STScI

Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14

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Chapter 8:
Calibration and
Data Reduction

8.1 Calibration Observations and Reference Data
8.2 Flat Fields
8.3 Dark Frames
8.4 Bias Frames
8.5 Data Products and Data Reduction
8.6 Pipeline Processing
8.7 On-The-Fly Reprocessing Systems
8.8 Fluxes and Standard Magnitudes
8.9 Color Transformations of Primary Filters
8.10 Calibration Plan Summary
8.11 Cycle 4 Calibration Plan
    8.11.1 Internal Monitors
    8.11.2 Photometric Monitors
    8.11.3 Earth Flats
8.12 Cycle 5 Calibration Plan
8.13 Cycle 6 Calibration Plan
8.14 Cycle 7 Calibration Plan
    8.14.1 Overview
8.15 Cycle 8 Calibration Plan
    8.15.1 Introduction
    8.15.2 Overview
8.16 Cycle 9 Calibration Plan
8.17 Cycle 10 Calibration Plan
8.18 Cycle 11 Calibration Plan
8.19 Cycle 12 Calibration Plan
8.20 Cycle 13 Calibration Plan
8.21 Future Calibrations, Calibration by Observers, and Calibration Outsourcing
8.22 Calibration Accuracy

8.1 Calibration Observations and Reference Data

Standard calibration observations are obtained and maintained in the HST archive at the STScI, and can be retrieved by external users using StarView. This includes those flat field, dark, and bias reference files needed to operate the Post Observation Data Processing System (PODPS; now called OPUS, and usually just called the "pipeline"), photometric calibration derived from standard star observations and the measured filter profiles, and derived determinations of the plate scale, distortion, and so on. The first set of these calibrations was provided to the STScI by the WFPC2 IDT from the Servicing Mission Observatory Verification (SMOV) and System Level Thermal Vacuum (SLTV) testing periods, and has been maintained and updated thereafter by the STScI with assistance from the IDT as part of the long term calibration program. For measurements requiring more precise calibrations, special calibration observations may need to be obtained as part of the observing proposal. Please consult the STScI WFPC2 Contact Scientists for guidance if the routine calibration appears unlikely to support the requirements of a proposed observation, or email help@stsci.edu. Individual GO programs requiring special calibrations must directly request these observations as part of their Phase I proposal.

A database of laboratory characterizations of optical components, CCD sensors, filters, and the flat field channel has been collected to support the instrument calibration. On-orbit pointed calibrations require large HST resources, taking time that could otherwise be used for direct scientific observations. They can also be unsatisfactory due to the limitations of the available astronomical reference sources. For WFPC2, the inherent stability and uniformity of the CCD sensors, the well-calibrated filters, the internal flat field calibration system, and an archive populated with flat field images obtained in SLTV prior to launch improve the scientific data analysis and productivity. Hence the need for on-orbit calibrations has been minimized.

8.2 Flat Fields

The process of correcting for the effect of the variation in the sensitivity of the WFPC2 with field position is known as flat-fielding, or flattening. For ground based observations, usually a "flat field" (an exposure of a spatially uniform source) is observed through the telescope with the desired filter. Unfortunately, there is no uniformly illuminated target available on-orbit. Instead, several assets are available to estimate the flat field and monitor any changes -- these include pre-launch SLTV optical stimulus flats, Earth flats, calibration channel flats (VISFLATS), and internal flats (INTFLATS).

During SLTV (System Level Thermal Vacuum) ground tests of WFPC2, flat fields were obtained using both the calibration channel and the WFPC2 optical stimulus (HST simulator). The later provided a close approximation to a uniform target as viewed through HST, and are a prime ingredient for the final calibration flats.

The Earth is an imperfect flat field target because it is too bright for the WFPC2 in the broad-band green and red filters. In addition, the rapid motion of the HST creates streaks across the flat field images, though the streaks can be removed by combining multiple Earth observations with the streaks at different angles on the CCDs. An extensive discussion of the generation of Earth flat fields is available in Chapter 6 of the WF/PC-1 IDT OV/SV Report, as well as in the History records of the flat field reference files themselves.

While imperfect, Earth flats are an important part of the flat field calibration; they provide corrections to the SLTV flats for any differences between the SLTV optical stimulus illumination, and the OTA illumination pattern. Flat fields in narrow bandpass filters are obtained using the sunlit Earth (Target = EARTH-CALIB) as part of routine calibration. These are used primarily to remove the low spatial frequency effects in the calibration flats.

Flat field calibration files have been generated for all filters by combining information from the SLTV test flats (which are good for all but the lowest spatial frequencies), and on-orbit Earth flats obtained for a small subset of narrow band filters (F375N, F502N and F953N). These Earth flats are used to correct low spatial frequency errors in the ground based SLTV flats, which result from imperfect simulation of the HST OTA illumination pattern. These Earth flats taken regularly during available occultation time periods (i.e., no impact to science observations).

There are also two types of on-board flats available in WFPC2, which can be used to monitor changes in the flat field. The calibration channel (VISFLAT system) produces a reasonably flat illumination pattern down to about 1800Å. It works by imaging an illuminated diffuser plate onto the WFPC2 exit pupil (relay secondary) by means of an MgF2 lens. Two lamps provide optical and FUV illumination, yielding a flat field which resembles the input beam from the OTA between 1600Å and 10000Å. The system is mounted outside, but adjacent to, WFPC2, and light is directed into WFPC2 via a mechanically actuated flip mirror. A second system is much cruder, but provides a measure of redundancy: the internal flat system (INTFLAT system) consists of small lamps which, when commanded on, illuminate the back side of the shutter blade. The INTFLAT illumination pattern is not very uniform, but provides a robust backup capability.

The calibration channel data (VISFLATS) are used to monitor time dependent changes in the flat fields; only small changes have been seen to date in the visible filters. INTFLATS are also taken on a routine basis, and provide a redundant monitor capability. As of this writing (June 2001), both types on internal flats have been used only as monitors, with no corrections being made to the actual calibration files.

A major update of the flat field reference files for all standard filters redward of 300nm (F300W) was completed, using on-orbit data from Earthflat exposures covering the period from September 1995 to May 2001 (Koekemoer, Biretta & Mack 2002). The flat fields have been divided into epochs depending on the appearance of new dust spots, as well as long-term changes in existing features. The new correction flats are accurate for pixel-to-pixel variations down to an intrinsic level of approximately 0.3% for the PC and 0.1% for the WF chips, and they result in an improved rms noise of many of the flats by a factor of two or more. Since early 2002, the new flat fields are automatically applied to any WFPC2 data in relevant filters when the data are retrieved from the archive.

Note that the flat fields presently used in the pipeline are based on gain 14 data. The gain ratios are not constant from chip to chip, and therefore a small correction to photometric results derived from gain 7 data should be applied (see Table 4.4). (See Biretta 1995 for further discussion of WFPC2 flat fields; also see the HST Data Handbook.)

8.3 Dark Frames

Dark frames are long exposures that are taken with no light incident on the CCDs. They are used to detect CCD counts (the dark current) caused by thermal processes at the interfaces between the silicon and oxide layers, as well as charged particle and secondary radiation events. Estimated dark current and cosmic ray event rates are given in Dark Backgrounds and Cosmic Rays, respectively. Observers are cautioned that the calibration provided by the pipeline may not use the most up-to-date dark frames until several weeks after the observation is taken. The time delay is the time it takes for coeval dark frames to be taken, archived, and processed into dark reference files, and delivered for use in the pipeline and OTFC. Use of optimal darks can be important due to the new hot pixels continually being generated. Each week of observations typically has one applicable (optimum) dark reference file.

