Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14

8.17 Cycle 10 Calibration Plan

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As in previous cycles, the Cycle 10 calibration program is aimed at maintaining the calibration of WFPC2 via the internal and external monitoring programs as well as performing several new tests. The standard suite of calibrations will be continued, including those used to monitor the health of the instrument as well as the programs to collect data for calibration reference files. In addition, several new proposals will be implemented: a measurement of the effect of CTE on astrometry, a characterization of the PSF wings, a calibration check of the clocks ON mode, and a test of the methane quad filter throughput. The total spacecraft time required for the Cycle 10 plan is 61 externals orbits and 2294 occultation periods. This estimate does not include any calibrations associated with Servicing Mission (SM3b), which occurred in March 2002.

We also note that two "calibration outsourcing" programs are underway to improve the UV flatfields and test for a position-independent component of CTE. See Future Calibrations, Calibration by Observers, and Calibration Outsourcing for details.

Table 8.10: WFPC2 Cycle 10 Calibration Plan.

ID	Proposal Title	Frequency	Estimated Time (orbits)		Scheduling Required	Products	Accuracy Required	Notes
			"External"	"Internal"
Routine Monitoring Programs
8932- 8934	WFPC2 Decons & Associated Observations	1-2/4 wks	19 photmon 2 UV thru.put 2 UV flats	82	every 28 d	CDBS, IHB, Synphot, WWW reports	1-2%	Decons, phot. & focus monitor, internals, UV throughput, visflats and uvflats.
8935	Standard Darks	weekly		318	every 7 d	CDBS	1 e/hr	Also for WWW hot pixel lists.
8836- 8838	Supplemental Darks	0-3/day		1095	every day		n/a	For archive only, no analysis.
8839	Internal Monitor	weekly		76	every 7 d	CDBS	0.8e-/pix	Incl. intflats for preflash.
8840	Earth Flats	continuous		210	mid to late	CDBS	0.3%
8841	UV Earth Flats	continuous		400	early to mid	CDBS	3-10%	Outsourcing candidate.
8842	Intflat & Visflat Sweeps	1/cycle		61	mid-cycle	TIR	0.3%	Incl. filter rotation offset check.
Special Calibration Programs - Continuations
9253	Astrometric Monitor	2/cycle	2	2	early & late	ISR, STSDAS	0.05''	Cen as well as K-spots.
9254	CTE Photometric Monitor	2 x 3 orbits	6		mid & late	ISR	0.01 mag	Continuation of monitors.
9251	Photometric Characterization	1	4		mid-cycle	ISR, Synphot	2-3%	All four chips.
Special Calibration Programs - New
9255	Astrometric Effects of CTE	1	12		late	ISR	1-2 mas	Target kept on 1 chip.
9257	Super-PSF	1	6		mid-cycle	CDBS, STSDAS, ISR	10%	PSF wing characterization.
9252	Clocks ON Verification	1	1	50	early	ISR, Synphot	2-3%	Closure calibration.
9256	Methane Quad Filter Check	1	1		mid-cycle	Synphot, ISR	5%	Test of transmission curve across aperture (GO suggestion). Outsourcing candidate.
	~10% reserve		6					Placeholder for unexpected items.
TOTAL TIME (including all executions)			61	2294

8932, 8933, 8934: WFPC2 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: Decontamination: UV-blocking contaminants removed and hot pixels annealed by warming the CCDs to +20C for 6 hours.
  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 in a different chip each month.
  Focus Monitor: two PC, F555W observations of GRW+70D5824 will be taken during every photometric monitoring orbit (one at orbit start, one near orbit end).
  UV Throughput: PC & WF3 UV observations in most UV filters, popular UV filters in all chips, to verify that the UV spectral response curve is unchanged. In addition, two PC, F555W observations will be included as an extra focus monitor.
  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.
• Products: SYNPHOT, CDBS, Instr. 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. Focus measurement: 1.5 micron accuracy with a goal of 1 mic. UV throughput: better than 3%. Flatfield: 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%.

8935: WFPC2 Cycle 10 Standard Darks

• Purpose: Measure dark current & identify of 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.

8936, 8937, 8938: WFPC2 Cycle 10 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.

8939: WFPC2 Cycle 10 Internal Monitor

• Purpose: Verify the short-term instrument stability at both gain settings and provide intflats for calibrating preflashed observations.
• Description: Each set of internal observations consists 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. During the decon week, intflats in F502N will be taken, with each shutter blade and at a variety of exposure times to test for linearity. The F502N filter is likely to be the recommended filter for preflashing observations.
• Products: Superbiases delivered annually to CDBS; TIPS reports on possible buildup of contaminants on the CCD windows (worms) as well as gain ratio stability, based on intflats. A Technical Instrument Report will be issued if significant changes occur. Preflash correction images may be generated.
• Accuracy Goals: Approximately 120 bias frames are used for each superbias pipeline reference file, generated once a year; accuracy is required to be better than 1.5 e^-/pixel, and is expected to be 0.8 e^-/pixel.

