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WFPC2 STAN

S T A N / W F P C 2 - Number 1, November 1994

CONTENTS:

PAPER ON "THE PERFORMANCE AND CALIBRATION OF THE WFPC2"

The WFPC2 Investigation Definition Team has written a paper for publication in PASP that describes the on-orbit performance of the WPFC2. The paper, by Jon A. Holtzman et al., is not yet in final form and is still subject to revision, but nonetheless we highly recommend it to all WFPC2 observers. It discusses many instrumental effects including the charge transfer problem (which can affect photometry), the hot pixel growth rate, photometric performance in the visible and ultraviolet, and astrometric calibration.

The plate scales derived for the 4 WFPC2 CCDs are better than values currently stored in the image headers. The plate scales from the paper are 0.0455 (PC1), 0.09963 (WF2), 0.09953 (WF2), 0.09967 (WF4) arcsec per pixel.

The paper is available in PostScript through anonymous ftp from ftp.stsci.edu in instrument_news/wfpc2/wfpc2-status.ps. You can save time in the data transfer by using the ftp server's ability to compress files on-the-fly. Set the transfer mode to binary and enter "get wfpc2-status.ps.Z".

WHERE TO GET WFPC2 CALIBRATION INFORMATION

WFPC2 data are automatically corrected for various instrumental effects using the best calibration files available at the time of the observation. It often happens, however, that better calibration files become available later. This is especially true for dark current frames, since new hot pixels that appeared shortly before the time of observation will usually not be in the pipeline dark frame.

The WFPC2 reference file memo on STEIS has a list of all currently available calibration files. It can be retrieved via ftp from ftp.stsci.edu, directory instrument_news/wfpc2, file wfpc2_referencefiles. It is also accessible through WWW from the WFPC2 home page (see the Appendix) by following links "Instrument Status and Calibration Products" -> "Reference files".

A list of calibration files for a particular observation can also be retrieved directly from the calibration database either through Starview or, for users on STScI computers, with the getreffile task in the stsdas.stlocal.cdbsutil package. In Starview, select "WFPC2 Calibration" on the Searches menu. The getreffile task provides a simple interface to access the same calibration information.

WFPC2 DECONTAMINATION HISTORY

by Krista Rudloff & Sylvia Baggett

About once per month the WFPC2 CCDs are decontaminated by warming them to +20 C. This serves two purposes: it improves UV sensitivity by removing the contaminants that collect on the cold CCDs, and it restores most of the hot pixels to their normal (low) dark current. For many observations it is important to know when the most recent WFPC2 decon occurred (e.g. the UV throughput drops by about 1%/day at the current -88 C operating temperature for the CCDs.) Since the servicing mission there have been 10 decontaminations: Feb 22, Mar 25, Apr 23, May 23, Jun 13, Jul 10, Jul 28, Aug 27, Sep 24, Oct 20. The Apr 23 decontamination marked the change to colder (-88 C) CCD operating temperatures.

The next decon is scheduled for the week of Nov 14. The decontamination history can be found in the WFPC2 reference files memo (see "Where to Get WFPC2 Calibration Information".)

SUPERDARKS IN THE CALIBRATION DATABASE

by Krista Rudloff

The WFPC2 Instrument Team has recently added a set of "superdarks" to the Calibration Database. The superdarks were each generated from 100 individual dark frames taken from April 23, 1994 to October 1, 1994. Though not applied to data in the pipeline software, the superdarks are available to GOs who wish to recalibrate with them.

Please note that the superdarks can only be used for data taken after April 23, 1994, the date that the WFPC2 chips were cooled from -76 degrees to -88 degrees.

The important factors in deciding whether to recalibrate with the superdarks are the number and duration of the exposures taken. Use of the superdarks will improve the dark calibration of high signal to noise images. Their use should then be considered in the case of long (> 30 min) exposures and/or many (>4) exposures that are averaged together. Since the superdarks consist of many frames over a long period of time averaged together, they do not include new hot pixels, which might change on a much shorter time scale. To identify these hot pixels, one should retrieve from the archive the dark frame that is closest in time *after* the date of observation.

All calibration data, including the superdarks, can be retrieved from the HST Data Archive. To find the superdark that is appropriate for your data, consult the WFPC2 reference file memo on STEIS (see "Where to Get WFPC2 Calibration Information" above for details.) There is one superdark for each observing mode. Once you know the name of the appropriate superdark, use the Data Files screen in Starview to retrieve it from the archive.

GETTING THE LATEST SCHEDULING INFORMATION

by Mark Johnston PRESTO Project Lead

You can check the implementation and scheduling status of your observations by using the PRESTO (Project to Re-Engineer Space Telescope Observing) public page available via the World-Wide Web. This is most easily accessed through the Mosaic client program (as described below), although other WWW browsers are also available. The PRESTO page provides a wealth of information about the status of the HST observing program, including the weekly timelines. Once the developing long-range observing plan is in place, it will also be available in this page.

The PRESTO homepage is at http://presto.stsci.edu/public/propinfo.html. An alternative way into PRESTO is through the Space Telescope Electronic Information Service (STEIS) (http://www.stsci.edu/top.html) by going to the OBSERVER INFORMATION paragraph and clicking on PROGRAM STATUS.

To get scheduling information on your proposal, enter your program ID in the box and click on "Get Program Information"; then select "Visit Status Information" to get detailed reports on each observation in your proposal.

THE ANALYSIS TEAMS: A PILOT PROJEST TO IMPROVE POST-OBSERVATION USER SUPPORT

by Brad Whitmore, A-Team Project Coordinator

We are pleased to announce a new initiative designed to improve post-observation user support at STScI. The Analysis Team (= A-Team) is a pilot project developed to address your main concerns, as determined from the post-observation user support survey circulated last spring. We plan to focus on:

Each Analysis Team consists of an active research astronomer (the Analysis Scientist), one or two research assistants (the Analysis Specialists), and a scientific programmer (the Analysis Programmer).

Since this is a pilot project we are not able to support all Cycle 4 programs. Hence, only WFPC2 and FOS programs are being covered at present. In addition, very experienced observers and observers with other means of support (e.g., Guaranteed Time Observers, ESA observers, carryover proposals, most STScI staff, etc.) and programs that completed before 1 October 1994 will not be covered in the initial plan. However, information of general utility will be distributed to a wider audience via this newsletter.

If you feel that our services would be especially useful for your program, and you have not recently been contacted by the A-Team, please contact Krista Rudloff at analysis@stsci.edu (410-338-1082) and we will add you to our list of covered proposals.

APPENDIX: WFPC2 CONTACTS

Any questions about the scheduling of your observations should be addressed to your PRESTO contact. If you do not know who this person is, PRESTO's Web page (http://presto.stsci.edu/public/propinfo.html) contains that information.

Post-Observation questions should be addressed to the WFPC2 Analysis Team, if your program is covered by this pilot project (see article above) or help@stsci.edu.

Comments, questions, requests for issues, additions or deletions to the mailing list, etc. can be e-mailed to analysis@stsci.edu.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.