8.4 Bias Frames

The WFPC2 bias correction is performed in the pipeline in two steps: a pedestal level is removed and a bias image subtracted. The pedestal level is determined from the overscan columns in each science image; the specific values subtracted are documented in the bias-even / bias-odd science image header keywords. However, the value of the pedestal can also vary with position across the chip. Therefore, after the pedestal correction is performed, the pipeline removes any position-dependent bias pattern by subtracting a bias reference file. This reference file is typically generated from a stack of 120 bias frames (CCD readouts without an exposure); new bias reference files are usually installed in the pipeline about once a year.

8.5 Data Products and Data Reduction

The routine processing of WFPC2 science data consists of the pipeline functions described below. The resulting images will be available in FITS format on magnetic tape or via FTP transfer, and as grey scale images in PDF format. The reformatted raw data will also be available, along with the relevant calibration data. The HST Data Handbook or STSDAS Calibration Guide should be consulted for a more complete description than the summary presented here.

The following data are supplied to observers on FITS tapes:

In addition, a histogram file used for monitoring of the signal chain (.c2h file), and a calibration table containing the throughput curve (.c3t file) used in populating the photometric keywords are included.

Further data reduction and analysis can be performed under the STScI's science data analysis software system (STSDAS). Standard routines are available, operating under IRAF, for the analysis of data for image photometry, spectral analysis, astrometry, and the generation of the calibration data files.

8.6 Pipeline Processing

The pipeline processing of WFPC2 data sets reformats the telemetry received from HST into group FITS format images, generates headers containing a large number of keywords taken from both the HST and WFPC2 telemetry streams, in addition to various STScI ground system databases, and applies the corrections described below. This calibration is done with a software module known as "CALWP2" which is written in the IRAF SPP language and is available, in identical form, to users of the STSDAS system. Therefore, off-line recalibration of observations is fairly easy, and will use the same program as the OPUS system. Documentation is available in the HST Data Handbook, and the STSDAS User's Guide.

CALWP2 performs the following operations if required by the observation:

In addition, the following conditions are flagged in the Data Quality File (DQF):

8.7 On-The-Fly Reprocessing Systems

The On-The-Fly Calibration (OTFC) system, publicly released in Dec. 1999, calibrated data at the time a user requested data from the archive. The advantages to using OTFC included the automatic application of improved calibration files and switches, use of most recent calibration software (allowing for rapid access to improved algorithms, new capabilities, and software fixes), and correction of header keywords if needed. An additional benefit is that only the uncalibrated data needs to be stored in the archive.

The On-The-Fly Reprocessing (OTFR) system replaced OTFC on May 16, 2001. The change is transparent to most HST archive users. Requests for data are submitted as usual via StarView or WWW; raw and freshly-calibrated data will be delivered. There is no need to explicitly ask for OTFR: all requests for WFPC2 data are handled by the OTFR system.

The primary difference between the two systems is that OTFR begins earlier in the data path. It uses the original telemetry files ("POD" files) received from Goddard Space Flight Center and performs all pipeline processing steps; OTFC performed only the last pipeline processing step (calibration), on raw files retrieved from the archive. An overview of the data flow for both systems is summarized in the table below. The benefits of the OTFR system encompass the benefits in the OTFC system; in addition, OTFR data needs fewer header corrections (most problems are fixed as part of the pre-calibration pipeline processing) and the system as a whole requires significantly less maintenance effort than OTFC.

UCHCOORD and OTFR.

Improved knowledge of the detector plate scales and chip rotations, as well as changes in reference pixel locations, have resulted in periodic changes to the pointing parameters, especially early in the instrument's lifetime. These header parameters, which define the mapping between the pixel and world coordinate systems, can be updated using the STSDAS task UCHCOORD. The keywords affected include the reference pixel locations (crpix*), the values of the world coordinate system at the reference location (crval*), the partial derivatives of the world coordinate system with respect to the pixel coordinates (cd*), and the orientation of the chip (orientat).

Prior to OTFR (released to the public on May 16, 2001), observers requiring the most up-to-date pointing information in their science image headers ran UCHCOORD on their calibrated images. Since the implementation of OTFR in May 2001, all WFPC2 data retrieved from the archive, regardless of its observation date, has had these corrections applied automatically before being delivered, thus we have discouraged running the UCHCOORD task on OTFR data since it is no longer needed (as described in WFPC2 STAN 45, March 2001 by Baggett, Hsu & Gonzaga). In fact, running UCHCOORD (versions prior to September 2003) on OTFR data would apply unnecessary corrections and corrupt the astrometry (for example, Section 4.3.3 in WFPC2 Data Handbook, Version 4.0, 2002, S. Baggett et al.).

The new version of UCHCOORD in the September 2003 STSDAS release will correctly check whether or not the images have been processed through OTFR, and will no longer modify the header astrometric keywords in such cases. We remind users that it is no longer necessary to run the UCHCOORD task on any WFPC2 data that is retrieved via OTFR, and we recommend that any old WFPC2 data should rather be re-retrieved via OTFR since many other calibrations are also improved.


Table 8.1: Comparison of Dataflow in On-The-Fly Systems.
OTFC OTFR
Request for data is submitted to the archive via StarView or WWW interface; archive responds with acknowledgement email. Same as OTFC.
Raw files are retrieved from the HST archive and passed to the OTFC system. For WFPC2, the raw files include d0, q0, q1, x0, and trl files. POD file (original telemetry file) is retrieved from HST archive and passed to OTFR system. For WFPC2, there is typically 1 POD file for each image. Pre-calibration OPUS processing is performed: data partitioning, data editing, and generic conversion; these steps generate the raw files (d0, q0, q1, x0, and trl files).
Any problems in the header keywords are fixed by special lookup table. Same as OTFC although the OPUS pre-calibration processing will have fixed the majority of keyword problems automatically (i.e., significantly fewer header corrections required in OTFR).
The best calibration files & switches are determined by separate standalone task, and header keywords updated accordingly. Not needed. The best calibration files & switches are set by the pre-calibration OPUS code (generic conversion).
Images are calibrated by STSDAS calxxx module and sent back to the archive system. Same as OTFC.
Archive delivers raw + calibrated data and emails completion notification to the requestor. Same as OTFC.

8.8 Fluxes and Standard Magnitudes

The pipeline calibrated data are not flux calibrated and the data are in units of Data Numbers (DN). However, a flux calibration is supplied in the header keywords. To obtain the flux density, multiply DN by the value of the keyword PHOTFLAM in the calibrated (.c0h) science header file, and divide by the value of the keyword EXPTIME.

The magnitude of an object can be determined using the photometric zero-point keyword PHOTZPT as:

where m is in the STMAG system which is based on a spectrum with constant flux per unit wavelength set to roughly match the Johnson system at V. The more conventional systems are based on Vega's spectrum. Table 8.2 was generated using SYNPHOT to provide rough conversions from STMAG to the Johnson UBVRI and Cousins RI systems. Typical uncertainties are 5%, and probably much worse for the U filter. The correction depends on the spectrum of the object, hence the table was generated using a wide range of Bruzual models.

For example, to convert to the Cousins I band for an object on WF4, get PHOTZPT=-21.1 and PHOTFLAM=2.6044 x 10-18 from the header. Then convert from WFPC2 counts to magnitudes in Cousins I using:

Note that the Cousins I filter is much closer to the F814W filter than Johnson I, as shown by the nearly constant correction as a function of spectral type (i.e. color term).


Table 8.2: Conversion from STMAG to Johnson UBVRI and Cousins RI.
 