8940: WFPC2 Cycle 10 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. These Earth flats will complement the Earth flat data obtained during SMOV and Cycles 4-9.
• Description: Observations of the bright Earth (Earthcals) are obtained in a variety of filters. Approximately 200 exposures in each of four narrowband filters (F375N, F502N, F656N, F953N) are required, as well as about 50 exposures in other filters (F160BW, F336W, F343N, F390N, F437N, F469N, F487N, F631N, F658N, F673N -- the F160BW filter is included to provide pinhole information).
• 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%.

8941: WFPC2 Cycle 10 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. These Earth flats will complement the UV Earth flat data obtained during Cycles 8-9.
• Description: Earth streak-flats are taken in UV filters (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. Earth flats required: 100 for each of the 7 UV filters plus 20 with each of the crossed filter sets (16 combinations). The entire proposal should be done within 7 months, with the observations evenly distributed over that period of time. The observations are divided into 10 batches, with each batch done 21 days apart.
• Products: Updated flatfields for pipeline via CDBS.
• Accuracy Goals: 3-10%. Outsourcing candidate.

8942: WFPC2 Cycle 10 Intflat Sweeps and Linearity Test

• 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 9 program 8817. 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. New this cycle will be extra visflat exposures to test the repeatability of filter wheel motions.
• Description: Intflat sweep -- flatfields are obtained with a variety of filters (F336W, F439W, F547M, F555W, F569W, F606W, F622W, F631N, F502N, F656N, F675W, F673N, F702W, F785LP, F814W, F1042M) using shutters A and B, and gains 7 and 15; the BLADE optional parameter is used throughout. A smaller set is obtained only at gain 7 using any shutter blade (F160BW, F300W, F380W, F390N, F410M, F437N, F450W, F469N, F487N, F467M, F588N, F658N, F791W, F850LP, F953N).
  Linearity test -- flatfields are taken with F555W at a variety of exposure times, using shutters A & B, and gains 7 & 15. In addition, a set is done with clocks ON (only gain 7, shutter B; gain 7 shutter A set was taken during Cycle 9). 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 -- 10 visits of visflats will be taken to test the repeatability of the filter wheel positioning. A problem is known to exist in FR533N; other filters will be used, to determine whether the problem is limited to the one filter or is present in other filters/wheels.
• Products: TIPS. TIR/ISR if any significant variations are observed or if any new filter problems are noted.
• Accuracy Goals: Intflats: Stable to better than 1%. (intflats will provide a baseline comparison of intflat vs visflat (taken in decon proposal) if the CAL channel system fails.)

9253: WFPC2 Cycle 10 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, every ~six months. 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.

9254: WFPC2 Cycle 10 CTE Photometric Monitor

• Purpose: Monitor CTE changes during Cycle 10.
• Description: Obtain observations of Cen (NGC 5139) to continue tracking changes in the CTE (charge transfer efficiency) losses in WFPC2. A continuation of proposals in earlier cycles (7629, 8447, and 8821), the principal observations will be at gain 7, in F814W and F555W, taken with and without a variety of preflash (background) levels (20 to 1000 e^-). The same pointing is used at WF2 and WF4; along with the relative orientation of the chips, this results in stars at the bottom of one chip falling near the top of the other chip.
• Products: ISR and updates to published CTE correction formulae.
• Accuracy Goals: 0.01 magnitudes.

9251: WFPC2 Cycle 10 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 (F380W, F410M, F450W, F467M, F547M, F569W, F606W, F622W, F702W, F785LP, 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 if necessary.
• Accuracy Goals: 2-3% photometry.

