Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14

8.18 Cycle 11 Calibration Plan

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Due to the significant decrease in WFPC2 observations in Cycle 11 (with the advent of ACS becoming operational), the WFPC2 calibration plan for Cycle 11 has been scaled back. Forty external orbits will be used for WFPC2 calibration this cycle, compared to 61 orbits used in Cycle 10.

Routine calibrations from previous cycles, such as the photometric monitor, darks, and flat fields, will continue in Cycle 11. There is, however, one significant change in these routine monitors: the interval between decontaminations has been increased from 28 days to 49 days. A requirement for WFPC2 UV observations is that the decrease in throughput due to contamination never drops below 70% of the total (post-decon) throughput. Over the past few years, UV photometric monitor data of the standard star GRW+20D5824 has shown that the contamination rates have been decreasing, making it safer to implement longer intervals between decons.

Other scaled-back programs include the Earth flats and astrometric monitor. There was a major update to flat field reference files, as documented by Koekemoer et al. in ISR-2002-02: Updated WFPC2 Flatfield Reference Files for 1995 - 2001. For Cycle 11, fewer Earth flats are being executed, and these will be used primarily to track possible small-scale changes in order to update the current flat field reference files.

Astrometric monitor observations will be done once a year, compared to twice a year in past cycles. Since the astrometric properties of the chips are changing more slowly with time, decreasing the frequency to once per year is sufficient to track the changes. An improved astrometric solution for F555W was published by Casertano et al. in ISR-2001-10: An Improved Geometric Solution for WFPC2. The WFPC2 group expects to derive additional solutions for other filters in the future.

The CTE characterization program continues the CTE monitor of previous cycles. In addition, two new components have been added. One is to better characterize the "long-vs-short" anomaly since there are indications that this effect is only relevant for very crowded fields. The second component is a test of 2x2 pixel-binned observations to see if binning reduces CTE. A test of this technique could also be useful for ACS observations.

Data from the Photometric Characterization program will, as in past cycles, be used to verify photometric stability to 1-2%. Observations from this and previous cycles will also be used to update photometric zeropoints used in SYNPHOT.

A new program added to the calibration plan is the WFPC2-ACS Photometric Cross-Calibration. Observations of the primary ACS standard star, two globular clusters spanning a wide range of metallicities, as well as Sloan standard stars, will be taken in WFPC2 with a wide range of the more commonly-used filters.

The WFPC2 group is also beginning to lay out plans for closure of WFPC2 calibration work, making sure all major aspects of WFPC2 are well-characterized. A better idea of what needs to be done will emerge during the WFPC2 session of the upcoming Calibration Workshop (October 2002), where user feedback will provide additional guidance. One primary item for closure will be an accurate cross-calibration between WFPC2 and ACS, an effort that is being coordinated with ACS starting this cycle.

Details about the Cycle 11 calibration plan are outlined in the following pages. For additional details on the WFPC2 calibration plan, please email to help@stsci.edu.

Table 8.11: WFPC2 Cycle 11 Calibration Plan.

ID	Proposal Title	Frequency	Estimated Time (orbits)		Scheduling Required	Products	Accuracy Required	Notes
			"External"	"Internal"
Routine Monitoring Programs
9589	WFPC2 Decons & Associated Observations	Decons every 49d (from 28d)	20 (13 photmon, 5 UV thruput, 2 UV flats)	124	every 49d, UV thruput early	CDBS, Inst Hbk, Synphot, WWW reports	1-2%	Decons, phot.monitor, internals, UV throughput, VISFLATS and UVFLATS.
9592	Standard Darks	weekly, except decon week		294	every 7 days, except decon wk.	CDBS	1 e-/hr	Also for WWW hot pixel lists.
9593, 9594, 9595	Supplemental Darks	0-3/day		1182	every day		n/a	For archive only, no analysis.
9596	Internal Monitor	weekly, except decon week		53	every 7 days, except decon week	CDBS	0.8e-/pix
9598	Earth Flats	continuous		150	mid-to-late	CDBS	0.3%
9599	UV Earth Flats	continuous		200	early-to-mid	CDBS	3-10%
9597	Intflat & Visflat Sweeps, Filter Anomaly Check	1/cycle		166	mid-cycle	TIR	0.3%
Special Calibration Programs
9600	Astrometric Monitor	1/cycle	1	1	mid-cycle	ISR, STSDAS	0.05''	Cen as well as K-spots.
9591	CTE Characterization	1/cycle	5		early, late	ISR	0.03 mag	Continuation of monitors, new tests (2x2 binning & long vs. short).
9590	Photometric Characterization	1/cycle	3		early	ISR, Synphot	1%	Std. star in all 4 chips. Non-monitor broadband filters, not UV.
9601	WFPC2-ACS Photometric Cross-Calibration	1/cycle	8		early	ISR, Synphot	1%	Cross-calibration with ACS, using NGC2419, 47 Tuc, ACS calib star, & Sloane Calibration Field.
	~10% reserve		3					Placeholder for unexpected items.
TOTAL TIME (including all executions)			40	2171