U-F336W
B-F439W
V-F555W
RJ-F675W
IJ-F814W
RC-F675W
IC-F814W
O5V
0.53
0.67
0.05
-0.67
-1.11
-0.71
-1.22
B0V
0.46
0.66
0.05
-0.67
-1.13
-0.70
-1.22
A0V
-0.08
0.67
0.02
-0.68
-1.22
-0.67
-1.21
F2V
-0.03
0.62
-0.00
-0.69
-1.28
-0.63
-1.22
G0V
-0.02
0.58
-0.01
-0.70
-1.31
-0.60
-1.23
K0V
-0.10
0.53
-0.01
-0.69
-1.32
-0.58
-1.23
M0V
-0.04
0.43
-0.00
-0.78
-1.48
-0.54
-1.22
M6V
0.05
0.29
-0.03
-1.05
-1.67
-0.56
-1.21

This procedure will provide typical accuracies of about 0.05 mag, worse in the UV. More accurate photometry will require a variety of corrections (e.g., CTE effect, contamination and red leaks for the UV filters, variable gains on different chips, color terms, geometric distortions) which are discussed in detail in Holtzman et al. (P.A.S.P., 1995b) and in the HST Data Handbook.

8.9 Color Transformations of Primary Filters

The WFPC2 UBVRI system is fairly close as regards effective wavelengths to the Johnson UBVRI system, but cross-calibration is necessary to convert to Johnson magnitudes. See the IDT OV/SV Report and Harris, et al., A.J. 101, 677 (1991) for examples in the case of WF/PC-1. Figure 8.1 through Figure 8.5 show the results of regression fits between these two systems on the main sequence stars in the Bruzual, Persson, Gunn, Stryker atlas that is installed in the calibration database system (CDBS). This atlas, and others, are available via the WWW from the WFPC2 Documentation page, or directly via 

 http://www.stsci.edu/instruments/observatory/cdbs/
astronomical_catalogs.html
Figure 8.1: F336W-F439W against Johnson U-B for the BPGS atlas of MS dwarf spectra. The change in slope in the transformation for U-B greater than about 0.1 is due to red leak in the F336W filter. For hotter stars, the transformation is quite linear. 

 
 
Figure 8.2: F439W-F555W against Johnson B-V. The residuals from the best linear fit are quite similar to those that apply if F569W (instead of the preferred F555W) is chosen for a WFPC2 passband. 

 
 
Figure 8.3: F555W-F814W against Johnson V-IC. The residuals from the best linear fit are generally very small. This particular color combination is widely used. 

 
 
Figure 8.4: F555W-F675W against Johnson V-RC. The residuals from the best linear fit are somewhat larger for blue stars than those that apply if F569W is chosen. 


 

These fits should be used with caution for quantitative work. The zero-points in all cases are defined so that Vega's spectrum integrated over the bandpass is exactly magnitude zero (VEGAMAG in XCAL). The zero-points of the canonical Johnson-Cousins system differ from this by up to 0.02 magnitudes. The zero-points thus defined for the HST filters do not coincide with the STMAG definition used in the previous section. In addition, the ground based filter curves used, which are taken from Bessel (P.A.S.P. 102, 1181), give a good approximation to the standard Johnson-Cousins system, but are not as accurate as taking Landolt's curves and applying his color corrections to transform to the standard system. The latter procedure was used to derive the transformations given in Holtzman et al. (P.A.S.P., 1995b), which also discusses the changes in the transformations that result from source spectrum variations (such as metallicity and gravity effects).

Figure 8.5: F675W-F814W against Cousins RC-IC. The residuals from the best linear fit are similar to those that apply if F791W is chosen for a WFPC2 I passband. For spectral type M8V and later (not shown) the transformation will not work as well.


 

8.10 Calibration Plan Summary

Table 8.3 summarizes the nominal proposal cycle boundaries. The dates are intended as a rough guideline only, since in reality, of course, there are no sharp cycle boundaries. Some GO proposals are identified as candidates for early execution while other proposals take longer to complete due to various scheduling constraints. Additional observatory restrictions factor in as well, for example, the acceleration of the NICMOS observations in Cycle 7 (due to the limited lifetime of the NICMOS cryogen) caused many WFPC2 and other programs to be delayed. Cycle 7, initially set to span July 1997 to July 1998 was initially extended to October 1998 (due to Servicing Mission requirements), then later extended to July 1999, due to acceleration of NICMOS programs and the additional NICMOS programs solicited during the "Cycle 7N - NICMOS only" call for proposals.


Table 8.3: Nominal Proposal Cycle Boundaries.
cycle
start date
end date
4
May 28, 1994
July 1, 1995
5
July 1, 1995
July 1, 1996
6
July 1, 1996
July 1, 1997
7
July 1, 1997
July 1, 1999
8
July 1, 1999
July 1, 2000
9
July 1, 2000
July 1, 2001
10
July 1, 2001
July 1, 2002
11
July 1, 2002
October 1, 2003
12
October 1, 2003
October 1, 2004
13
October 1, 2004
October 1, 2005

8.11 Cycle 4 Calibration Plan

The primary goal of the Cycle 4 calibration plan was to provide an instrument calibration for Cycle 4 GO and GTO science programs as well as monitor internal health, photometric and optical stability of the instrument. This report briefly summarizes the WFPC2 calibration proposals as they were implemented during Cycle 4. Table 8.4 outlines the proposal contents as well as the approximate frequency of execution while the following section presents more details of the justification and intent for each proposal.


Table 8.4: Summary of Cycle 4 Calibration Plans.
ID
Title
Frequency
notes
Internals
5568 Decontaminations 1 x month internals plus decon itself
5560 Internal Monitor 1. 2 x week intflats, biases, kspots
5561 5655 Internal Monitor 2. - Flats ~2 x month darks, visflats. UVflats suspended after June 12; one set of uvflats run as a test (Oct. 1994)
5562 Internal Monitor 3. - Darks 5 per week serials=ON taken in 5561/5655
5569 5764 Internal Flats once
intflats & visflats, in variety of filters
6140 Ramps: Internals & Earthcals once internals to aid in LRF calibration
Photometry
5563 Photometric Monitor - UV Std. 2 x month GRW+70D5824, std.filters. Clocks OFF except Dec. 1994, 1994 (ON).
5565 5663 Photometric Monitor - Fields 2 x month Usually NGC5139. Clocks OFF except Dec. 1994 (ON).
5629 Photometric Monitor - Four Chip Std. 1 x week GRW+70D5824; 4 CCDs, F170W
5572 Photometric Filter Calibration
run twice during Cycle 4 GRW+70D5824 in all filters used by GO/GTOs during Cycle 4; NGC5139 done in subset of broadbands.
5646 CTE Dither Test run once Omega Cen, nine 40 sec images in F555W, offset in 15" steps
5659 CTE Dither Test - Part 2
run once same as 5646 but in F439W, F814W
5564 Photometric Calibration: 4-CCD to be run once GRW+70D5824, in 4 CCDs, standard broadband filters
Earth Flats
5570 Earth flats - Large Set ~ 40 per week 200 Earthflats in each of four filters: F375N, F502N, F656N and F953N
5571 Earth flats - Small Set ~14-20 per week 20 Earthflats in each of ~20 filters
Other
5643 Partially Rotated Filters done once GRW+70D5824 & visflats, FQUVN, FQCH4N and partial rotations
5632 Preflash Test done twice Omega Cen in broadband filters; includes preflash done with intflat; replaced 5565 during Apr. 1994
5778 Lyman Alpha Throughput Check do twice target: BD+75D325
5573 Ramp Calibration

5574 Polarization Calibration done once G191B2B, BD+64D106, RMon, NGC 4147, and visflats
Placeholders/Contingency (no plans to execute, may not be run at all)
5627 Photometric Monitor 1a: Std. in UV
Not needed.
5628 Photometric Monitor 1b: Std. in Vis.
Not needed.
5645 Optimum Preflash Test
Not needed.
5648 Charge Transfer Test
Withdrawn.
5566 5567 UV Campaign Photometry UV Campaign Flats
Not needed; no UV campaigns run
5575 Dither Test
Not needed.

