Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
8.15 Cycle 8 Calibration Plan
The Cycle 8 calibration program is aimed at maintaining the calibration of WFPC2 via monitoring programs, as well as continuing some proposals from previous Cycles into Cycle 8 and performing new tests to improve our understanding in several key areas. A brief overview of the Cycle 8 program as a whole is provided in the next section, followed by a table summarizing the proposals, and finally, detailed descriptions of each program (including proposal numbers, statement of purpose, observing description, products, and accuracy expected).
Standard Monitoring Programs
As in previous cycles, a substantial part of the program consists of the routine monitors and decontamination (decon) procedures. In Cycle 8, the decons will continue to be performed on a monthly basis, to remove the UV contaminants and anneal hot pixels. The monitoring observations associated with the decons are similar to those from previous Cycles, allowing us to efficiently track the overall long-term photometric throughput of the camera, the monthly throughput decline rates due to contaminant buildup on the CCD windows, the return to nominal throughput after the decons, the PSF properties at different wavelengths, the OTA focus, and the general health and performance of the cameras. A new aspect this cycle is that a handful of programs tied to a decon (internals, photometric monitor, focus check, UV throughput) have been combined into the decon proposal, to help minimize scheduling problems. For convenience, the resulting decon proposal was split into two pieces (8441, 8459; see Table 8.8), to run before and after SM3a in Oct. 1999.
In addition to the decon proposal and its associated observations, we will continue the standard darks program (six darks per week, used for reference files), the supplemental darks program (0-3 darks per day, low priority, for archive only), and the weekly internal monitor (biases and kspots). The Earth flat program will also be continued, to allow tracking and correction for changes in the flat field. Following the general plans of previous cycles, streak flats in a subset of filters will be obtained to construct superflats which are used to generate the pipeline flats.
The other monitoring proposals include the astrometric monitor, the CTE monitor, the INTFLAT/VISFLAT sweeps, and the UV internal flats. The astrometry program, along with the internal kspots, will allow measurement of any chip position shifts or changes in the astrometry. The CTE monitor program will allow tracking of the CTE problem, which continues to worsen with time. The internal flats programs will provide verification of the pixel-to-pixel flat field response; as in Cycle 7, the emphasis will be on the INTFLATS, so as to minimize shortening the VISFLAT lamp lifetime.
Special Programs - Continuations from Previous Cycles
The remaining proposals planned for Cycle 8 will be used to verify and improve the existing WFPC2 calibration in key areas. Several special programs which were executed in previous cycles will be run as shorter versions in Cycle 8: the photometric and PSF characterization proposals, the polarization check, and the linear ramp filter proposal. Cycle 7 included a thorough test of the photometric zeropoints and contamination rates; the Cycle 8 proposal will be a spot-check of those results, with a comparison to the baseline observations to identify any time dependencies. The PSF characterization proposal will be similar to that of Cycle 6, but only two filters will be checked (F555W and F814W), instead of the full suite of filters. The polarization proposal will allow us to verify the stability of the polarization calibration from Cycle 5 via observations of polarized and unpolarized standards; a small set of VISFLATs will be obtained to check for flat field changes. The linear ramp filter proposal is at present only a placeholder, pending receipt and analysis of the Cycle 7 data; however, the plan is to merely spot-check a subset of wavelengths in Cycle 8.
Special Programs - New
There are five new special programs, designed to address the remaining photometric issues (CTE and long vs. short) as well as user concerns from previous cycles.
The noiseless preflash proposal will test if illuminating the detectors prior to an exposure reduces the impact of the CTE and long vs. short anomalies. The preflash will be accomplished via INTFLATs which will be read out prior to the external exposures, thereby minimizing additional noise in the observations. Darks will be taken before the visit and during occultations, to insure that no prior exposures will effectively preflash the non-preflashed images.
The CTE for extended sources proposal will, for the first time, allow a direct measurement of the CTE effect on small (2"-3") extended sources; the tentative target, selected from the archive, is galaxy cluster 135951+621305. The cluster will be positioned at a variety of chip locations; images will be obtained in F606W and F814W to match those in the archive, thereby allowing an assessment of any temporal changes in the CTE.
