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Hubble Space Telescope
WFPC2 STAN

S T A N / W F P C 2 - Number 45, March 2001

CONTENTS:

Preparation of the WFPC2 Cycle 10 Calibration Program: An invitation for user input:

The WFPC2 team is in the process of preparing the Cycle 10 Calibration Program. The basis of the plan will be the current Cycle 9 Calibration Program, which is presented in datail in Instrument Science Report 00-01, available online at the WFCP2 Website.

We plan to continue the Cycle 9 suite of calibrations that are used to monitor the instrument (e.g., decontaminations, photometric and astrometric monitors) as well as the progams to collect data used in making calibration reference files (darks, biases, internal flats, and Earthflats). In addition, a small number of orbits will be available for other, more specific, calibrations; given the upcoming installation of ACS, these programs will be developed with a view towards closure calibrations for WFPC2.

We hereby invite the HST user community to provide any suggestions for additional WFPC2 calibration observations that are currently unavailable or inadequate, and which can be expected to serve a significant fraction of WFPC2 users (say of order 10% or higher). Suggestions for calibration programs in Cycle 10 will be accepted until April 6th. These should also include details of the suggested exposures and estimates of the telescope resources (orbits) required. Please send your suggestions to Sylvia Baggett (sbaggett@stsci.edu).

While your suggestions will be carefully considered against other necessary calibrations, the WFPC2 team cannot guarantee that your suggested calibrations will become part of the final Cycle 10 Calibration Program. Please be aware that there will also be an opportunity to submit Calibration Outsourcing proposals as part of the Cycle 11 GO program; the Calibration Outsourcing information made available during the previous Cycle 10 call for proposals is online.

Possible small errors in orientation header information in some images taken prior to Sept 1997:

by Reinhart, M., Lallo, M., Schultz, A., Baggett, S., Biretta, J.

All HST data (including WFPC2 images) taken prior to Sept 15, 1997 may have slightly incorrect orientation information in the headers (keywords PA_V3 and ORIENTAT). For most images, the errors are very small (<0.1 degrees), but for images taken during long visits, the errors can accumulate and reach many tenths of a degree. These errors affect only rotation of the field around the proposed aperture location, and otherwise have no impact on the target position in the image.

The problem manifests itself in data taken during a long, multi-orbit visit containing one or more pos-targs, slews, or other small telescope motions, the position angle information reported in the header changes slightly after each motion when in fact there was no change in the telescope orientation during the visit. The first image in a long visit contains correct header information; however, orientation keywords in subsequent observations begin to deviate, with each slew, from the correct orientation value at a rate of at most 1 degree per day (~7e-04 deg/min). The error also depends on the target's position in the sky (sine of the declination); thus, keywords in images of targets near the equator will have almost no error while images taken near the poles will show larger errors.

A search of the archive indicates that ~3% of the visits prior to Sept 15, 1997 are longer than 5 orbits and contain at least one small telescope maneuver. This corresponds to images where the problem can potentially exceed 0.33 degrees in the ORIENTAT and PA_V3 header keywords. Tables listing possibly-affected visits and exposures have been posted to the WFPC2 Website.

In compiling these tables no allowances have been made for target position in the sky or for the number and timing of slews in each visit. Hence, for many of these images the error will be much less than 0.33 degrees. This list of potentially affected visits should serve as a starting point for observers to assess the accuracy of information they may have used in developing Phase II proposals, etc. Note: Moving target images have not been included in the lists, as these type of observations appear to be unaffected by the problem; further investigation is required to confirm this.

We recommend that observers using orientation information from any WFPC2 images on this list check the the jitter data in addition to the science header. For advice on how to perform the check, please see the section on "How do I best determine an observation's actual orientation?" in the WFPC2 FAQ.

If you have any additional questions or require further assistance, please email help@stsci.edu.

Noiseless Preflashing of the WFPC2 CCDs:

by A. B. Schultz, I. Heyer, and J. Biretta

Abstract: We present results from analysis of WFPC2 images obtained prior to and following a "noiseless" pre-flash of the CCDs (WFPC2 CAL program ID: 8450). In this method, the CCDs are preflashed and then cleared immediately prior to the science exposure, hence contributing zero photon noise. A crowded field in the globular cluster Omega Cen (HD116790) was observed, and the WFPC2 calibration lamp was used as the preflash source for the CCDs. Only a modest enhancement of ~3% in the detected counts was measured for point sources far from the horizontal shift register (at Y=800) following the preflash exposure. For preflashed exposures, the second image after the preflash showed a decrease in the detected counts; the CCDs essentially returned to a previous sensitivity level.

Switch from OTFC to OTFR planned for April 2001 - and a note about using uchcoord:

by S. Baggett, J. C. Hsu, and S. Gonzaga

The On-The-Fly reprocessing (OTFR) system will be replacing the On-The-Fly calibration (OTFC) system in April 2001. OTFR starts with the original telemetry files ("POD" files) received from Goddard Space Flight Center and performs all pipeline processing steps. The older OTFC system performs only the last pipeline step (calibration), on raw files retrieved from the archive. The switch to OTFR should be transparent to users: requests for data will be submitted as usual via Starview or WWW and raw plus best-calibrated data will be delivered.

Observers should, however, be aware of one change. In the near-term, observers should not run STSDAS task UCHCOORD on data processed through the new OTFR system. This task updates the header keywords which define the mapping between the pixel and world coordinate systems. These keywords 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 (orientat). Data processed through OTFR will already have all necessary correction performed, and it is possible that UCHCOORD would make additional, un-needed (and incorrect) corrections.

To address this issue, a new header keyword (PROCTIME) has been added to OTFR data to record when the system processed the data. A new version of UCHCOORD will be part of this summer's STSDAS release, and it will correctly handle both old data and new OTFR data by detecting the PROCTIME keyword.

The current version of UCHCOORD can, of course, still be run on any of the older OTFC or tape data observers already have (i.e., images without the PROCTIME keyword). Only new data processed through OTFR and having the PROCTIME keyword are affected. WFPC2 observers with old data are still advised to execute the STSDAS task UCHCOORD to update the position-related group header keywords.

RECENT PREPRINTS:

We draw your attention to these papers, based on WF/PC and WFPC2 data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list.

STOCKTON, A.; RIDGWAY, S.E. "The Host Galaxy and
the Environment of the z = 1.195 Quasar 3C 190" ApJ accepted

BIANCHI, L.; SCUDERI, S.; MASSEY, P.; ROMANIELLO,
M. "The Massive Star Content of NGC 6822.
Ground-Based and HST Photometry" AJ accepted

O'DEA, C.P.; KOEKEMOER, A.M.; BAUM, S.; SPARKS,
W.B.; MARTEL, A.R.; ALLEN, M.G.; MACCHETTO, F.D.;
MILEY, G.K. "3C 236: Radio Source, Interrupted?" AJ
4-01

FELTZING, S.; WYSE, R.; HOUDASHELT, M.; GILMORE, G.
"The Low-Mass IMF -- Deep Star Counts in the dSph
Galaxy Ursa Minor" Modes of Star Formation

APPENDIX: WFPC2 Contacts:

Any questions about the scheduling of your observations should be addressed to your Program Coordinator. Post-Observation questions can be addressed to your Contact Scientist. If you do not know who these persons are, you can find the information on the WWW at www.stsci.edu/public/propinfo.html.

Analysis, STSDAS or any other questions can also be addressed to help@stsci.edu.

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