8.11.1 Internal Monitors

These proposals monitor the instrument's health throughout the cycle, including the stability of the cameras, their signal chain electronics, and the internal optical alignment of the WFPC2. The internal flat fields are intended as delta-flats, in order to monitor the stability for the main photometric filter set as well as provide data for the generation of high S/N flat fields. The darks are required not only for the generation of darks for the pipeline but also for tracking the evolution of hot pixels and mapping low-level CCD defects such as traps.

The internal visflats obtained in most of the visible WFPC2 filters will be used with the thermal vacuum test flat fields and the Earth flats to generate "superflats"; the intflats are obtained as a backup for the visflats (for example, if the cal-channel should fail).

8.11.2 Photometric Monitors

These proposals provide a regular monitor of the instrument's QE stability from the FUV to the near-IR, allow tracking of the UV throughput decline due to contaminant buildup, and provide observational PSFs. The standard star observations provide a baseline for the standard field photometry and allow updates to SYNPHOT. The standard field measurements will allow calibration of the filter sets' color terms, as well as enabling a mapping of the geometric distortions across the field of view. The four CCD photometry program provides the relative CCD-to-CCD sensitivities for the filter set and allows the regular PC1/WF3 standard star photometry to be applied to WF2 and WF4.

8.11.3 Earth Flats

These are divided into two proposals: the first to obtain a large number of Earth cals (observations of the bright Earth) in just four filters and the second to obtain a small set of Earth cals in a larger number of (primarily narrowband) filters. The superflats generated from the first proposal's data map the OTA illumination pattern and are combined with the thermal vacuum test data flats (and possibly cal-channel flat fields) to provide a set of flat fields which remove both the OTA illumination pattern and the pixel-to-pixel response of the cameras. The images from the second proposal will also help determine the OTA illumination pattern, but used in conjunction with the Earth superflats (and cal-channel flats), will provide delta corrections to the superflats applicable to the narrow- and medium- band filters.

8.12 Cycle 5 Calibration Plan

A summary of the Cycle 5 calibration plan follows as a general guide to the calibration and monitoring program in place for WFPC2. The full proposals are available through STScI's proposal status web page. 

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

The data that the calibration and monitoring program produced have no proprietary period and are immediately available through the HST archive.

Calibration information obtained by the start of Cycle 5 consisted primarily of the System Level Thermal Vacuum (SLTV) tests, the initial on-orbit tests conducted in SMOV, and the Cycle 4 calibration. These tests have shown that the instrument is stable with some important exceptions and have provided an initial calibration sufficient for routine processing of most data.

The Cycle 5 calibration was designed to enable users to maximize the scientific usefulness of their data, while at the same time minimizing the use of spacecraft time. This was done by designing efficient proposals that:

A. Improved the existing calibration - in particular towards the goal of 1% absolute photometric accuracy.
 B. Assessed the accuracy of the existing and new calibrations.
 C. Recalibrated important known time variable features of the instrument.
 D. Calibrated some important instrumental effects that are not well understood.
 E. Monitored the instrument and telescope to ensure that no new problems or variability in their performance are missed.
 F. Maintained the instrument in a healthy state and ensured that in the event of partial instrumental failures, the calibration can be maintained when possible.

The calibration of the instrument is seen in a larger context than simply preparing reference files for a pipeline reduction and assessing the errors in them. Several calibrations (such as geometric distortion, CTE correction, PSF calibration, chip-to-chip alignments, polarization calibration) are very important to some observers, yet are not included in the pipeline. Other corrections frequently need to be done to the data after it is ADC, bias, dark, and flat field corrected, with a photometric calibration included in the header. These other calibrations are made available to users through this Instrument Handbook, journal publications, instrument science reports, and postings linked to the Institute's WFPC2 WWW home page. The address is: 

 http://www.stsci.edu/instruments/wfpc2/wfpc2_top.html

A list of the most important calibrations consists of the following items:

  1. Photometric zero-point: converting count rates to flux units.
  2. Photometric transformations: converting DN values to magnitudes in standard systems. Two separate photometric calibrations can be used for this, a direct approach and a synthetic approach.
  3. Photometric temporal variations: particularly important in the UV where significant variability is seen.
  4. Photometric spatial variation: flat fields and charge transfer efficiencies.
  5. Dark current: including its time variability and hot pixels.
  6. Bias.
  7. Analog-to-Digital converter errors.
  8. PSF: crucial for PSF fitting photometry, PSF subtraction, PSF modeling, and deconvolution efforts. Because PSF subtraction of very saturated sources is specialized to a few very diverse programs, PSF calibration in the image halo (beyond about 0.5 arcsecond) is not supported and must be requested with the program as a special calibration.
  9. Polarization and Linear Ramp Filter calibrations.
  10. Geometric calibration.

The Cycle 5 program consisted of 15 proposals which used a total of 63 orbits of spacecraft time (compared to a total of about 1550 orbits of approved Cycle 5 GO time). The proposal summaries and their associated Phase II files largely speak for themselves. Table 8.5 lists all of the proposal numbers, titles, the schedule for the calibration execution, an indication of whether the output forms part of the pipeline data reduction (CDBS) or provides other information, usually documented in Instrument Science Reports (ISRs), the approximate calibration accuracy expected (see the summary forms for the interpretation of these numbers), the primary areas from the above 10 calibration types they address and in what ways (A-F from the above list). Following the table there are more details on each proposal individually, including purpose, observing description, accuracy, and data products. For a report on the final results from these programs, please see ISR WFPC2 97-02 ("Cycle 5 Closure Report") to be found at: 

  http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr9702.html.



   Table 8.5:   Summary of Cycle 5 Calibration Plans.
  
    
 

ID



    
 

Proposal Title



    
 

Schedule



    
 

Results



    
 

Accuracy



    
 

External Time (orbits)



    
 

Notes1





  
     6179
     Photomet. Zero.
     Late 95
    
 CDBS

    
 1%

    
 8

     1ABE, 2AB


  
     6182
     Photomet. Trans.
     9/95, 3/96
    
 CDBS

    
 2%

    
 6

     2ABE


  
     6183
     Decontamination
     1x per 4 wks.
    
 ISR

    
 N/A

    
 0

     F


  
     6184
     Photometric Mon. 
     2x per 4 weeks
    
 ISR

    
 1%

    
 24

     3E


  
     6186
     UV Throughput
     Early in Cyc. 5
    
 CDBS

    
 10%

    
 6

     1AB, 3C


  
     6187
     Earth Flats
     Continuous
    
 CDBS

    
 1%

    
 0

     4ABE


  
     6188
     Darks
     Weekly
    
 CDBS

    
 6%

    
 0

     5ABC


  
     6189
     Visflat Monitor
     2x per 4 weeks
    
 ISR

    
 0.6%

    
 0

     4E


  
     6190
     Internal Flats
     Early Cyc. 5
    
 CDBS

    
 0.6%

    
 0

     4F,7E


  
     6191
     UV flats
     2x in Cyc. 5
    
 ISR

    
 2%

    
 0

     4ABE


  
     6192
     CTE Calibration
     Early Cyc. 5
    
 TIPS

    
 <1%

    
 4

     4ABD


  
     6193
     PSF
     CTE+2m
    
 TIM

    
 10%

    
 5

     8ABD


  
     6194
     Polarizers+Ramps
     TBD
    
 CDBS

    
 3%+2%

    
 8

     9DE, 1AB


  
     6195
     Flat field Check
     Late 95
    
 CDBS

    
 1%

    
 2

     4B


  
     6250
     Internal Monitor
     2x per week
    
 ISR

    
 N/A

    
 0

     5,6,10F


  
     
     TOTALS
     
    
    
    
 63

     







  

     1Letters and numbers are keyed to lists in text.

