8.14 Cycle 7 Calibration Plan

8.14.1 Overview

The main goals of the WFPC2 Calibration Plans for Cycle 7 are:

• verify that the instrument remains stable in its main characteristics.
• address its photometric accuracy.

These goals are achieved by a mix of monitoring programs, which verify the stability and continued performance of the camera by repeating routine observations on a regular basis, and special calibrations, which have the goal to enhance the WFPC2 calibration in specific areas.

Standard Monitoring Programs

The stability of WFPC2 is mainly verified through the Photometric Monitoring program and the set of internal monitoring programs. The Photometric Monitoring program (7618) consists of regular one-orbit visits of our photometric standard GRW+70D5824, executed immediately before and after decontaminations. These observations allow us to monitor efficiently four main areas: the overall photometric throughput of the camera, the contamination of the CCD windows, especially in the UV, the PSF properties at different wavelengths, and the OTA focus. We continue to rely heavily on internal observations for some instrument maintenance and for many other types of monitoring: decontaminations (7619) to clear the contaminants from the CCD windows and to limit the growth in hot pixels; darks (7620) in order to produce up-to-date, high-quality dark reference files and to identify new hot pixels in a timely manner; biases, INTFLATs, and K-spots (decontamination programs, plus 7622, 7623), to verify the integrity of the camera's optics and electronics chain, and the pixel-to-pixel response in the visible; Earth Flats (7625) to follow variations in the large-scale flat field; and UV flats using the internal UV lamp (7624), to monitor the pixel-to-pixel response in the UV. The planned observations have remained largely the same as in previous cycles, except that internal flats place an increasing emphasis on the INTFLAT channel because of the continuing degradation of the VISFLAT channel.

Other Monitoring Programs

Some additional monitoring programs introduced in Cycles 7 deserve special mention. The Supplemental Darks program (7621, 7712, 7713) aims at obtaining a large number of relatively short darks on a very frequent basis, with the main goal of helping users identify hot pixels in their observations. The program has been designed to place the least possible burden on the scheduling system; that these additional darks have a low priority, and are scheduled whenever feasible. Under normal circumstances, this program provides up to 21 additional 1000s darks per week, providing users a good chance of having a dark within half a day of their observations. The Astrometric Monitor program (7627) measures the relative placement of the four WFPC2 CCD in the focal plane; we have evidence that shifts of up to 150 mas have occurred since 1994, and the Astrometric Monitor program is designed to track the continuing motion of the detectors. Finally, the CTE Monitor (7929), introduced late in Cycle 7, measures the photometric impact of the loss in charge transfer efficiency of the WFPC2 detectors, which continues to increase with time.

Special Calibration Programs

Special calibration programs address specific areas of WFPC2 calibration that require dedicated calibration measurements. They include substantial photometric, CTE, and PSF characterization programs, of interest to the majority of WFPC2 users, as well as a number of smaller programs which address areas of more limited interest.

The Photometric Characterization program, a continuation of the Cycle 6 program of the same name, is designed to improve the link between WFPC2 and ground based photometry. The Cycle 7 program (7628) includes additional observations of NGC 2100, a young LMC cluster, and NGC 2419, a very distant globular cluster in the Milky Way, which allows good coverage of the bright red giants, too bright and rare in nearby clusters. We also carry out a filter sweep on both our primary standard, GRW+70D5824, and our reference rich field in Cen. In Cycle 8 (8451) we repeat the filter sweep of the primary standard.

The Cycle 7 CTE Characterization program (7630) has provided a thorough exploration of the various parameters that could affect the so-called "long vs. short" anomaly, that is, the observed difference in count rates between long and short exposures. This extensive set of dedicated observations, in which each of the potentially critical parameters is varied in turn, has enabled us to characterize the anomaly and to suggest a correction formula that removes its impact almost completely.

The PSF Characterization program (7629) continues our accumulation of data for the WFPC2 PSF library, by addressing often-used filters such as F300W, F450W, F702W which were not included in previous cycles.

