Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
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| Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14 | ||||
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8.13 Cycle 6 Calibration Plan
The Cycle 6 calibration plan is similar to that for Cycle 5, and is summarized in Table 8.6. Important differences include the addition of programs to check the astrometric calibration (6941), more detailed checking of the camera electronics (6942), and measurements of narrow band filter throughputs (6943). Also, it is expected that there will be reduced usage of the calibration channel VISLAMP, so as to prolong the lamp lifetime. More detailed program descriptions are given below. For a report on the final results from these programs, please see ISR WFPC2 98-01 ("Cycle 6 Closure Report") at:
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9801.html
6902: Photometric Monitor
- Purpose: Monthly external check of instrumental stability.
- Description: The standard star GRW+70D5824 is observed before and after each decontamination (i.e. twice in a four-week period). Each observation consists of three sequences: (1) F170W in all four chips to monitor contamination in the far UV; (2) F439W, F555W, F814W on the PC to monitor focus; and (3) F160W, F185W, F218W, F255W, F300W, F336W, F439W, F555W, F675W, F814W in a different chip each month. Some filters may be cut because of lack of time (F185W cut first, then F300W, then F675W, then F218W). This proposal is based largely on Cycle 5 program 6184; focus monitoring in F439W and F814W is added at the expense of some UV filters.
- 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. The point of the test is to measure this variation.
- Products: Documents produced are Instrument Handbook, TIPS, WWW (sensitivity trends). Updates in UV sensitivity variation used in SYNPHOT are provided.
6903: Decontamination
- Purpose: UV blocking contaminants are removed by warming the CCDs.
- Description: The decon itself is implemented via use of 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; continuation of 6-hour decons is anticipated for Cycle 6. 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 kspots (both at gain 15, one short and one long exposure, optimized for PC and WF), and finally, five darks (gain 7, CLOCKS=NO). 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 than 12 hours after the DECON. To protect against residual images in the darks (which results in the irretrievable loss of the critical pre-DECON hotpixel status), the darks will be executed NON-INT and requested to be done at least 30 minutes after any WFPC2 activity.
- Special Requirements: This requires scheduling at four-week intervals. It prevents WFPC2 from being used for several hours, although other instruments can be used most of that time. Dark frames taken before decontaminations need to be protected from possible residual images from overexposed sources.
- 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.
- Products: Those obtained from use of darks, biases and other internals (see Proposals 6904 and 6905).
6904: Darks
- Purpose: Measure dark current on individual pixels and identify hot pixels at frequent intervals.
- Description: Every week, six 1800s exposures are taken with the shutter closed (five with clocks=OFF, and one with clocks=ON). The length of the exposures is chosen to fit within an occultation period. The weekly frequency is required because of the high formation rate of new hot pixels (about 70/CCD/day). Five darks (clocks=OFF) 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, there are some weeks when 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 NON-INT and at least 30 minutes after any WFPC2 activity (see Proposal 6903). Non-decon-week darks do not need to be protected in this fashion.
- Accuracy: Superdarks should be accurate to better than 1 e-/hour and are expected to reach errors of about 0.05 e-/hour (single-pixel rms). Systematic errors due to dark glow (a spatially and temporally variable component of dark signal) and hot pixels may exceed these limits significantly.
- Products: Weekly dark reference files are delivered to CDBS and monthly tables of hot pixels are posted on the Web.
6905: 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 5 Internal Monitor (6250), except that the K-spot images have been removed (these are being taken with the DECON Proposal). The execution frequency during Cycle 6 has also been reduced, from twice a week to once a week, although the total number of biases has been increased to continue to provide an adequate number of frames for pipeline reference file generation.
- Accuracy: Approximately 120 bias frames will be used for each pipeline reference file; accuracy is required to be better than 1.5 e-/pixel, and is expected to be 0.8 e-/pixel.
- Products: Superbiases are delivered every 6-12 months to CDBS. TIPS reports are made on possible buildup of contaminants on the CCD windows (worms) as well as gain ratio stability, based on INTFLATs. ISRs document any changes.
6906: Visflat Monitor
- Purpose: Monitor the stability of the camera and filter responses via the VISFLAT channel.
- Description: Twice a month, internal flat fields (VISFLATs) will be obtained using the visible calibration channel lamp with the photometric filter set plus a couple of narrow-band filters. The images will be used to monitor WFPC2's flat field response as well as to build a high S/N flat field database, which will provide information on the pixel-to-pixel response in the cameras and any possible long-term contamination-induced changes. The LRF (FR533N) exposures, one at each gain, taken after DECON will provide a monitor of the ADC's performance. Histograms generated from the ramp filter flats will be used to trace the ADC transfer curve. ON HOLD: In addition to the monitor observations, an initial filter-sweep is done to obtain VISFLATs in all visible filters. These will be compared to the Cycle 5 filter-sweep data to verify that none of the filters are developing any problems, and to provide a check of the calibration channel's long-term stability.