6179: Photometric Zero-point

6182: Photometric Transformation

6183: Decontamination

6184: Photometric Monitor

6186: UV Throughput

6187: Earth Flats

6188: Darks

6189: VISFLAT Monitor

6190: Internal Flats

6191: UV Flats

6192: CTE Calibration

6193: PSF Characterization

6194: Polarization and Ramps

6195: Flat field Check

6250: Internal Monitor

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


Table 8.6: Summary of Cycle 6 Calibration Plan.
ID
Proposal Title
Schedule
Results
Accuracy
External Time (orbits)
Notes
Routine Monitoring Programs
6902 Photometric Monitor
2x per 4 weeks
SYNPHOT
2%
26
6903 Decontamination
1x per 4 weeks
CDBS
n/a
0
 (darks, internals)
6904 Darks
Weekly
CDBS
1 e/hr
0
 WWW hot pixel lists
6905 Internal Monitor
Weekly
CDBS
0.8 e
0
6906 Visflat Monitor
2x per 4 weeks
ISR
0.3%
0
 (monitor lamp health)
6907 Intflat Monitor
1x per 4 weeks
ISR
0.3%
0
6908 UV Flat Field Monitor
2x in Cyc. 6
ISR
2-8%
0
6909 Earth Flats
Continuous
CDBS
0.3%
0
Special Calibration Programs
6934 Photometric Zeropoint
1x in Cyc. 6
SYNPHOT
2%
6
 Add 2 new standards
6935 Photometric Trans.
2x in Cyc. 6
ISR
2-5%
9
 Three targets
6936 UV Throughput & Ly
2x in Cyc. 6
SYNPHOT
3-10%
12
 Include BD+75D325
6937 CTE Calibration
1x in Cyc. 6
ISR
1%
2
6938 PSF Characterization
1x in Cyc. 6
CDBS
10%
7
6939 Linear Ramp Filters
1x in Cyc. 6
CDBS
3%
4
6940 Polarizers
1x in Cyc. 6
CDBS
3%
4
6941 Astrometry Verification
1x in Cyc. 6
STSDAS
0.01"
4
6942 Camera Elect. Verification
1x in Cyc. 6
ISR
0.5%
1
6943 Narrow Band Throughput
SNAP
SYNPHOT
3%
8
 SNAP

TOTALS
83

6902: Photometric Monitor

6903: Decontamination

6904: Darks

6905: Internal Monitor

6906: Visflat Monitor

6907: Intflat Monitor

6908: UV Flat Field Monitor

6909: Earth Flats

6934: Photometric Zeropoint

6935: Photometric Transformation

6936: UV Throughput and Lyman- Verification

6937: CTE Calibration

6938: PSF Characterization

6939: Linear Ramp Filters

6940: Polarizers

6941: Astrometry Verification

6942: Camera Electronics Verification

6943: Throughput Verification for Narrow Band Filters

8.14 Cycle 7 Calibration Plan

8.14.1 Overview

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

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

7619: WFPC2 Cycle 7: Decontamination

7620: WFPC2 Cycle 7: Standard Darks

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

7622: WFPC2 Cycle 7: Internal Monitor

7623: WFPC2 Cycle 7: Internal Flats

7624: WFPC2 Cycle 7: UV Flat field Monitor

7625: WFPC2 Cycle 7: Earth Flats

Proposal ID 7626: WFPC2 Cycle 7: UV Throughput

7627: WFPC2 Cycle 7: Astrometric Monitor

7628: WFPC2 Cycle 7: Photometric Characterization

7629: WFPC2 Cycle 7: PSF Characterization

7630: WFPC2 Cycle 7: CTE Calibration

7929: WFPC2 Cycle 7: CTE Monitor

8053: WFPC2 Cycle 7: Supplemental Earth Flats

8054: WFPC2 Cycle 7: LRF Calibration

8.15 Cycle 8 Calibration Plan

8.15.1 Introduction

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).

8.15.2 Overview

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
Accuracy
Required
Notes
"External"
"Internal"
Routine Monitoring Programs
8441 8459 WFPC2 Decons & Associated Observations 1-2/4 weeks
32
72
every 28 days
Synphot, CDBS
1-2%
 Includes decons, photometric monitor, focus monitor, internals, UV throughput.
8442 Standard Darks weekly
324
every 7 days
CDBS
1 e/hr
 Also hot pixel lists on WWW.
8443 Supplemental Darks (8460, 8461) 0-3/day
1308
anytime
n/a
 For archive only, no analysis provided.
8444 Internal Monitor 3/4 weeks
45
every 7days
CDBS
0.8e/pixel
 New superbiases, not run on decon weeks.
8445 Earth Flats continuous
442
mid-cycle
CDBS
0.3%
 Also LRF, Methane quads.
8446 Astrometric Monitor 2/cycle
2
early & late
ISR
0.05''
 Also K-spots & plate scale check in red.
8447 CTE Monitor 2/cycle
4
mid- & late
ISR
0.01 mag
8448 Intflat and Visflat Sweeps 1/cycle
43
mid-cycle
TIR
0.3%
 Mostly intflats.
8449 UV Flats Internal Monitor 1/cycle
2
mid-cycle
TIR
2-8%
 Uses UV cal channel lamp.
Special Calibration Programs
8451 Photometric Characterization 1
2
mid-cycle
ISR
2.5%
 Subset of Cycle 7 proposal, as check.
8452 PSF Characterization 1
2
late in cycle
CDBS
10%
 Subset of standard broadband filters.
8453 Polarization 1
6
10
early in cycle
CDBS
3-5%
 Subset of Cycle 6, as check.
8454 Linear Ramp Filters 1
4
late in cycle
CDBS
3%
 Placeholder, pending results from Cycle 7.
8450 Noiseless Preflash 1
5
early in cycle
TIR
0.01 mag
 Test scheme to reduce CTE problem.
8455 Photometry of Very Red Stars 1
2
mid-cycle
ISR
2-5%
 Outsourcing candidate.
8456 CTE for Extended Sources (2-3") 1
4
mid-cycle
ISR
0.01 mag
 Outsourcing candidate.
8457 UV Earth Flats continuous
720
early in cycle
CDBS
10%
 Outsourcing candidate.
8458 Plate Scale Verification 1
1
mid-cycle
ISR
0.05%
 Outsourcing candidate.

~10% reserve for unexpected items
7
 Placeholder.
TOTAL TIME (including all executions)
73
2964

8441, 8459: WFPC2 Cycle 8: Decontaminations and Associated Observations

8442: WFPC2 Cycle 8: Standard Darks

8443, 8460, 8461: WFPC2 Cycle 8: Supplemental Darks

8444: WFPC2 Cycle 8: Internal Monitor

8445: WFPC2 Cycle 8: Earth Flats

8446: WFPC2 Cycle 8: Astrometric Monitor

8447: WFPC2 Cycle 8: CTE Monitor

8448: WFPC2 Cycle 8: Intflat and Visflat Sweeps

8449: WFPC2 Cycle 8: UV Flats Internal Monitor

8451: WFPC2 Cycle 8: Photometric Characterization

8452: WFPC2 Cycle 8: PSF Characterization

8453: WFPC2 Cycle 8: Polarization

8454: WFPC2 Cycle 8: Linear Ramp Filter

8450: WFPC2 Cycle 8: Noiseless Preflash

8455: WFPC2 Cycle 8: Photometry of Very Red Stars

8456: WFPC2 Cycle 8: CTE for Extended Sources

8457: WFPC2 Cycle 8: UV Earth Flats

8458: WFPC2 Cycle 8: Plate Scale Verification

8.16 Cycle 9 Calibration Plan

The Cycle 9 calibration program is aimed at maintaining the calibration of WFPC2 via monitoring programs, as well as continuing two proposals from previous Cycles into Cycle 9 (photometric and PSF characterizations) and performing several new tests (on-orbit red leak check, CTE, wavelength stability check of narrowbands and linear ramp filters). Table 8.9 summarizes the programs proposed for calibrating WFPC2 in Cycle 9, followed by detailed descriptions of each program (including proposal numbers, statement of purpose, observing description, products, and accuracy expected).