The Cycle 8 special programs also include a check of the photometric calibration for very red stars (two late M dwarfs, VB8 and VB10) in BVRI. The current zeropoints (based on a white dwarf UV standard and verified via solar analog data) and the color transformations from HST BVRI to ground based BVRI are highly uncertain for stars this red; this program will provide straightforward empirical calibration. In addition, a short single-orbit program will allow us to measure variations in the plate scale with wavelength, particularly in the UV, where the index of refraction in the MgF window increases rapidly. Finally, a special program is being developed to help improve the quality of the UV flat fields: the Earth flats will be obtained in a variety of UV filters as well as some crossed filter combinations to account for any read leak contributions. Several of these special programs have been designated as candidates for "calibration outsourcing", where external groups would be funded to perform the analysis.Table 8.8: WFPC2 Cycle 8 Calibration Plan.
ID Proposal Title Frequency Estimated Time (orbits) Scheduling Required Products AccuracyRequired Notes "External" "Internal" Routine Monitoring Programs 8441 8459 WFPC2 Decons & Associated Observations 1-2/4 weeks Includes decons, photometric monitor, focus monitor, internals, UV throughput. 8442 Standard Darks weekly Also hot pixel lists on WWW. 8443 Supplemental Darks (8460, 8461) 0-3/day For archive only, no analysis provided. 8444 Internal Monitor 3/4 weeks New superbiases, not run on decon weeks. 8445 Earth Flats continuous Also LRF, Methane quads. 8446 Astrometric Monitor 2/cycle Also K-spots & plate scale check in red. 8447 CTE Monitor 2/cycle 8448 Intflat and Visflat Sweeps 1/cycle Mostly intflats. 8449 UV Flats Internal Monitor 1/cycle Uses UV cal channel lamp. Special Calibration Programs 8451 Photometric Characterization 1 Subset of Cycle 7 proposal, as check. 8452 PSF Characterization 1 Subset of standard broadband filters. 8453 Polarization 1 Subset of Cycle 6, as check. 8454 Linear Ramp Filters 1 Placeholder, pending results from Cycle 7. 8450 Noiseless Preflash 1 Test scheme to reduce CTE problem. 8455 Photometry of Very Red Stars 1 Outsourcing candidate. 8456 CTE for Extended Sources (2-3") 1 Outsourcing candidate. 8457 UV Earth Flats continuous Outsourcing candidate. 8458 Plate Scale Verification 1 Outsourcing candidate. ~10% reserve for unexpected items Placeholder. TOTAL TIME (including all executions)
8441, 8459: WFPC2 Cycle 8: Decontaminations and Associated Observations
- Purpose: Monthly WFPC2 decons. Instrument monitors tied to decons: photometric stability check, focus monitor, pre- and post-decon internals -- bias, intflats, K-spots, & darks, UV throughput checks.
- Description: Decontamination: UV-blocking contaminants removed and hot pixels annealed, by warming the CCDs to +20C for 6 hours. Internals: intflats, biases, darks & K-spots, before/after decons. Photometric and Focus Monitor: Standard star GRW+70D5824 is observed after each decon and before every other decon: (1) F170W in all chips to monitor far UV contamination. (2) PC focus monitor observations in F439W, F555W, F814W. (3) F160BW, F218W, F255W, F336W, F439W, F555W, F814W observed in a different chip each month.
UV Throughput: PC & WF3 UV observations in all UV filters, popular UV filters in all chips, to verify that the UV spectral response curve is unchanged. Also check Methane quads.
- Products: SYNPHOT, CDBS, Instrument Handbook, TIPS meetings, WWW reports, TIR, ISR.
- Accuracy: Photometry: less than 2% discrepancy between results, 1% rms expected. Focus measurement: 1.5 µ accuracy, with a goal of 1 µ. UV throughput: better than 3%. Flat Field: temporal variations monitored at 1% level. Gain ratios: stable to better than 0.1%.