9589: WFPC2 Cycle 11: Decontaminations and Associated Observations

• Purpose: Monthly WFPC2 decons. Other programs tied to decons are also included: photometric stability check, focus monitor, pre- and post-decon internals, UV throughput checks, visflat sweep, and internal UV flat check.
• Description: UV-blocking contaminants removed and hot pixels annealed by warming the CCDs to +20C for 6 hours. Done every 49 days.
  Internals: intflats, biases, darks & kspots, before/after decons.
  Photometric Monitor: GRW+70D5824 is observed after each decon and before every other decon: (1) F170W in all chips to monitor far UV contamination. (2) As many as possible of F160BW, F218W, F255W, F336W, F439W, F555W, F814W will be observed within 1 orbit in a different chip each month.
  UV Throughput: pre-decon PC & WF3 UV observations in most UV filters, post-decon observations of same filters in all 4 chips. Used to verify that the UV spectral response curve is unchanged.
  Internal UV flatfields: obtained with the CAL channel's UV lamp using the filters F122M, F170W, F160BW, F185W, & F336W. The uvflats are used to monitor UV flatfield stability and the stability of the F160BW filter by using F170W as the control. The F336W ratio of visflat to uvflat provides a diagnostic of the UV flatfield degradation & ties the uvflat and visflat flatfield patterns. Two supplemental dark frames must be obtained immediately after each use of the lamp to check for possible after-images.
  VISFLAT mini-sweep: Taken before and after a decon, once during the cycle. VISFLATs will also be taken with a ramp filter, one at each gain, to be done at the post-decon visit, to provide a check of the A-to-D correction.The F336W ratio of VISFLAT to UVFLAT provides a diagnostic of the UV flatfield degradation & ties the UVFLAT and VISFLAT flatfield patterns.
• Products: SYNPHOT, CDBS, Instrument Handbook, TIPS meetings, WWW reports, TIR, ISR; new UV flatfields if changes are detected.
• Accuracy Goals: Photometry: less than 2% discrepancy between results, 1% rms expected.
  UV throughput: better than 3%.
  Flatfields: temporal variations monitored at 1% level. Gain ratios: stable to better than 0.1%.
  UV flats: About 2-8% pixel-to-pixel expected (filter dependent). Visflats: stable to better than 1% in overall level and spatial variations (after correcting for lamp degradation). Contamination effects should be < 1%.

9592: WFPC2 Cycle 11: Standard Darks

• Purpose: Measure dark current & identify hot pixels.
• Description: Six 1800s exp/week with the shutter closed, five with clocks off, one with clocks on. This frequency is required due to the high formation rate of new hot pixels (several tens/CCD/day). Five darks per week are required for cosmic ray rejection, counterbalancing losses due to residual images, & improving the noise of individual measurements. Sometimes, no usable darks are available for a given week due to residual images, resulting in a longer-than-usual gap in the hot pixel lists, but in a decon week, information on hot pixels that became hot and then annealed would be lost irretrievably. As a result, pre-decon darks (see Decon proposal) are executed NON-INT and at least 30 min. after any WFPC2 activity.
• Products: Weekly darks delivered to CDBS and monthly tables of hot pixels on the WWW. Superdarks for use in generating pipeline dark reference files.
• Accuracy Goals: Require ~1 e-/hr (single-pixel rms) accuracy for most science applications. Expected accuracy in a typical superdark is 0.05 e-/hour for normal pixels. The need for regular darks 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.