Table 8.9: WFPC2 Cycle 9 Calibration Plan.
ID
Proposal Title
Frequency
Estimated Time (orbits)
Scheduling Required
Products
Accuracy
Required
Notes
"External"
"Internal"
Routine Monitoring Programs
8822- 8825 WFPC2 Decons & Associated Observations
1-2/4 wks 30 82 every 28 d Synphot, CDBS 1-2% Decons, phot. & focus monitor, internals, UV throughput, VISFLATs, and UV FLATs
8811 Standard Darks weekly
366 every 7 d CDBS 1 e-/hr Also hot pixel lists on WWW.
8826- 8828 Supplemental Darks (8460, 8461) 0-3/day
1282 anytime
n/a For archive only, no analysis provided
8812 Internal Monitor weekly
76 every 7d CDBS 0.8e/pixel Includes INTFLAT monitor, for possible future preflashed observations.
8815 Earth Flats continuous
210 mid-cycle CDBS 0.3%
8816 UV Earth Flats continuous
400 mid-cycle CDBS 3-10% Outsourcing candidate.
8813 Astrometric Monitor 2/cycle 2
early & late ISR 0.05'' Omega Cen as well as K-spots.
8817 Intflat Sweep and Linearity Test 1/cycle
21 mid-cycle TIR 0.3%
Special Calibration Programs
8818 Photometric Characterization 1 2
mid-cycle ISR 2-3% GRW+70D5824; nonstandard filters.
8819 PSF Characterization 1 6
late in cycle CDBS 10% Omega Cen; standard broadband filters.
8814 Read Leak Check 1 2
mid-cycle Synphot, CDBS 2% Solar analogs used to measure UV filter read leaks.
8821 CTE - Monitor and Absolute Calibration 1 15
mid/late ISR 0.01 mag Includes monitor as well as follow up to ground based observations. Outsourcing candidate.
8820 Wavelength Stability of Narrowband and Linear Ramp Filters 1 4 15 mid/late CDBS, ISR 2Å Check of wavelength/aperture mapping and test for changes in LRFs.

~10% reserve
6



Placeholder.
TOTAL TIME (including all executions)
67
2452

8822, 8823, 8824, 8825: Decontaminations and Associated Observations

8811:WFPC2 Cycle 9 Standard Darks

8826, 8827, 8828: WFPC2 Cycle 9 Supplemental Darks

8812: WFPC2 Cycle 9 Internal Monitor

8815: WFPC2 Cycle 9 Earth Flats

8816: WFPC2 Cycle 9 UV Earth Flats

8813: WFPC2 Cycle 9 Astrometric Monitor

8817: WFPC2 Cycle 9 Intflat Sweeps and Linearity Test

8818: WFPC2 Cycle 9 Photometric Characterization

8819: WFPC2 Cycle 9 PSF Characterization

8814: WFPC2 Cycle 9 Red Leak Check

8821: WFPC2 Cycle 9 CTE - Monitor and Absolute Calibration

8820: WFPC2 Wavelength Stability of Narrowband and Linear Ramp Filters

8.17 Cycle 10 Calibration Plan

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

8935: WFPC2 Cycle 10 Standard Darks

8936, 8937, 8938: WFPC2 Cycle 10 Supplemental Darks

8939: WFPC2 Cycle 10 Internal Monitor

8940: WFPC2 Cycle 10 Earth Flats

8941: WFPC2 Cycle 10 UV Earth Flats

8942: WFPC2 Cycle 10 Intflat Sweeps and Linearity Test

9253: WFPC2 Cycle 10 Astrometric Monitor

9254: WFPC2 Cycle 10 CTE Photometric Monitor

9251: WFPC2 Cycle 10 Photometric Characterization

9255: WFPC2 Cycle 10 Astrometric Effects of CTE

9257: WFPC2 Cycle 10 Super-PSF

9252: WFPC2 Cycle 10 Clocks ON Verification

9256: WFPC2 Cycle 10 Methane Quad Filter Check

8.18 Cycle 11 Calibration Plan

Due to the significant decrease in WFPC2 observations in Cycle 11 (with the advent of ACS becoming operational), the WFPC2 calibration plan for Cycle 11 has been scaled back. Forty external orbits will be used for WFPC2 calibration this cycle, compared to 61 orbits used in Cycle 10.

Routine calibrations from previous cycles, such as the photometric monitor, darks, and flat fields, will continue in Cycle 11. There is, however, one significant change in these routine monitors: the interval between decontaminations has been increased from 28 days to 49 days. A requirement for WFPC2 UV observations is that the decrease in throughput due to contamination never drops below 70% of the total (post-decon) throughput. Over the past few years, UV photometric monitor data of the standard star GRW+20D5824 has shown that the contamination rates have been decreasing, making it safer to implement longer intervals between decons.

Other scaled-back programs include the Earth flats and astrometric monitor. There was a major update to flat field reference files, as documented by Koekemoer et al. in ISR-2002-02: Updated WFPC2 Flatfield Reference Files for 1995 - 2001. For Cycle 11, fewer Earth flats are being executed, and these will be used primarily to track possible small-scale changes in order to update the current flat field reference files.

Astrometric monitor observations will be done once a year, compared to twice a year in past cycles. Since the astrometric properties of the chips are changing more slowly with time, decreasing the frequency to once per year is sufficient to track the changes. An improved astrometric solution for F555W was published by Casertano et al. in ISR-2001-10: An Improved Geometric Solution for WFPC2. The WFPC2 group expects to derive additional solutions for other filters in the future.

The CTE characterization program continues the CTE monitor of previous cycles. In addition, two new components have been added. One is to better characterize the "long-vs-short" anomaly since there are indications that this effect is only relevant for very crowded fields. The second component is a test of 2x2 pixel-binned observations to see if binning reduces CTE. A test of this technique could also be useful for ACS observations.

Data from the Photometric Characterization program will, as in past cycles, be used to verify photometric stability to 1-2%. Observations from this and previous cycles will also be used to update photometric zeropoints used in SYNPHOT.

A new program added to the calibration plan is the WFPC2-ACS Photometric Cross-Calibration. Observations of the primary ACS standard star, two globular clusters spanning a wide range of metallicities, as well as Sloan standard stars, will be taken in WFPC2 with a wide range of the more commonly-used filters.

The WFPC2 group is also beginning to lay out plans for closure of WFPC2 calibration work, making sure all major aspects of WFPC2 are well-characterized. A better idea of what needs to be done will emerge during the WFPC2 session of the upcoming Calibration Workshop (October 2002), where user feedback will provide additional guidance. One primary item for closure will be an accurate cross-calibration between WFPC2 and ACS, an effort that is being coordinated with ACS starting this cycle.

Details about the Cycle 11 calibration plan are outlined in the following pages. For additional details on the WFPC2 calibration plan, please email to help@stsci.edu.