- Special Requirements: Uses the VISFLAT calibration channel, whose Welch-Allyn bulb is apparently wearing out. (A back-up exists for the Welch-Allyn bulb.) The Cycle 6 proposal has been redesigned to limit the number of ON/OFF cycles placed on this channel to a level believed safe over 10-15 years. The sweep part of the proposal, which puts the heaviest usage on the lamp, is on hold, pending verification of the lamp health from the short monthly executions. The INTFLAT Monitor (Proposal 6907) can obtain similar information if necessary.
- Accuracy: The VISFLAT response is stable to about 0.3%, both in overall level (lamp degradation aside) and in spatial variations. The point of this proposal is to verify this stability on a regular basis and to monitor the lamp degradation.
- Products: ISR and TIPS reports will be prepared.
6907: Intflat Monitor
- Purpose: Provide backup database of INTFLATS in case VISFLAT channel fails.
- Description: This proposal consists of two parts: 1) an INTFLAT filter sweep and, 2) a series of exposure to test the linearity of the camera. 1) The sweep is a complete set of internal flats cycling through both shutter blades and both gains. Signal-to-noise per pixel is estimated to be similar to the VISFLATs (0.6%) 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 calibration channel system failure and temporal variations in the flat fields at the 1% level. In addition, these images will provide a good measurement (better than 1%) of the stability of the gain ratios. 2) The linearity test portion is aimed at obtaining a series of INTFLAT with both gains and both shutters. Since the INTFLATs have significant spatial structure, any non-linearity would appear as a non-uniform ratio of INTFLATs with different exposure times. A set of exposures is also taken with gain 7, shutter B, and CLOCKS=YES.
- Accuracy: The signal-to-noise per pixel is similar to that obtained in the VISFLAT program (0.6%) but there are much larger spatial and wavelength variations in the illumination pattern. As a result, this dataset will not form any part of the pipeline calibration. This baseline is necessary in case the VISFLAT channel fails and there are temporal variations in the camera flat fields at the 1% level.
- Products: TIPS reports and ISRs will be prepared if any significant variations are observed.
6908: UV Flat Field Monitor
- Purpose: Monitor the stability of UV flat field.
- Description: UV flat fields will be obtained with the calibration channel's ultraviolet lamp (UVFLAT) using the UV filter set (F122M, F170W, F160BW, F185W, and F336W). The UV flats will be used to monitor UV flat field stability and the stability of the Woods filter (F160BW) by using F170W as the control. The F336W ratio of VISFLAT (Cycle 6 proposal 6906)/UVFLAT ratio 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 afterimages.
- Special Requirements: This uses the limited life UV lamp. In order to prevent excessive degradation of the lamp, the SU duration for each UVFLAT visit should be kept the same as the durations used during Cycle 5 (proposal 6191); the lamp should not remain on for periods of time longer than those used in Cycle 5. To be executed once before and once after the refurbishment mission, shortly after a decontamination.
- Accuracy: About 2-8% pixel-to-pixel are expected (depending on filter).
- Products: New UV flat fields are made if any changes are detected.
6909: Earth Flats
- Purpose: Monitor flat field stability.
- Description: As in Cycle 5 program 6187, four 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 flat field. The Cycle 4 plan is being largely repeated except: (1) UV filters are dropped because measurement is generally only of the read leak; (2) F160BW is retained in order to check for developing pinholes; and (3) Crossed filters used as neutral densities are eliminated (illumination pattern is wrong). Specific observations for the Methane Quad filters will also be included.
- Accuracy: The single-pixel noise expected in the flat field is 0.3%.
- Products: New flat fields to CDBS if changes detected.
6934: Photometric Zeropoint
- Purpose: Verify synthetic zeropoint of WFPC2 filters.
- Description: Standard stars are observed through all filters longward of F336W (inclusive) with the limit of one orbit per target. Targets include: the spectrophotometric standard GRW+70D5824 in PC and WF3; two stars chosen as standards by other instruments on HST; and two standard star fields, containing 3-4 stars each, commonly used in ground based photometry. Observations of GRW+70D5824 will be directly comparable to the corresponding observations for Cycles 4 and 5 (programs 5572 and 6179) and will verify the stability of the filters as well as improve the accuracy of the calibration. The other standards are observed to provide cross-instrument calibration and in order to increase the range of colors used for photometric verification. This proposal will help all observers who want to do quantitative photometry at the 2% level.