8442: WFPC2 Cycle 8: Standard Darks
- Purpose: Measure dark current and identify hot pixels.
- Description: Six 1800s exposures/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, and 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. 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 in a non-interruptible sequence, at least 30 min after any WFPC2 activity.
- Products: Weekly darks delivered to CDBS and monthly tables of hot pixels on the WWW. Superdark reference files.
- Accuracy: Require ~1 e-/hour (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.
8443, 8460, 8461: WFPC2 Cycle 8: Supplemental Darks
- Purpose: Obtain very frequent monitoring of hot pixels.
- Description: This program (a continuation of Cycle 7 programs 7621, 7712, and 7713) 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 the higher priority Standard Darks proposal that has longer individual observations for producing high-quality pipeline darks and superdarks. Note that 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.) These observations will be made available as a service to the GO community; there is no plan to use them in our standard analysis and products.
- Products: None.
- Accuracy: n/a
8444: WFPC2 Cycle 8: Internal Monitor
- Purpose: Verify the short-term instrument stability at both gain settings.
- Description: Each set of internal observations consists 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.
- 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.
- Accuracy: 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.
8445: WFPC2 Cycle 8: Earth Flats
- Purpose: Monitor flat field stability.
- Description: As in Cycle 7 programs 7625 and 8053, sets of 200 Earth-streak flats are taken to construct high quality narrow-band flat fields with the filters 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 are 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 plans of Cycles 5, 6, and 7 are repeated.
- Products: New flat fields generated and delivered to CDBS if changes detected.
- Accuracy: The single-pixel signal-to-noise ratio expected in the flat field is 0.3%.
8446: WFPC2 Cycle 8: Astrometric Monitor
- Purpose: Verify relative positions of WFPC2 chips with respect to one another.
- Description: The rich field in Cen (same positions as Cycle 7 proposal 7627) is observed with large shifts (35") in F555W only, at two different times during Cycle 8. This will indicate whether there are 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. The K-spots data and some external data indicate that shifts of up to 1 pixel may have occurred since mid-1994.
- Products: TIPS, Technical Instrument Report, update of chip positions in PDB and of geometric solution in STSDAS tasks metric and wmosaic if significant changes are found.
- Accuracy: relative positions determined to 0.05"; variations to 0.01".
8447: WFPC2 Cycle 8: CTE Monitor
- Purpose: Monitor CTE changes during Cycle 8.
- Description: Observations of Cen (NGC 5139) are taken every six months during Cycle 8 to monitor changes in Charge Transfer Efficiency (CTE) of the WFPC2 (extension of Cycle 7 proposal 7929). The principal observations will be in F814W at gain 15 in WF2 and WF4. Supplemental observations at gain 7, in WF3, and with a preflash will be performed if time permits, along with observations in F439W and F555W. For each visit, observations will be done in single guide star mode.
- Products: Instrument Science Report
- Accuracy: 0.01 magnitudes.
8448: WFPC2 Cycle 8: Intflat and Visflat Sweeps
- Purpose: Monitor the pixel-to-pixel flat field response and the VISFLAT lamp degradation, as well as detect any possible changes due to contamination. The linearity test obtains a series of INTFLATs with both gains and both shutters. Since the INTFLATs have significant spatial structure, any nonlinearity would appear as a non-uniform ratio of INTFLATs with different exposure times.
- Description: VISFLAT mini-sweep: pre- and post-decon observations using the photometric filter set at gain 7, and FR533N at both gains to test the camera linearity. INTFLAT sweep: taken within a two-week period. Almost all filters used, some with both blades and gains, others with just one blade and gain. Linearity test: done at both gains and blades using F555W, and an additional set with one blade and gain with clocks=on.
- Products: TIPS, TIR if any significant variations are observed.