Table 8.7 summarizes the relevant data for Cycle 7 programs, followed by summary descriptions of each program. For a report on the final results from these programs, please see ISR WFPC2 99-05 (Cycle 7 Closure Report) at:

http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9905
 

Details on individual proposals can be found through the HST Program Information page at URL

http://presto.stsci.edu/public/propinfo.html
 

Table 8.7: WFPC2 Cycle 7 Calibration Plan.

ID	Proposal Title	Frequency	Estimated Time (orbits)		Products	Accuracy	Notes
			"External"	"Internal"
Routine Monitoring Programs
7618	Photometric Monitor	1-2/4 weeks	36		SYNPHOT	1-2%	Also focus monitor
7619	Decontamination	1/4 weeks		288	CDBS	n/a	Used together with darks, internals
7620	Standard Darks	weekly		360	CDBS, WWW	1 e/hr	Also hot pixel lists on WWW
7621, 7712, 7713	Supplemental Darks	weekly		2016		n/a	No analysis provided
7622	Internal Monitor	2/4 weeks		72	CDBS, TIR	0.8 e/pixel	New superbias (with 7619)
7623	Internal Flats	1/4 weeks		75	TIR	0.3%	Mostly INTFLATs
7624	UV Flat Field Monitor	2/cycle	4	8		2-8%	Before and after decon
7625	Earth Flats	continuous		155	CDBS	0.3%	Also LRF, Methane quads
7626	UV Throughput	2/cycle	4		SYNPHOT	3-10%
7627	Astrometric Monitor	2/cycle	2	2	TIPS, TIR	0.05	Also K-spots
Special Calibration Programs
7628	Photometric Characterization	1	10		ISR	2-5%	Also test zeropoint differences between chips, UV vignetting, astrometry
7629	PSF Characterization	1	5		WWW	10%	Covers widely used, high-throughput filters
7630	CTE Characterization	1	14		ISR	0.01 mag	Extensive coverage of preflash levels and exposure times in F555W, spot checks in F555W, F300W, hysteresis, CTE ramp
7929	CTE Monitor	4	4		ISR	0.02 mag	Measure changes in CTE ramp
8053	Supplemental Earth Flats	1		155	CDBS	0.3%	Repeat Earth Flats towards end of cycle
8054	LRF Calibration	1	10		ISR	3-5%	Complete LRF calibration, test stability
TOTAL TIME (including all executions)			89	3131

7618: WFPC2 Cycle 7: Photometric Monitor

• Purpose: Regular external check of instrumental stability. Based on Cycle 6 program 6902.
• Description: The standard star GRW+70D5824 is observed before and after a decontamination using three different strategies:
• F170W in all four chips to monitor contamination in the far UV.
• F439W, F555W, F814W on the PC to monitor focus.
• F160BW, F218W, F255W, F300W, F336W, F439W, F555W, F675W, F814W in a different chip each month. Some filters may be cut because of lack of time.
• Observations are taken after each decontamination and before every other decontamination, resulting in 36 orbits for 24 decontamination cycles.
• 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. This has been the case in Cycles 4 through 6. The point of the test is to measure this variation. Focus measurements have an expected accuracy of 1.5 micron, and a goal of 1 micron; the uncertainty in the focus determination is dominated by external factors, such as OTA breathing.
• Products: Instrument Handbook, reports at monthly TIPS meetings, WWW (sensitivity trends); updates in UV sensitivity variation used in SYNPHOT.

7619: WFPC2 Cycle 7: Decontamination

• Purpose: UV blocking contaminants are removed, and hot pixels cured, by warming the CCDs to +20C for six hours.
• Description: The decontamination itself is implemented via the DECON mode, in which the TECs are turned off and the CCD and heat pipe heaters are turned on to warm the detectors and window surfaces. Keeping WFPC2 warm for ~6 hours has been shown in previous Cycles to be sufficient to remove the contaminants and anneal many hot pixels.
• The internal observations taken before and after each decontamination consist of: four biases (two at each gain setting), four INTFLATs (two at each gain setting), two K-spots (both at gain 15, one short and one long exposure, optimized for PC and WF), and finally, five darks (gain 7, clocks off). To minimize time-dependent effects, each set of internals will be grouped within two days and performed no more than one 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 other WFPC2 activity.
• 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 7620 and 7622).
• Products: Those obtained from use of darks, biases and other internals (see Proposals 7620 and 7622).