- Special Requirements: Specific orientations will be required for the two fields of standards in order to fit the maximum number of stars. All observations should be executed within a week after decontamination.
- Accuracy: 2% required, 1% expected for our main spectrophotometric standard GRW+70D5824. Expected accuracy for the other standards is between 2% and 5%, depending on spectral type and filter; most of the error derives from limited knowledge of the transformations between ground based and WFPC2 photometric systems.
- Products: TIPS reports, SYNPHOT updates if necessary, and ISRs will be prepared.
6935: Photometric Transformation
- Purpose: (1) Update photometric transformations to Johnson-Cousins system and Strömgren system; (2) Determine spatial dependence of contamination; (3) Check the astrometric solution using M67; (4) Spot check of gain=7 vs. gain=15 ratios; 5) Spot check short vs. long exposure zeropoints.
- Description: Three photometric standard star fields in NGC 5139 (
Cen; metal rich), NGC 2682 (M67; metal poor), and NGC 2100 (young cluster) are observed before and after a decontamination. Four different filter sets are used: (1) The five filters generally used to match the Johnson-Cousins system (F336W, F439W, F555W, F675W, F814W); (2) The wide-band equivalents for the Johnson-Cousins system (F300W, F380W, F450W, F606W, F702W); (3) The Strömgren equivalents (F336W, F410M, F467M, F547M); and (4) Two filters farther toward the UV (F255W, F170W), so that contamination over the full field of view can be measured. F255W is not used for the reddest cluster (NGC 5139). F170W is only used for the bluest cluster (NGC 2100). For the brighter clusters (NGC 2682 and NGC 2100) long and short exposures are taken in the UBVRI equivalents both to extend the dynamic range and to check for differences in photometric zeropoints. A spot check is included to compare gain=7 and gain=15 is also included for NGC 5139 and NGC 2682.
- Special Requirements: The first visit for each target must be taken within 3 days after a decontamination. The second visit, including only the UV filters, must be taken more than 25 days after the first visit, but before the next decontamination.
- Accuracy: The photometric transformations should be accurate to 2-5%. The stability of these transformations will be measured to the 1% level. The astrometry should be good to 0.1" (absolute) and 0.05" (relative).
- Products: ISR and Instrument Handbook. It will also be part of a planned paper on the possibility to do 1% photometry.
6936: UV Throughput and Lyman-
Verification
- Purpose: Verify throughput for all UV filters, including Lyman-
- Description: Spectrophotometric standards are observed shortly before and after a DECON through all the UV filters in each chip and through F160BW crossed with F130LP, F185LP, and F165LP to determine the wavelength dependence of the throughput across the bandpass (for color terms). This proposal is based on the Cycle 5 UV Throughput proposal (6186) but includes also the standard BD+75D325 used in Cycle 4 (proposal 5778) to establish the Lyman-
- Special Requirements: Timing requirements with respect to decontaminations.
- Accuracy: The UV throughput will be measured to better than 3%. Accuracy in Lyman-
- Products: TIPS reports, SYNPHOT updates, and ISRs will be prepared if necessary.
6937: CTE Calibration
- Purpose: (1) Test if the exposure-time dependence of the photometric calibration is due to CTE (long vs. short exposure problem); (2) refine flux and background-level dependent aperture corrections.
- Description: The globular cluster NGC 2419 will be observed through F555W with a combination of exposure times (between 5 and 1400 s) and preflash levels (0 to 500 e-). Analysis of Cycle 5 CTE calibration (proposal 6192) suggests that magnitude errors due to charge transfer effects are greatly reduced (if not entirely eliminated) at background levels of 160 e- or greater. CTE effects have been proposed as the solution to reported differences in the magnitudes of stars measured on frames with short exposures vs. long exposures. If CTE is the cause, then the differences should disappear with preflash. We will re-observe the cluster NGC 2419 with and without preflash to test this hypothesis. This dataset will also provide a large number of stars with which to refine existing measurements of the effects of CTE on the wings of the PSF. A range of preflash levels will be explored at the 60 sec exposure time.
- Special Requirements: Observations should be made at the same position and roll angle as the previous NGC 2419 LONG exposures (proposal GO-5481).
- Accuracy: The reported short vs. long effect is ~0.05 mag. We wish to reduce this to less than 0.01 mag.
