
instrument_news/WFPC2/wfpc2_warmpix.html)

Guide to WFPC2 Synphot Tables, Baggett, May 1997 (http://www.stsci.edu/ftp/ instrument_news/WFPC2/Wfpc2_phot/wfpc2_synphot.html) Table of SYNPHOT Zeropoints, Baggett and Casertano, May 1997, (http://
www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_phot/ wfpc2_photlam.html)

WFPC2 Photometry Cookbook, Whitmore, (http://www.stsci.edu/ftp/ instrument_news/WFPC2/Wfpc2_phot/wfpc2_cookbook.html) The Performance and Calibration of WFPC2, Holtzman, J.A., et al., 1995a, PASP, 107, 156 (also at http://www.stsci.edu/ftp/instrument_news/WFPC2/ wfpc2_doc.html#Wcal). Photometric Performance and Calibration of WFPC2, Holtzman, J.A., et al., 1995b, PASP, 107, 1065 (also at http://www.stsci.edu/ftp/instrument_news/ WFPC2/wfpc2_doc.html#Wcal). The CTE effect as a function of preflash levels, Ferguson, May 96 (http://
www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_cte/cte.html)

The long vs. short exposure time problem, Casertano, May 96 (http://www.stsci.edu/

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ftp/instrument_news/WFPC2/Wfpc2_cte/shortnlong.html)

Tiny Tim, Krist, (http://scivax.stsci.edu/~krist/tinytim.html) Modelling HST Focal-Length Variations, Hershey, November 1997 (copy available at http://www.stsci.edu/ftp/instrument_news/Observatory/focus/focus2.html) HST Focus History, Lallo, November 1997 (http://www.stsci.edu/ftp/instrument_news/ Observatory/focus/focus2.html) ISR OTA-18: WFPC2 Focus Report, Casertano, January 1995 (http://www.stsci.edu/ ftp/instrument_news/Observatory/postscript/ota18.ps) HST Focus Page, Lallo (http://www.stsci.edu/ftp/instrument_news/Observatory/services.html)

WFPC2 Instrument Science Reports (Postscript copies are available online at
http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_bib.html)

97-11: WFPC2 Polarization Calibration, Biretta and McMaster 97-10: WFPC2 Synphot Update, Baggett, et al., 10/97 97-09: Results of WFPC2 Post-Servicing Mission 97 Calibration, Biretta et al., 09/97 97-08: New Results on Charge Transfer Efficiency and Constraints on the Flat-Field, Whitmore and Heyer, 09/97 97-07: WFPC2 Electronics Verification, Stiavelli and Mutchler, 07/97 97-06: WFPC2 Cycle 7 Calibration Plan, Casertano, et al., 08/97 97-05: WFPC2 CTE/Residual Images, Biretta and Mutchler (in progress) 97-04: Properties of WFPC2 Bias Frames, O'Dea et al., 06/97 97-03: Summary of WFPC2 SM97 Plans, Biretta et al., 02/97 97-02: WFPC2 Cycle 5 Calibration Closure Report, Casertano and Baggett, 02/97 97-01: Impact of Focus Drift on Aperture Photometry, Suchkov and Casertano, 01/97 96-08: WFPC2 Cycle 6 Calibration Plan, Casertano, et al., 07/96 96-07: WFPC2 Throughput Stability in the Ultraviolet, MacKenty and Baggett, 05/96 96-06: Photometric Calibration of WFPC2 Linear Ramp Filter Data in SYNPHOT, Biretta, Baggett, and Noll, 07/96 96-05: Wavelength / Aperture Calibration of the Linear Ramp Filters, Biretta, et al., 05/96 96-04: Effects of Contamination on WFPC2 Photometry, Whitmore, et al., 06/96 96-02: Contamination Correction in SYNPHOT for WFPC2 and WF/PC-1, Baggett, et al., 02/96 96-01: Internal Flat Field Monitoring, Stiavelli and Baggett, 01/96 95-07: WFPC2 Cycle 4 Calibration Summary, Baggett, Casertano, and Biretta, 12/95

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95-06: A Field Guide to WFPC2 Image Anomalies, Biretta, Ritchie, and Rudloff, 08/95 95-04: A Demonstration Analysis Script for Performing Aperture Photometry, Whitmore and Heyer, 07/95 95-03: Charge Transfer Traps in the WFPC2, Whitmore and Wiggs, 07/95 95-02: The Geometric Distortion of the WFPC2 Cameras, Gilmozzi, et al., 06/95 95-01: WFPC2 Polarization Observations: Strategies, Apertures, and Calibration Plans., Biretta and Sparks, 02/95 94-03: WFPC2 Pipeline Calibration, Burrows, 12/94 94-01: Large Angle Scattering in WFPC2 and Horizontal "Smearing" Correction, Krist and Burrows, 10/94 93-01: Polarizer Quad Nomenclature, Mark Clampin, 3/93 92-06: WFPC2 CCDs, Mark Clampin, 12/92 92-05: WFPC2 AFM and POMM Actuation Algorithm, J.T. Trauger et al., 7/92 92-03: WFPC2 Science Observation and Engineering Modes, Trauger and Brown, 10/92 92-02: System Level Contamination Issues for WFPC2 and COSTAR, M. Clampin, 9/92 WFPC2 Technical Instrument Reports These are informal, internal reports; paper copies available on request. 97-11: Long-term Study of Bias Jumps, O'Dea et al., 11/97 97-10: WFPC2 Photometry from Subtraction of Observed PSFs, Surdej et al., 10/97 97-09: WFPC2 PSF Library, Wiggs et al., 9/97 97-08: SMOV Flat Field Check (prop 7019), Biretta and Wiggs, 9/97 97-07: WFPC2 SM97 Internal Monitoring (prop 7022), Mutchler and Stiavelli, 7/97 97-06: SMOV2 Check of WFPC2 PSF Stability, Fruchter and McMaster, 9/97 97-05: Results of the WFPC2 SM-2 Lyman- Throughput Check (prop 7018 and 7029), O'Dea, Baggett, and Gonzaga, 6/97 97-04: VISFLAT Channel Monitoring, Stiavelli, 5/97 97-03: OTA Focus During SMOV, Casertano et al., 5/97 97-02: SM-2 UV Monitoring and Cool-down Procedure (prop 7016 , 7122), Stiavelli et al., 5/97 97-01: Results of the WFPC2 SMOV-97 Relative Photometry Check (prop 7020), Whitmore, Gonzaga, and Heyer 4/97

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