7620: WFPC2 Cycle 7: Standard Darks

• Purpose: Measure dark current on individual pixels and identify hot pixels at frequent intervals.
• Description: Every week, six 1800s exposures are taken (five with clocks=OFF and one with clocks=ON) 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 in a non-interruptible sequence, at least 30 minutes after any WFPC2 activity (see Proposal 7619). Normal darks do not need to be protected in this fashion. The Supplemental Darks program (7621, 7712, 7713) will provide additional information on hot pixels.
• 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 the 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.

7621, 7712, 7713: WFPC2 Cycle 7: Supplemental Darks

• Purpose: Obtain very 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 observations can fit into almost any occultation period, making automatic scheduling feasible. Supplemental darks will be taken at low priority, and only when there is no other requirement for that specific occultation period. This program is complementary with 7620, Standard Darks, whose longer individual observations are better suited to produce high-quality pipeline darks and superdarks, and are also carried out at higher priority. Note that hot pixels are often a cause of concern for relatively short science programs, since they can mimic or mask key features of the observations, and about 400 new hot pixels per CCD are formed between executions of the Standard Darks program (7620). These observations will be made available as a service to the GO community, and there is no plan to use them in our standard analysis and products. This program has become feasible starting in Cycle 7, due to the placement of a solid state recorder on-board HST.
• Accuracy: N/A
• Products: None

7622: WFPC2 Cycle 7: Internal Monitor

• Purpose: Verification of short-term instrument stability for both gain settings.
• Description: The internal observations will consist of eight biases (four at each gain) and four INTFLATs (two 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 6 Internal Monitor (6905).
• Accuracy: Approximately 120 bias frames will be 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.
• Products: Superbiases delivered yearly 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.

7623: WFPC2 Cycle 7: Internal Flats

• Purpose: Monitor the pixel-to-pixel flat field response and the VISFLAT lamp degradation as well as detect any possible changes due to contamination. This program is a combination and continuation of the Cycle 6 VISFLAT Monitor and INTFLAT Monitor proposals (6906, 6907, respectively). The VISFLAT portion has been minimized to conserve the lifetime of the CAL channel lamp.
• Description: This proposal contains an INTFLAT filter sweep, a VISFLAT mini-sweep, linearity tests, and monitoring images. Monitoring is carried out by taking INTFLATs with the photometric filter set after each decon. The VISFLAT mini-sweeps (before and after decon, twice during the cycle) will include the photometric filter set at gain 7, plus the linear ramp filter FR533N at both gains to test the camera linearity. The INTFLAT sweep, taken within a two-week period, includes almost all filters, some with both blades and gains (F336W, F439W, F547M, F555W, F569W, F606W, F622W, F631N, F502N, F656N, F675W, F673N, F702W, F785LP, F814W, F1042M), others with just one blade and gain (F487N, F467M, F588N, F380W, F658N, F791W, F850LP, F953N, F450W, F300W, F390N, F410M, F437N, F469N, and F160BW). The linearity test is done at both gains and blades using F555W, and an additional set with one blade and gain with clocks on.
• Accuracy: Assuming Cycle 7 results will be similar to those from previous cycles, the VISFLATs should be stable to better than 1%, both in overall level and spatial variations (after correcting for lamp degradation), and contamination effects should be < 1%. For the INTFLATs, the signal-to-noise ratio per pixel is estimated to be similar to the VISFLATs, 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 complete failure of the CAL channel system. Temporal variations in the flat fields can be monitored at the 1% level. Gain ratios should be stable to better than 0.1%.
• Products: TIPS report, Technical Instrument Report if any significant variations are observed.