- Accuracy: VISFLATs: stable to better than 1% in overall level and spatial variations (after correcting for lamp degradation). Contamination effects should be < 1%. INTFLATs: signal-to-noise ratio per pixel similar to the VISFLATs, but spatial and wavelength variations in the illumination pattern are much larger. (INTFLATs will provide a baseline comparison of INTFLAT vs. VISFLAT if the CAL channel system fails.)
8449: WFPC2 Cycle 8: UV Flats Internal Monitor
- Purpose: Monitor the stability of UV flat field.
- Description: UV flat fields obtained with the CAL channel's ultraviolet lamp (UVFLAT) using the UV filters F122M, F170W, F160BW, F185W, and F336W. The UV flats are used to monitor UV flat field 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 flat field degradation and ties the UVFLAT and VISFLAT flat field patterns. Two supplemental dark frames must be obtained immediately after each use of the lamp to check for possible after-images.
- Products: New UV flat fields if changes are detected.
- Accuracy: About 2-8% pixel-to-pixel expected (depending on filter).
8451: WFPC2 Cycle 8: Photometric Characterization
- Purpose: Determine if any changes in the zeropoint, the spatial dependence of the zeropoint, or contamination rates have occurred, by comparing with the baseline measurements for GRW+70D5824 (single photometric standard with 13 filters)
- Description: Observe the standard star GRW+70D5824 in PC1 and WF3 using filters F380W, F410M, F450W, F467M, F547M, F569W, F606W, F622W, F702W, F785LP, F791W, F850LP, and F1042M. Observations should be done within seven days after a decon. These observations will be compared with data from the Cycle 7 program 7628.
- Products: TIR, SYNPHOT update if necessary.
- Accuracy: 2% photometry.
8452: WFPC2 Cycle 8: PSF Characterization
- Purpose: Provide a check of the subsampled PSF over the full field.
- Description: Observations using only two of the standard broadband filters (F555W and F814W). With one orbit per photometric filter, DITHER-LINE and POS TARG observations are performed in a 4x4 parallelogram. The dither-line-spacing is 0.177, and POS TARG steps are 0.125; this yields a critically sampled PSF over most of the visible range. Each star is measured 16 times per filter at different pixel phase, providing a high S/N, critically sampled PSF. This will improve the quality of PSF fitting photometry.
- Products: PSF library (WWW). Updates for TIM and TinyTIM. Accurate empirical PSFs to be derived for PSF fitting photometry.
- Accuracy: Results will be limited by breathing variations in focus, so predicting PSF accuracy is difficult. (For breathing < 5 micron peak-to-peak, PSFs should be good to ~10% in each pixel.) Proposal provides a measurement of pixel phase effect on photometry (sub-pixel QE variations exist), and gives a direct measurement of sub-pixel phase effects on photometry, measured to better than 1%.
8453: WFPC2 Cycle 8: Polarization
- Purpose: Verify stability of polarization calibration.
- Description: The data from this proposal will be used to identify any changes that may have occurred since the polarizer calibration in Cycle 5. Two stars will be observed, G191B2B and BD+64D106, a non-polarized and polarized standard star, respectively. The unpolarized star will be observed in two visits with the ORIENT changed by 90 degrees between visits, so as to sample any residual polarization of the star. The polarized star will be observed in four visits with the ORIENT changed by 45 degrees between visits, so as to fully sample the properties of each polarizer quad. Each visit consists of F555W exposures in PC1 and WF3, followed by F555W+POLQ exposures in PC1, WF2, WF3, and WF4. Other popular broadband filters (F300W, F439W, F675W, and F814W) will be checked using only the unrotated polarizer. Finally, a small set of VISFLATs (with a minimum of lamp cycles) will be included to check for flat field changes.
- Products: TIR or ISR report. If necessary, update SYNPHOT tables, WWW polarization calibration tools, and CDBS flat fields.
- Accuracy: Expected accuracy is <3%.
8454: WFPC2 Cycle 8: Linear Ramp Filter
- Purpose: Check wavelength and throughput calibration for LRFs at selected wavelengths.
