8.20 Cycle 13 Calibration Plan

The overall goals of the Cycle 13 WFPC2 Calibration Programs are to monitor health and safety of the instrument and to maintain required calibration accuracies for the science modes used in Cycle 13. We will also streamline routine monitoring programs and remove unnecessary observations. Specifically, we are discontinuing the supplemental darks program as they were little used. Eliminating them will help prolong the life of the data transmitter aboard HST. We will further implement special close-out calibration programs to increase the value of the WFPC2 archive scientific legacy, such as photometric cross-calibration with other instruments.

Until the end of Cycle 10, WFPC2 was most heavily used in prime mode. During Cycles 11-13, WFPC2 prime orbits have decreased dramatically and parallel usage is now the dominant mode for WFPC2 science operation. Table 8.15 shows the recent history of science program and calibration program usage.

WFPC2 Cycle 13 Calibration Plan.

ID	Proposal Title	Frequency	Estimated Time (orbits)		Scheduling Required	Products	Accuracy Required	Notes
			"External"	"Internal"
Routine Monitoring Programs
10356	WFPC2 Decons & Associated Observations	Decons every 49d	8	124	every 49d	CDBS, IHB, Synphot, WWW reports	1-2%	Decons, phot.monitor, internals, UV throughput, VISFLATS and UVFLATS.
10359	Standard Darks	weekly, exc. decon wk		264	every 7 days, exc.decon wk	CDBS	1 e-/hr	CDBS updates and weekly WWW hot pixel lists.
10360	Internal Monitor	weekly, exc. decon wk		44	every 7 days, exc.decon wk	CDBS	0.8e-/pix	BIAS, INTFLATS in F555W for gain and throughput stability measurements
10361	Visible Earth Flats	continuous		50	mid-to-late	CDBS	0.3%	F502N only (time dependence only)
10362	UV Earth Flats	continuous		20	mid-to-late	CDBS	0.3%	F255W only
10363	Intflat & Visflat Sweeps, Filter Anomaly Check	1/cycle		80	mid-cycle	TIR	0.3%	Flats in all the filters used in Cycle 12, both gain settings/shutters.
10364	CTE Monitor	1/cycle	2		mid-to-late	ISR	0.03 mag	Continue CTE monitor.
10365	Photometric Monitor	1/cycle	3		mid-cycle	ISR, Synphot	1%	GRW+70D5824 in filter/chip combos used for science in Cycle 13.
Close-Out Calibration Programs
10366	Photometric Cross-Calibration	once	2		mid-cycle	ISR, Synphot	1%	T-dwarf star in a range of WFPC2 filters, for ACS & WFC3 cross-calibr.
	~10% reserve		2					Placeholder for unexpected items.
TOTAL TIME (including all executions)			17	582

10356: WFPC2 Cycle 13: Decontaminations and Associated Observations

• Purpose: WFPC2 decons every 49 days. Other programs tied to decons are also included: photometric stability check, focus monitor, pre- and post-decon internals, visflat sweep, and internal UV flat check.
• Description: Decontamination: 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 alternately before or after each decon, rotating chips: (1) F170W in all chips to monitor far UV contamination. (2) As many as possible of F122M, F185W, F218W, F255W, F300W, F336W, F343N, F375N, F390N, and F555W will be observed within 1 orbit in a different chip each time.
  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%.

10359: WFPC2 Cycle 13: 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.

10360: WFPC2 Cycle 13: Internal Monitor

• Purpose: This calibration proposal is the Cycle 13 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 13 (early August 2004), replacing Cycle 12 Internal Monitor proposal 10072. 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.
• Products: CDBS (superbias created annually)
• Accuracy Goals: 0.8 e-/pix for superbias reference file.

10361: WFPC2 Cycle 13: Visible 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 F502N to monitor time-dependence of flatfield features. In Cycle 13, we will continue monitoring only F502N (as was done in Cycle 12; prior to Cycle 12, all 12 narrow-band filters have been used). A detailed study of Earthflats (Koekemoer 2001) showed that there was no time evolution in the color dependence of flatfields, i.e. the time evolution of flatfields is monochromatic.
• Products: New flatfields generated and delivered to CDBS if changes detected.
• Accuracy Goals: The single-pixel signal-to-noise ratio expected in the flatfield is 0.3%.

10362: WFPC2 Cycle 13: 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 filter F255W, in order to measure time dependence of flatfield due to changes in the camera geometry as well as any possible long-term changes from permanent evaporation of contaminants.
• Products: Updated flatfields for pipeline via CDBS.
• Accuracy Goals: 3-10%.

10363: WFPC2 Cycle 13: 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 VISLFAT 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 12, we plan to continue to take extra VISLFAT, 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 and Earthflats 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%

10364: WFPC2 Cycle 13: CTE Monitor

• Purpose: Monitor CTE changes during Cycle 13; 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, 8821, 9254, 10076), 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 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

10365: WFPC2 Cycle 13: Photometric Monitor

• 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 Cycles 12 and 13. The filters are PC1: F439W, F450W, F467M, F487N, F502N, F547M, F555W, F588N, F606W, F631N, F656N, F658N, F673N, F675W, F791W, F814W, F850LP, F953N, F1042M, and FQCH4N15; WF2: same as PC1 but omitting F467M, F675W, F791W, F850LP, F1042M, and FQCH4N15 -- and adding F410M; WF3 and WF4: same as WF2. 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.

10366: WFPC2 Cycle 13: Close-Out Photometric Cross-Calibration

• Purpose: Tie WFPC2 photometry to other systems (ACS, WFC3, STIS, NICMOS, and ground based) for extremely red targets.
• Description: Observe T-dwarf star (2M0559-14) in as many WFPC2 filters as possible, essentially replicating ACS calibration program 10056 visit 4. The plan is to spend 2 orbits exposing on PC1 in single chip read-out mode (to minimize overheads), cycling through filters F606W, F622W, F675W, F702W, F785LP, F791W, F814W, F850LP, F1042M. Images should be taken within 7 days after a Decon, to minimize any contamination effects.
• Products: ISR, Synphot update
• Accuracy Goals: 2-3% photometry