7624: WFPC2 Cycle 7: UV Flat field Monitor

• 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 Wood's 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 after-images.
• Accuracy: About 2-8% pixel-to-pixel expected (depending on filter).
• Products: New UV flat fields if changes are detected.

7625: WFPC2 Cycle 7: Earth Flats

• Purpose: Monitor flat field stability.
• Description: As in Cycle 6 program 6909, 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 de-streaking. 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-to-pixel variations in the flat fields. The general plan of Cycles 5 and 6 is repeated.
• Accuracy: The single-pixel signal-to-noise ratio expected in the flat field is 0.3%.
• Products: New flat fields to CDBS if any changes are detected.

Proposal ID 7626: WFPC2 Cycle 7: UV Throughput

• Purpose: Verify throughput for all UV filters. Loosely based on the Cycle 5 and 6 UV throughput proposals (6186, 6936).
• Description: GRW+70D5824 will be observed shortly before and after a DECON through all the UV filters in PC and WF3. Observations should be taken roughly mid-way through the cycle.
• Accuracy: The UV throughput will be measured to better than 3%.
• Products: TIPS, SYNPHOT update if necessary, Technical Instrument Report to document any changes if necessary.

7627: WFPC2 Cycle 7: Astrometric Monitor

• Purpose: Verify relative positions of WFPC2 chips with respect to one another. Repeats parts of Cycle 6 proposal 6942 twice during Cycle 7.
• Description: The rich field in Cen used for the Astrometry Verification (6942) is observed with large shifts (35) in F555W only, at two different times during Cycle 7. This will indicate whether there are shifts in the relative positions of the chips or changes in the astrometric solution at the subpixel level. Kelsall spot images will be taken in conjunction with each execution. The K-spots data and some external data indicate that shifts of up to 1 pixel may have occurred since mid-1994.
• Accuracy: At least 0.1 in the relative shifts, with a goal of 0.02-0.05.
• Products: TIPS, Technical Instrument Report; update of chip positions in PDB and of geometric solution in STSDAS task metric if any changes are found.

7628: WFPC2 Cycle 7: Photometric Characterization

• Purpose: (1) Determine if any changes in the zeropoint, or in the spatial dependence of the zeropoint or contamination, have occurred; (2) include another globular cluster (NGC 2419) in order to extend the parameter space for determinations of photometric transformation. Combines and continues Cycle 6 proposals 6934, 6935.
• Description: Observations of the primary photometric standard GRW+70D5824 will be compared against baseline observations. The cluster fields in Cen and NGC 2100 will be compared to previously obtained data in order to test for spatial variations in the throughput, using most broad-band and intermediate-width filters, including the far UV set for NGC 2100 (very young, many blue stars). A contamination test using UV filters will also be performed for NGC 2100. New observations of the Galactic globular cluster NGC 2419 will be compared with good ground based photometry; this cluster is very distant (100 kpc) and will provide a large color spread on giant branch and HB.
• Accuracy: Photometric stability expected to be better than 2%. Photometric transformations to be defined to 2-5%, depending on filter; most of the error derives from limited knowledge of the transformations between ground based and WFPC2 photometric systems.
• Products: ISR; SYNPHOT updates if necessary.

7629: WFPC2 Cycle 7: PSF Characterization

• Purpose: Provide a subsampled PSF over the full field to allow PSF fitting photometry, test PSF subtraction as well as dithering techniques. Based on Cycle 6 program 6938.
• Description: Measure the PSF over the full field in often-used, high-throughput filters in order to update the Tim and TinyTIM models and to allow accurate empirical PSFs to be derived for PSF fitting photometry. Compared to Cycles 5 and 6, we will repeat F814W to provide a continuing baseline, and will replace the other filters with F300W, F450W, F606W and F702W, which are often used because of their high throughput but are not as well characterized as the photometric set (F336W, F439W, F555W, F675W) used in previous Cycles. 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 4 x 4 grid on the CCD. The step size is 0.025 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. The proposal also allows a check for sub-pixel phase effects on the integrated photometry.
• 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 will improve the quality of PSF fitting photometry for the filters used. 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. 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 (WWW).