- Description: A thorough check of the linear ramp filters (LRFs) is being done as part of the Cycle 7 calibration program, where the UV spectrophotometric standard (GRW+70D5824) is observed at 75 different wavelengths and an extended source (Orion Nebula) is observed for one orbit as well. This proposal is currently a placeholder, pending data analysis results from the Cycle 7 program. We anticipate requiring four orbits to spot-check some of the more popular wavelengths as well as cover any wavelengths requested by Cycle 8 GOs that were not observed as part of the Cycle 7 calibration program.
- Products: Updates to SYNPHOT tables if necessary and an ISR.
- Accuracy: Throughput accuracy should be better than 3%.
8450: WFPC2 Cycle 8: Noiseless Preflash
- Purpose: Test effectiveness of "Noiseless" preflash in reducing CTE and long vs. short Photometric effects.
- Description: A globular cluster is observed both before and after a preflash that has been read out (i.e. noiseless). The preflash will be tailored to expose the CCDs to about 3000 DN without saturation. The hypothesis is that the traps in the CCD will remain filled even though the preflash has been read out, thereby minimizing the effects of CTE. The observation sequence is repeated at two detector positions and exposure times, so as to test for CTE and long vs. short effects. The four orbits are done in one non-interruptible visit, which is preceded by a pair of 1800s darks and includes single darks during occultation periods, to insure that no prior exposures will effectively preflash the non-preflash exposures.
- Products: Improved observing strategies; ISR.
- Accuracy: 1% photometry
8455: WFPC2 Cycle 8: Photometry of Very Red Stars
- Purpose: Verify the photometric calibration of WFPC2 filters and obtain estimated color terms (HST to Johnson) for late M stars.
- Description: WFPC2 imaging (F439W, F555W, F675W, F814W) of two well-known M dwarfs, VB8 and VB10, for which ground based measurements in the Johnson filters exist. Use two different y positions to account for CTE. The current calibration is based on white dwarf and solar analog data, which are insufficient to produce an accurate calibration for cool red stars (late K and M) in broad-band filters. The calibration of cool stars is especially difficult at the red end (F814W), because their spectra can rise quickly where the DQE drops substantially (increasing the uncertainty in the synthetic magnitude calibration). The observations of two well-studied late M stars, VB8 and VB10, will provide a direct empirical calibration of these effects and reduce the uncertainties in the photometric response of WFPC2 for very red stars.
- Products: N/A
- Accuracy: Better than 0.03 mag.
8456: WFPC2 Cycle 8: CTE for Extended Sources
- Purpose: Determine the effect of Charge Transfer Efficiency (CTE) on small extended sources.
- Description: Previous CTE proposals have all focused on stellar targets. This proposal is aimed at observing small (~2-3") extended sources in a suitable galaxy cluster. The target (tentatively cluster 135951+621305, at z=0.3) will be observed in WF2 and WF4, in F606W and F814W. The filter F606W is chosen instead of the F555W used for stellar CTE measurements, to allow a comparison to archival images for estimation of any possible time-dependence. One orbit is needed for each pointing for each filter, for a total of four orbits.
- Products: ISR
- Accuracy: 10%
8457: WFPC2 Cycle 8: UV Earth Flats
- Purpose: Improve quality of pipeline UV flat fields.
- 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 seven UV filters plus 20 with each of the crossed filter sets (16 combinations).
- Products: Updated flat fields for pipeline via CDBS.
- Accuracy: 10%
8458: WFPC2 Cycle 8: Plate Scale Verification
- Purpose: Check of the WFPC2 plate scale in the UV and red.
- Description: UV and F953N observations of the bright cluster NGC2100. Data will be taken in F170W, F218W, F300W, F555W (to allow tie-in to previous observations) and F953N. To minimize orbits required, the program is designed around short exposures in the filters listed above; the data will provide a verification the plate scale in the UV but exposure times will not be long enough to allow a full distortion solution.
- Products: ISR
- Accuracy: Better than 0.05% (0.4 pixels over 1 chip), or 0.05 mas/pixel in WF.
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