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Outsourcing Candidates for WFPC2

Outsourcing is an STScI initiative to fund external groups to perform and document some aspects of HST calibration. These funded groups would be contracted to analyze the calibration data and document the results, which would then be made available to the GO community by STScI. The projects to be outsourced could rely on archival data or could require obtaining new calibration data; the aim is not to replace the current calibrations but to complement and extend the current programs.

At STScI, specialized modes or specific concerns which impact only a small segment of the GO community have, by necessity, been given lower calibration priority than the more commonly used modes due to lack of resources. One primary benefit of outsourcing calibration work is to tap the expertise of external scientists with significant experience in a particular area, making the calibration information about these infrequently-used modes available more quickly. With relatively small investments in calibration, the HST archival legacy would be enhanced.

As a test of the concept, two programs have already been outsourced as part of a pilot program. The first project, improving the UV flatfields, is nearing completion. While sufficient for most WFPC2 data, the standard pipeline UV flatfields did occasionally limit the signal-to-noise ratio in images with extended, bright targets, such as planets. The new flats, to be made available via the archive, reduce the noise in certain observations by up to a factor of ~3. The second outsourced project will provide a direct verification of the absolute photometric calibration of WFPC2 in observations that may be affected by CTE, and therefore a more robust determination of the zero point for many WFPC2 observations. While there is no evidence that the current WFPC2 zero point is inapplicable to faint sources, enough corrections need to be applied that a direct verification is extremely desirable. Due to the success of the initial outsourcing programs, STScI is now formally soliciting future outsourcing proposals from the entire scientific community.

Outsourcing proposals for Cycle 11 will follow the normal GO proposal process, requiring evaluation and approval by the TAC. Projects can involve the acquisition of new data or rely upon the existing archival data. The list below is intended to highlight some areas identified by STScI as potential outsourcing projects. Note, however, that this list is not meant to be complete and no guarantee of approval by the TAC is intended or implied.

Any archival datasets discussed below can be accessed via the HST archive at:

http://archive.stsci.edu.

Phase II versions of the proposals can be viewed online at:

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

or by simply clicking on the proposal number listed.

CTE/Long versus Short: Much work on the CTE and long versus short issues has already been completed and published by many users, both internal and external to STScI; additional work (e.g., effects on extended targets, noiseless preflash test, residual images, etc.) is currently underway. Past CTE proposals include

5646 Cycle 4 CTE Dither Test,
5659 Cycle 4 CTE Dither Test, Part II,
6192 Cycle 5 CTE Calibration,
6937 Cycle 6 CTE Calibration,
7630 Cycle 7 CTE Characterization,
7929 Cycle 7 CTE Monitor,
8447 Cycle 8 CTE Monitor,
8456 Cycle 8 CTE for Extended Sources,
8821 Cycle 9 CTE Monitor,
9254 Cycle 10 CTE Monitor, and
9255 Cycle 10 WFPC2 Astrometric Effects of CTE.

Photometric Transformations: Most broad and medium-width filters should have transformations accurate to 2-5%; many filters will be at the 1-2% level but the accuracy could be worse in e.g., F336W (due to redleak). A detailed comparison between the available archival data (e.g., NGC 2100, M67, NGC 2419, in HST Johnson-Cousins and Stromgren equivalents) and existing groundbased data would provide a check of these accuracy levels. See proposals 6182, 6935, and 7628 for examples of existing datasets relevant to this item.

Calibration for Cool Stars: Archival data exists for two well-known M dwarfs, VB8 and VB10, for which ground-based measurements in the Johnson filters exist as well. The calibration of cool stars is particularly difficult at the red end (F814W) because their spectra rise quickly in the region where the WFPC2 DQE drops substantially, thus increasing the uncertainty in the synthetic magnitude calibration. The observations of these late M stars would provide a direct empirical calibration of these effects and reduce the uncertainties in the photometric response of WFPC2 for very red stars. See proposal 8455 for details of the calibration observations taken.

UV Platescale: Archival observations exist of the bright cluster NGC2100 in a range of UV filters. The data could provide a measurement of the plate scale changes in the UV, where the MgF2 window introduces significant wavelength dependencies. Note that the exposure times of the archival images are not long enough to allow a full distortion solution. See proposal 8458 for details of the calibration observations taken.

Astrometry: The current relative accuracy is 0.005" in the same chip (after geometric correction) and 0.15" across chips; absolute astrometry accuracy is estimated at ~1" rms (limited by guide star positions). Possible future projects could include an investigation of wavelength and time dependencies of the astrometric solution using archival data. See proposals 8446, 7627, and 6941 for examples of existing datasets relevant to this item. The Cycle 9 astrometry proposal (8813) is on the long-range plan for execution ~August 2000 and ~March 2001.

Wavelength Check of Linear Ramp and Narrow Band Filters: On-orbit VISFLATs will be taken in Cycle 9 using the ramps alone and the ramps crossed with narrow band filters. The crossed VISFLATs will constrain the wavelength calibration of the ramp filters relative to the narrow band filters: comparison with similar Cycle 4 data will reveal whether the filter properties have evolved with time due to annealing/shrinkage of the thin film materials. The uncrossed VISFLATs can be used to constrain the transverse (cross-wavelength) placement of the ramp filters. In addition, observations of an extended line emission source will be taken, in order to provide an absolute test for wavelength changes in the ramp filters. See proposals 8820 and 6140 for details of the calibration observations.

Polarization: A variety of archival data exists and the last two visits of the Cycle 8 program are on the long-range plan for Fall 2000. Open issues include determining the cause of occasional strange results and testing for longterm evolution that may have affected data already taken. The current accuracy for polarization data is between 1.5-3% and many calibration exposures indicate the 1.5% accuracy is met. However, there are a few images where 3-5% errors are seen. The cause of these outliers is unknown at this time: no obvious correlations with spectral filter, polarizer quad, spacecraft orientation, etc., have been found. See proposals 5574, 6194, and 8453 for details of the calibration observations.

Hot Pixel Analysis: A substantial amount of archival data now exists, in the form of images as well as the STScI-produced hot pixel lists, that could be used to investigate issues such as the evolution over time of the hot pixel population, their persistence, growth and annealing rates.

Methane Quad Filter Check: A potential outsourcing project for Cycle 10, proposal 9256 will provide data to check the methane filter transmission across the field of view. This program would benefit all FQCH4N-D archival observations (~1% of external images in the archive).

More details on these or other specific topics can be found in the WFPC2 Instrument Handbook, and the WFPC2 Instrument Science Reports, available online at:

WFPC2 Instrument Handbook

WFPC2 ISRs