Table 8.11: WFPC2 Cycle 11 Calibration Plan.
ID Proposal Title Frequency Estimated Time (orbits) Scheduling Required Products Accuracy Required
Notes
"External" "Internal"
Routine Monitoring Programs
9589 WFPC2 Decons & Associated Observations Decons every 49d (from 28d) 20 (13 photmon, 5 UV thruput, 2 UV flats) 124 every 49d, UV thruput early CDBS, Inst Hbk, Synphot, WWW reports 1-2% Decons, phot.monitor, internals, UV throughput, VISFLATS and UVFLATS.
9592 Standard Darks weekly, except decon week
294 every 7 days, except decon wk. CDBS 1 e-/hr Also for WWW hot pixel lists.
9593, 9594, 9595 Supplemental Darks 0-3/day
1182 every day
n/a For archive only, no analysis.
9596 Internal Monitor weekly, except decon week
53 every 7 days, except decon week CDBS 0.8e-/pix
9598 Earth Flats continuous
150 mid-to-late CDBS 0.3%
9599 UV Earth Flats continuous
200 early-to-mid CDBS 3-10%
9597 Intflat & Visflat Sweeps, Filter Anomaly Check 1/cycle
166 mid-cycle TIR 0.3%
Special Calibration Programs
9600 Astrometric Monitor 1/cycle 1 1 mid-cycle ISR, STSDAS 0.05'' Cen as well as K-spots.
9591 CTE Characterization 1/cycle 5
early, late ISR 0.03 mag Continuation of monitors, new tests (2x2 binning & long vs. short).
9590 Photometric Characterization 1/cycle 3
early ISR, Synphot 1% Std. star in all 4 chips. Non-monitor broadband filters, not UV.
9601 WFPC2-ACS Photometric Cross-Calibration 1/cycle 8
early ISR, Synphot 1% Cross-calibration with ACS, using NGC2419, 47 Tuc, ACS calib star, & Sloane Calibration Field.

~10% reserve
3



Placeholder for unexpected items.
TOTAL TIME (including all executions) 40 2171    

9589: WFPC2 Cycle 11: Decontaminations and Associated Observations

9592: WFPC2 Cycle 11: Standard Darks

9593, 9594, 9595: WFPC2 Cycle 11: Supplemental Darks

9596: WFPC2 Cycle 11: Internal Monitor

9598: WFPC2 Cycle 11: Earth Flats

9599: WFPC2 Cycle 11: UV Earth Flats

9597: WFPC2 Cycle 11: Intflat and Visflat Sweeps, and Filter Rotation Anomaly Monitor

9600: WFPC2 Cycle 11: Astrometric Monitor

9591: WFPC2 Cycle 11: CTE Characterization

9590: WFPC2 Cycle 11: Photometric Characterization

9601: WFPC2 Cycle 11: WFPC2-ACS Photometric Cross-Calibration

8.19 Cycle 12 Calibration Plan

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


Table 8.12: WFPC2 Science Program Usage During Cycles 10 - 12.
WFPC2 Science Program Usage
Cycle 10
Cycle 11
Cycle 12
Primary Orbits
1080
153
56
Coordinated Parallel Orbits
40
200
381
Pure Parallel Orbits
300
500
844

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

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

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

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

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


Table 8.13: WFPC2 Calibration Program Orbit Allocations During Cycles 10 - 12.
WFPC2 Calibration Program Usage
Cycle 10
Cycle 11
Cycle 12
Ext.
Int.
Ext.
Int.
Ext.
Int.
Monitors: Decons, darks, internal flats
23
2242
19
2170
12
1677
Special / Close-Out Programs
32
52
17
1
13
4
Reserve (Unexpected Items)
6
3
2
Total
61
2294
40
2171
25
1681


Table 8.14: WFPC2 Cycle 12 Calibration Plan.
ID Proposal Title Frequency Estimated Time (orbits) Scheduling Required Products Accuracy Required
Notes
"External" "Internal"
Routine Monitoring Programs
10067 WFPC2 Decons & Associated Observations Decons every 49d 8 124 every 49d CDBS, IHB, Synphot, WWW reports 1-2% Decons, phot.monitor, internals, UV throughput, VISFLATS and UVFLATS.
10068 Standard Darks weekly, exc. decon wk
264 every 7 days, exc.decon wk CDBS 1 e-/hr CDBS updates and weekly WWW hot pixel lists.
10069, 10070, 10071 Supplemental Darks 0-3/day
1195 every day
n/a For archive only, no analysis. Schedule at a low priority. Useful for calibrating WFPC2 parallels.
10072 Internal Monitor weekly, exc. decon wk
44 every 7 days, exc.decon wk CDBS 0.8e-/pix BIAS, INTFLATS in F555W for gain and throughput stability measurements
10073 Visible Earth Flats continuous
50 mid-to-late CDBS 0.3% Reduce to 1 filter (time dep. only)
10074 UV Earth Flats continuous
20 mid-to-late CDBS 0.3%
10075 Intflat & Visflat Sweeps, Filter Anomaly Check 1/cycle
80 mid-cycle TIR 0.3% Flats in all the filters used in Cycle 12, both gain settings/shutters.
10076 CTE Monitor 1/cycle 2
mid-to-late ISR 0.03 mag Continue CTE monitor.
10077 Photometric Monitor 1/cycle 2
mid-cycle ISR, Synphot 1% GRW+70D5824 in filter/chip combos used for science in Cycle 12.
Close-Out Calibration Programs
10078 Photometric Cross-Calibration once 6
mid-cycle ISR, Synphot 1% Several standard stars in a range of WFPC2 filters, for ACS & WFC3 cross-calibration.
10079 UV Astrometric Characterization once 3 1 mid-cycle ISR, Synphot 0.05" Determine astrometric solution in new UV filters (not done before), and K-spots.
10080 Narrow-Band/LRF Characterization once 2 3 mid-to-late ISR, Synphot 1-2% Check filter wavelength stability: observe emission-line source in narrow-band filters, and crossed with LRFs

~10% reserve
2



Placeholder for unexpected items.
TOTAL TIME (including all executions) 25 1601    

10067: WFPC2 Cycle 12: Decontaminations and Associated Observations

10068: WFPC2 Cycle 12: Standard Darks

10069, 10070, 10071: WFPC2 Cycle 12: Supplemental Darks

10072: WFPC2 Cycle 12: Internal Monitor

10073: WFPC2 Cycle 12: Visible Earth Flats

10074: WFPC2 Cycle 12: UV Earth Flats

10075: WFPC2 Cycle 12: Intflat and Visflat Sweeps, and Filter Rotation Anomaly Monitor

10076: WFPC2 Cycle 12: CTE Monitor

10077: WFPC2 Cycle 12: Photometric Monitor

10078: WFPC2 Cycle 12: Close-Out Photometric Cross-Calibration

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

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

8.20 Cycle 13 Calibration Plan

The overall goals of the Cycle 13 WFPC2 Calibration Programs are to monitor health and safety of the instrument and to maintain required calibration accuracies for the science modes used in Cycle 13. We will also streamline routine monitoring programs and remove unnecessary observations. Specifically, we are discontinuing the supplemental darks program as they were little used. Eliminating them will help prolong the life of the data transmitter aboard HST. We will further implement special close-out calibration programs to increase the value of the WFPC2 archive scientific legacy, such as photometric cross-calibration with other instruments.

Until the end of Cycle 10, WFPC2 was most heavily used in prime mode. During Cycles 11-13, WFPC2 prime orbits have decreased dramatically and parallel usage is now the dominant mode for WFPC2 science operation. Table 8.15 shows the recent history of science program and calibration program usage.