9255: WFPC2 Cycle 10 Astrometric Effects of CTE

• Purpose: This proposal attempts to quantify the astrometric effects of CTE by measuring (1) the relative separation of a bright source vs. a faint target at different positions on the PC1 CCD, and (2) the relative motion of a source on the CCD compared to very precise slews performed with the FGSs. These tests will be conducted for point and extended targets at several different intensity levels.
• Description: While the photometric effects of CTE have been well studied, and correction algorithms have been developed, very little is known about the astrometric effects of CTE. Riess (2000; WFPC2 ISR 00-04) has shown that extended sources suffer some degree of distortion due to CTE, indicating that the astrometry of sources must also be affected. E.g., the relative separation of a faint source from a bright source may depend on all the factors that influence CTE (position on detector, observing epoch, brightness in electrons, and image background).
  Targets will be observed in PC, 2x2 grid, 20" on a side, in the first orbit; the second orbit is a repeat with a small pointing offset (dither of N+1/2 PC pixels). The 2-orbit sequence will be done at different background light levels, using exposures from 100s in F450W to 1200s in F622W, repeated three times for two targets (total ~12 orbits).
  Targets will be the dense star field in Cen, and a field of faint galaxies (from extended target CTE proposal 8456). Both fields in both targets will be chosen to have a bright star surrounded by fainter objects. While most of the test is performed on the PC, the WFC CCDs will also be important, as they can provide a sanity check on the motions made with the FGSs. The motions on the WFC CCDs will be a smaller number of pixels, and hence less subject to CTE variations. (Though the larger WFC pixels will make astrometry more difficult). Hence some care should be taken to have stars available on the WFC CCDs, though PC1 is the detector of primary interest.
• Products: ISR.
• Accuracy Goals: The relative separations of the faint targets from the bright star will be measured with millarcsecond accuracy as a function of target position on the CCD and background level. The measured sizes of the large slews (~20") will also be compared to the sizes of commanded slews as a function of background intensity. It will be necessary to correct for image scale effects (i.e. geometric distortion) with care to insure that optical and CTE effects do not become confused.

9257: WFPC2 Cycle 10 Super-PSF

• Purpose: Obtain deep images of the WFPC2 PSF in several broadband filters in order to investigate the 2-dimensional structure in the PSF wings and characterize the change in structure with varying focus and target color.
• Description: This program will provide deep observations of the PSF wings by obtaining highly saturated images which are stepped in exposure time, allowing the creation of a "Super-PSF". STScI is currently developing software that will be able to accurately blend these larger-scale empirical PSF wings together with model PSF cores from TinyTIM. The new software will then perform 2-dimensional fitting and subtraction of the blended PSF from science images. The deep images proposed here are crucial for the new software to be effective: the majority of PSFs in the library have poor S/N in the wings and the TinyTIM models, while excellent for PSF cores, are inadequate beyond ~2", primarily due to scattering by the WFPC2 detectors.
  Since the shape and width of the PSF varies over time due to the change in telescope focus, the observations are split into 3 visits, 2 orbits each. The majority of archival programs which would benefit from these observations have placed the target on the PC chip, therefore, the PC1-FIX aperture will be used. Targets will be an A0V and a G0V star (TBD), to allow characterization of the color dependence of the PSF. Each star will be observed for an entire orbit, cycling through several broadband filters (F450W, F606W, F702W and F814W), 2 images per filter, at low and high saturation levels.
• Products: Accurate empirical PSFs to be derived for PSF fitting photometry, intended for use with STSDAS software currently under development. Observed PSFs and blended PSFs to be archived in on-line PSF library in CDBS. ISR.
• Accuracy Goals: Enable PSF subtraction with ~10% residuals at ~2".

9252: WFPC2 Cycle 10 Clocks ON Verification

• Purpose: Closure calibration for clocks ON mode.
• Description: This proposal will provide a check of the existing photometric and dark calibration for the clocks ON mode. An initial analysis was performed on photometric data taken Dec 1994; additional standard star observations using a small number of filters were taken and checked in Nov. 2000. The Cycle 10 external orbit will be used to observe a standard star in as many of the most frequently used filters and apertures not covered by prior observations. The results will be compared with clocks OFF data, in order to determine whether any significant differences exist between the two modes (none are expected). The new images will serve as a final verification of the calibration of this mode.
  The requested occultation periods will be used to obtain sufficient clocks ON darks over a time span of a few months to allow for generation of a clocks ON superdark.
• Products: SYNPHOT update; new clocks ON superdark and reference files, if necessary. ISR.
• Accuracy Goals: Photometric accuracy 2-3%. Expected accuracy in resulting superdark ~0.08 e^-/hour for normal pixels.

9256: WFPC2 Cycle 10 Methane Quad Filter Check

• Purpose: Verify FQCH4N-D methane filter characteristics.
• Description: Based on results from Jupiter and Uranus archival WFPC2 data, the extended wings of the methane filter transmission curve appear to vary across the field of view (Karkoshka, priv. comm.). While this is unimportant for objects with flat spectra, it can have a major impact on photometry of objects with methane bands, where a significant fraction of photons comes from the wings. To provide data to check the methane filter, a set of eight 40-sec Saturn images will be taken in a 3x3 grid around the FQCH4W3 methane quad filter aperture (one of the 9 positions falls outside of the filter). The magnitude and direction of the effect will be quantified by comparing results from the rings of Saturn (flat spectrum) to results from Saturn itself (deep methane band spectrum).
• Products: Outsourcing candidate. Expected product would be an updated filter transmission curve, to be installed in SYNPHOT and an ISR, if the project is not outsourced.
• Accuracy Goals: 5%.