- Products: An ISR will be prepared. If appropriate, a special task to correct the CTE effect will be generated.
6938: PSF Characterization
- Purpose: Provide a sub-sampled PSF over the full field to allow PSF fitting photometry, test PSF subtraction as well as dithering techniques (c.f. effects of the OTA breathing and CCD gain).
- Description: Measure PSF over full field in photometric filters in order to update the TIM and TinyTIM models and to allow accurate empirical PSFs to be derived for PSF fitting photometry. 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 4X4 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
- Special Requirements: This needs the same pointing and orientation as Cycle 5 observations for proposal 6193, thus should be scheduled within a similar time frame.
- Accuracy: It 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 phases. The proposal therefore provides, in principle, a high signal-to-noise, critically sampled PSF. This would leave PSF fitting photometrists in a much better position than now, where pixel undersampling clearly limits the results. The result will be largely limited by breathing variations in focus. It is hard to judge 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. Breathing effects will be investigated (one additional orbit) as well as the gain dependence (one additional orbit). PSF fitting results using this calibration would of course be much more accurate. In addition, the test gives a direct measurement of sub-pixel phase effects on photometry, which should be measured to better than 1%.
- Products: A PSF library (CDBS) will be assembled, and an ISR will be issued as needed.
6939: Linear Ramp Filters
- Purpose: Verify throughput calibration for Linear Ramp Filters at selected wavelengths.
- Description: Throughput calibration is obtained by observing the spectrophotometric standard GRW+70D5824 at several filter rotations and wavelengths. This completes the program carried out in Cycle 5, in which some wavelength ranges and rotations could not be covered.
- Accuracy: Throughput accuracy should be verified to better than 3%; 1-2% can be achieved.
- Products: SYNPHOT throughput tables will be updated if necessary.
6940: Polarizers
- Purpose: Verify stability of polarization calibration.
- Description: The goal of this proposal is to check for any changes in the polarization calibration since Cycle 5. Observations are made in F555W+POLQ of both polarized and unpolarized stars, in addition to VISFLATs. Data are taken in all four quads of the polarizer, as well as in three rotated positions of the POLQ.
- Special Requirements: Requires specific orientations.
- Accuracy: 3%.
- Products: Update throughput tables if necessary
6941: Astrometry Verification
- Purpose: Verify accuracy and stability of geometric transformation and relative astrometry solution.
- Description: A very rich star field in
- Accuracy: We expect better than 0.01" and require better than 0.05" (full field of view).
- Products: Improvements will be noted in METRIC and in the aperture reference file if required. An ISR will be issued as needed.
6942: Camera Electronics Verification
- Purpose: Verify several aspects of the WFPC2 camera electronics: linearity, gain ratios, effect of CLOCKS, and effect of CTE on extended sources.
- Description: Observing a very extended non-uniform target represented by the giant elliptical galaxy NGC 4472. The linearity test is carried out by taking exposures of NGC 4472, centered in WFALL, with a variety of exposure times. Since the galaxy is non-uniform, the ratio of these exposures is directly related to the camera linearity. The exposures will be taken with GAIN=7. However, one exposure will also be taken with GAIN=15. Additional exposures will be taken with CLOCKS=YES and with a preflash. These observations complement those of the internal calibration proposal 6907. The two major advantages of these observations compared to the 6907 ones are the possibility of studying the effect of preflash (since the light distribution of the preflash is different from that of NGC 4472) and the possibility of measuring an absolute response curve, since NGC 4472, unlike the INTFLAT lamp, does not have variations in luminosity. NGC 4472 has been chosen as target galaxy because it is large enough to produce significant signal in all chips and bright enough to allow us to explore the highest counts without excessive integration times.
- Accuracy: We expect 0.5% for linearity and CTE, 0.1-0.2% for gain ratios and CLOCKS. We require less than 1% on each item.
- Products: ISRs and TIPS reports will be prepared as needed.
6943: Throughput Verification for Narrow Band Filters
- Purpose: Direct verification of throughput of narrow band filters through observations of emission line objects.
- Description: The current throughput calibration of narrow-band filters is based on filter profiles from data obtained before launch and on observations of continuum sources. This program will verify the accuracy of the calibration, and indirectly the stability of the filters, by observing eight planetary nebulae with strong lines and well-established ground based spectra. The observations can be executed in SNAPSHOT mode since they will be short and none is specifically required. Some planetary nebulae with existing Cycle 4 and 5 observations will be included for stability verification.
- Accuracy: We expect 2% and require 3%.
- Products: SYNPHOT tables will be updated and an ISR issued if required.
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