8.19 Cycle 12 Calibration Plan

Until Cycle 10, WFPC2 was the most heavily used instrument on HST (~40-60% of the total of ~3000 orbits available for science in a given Cycle), with much of the observing being carried out in prime mode. Since the installation of the Advanced Camera for Surveys (ACS) in Cycle 11, there has been a dramatic change in the usage pattern for WFPC2. The instrument is still used quite heavily by the community (~800 - 1200 orbits/Cycle), though somewhat less than before, but the main difference is that now almost all the WFPC2 observing is carried out in parallel mode. Thus about 40% of prime HST science orbits during Cycles 11 and 12 have WFPC2 observations of parallel targets, while the number of prime WFPC2 science observations have decreased to ~5% and 2% of the total awarded time in each of Cycles 11 and 12 respectively, as demonstrated in Table 8.14:.

Table 8.12: WFPC2 Science Program Usage During Cycles 10 - 12.

WFPC2 Science Program Usage	Cycle 10	Cycle 11	Cycle 12
Primary Orbits	1080	153	56
Coordinated Parallel Orbits	40	200	381
Pure Parallel Orbits	300	500	844

In addition to the change in emphasis by the community from prime to parallel observing, there has also been a change in emphasis on the types of WFPC2 science programs that are carried out. Specifically, WFPC2 remains competitive with ACS in the following three areas:

• A comparatively broad selection of filters in a total of 48 optical elements, consisting of 18 narrow-band and medium-band filters, 23 broad-band and long-pass filters, as well as 2 quad filters (each giving 4 different narrow-band wavelengths on the 4 WFPC2 chips), one polarizing filter, and 4 linear ramp filters (LRFs).
• A relatively high discovery efficiency in the near-UV (essentially the product of the instrument area and its UV throughput); while the ACS/HRC is ~3-4 times more sensitive than WFPC2 in the 2000-3500A wavelength range (i.e. short wards of the blue cutoff of ACS/WFC), the area of WFPC2 is ~30 times larger than the ACS/HRC.
• A long history of on-orbit performance (10 years as of December 2003) and well-characterized behavior, making WFPC2 suitable for long-term monitoring studies of objects that are variable in their photometric or astrometric properties.

Thus, the majority of Cycle 12 science programs making use of WFPC2 in prime mode have consisted of either narrow-band filter observations, or the continuation of long-term monitoring programs. On the other hand, the WFPC2 parallel science tends to consist mostly of observations through a subset of the broad-band filters, often in the near-UV.

• Considering both the reduced observing time for WFPC2 in Cycle 12, as well as the change in its usage patterns, this has necessitated a change in the calibration strategy for the instrument during this Cycle. The principal emphasis of routine calibration continues to be two-fold:
• Monitor and maintain the basic health and safety of the instrument in all its modes.
• Maintain the required calibration accuracies for science modes used in Cycle 12, while also streamlining the calibration process to remove unnecessary observations of modes that are not used during this Cycle.

In addition, during Cycles 11 and 12 we have begun implementation of a class of special Close-Out Calibration Programs, taking into account community input to carry out programs that will increase the value of the WFPC2 archival scientific legacy. In Cycle 12, these include the following programs:

• Photometric cross-calibration with other instruments and systems, in particular ACS and SDSS, by observing a range of different photometric standards through the widest possible range of WFPC2 filters.
• Improving our knowledge of the astrometric distortion of WFPC2 in the UV, by carrying out observations on our standard astrometric field Centauri using the near-UV F255W filter.
• Improving characterization of the long-term stability of the narrow-band / LRF filter set, by observing an emission-line source in a range of narrow-band filters, by themselves and crossed with LRFs. This will enable verification of the constancy of the central wavelength of the narrow-band filter, as well as providing a check on its throughput.

These changes to the calibration philosophy are reflected in the orbit allocations for the Cycle 12 calibration programs, as shown in Table 8.16:. The number of external orbits for routine monitoring programs are reduced compared with previous cycles, with the dominant remaining components being the verification of UV throughput before and after decontaminations, as well as a number of CTE and photometric monitoring exposures. Likewise, the internal orbit allocation is reduced somewhat to account for the fact that we are no longer calibrating filter modes that are not used for science during this Cycle. However, the Special and Close-Out programs remain at a level of ~50% of the entire external orbit allocation budget, reflecting our emphasis on carrying out these programs that are needed to provide final calibration data to enhance the long-term archival legacy of the instrument.

The total time allocated for Cycle 12 calibrations is 25 external orbits (compared to 40 orbits in cycle 11), and 1681 internal/occultation orbits. This allocation spans October 1, 2003 to September 30, 2004. As always, about 10% of the total external orbit time allocation has been set aside for calibration issues that arise during Cycle 12, amounting to 2 external orbits.

Table 8.13: WFPC2 Calibration Program Orbit Allocations During Cycles 10 - 12.

WFPC2 Calibration Program Usage	Cycle 10		Cycle 11		Cycle 12
	Ext.	Int.	Ext.	Int.	Ext.	Int.
Monitors: Decons, darks, internal flats	23	2242	19	2170	12	1677
Special / Close-Out Programs	32	52	17	1	13	4
Reserve (Unexpected Items)	6		3		2
Total	61	2294	40	2171	25	1681

10067: WFPC2 Cycle 12: 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, 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 either before or after a decon, rotating chips: (1) F170W in all chips to monitor far UV contamination. (2) As many as possible of F218W, F255W, F300W, F336W will be observed within 1 orbit in a different chip each time.
  UV Throughput: Observations in most UV filters (F122M, F185W, F343N, F375N, F390N) added to each the photometric monitor. 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%.

10068: WFPC2 Cycle 12: 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.

10069, 10070, 10071: WFPC2 Cycle 12: 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.

10072: WFPC2 Cycle 12: Internal Monitor

• Purpose: This calibration proposal is the Cycle 12 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 12 (early August 2003), replacing Cycle 11 Internal Monitor proposal 9596. 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.

10073: WFPC2 Cycle 12: 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 12, we will continue monitoring only F502N; 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. (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%.

10074: WFPC2 Cycle 12: 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%.

10075: WFPC2 Cycle 12: 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 11 program 9597. 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 11, 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%

10076: WFPC2 Cycle 12: CTE Monitor

• Purpose: Monitor CTE changes during Cycle 12; 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

10077: WFPC2 Cycle 12: Photometric Monitor

• Purpose: Provide a check of the zeropoints and contamination rates in non-standard, less-used, or WFPC2 filters F439W and long-ward.
• 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 12 (F439W, F450W, F547M, F555W, F569W, F606W, F675W, F791W, F814W, F850LP, and F1042M). F675W, F850LP and F1042M are observed in the PC1 chip only. 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.

10078: WFPC2 Cycle 12: Close-Out 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%

10079: WFPC2 Cycle 12: Close-Out UV Astrometric Characterization

• 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.

10080: WFPC2 Cycle 12: Close-Out Narrow-Band/LRF Characterization

• Purpose: This proposal is aimed at measuring the wavelength stability of the LRFs, which may have changed over time due to annealing or shrinkage of the thin films.
• Description: On-orbit VISFLATs taken through the LRFs crossed with narrow-band filters to constrain the LRF wavelength calibration, as well as uncrossed LFRF VISFLAT exposures to contain the transverse (cross-wavelength) placement of the LRFs. External observations of a planetary nebula (2 orbits) to provide an absolute test of the LRF wavelength calibration.
• Products: ISR, Synphot, WFPC2 and ACS IHB.
• Accuracy Goals: 1%.