7630: WFPC2 Cycle 7: CTE Calibration

• Purpose: Conduct a thorough examination of the variation in photometric zeropoint as a function of exposure length, background (via preflash), and position in the chip. Include spot checks for the dependence of zeropoint variations on filter, order of exposures, and camera shifts (CTE ramp).
• Description: A well-studied field in the globular cluster NGC 2419 will be observed through F814W with a combination of exposure times (10, 40, 100, 300, 1000s) and preflash levels (0, 5, 10, 100, and 1000 e^-). Completes Cycle 6 proposal 6937, which was shortened substantially because of SM constraints. Will also include several observations in reverse order (to test for hysteresis), in F555W and F300W (filter dependence), and after a pointing shift (to test for x, y dependence), as well as a series of equal-length exposures to test the effect of noiseless preflash. This proposal should improve substantially our understanding of CTE and of the long vs. short anomaly.
• Accuracy: The reported short vs. long effect is ~0.05 mag. We want to determine it to better than 0.02 mag, with a goal of 0.01 mag.
• Products: ISR, paper; if appropriate, a special task to correct the CTE effect will be generated.

7929: WFPC2 Cycle 7: CTE Monitor

• Purpose: Monitor variations in CTE ramp for bright and faint targets.
• Description: Analysis of Cycle 6 CTE data shows that the CTE ramp depends strongly on stellar magnitude and background, and that its amplitude varies in time for faint stars. However, most measurements have been taken so far under slightly different conditions from one another. This program will take four one-orbit measurements of the CTE at four month intervals, under the same conditions as the best data taken so far. It will provide an accurate and efficient tracer of changes in the CTE ramp, and show to what extent WFPC2 remains a photometric instrument for faint objects. Observations of the standard field in NGC 5139 ( Cen) will be taken at the same roll angle, but centered in each of the WF chips in turn, thus reversing the x and y positions of each star. No preflash test is included.
• Accuracy: The measurements will enable tracking of the CTE ramp with an accuracy requirement of 0.02 mag, and a goal of 0.01 mag.
• Products: ISR.

8053: WFPC2 Cycle 7: Supplemental Earth Flats

• Purpose: Repeat the sequence of Earth flats late in Cycle 7 to verify stability of flat field.
• Description: As in previous cycles and earlier in Cycle 7, 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 de-streaking. 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-to-pixel variations in the flat fields. A repeat is requested because of the length of Cycle 7 and the fact that low-level temporal variations are typically discerned on time scales of about a year.
• Accuracy: Large-scale flat field variations can be tracked to about 0.3%.
• Products: New flat fields will be generated and delivered.

8054: WFPC2 Cycle 7: LRF Calibration

• Purpose: Complete the analysis of LRF properties: throughput and wavelength scale.
• Description: The primary spectrophotometric standard GRW+70D5824 will be observed at several locations on the three most used Linear Ramp Filters to verify its throughput as a function of wavelength. In addition, exposures of the Orion Nebula at two different pointings will be used to verify the wavelength calibration of the LRF at the wavelengths of major nebular lines. Previous executions of the LRF calibration have demonstrated a throughput consistent with the expectations based on laboratory filter tracings, with a scatter of 8% rms. The series of observations of GRW+70D5824 will: 1) measure the temporal stability of the difference between measured and predicted throughput; 2) demonstrate whether the scatter is due to measurement errors or to intrinsic variations in the filter; 3) complete the wavelength coverage (some of the observations from previous programs were lost); and 4) and provide more closely spaced points in the most often used ramp filter. The observations of the Orion Nebula, at two carefully optimized pointings, will provide a direct test of the wavelength calibration and vignetting of the LRF at the wavelengths of H, H, [OIII], [NII] and [SII].
• Accuracy: Measure throughput to 5%, wavelength position to about 5-10 pixels.
• Products: ISR, new SYNPHOT tables.