9593, 9594, 9595: WFPC2 Cycle 11: Supplemental Darks

• Purpose: Images will allow for frequent monitoring of hot pixels.
• Description: This program is designed to provide up to three short (1000s) darks per day, to be used primarily for the identification of hot pixels. Shorter darks are used so that the observations can fit into almost any occultation period, making automatic scheduling feasible. These supplemental darks are low priority, and should be taken only when there is no other requirement for that specific occultation period. This program complements the higher priority Standard Darks proposal that has longer individual observations for producing high-quality pipeline darks and superdarks. Hot pixels are often a cause of concern for relatively short science programs, since they can mimic stars or mask key features of the observations: about 400 new hot pixels/CCD are formed between executions of the Standard Darks program. The supplemental darks are available to the GO community from the archive; there is no plan to use them in our standard analysis and products.
• Products: None, though some daily darks may occasionally be used for hot pixel lists if standard darks were lost.
• Accuracy Goals: For archive only, no STScI analysis provided.

9596: WFPC2 Cycle 11: Internal Monitor

• Purpose: This calibration proposal is the Cycle 11 routine internal monitor for WFPC2, to be run weekly to monitor the health of the cameras. A variety of internal exposures are obtained in order to provide a monitor of the integrity of the CCD camera electronics in both bays (gain 7 and gain 15), a test for quantum efficiency in the CCDs, and a monitor for possible buildup of contaminants on the CCD windows.
• Description: The internal observations consist of:
  at gain=7: 4 biases, 2 F555W intflats
  at gain=15: 4 biases, 2 F555W intflats
  The entire set should be run once a week (except on decon weeks), on a non-interference basis. Proposal should start near the beginning of Cycle 11 (early August 2002), replacing Cycle 10 Internal Monitor proposal 8939. This proposal should not be run during Decon weeks as the decon proposal will contain the necessary internal images for those weeks. Each visit should be somewhat evenly spaced, i.e. they should be scheduled about 1 week +/- 2 days apart. Due to the change in DECON schedule for Cycle 11, this program should start during the week beginning with August 5, 2002. All visits in 8939 after the start of this program will be withdrawn.
• Products: CDBS (superbias created annually)
• Accuracy Goals: 0.8 e-/pix for superbias reference file.

9598: WFPC2 Cycle 11: Earth Flats

• Purpose: Monitor flatfield stability. This proposal obtains sequences of Earth streak flats to construct high quality flat fields for the WFPC2 filter set. These flat fields will allow mapping of the OTA illumination pattern and will be used in conjunction with previous internal and external flats to generate new pipeline superflats.
• Description: Observations of the bright Earth (Earthcals) are obtained in a variety of filters: F375N, F502N, F656N, F953N, F160BW, F336W, F343N, F390N, F437N, F469N, F487N, F631N, F658N, and F673N.
• Products: New flatfields generated and delivered to CDBS if changes detected. (Most recent update was delivered to CDBS in early 2002.)
• Accuracy Goals: The single-pixel signal-to-noise ratio expected in the flatfield is 0.3%.

9599: WFPC2 Cycle 11: UV Earth Flats

• Purpose: Monitor flatfield stability. This proposal obtains sequences of Earth streak flats to improve the quality of pipeline flat fields for the WFPC2 UV filter set.
• Description: Earth streak-flats are taken in UV filteLrs (F170W, F185W, F218W, F255W, F300W, F336W, and F343N). Those UV filters with significant read leak will also be observed crossed with selected broadband filters (F450W, F606W, F675W, and F814W), in order to assess and remove the read leak contribution.
• Products: Updated flatfields for pipeline via CDBS. (Most recent delivery to CDBS made in early 2002.)
• Accuracy Goals: 3-10%.