Table 8.15: Recent Science and Calibrations Program Usage
WFPC2 Science Program Usage
Cycle 10
Cycle 11
Cycle 12
Cycle 13
Primary Orbits
1080
153
56
21
Coordinated Parallel Orbits
40
200
381
890
Pure Parallel Orbits
300
500
844
0
 
 
 
 
 
 
WFPC2 Calibration Program Usage
Cycle 10
Cycle 11
Cycle 12
Cycle 13
Ext.
Int.
Ext.
Int.
Ext.
Int.
Ext.
Int.
Monitors: decons, darks, internals, flats
23
2242
19
2170
12
1677
13
582
Special / Close-Out Programs
32
52
17
1
13
4
2
0
Reserve (Unexpected Items)
6
0
3
0
2
0
2
0
Total
61
2294
40
2171
25
1681
17
582

WFPC2 Cycle 13 Calibration Plan.

ID Proposal Title Frequency Estimated Time (orbits) Scheduling Required Products Accuracy Required
Notes
"External" "Internal"
Routine Monitoring Programs
10356 WFPC2 Decons & Associated Observations Decons every 49d 8 124 every 49d CDBS, IHB, Synphot, WWW reports 1-2% Decons, phot.monitor, internals, UV throughput, VISFLATS and UVFLATS.
10359 Standard Darks weekly, exc. decon wk
264 every 7 days, exc.decon wk CDBS 1 e-/hr CDBS updates and weekly WWW hot pixel lists.
10360 Internal Monitor weekly, exc. decon wk
44 every 7 days, exc.decon wk CDBS 0.8e-/pix BIAS, INTFLATS in F555W for gain and throughput stability measurements
10361 Visible Earth Flats continuous
50 mid-to-late CDBS 0.3% F502N only (time dependence only)
10362 UV Earth Flats continuous
20 mid-to-late CDBS 0.3% F255W only
10363 Intflat & Visflat Sweeps, Filter Anomaly Check 1/cycle
80 mid-cycle TIR 0.3% Flats in all the filters used in Cycle 12, both gain settings/shutters.
10364 CTE Monitor 1/cycle 2
mid-to-late ISR 0.03 mag Continue CTE monitor.
10365 Photometric Monitor 1/cycle 3
mid-cycle ISR, Synphot 1% GRW+70D5824 in filter/chip combos used for science in Cycle 13.
Close-Out Calibration Programs
10366 Photometric Cross-Calibration once 2
mid-cycle ISR, Synphot 1% T-dwarf star in a range of WFPC2 filters, for ACS & WFC3 cross-calibr.

~10% reserve
2



Placeholder for unexpected items.
TOTAL TIME (including all executions) 17 582    

10356: WFPC2 Cycle 13: Decontaminations and Associated Observations

10359: WFPC2 Cycle 13: Standard Darks

10360: WFPC2 Cycle 13: Internal Monitor

10361: WFPC2 Cycle 13: Visible Earth Flats

10362: WFPC2 Cycle 13: UV Earth Flats

10363: WFPC2 Cycle 13: Intflat and Visflat Sweeps, and Filter Rotation Anomaly Monitor

10364: WFPC2 Cycle 13: CTE Monitor

10365: WFPC2 Cycle 13: Photometric Monitor

10366: WFPC2 Cycle 13: Close-Out Photometric Cross-Calibration

8.21 Future Calibrations, Calibration by Observers, and Calibration Outsourcing

It is expected that the calibrations outlined here for recent cycles and the current Cycle 13 will be maintained for Cycle 14. However, it is possible that as the number WFPC2 users decreases further with the availability of ACS, some WFPC2 calibrations maybe curtailed. For example, monitoring observations may become less frequent, and special modes (e.g. ramp filters) may not receive further calibration. It is the intention of STScI to develop a calibration program that effectively balances the needs of the community for obtaining excellent science results from the instrument with the limited resources available (e.g., a nominal limit of 10% time available for calibration). As always, frequently used modes of the instrument will be fully calibrated.

In special situations it is possible that observers may find the STScI calibration programs do not meet their needs. For example, they may require an accuracy better than outlined in Table 8.16, or may require calibration of some unique mode or observation strategy. In these cases observers may propose to obtain their own calibration data. Such observations may be proposed in one of two ways.

The first type of special calibration would be to simply request additional orbits within a GO program for the purpose of calibrating the proposed science data (see section 4.3 of the CP). In this case the extra calibration would only need to be justified on the basis of the expected science return of the GO's program.

The second type of special calibration would be performed as a general service to the community via Calibration Proposals (section 3.7 of CP, sometimes called "Calibration Outsourcing"). These proposals are to obtain calibration data and/or support analysis of data (including archival data) for the purpose of improving calibrations. New observations obtained for calibration programs will generally be flagged as non-proprietary, and will be immediately released to the community. These proposals will generally be judged on their value to the scientific community and scientific impact they are likely to make (see the Call for Proposals for details). These programs, if approved, will usually carry a requirement to provide separately negotiated deliverables (e.g. results, reference files, documentation) so as to support other members of the community.

Proposers interested in obtaining either type of special calibration should consult with Instrument Scientists from the WFPC2 Group via the Help Desk at least 14 days before the proposal deadline in order to ascertain if the proposed calibrations would be done at STScI in the default program.

During Cycles 9 and 10, two WFPC2 Calibration Outsource programs were awarded. The first (Karkoshka, PI) sought to improve the signal-to-noise ratio of the UV flatfields. While the standard UV flats provided by STScI are adequate for most observers, people with bright targets (e.g. planets) have sometimes found their signal-to-noise ratio was limited by the calibration flats rather than photon noise. We anticipate this program will provide new flatfields in the UV which reduce noise in certain observations by up to a factor ~3. The second program (Saha, PI) tested for a position independent component of CTE by comparing ground based and WFPC2 observations of several fields containing faint standard stars. This program could potentially improve the photometric accuracy for faint targets, and impact scientifically important problems such as the extragalactic distance scale. Results of both programs will be disseminated via the WFPC2 WWW site, as results become available.

8.22 Calibration Accuracy

Table 8.16 summarizes the accuracy to be expected from WFPC2 observations in several areas. The numbers in the table should be used with care, and only after reading the relevant sections of this handbook and the documents referenced therein; they are presented in tabular form here for easy reference.


Table 8.16: Accuracy Expected in WFPC2 Observations.
Procedure Estimated Accuracy Notes
Calibration (flatfielding, bias subtraction, dark correction)
Bias subtraction 0.1 DN rms Unless bias jump is present
Dark subtraction 0.1 DN/hr rms Error larger for warm pixels; absolute error uncertain because of dark glow
Flatfielding <1% rms large scale Visible, near UV
0.3% rms small scale
~10% F160BW; however, significant noise reduction achieved with use of correction flats
Relative photometry
Residuals in CTE correction < 3% for the majority (~90%) of cases
up to 10% for extreme cases (e.g., very low backgrounds)
Long vs. short anomaly (uncorrected) < 5% Magnitude errors <1% for well-exposed stars but may be larger for fainter stars. Some studies have failed to confirm the effect
Aperture correction 4% rms focus dependence (1 pixel aperture) Can (should) be determined from data
<1% focus dependence (> 5 pixel)
1-2% field dependence (1 pixel aperture)
Contamination correction 3% rms max (28 days after decon) (F160BW)
1% rms max (28 days after decon) (filters bluer than F555W)
Background determination 0.1 DN/pixel (background > 10 DN/pixel) May be difficult to exceed, regardless of image S/N
Pixel centering < 1%
Absolute photometry
Sensitivity < 2% rms for standard photometric filters Red leaks are uncertain by ~10%
2% rms for broad and intermediate filters in visible
< 5% rms for narrow-band filters in visible
2-8% rms for UV filters
Astrometry
Relative 0.005" rms (after geometric and 34th-row corrections) Same chip
0.1" (estimated) Across chips
Absolute 1" rms (estimated)

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