9597: WFPC2 Cycle 11: Intflat and Visflat Sweeps, and Filter Rotation Anomaly Monitor

• Purpose: Using intflat observations, this WFPC2 proposal is designed to monitor the pixel-to-pixel flatfield response and provide a linearity check. The intflat sequences, to be done once during the year, are similar to those from the Cycle 10 program 8942. The images will provide a backup database in the event of complete failure of the visflat lamp as well as allow monitoring of the gain ratios. The sweep is a complete set of internal flats, cycling through both shutter blades and both gains. The linearity test consists of a series of intflats in F555W, in each gain and each shutter. As in Cycle 10, we plan to continue to take extra visflat, intflat, and earthflat exposures to test the repeatability of filter wheel motions.
• Description: This proposal contains the intflat filter sweep, linearity and filter rotation tests.
  Linearity test: flatfields are taken with F555W at a variety of exposure times, using shutters A & B, and gains 7 & 15. Since the intflats have significant spatial structure, any non-linearity will appear as a non-uniform ratio of intflats with different exposure times.
  Filter rotation check: visflats will be taken to test the repeatability of the filter wheel positioning. A problem is known to exist in several filters.
• Products: TIR, ISR
• Accuracy Goals: 0.3%

9600: WFPC2 Cycle 11: Astrometric Monitor

• Purpose: Verify relative positions of WFPC2 chips with respect to one another.
• Description: The positions of the WFPC2 chips with respect to each other appear to be shifting slowly (by about 1 pixel, since 1994). The rich field in Cen (same positions as cycle 9 proposal 8813) is observed with large shifts (35") in F555W only, once in the cycle. This will allow tracking of the shifts in the relative positions of the chips or changes in the astrometric solution at the sub-pixel level. Kelsall spot images will be taken in conjunction with each execution.
• Products: TIPS reports, ISR, update of chip positions in PDB and of geometric solution in STSDAS tasks metric and wmosaic if significant changes are found.
• Accuracy Goals: At least 0.01'' in relative shifts; 0.05" or better for absolute.

9591: WFPC2 Cycle 11: CTE Characterization

• Purpose: Monitor CTE changes during Cycle 11; test whether 2X2 binning affects CTE (may be relevant for ACS) and perform a high S/N long-vs-short test in an uncrowded field.
• Description: Obtain observations of Omega Cen (NGC 5139) to track changes in the CTE (charge transfer efficiency) in WFPC2. A continuation of proposals in earlier cycles (7629, 8447, and 8821, 9254), the principal observations will be at gains 7 and 15, in F814W and F555W. The same pointing is used on WF2 and WF4. The relative orientation of the chips then results in stars at the bottom of one chip falling near the top of the other chip, hence providing a measurement of the CTE loss. We will make similar observations using 2X2 binning. This may reduce CTE loss since the largest loss is to the adjoining pixel, which gets included in the 2X2 bin in 50% of the cases. This may be relevant for ACS. We will also obtain a high S/N measurement of the long-vs-short anomaly for uncrowded fields by taking 10 X 10s exposures in Omega Cen, in order to test the recent finding that the long-vs.-short problem is only relevant for crowded fields.
• Products: ISR and updates to published CTE correction formulae.
• Accuracy Goals: 0.03 magnitudes for the majority (90%) of cases

9590: WFPC2 Cycle 11: Photometric Characterization

• Purpose: Provide a check of the zeropoints and contamination rates in non-standard WFPC2 filters.
• Description: Observations of the standard star GRW+70D5824 in all four chips will be made using filters that are not routinely monitored but are still used in cycle 11(F467M, F547M, F569W, F606W, F675W, F791W, F850LP, and F1042M). Images should be taken within 7 days after a decon, to minimize any contamination effects. Results from this program will be compared with archival data from earlier cycles.
• Products: ISR, SYNPHOT update.
• Accuracy Goals: 2-3% photometry.

9601: WFPC2 Cycle 11: WFPC2-ACS Photometric Cross-Calibration

• Purpose: This proposal is aimed at providing photometric zeropoint cross-calibration between the commonly used WFPC2 photometric filter sets and those that will be used for ACS programs.
• Description: Observations of two globular clusters spanning a wide range in metallicity (NGC 2419 and 47 Tuc). Also WFPC2 observations of the primary ACS standard star, as well as observations of a Sloan Standard Field. These observations will produce a valuable tie-in between the WFPC2, ACS and Sloan filter photometric systems.
• Products: ISR, Synphot, WFPC2 and ACS Handbooks
• Accuracy Goals: 1%