S T A N / W F P C 2 - Number 23, January 1997

CONTENTS:

• WFPC2 NEWS:
• The New WFPC2 PSF Library Search Tool
• Single-chip WFPC2 Images: New Calibration Code Available
• Anomalies in Some Far UV PSFs
• Second Update on WFPC2 Polarization Calibration
• New Version of the Data Handbook is Planned
• New WFPC2 Documentation
• RECENT PREPRINTS
• APPENDIX: WFPC2 CONTACTS

The New WFPC2 PSF Library Search Tool:

by M. S. Wiggs, J. Surdej, S. Baggett, C. Tullos, T. Kimball

A library of WFPC2 PSFs is now available as a resource for projects where PSF characterization or PSF subtraction is required. A WWW interface provides a means of sorting through and selectively choosing and retrieving only those PSFs of interest.

The PSFs in the Library were extracted from observations taken as part of STScI's routine calibration programs. The observations used were taken from the HST Archive, with no processing other than standard pipeline calibration. Most of the observations were not run through CRREJ or any other cosmic ray removal program, but there are plans to provide composite PSFs and CR rejected images in cases where more than one image was taken.

The PSF images were generated by extracting a square region centered on a given star. The width of this box was determined by the following criteria: 128x128 if the peak counts were greater than 3500 DN, and 64x64 if less than 3500 DN. Only those stars with counts greater than 150 DN were selected for extraction.

There are currently over 2000 PSFs in the Library, primarily for the F336W, F439W, F555W, F606W, F675W, and F814W filters.

The WFPC2 PSF Library Search Tool allows the user to search the database on many different criteria including:

* Search based on filter, detector, exposure time, RA & Dec, etc.
* Search over a range of dates.
* Search within a user defined radius around a given RA and Dec.
* Search on proposal number, target name, etc.
* Search on PSF characteristics such as intensity, relative focus, X & Y centers, etc.
* Ability to sort the output table based on up to 3 search keywords.

In the coming months we will be upgrading and improving the features of the PSF Library, including:

* Routine addition of more PSFs.
* Adding relative focus information (populate RELFOCUS keyword, currently
  set to 0).
* Providing means to plot some critical data (e.g., X and Y centers on
  chip, or PSF peak intensities as a function of time).
* Incorporating PSFs from GO observations.
* Adding capability of custom output table.
* Addition of composite PSFs.

Note that simulated PSFs using the TinyTim software can also be used in many cases.

Single-chip WFPC2 Images: New Calibration Code Available:

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

The calwp2 bug reported in December's STAN has been fixed and tested. The bug appeared in Dec 1994 (calwp2 version 1.3.0.7) and affects the processing of all non-PC, single-chip readout images: they were processed as if they were PC images. The new calwp2 is now running in the routine HST pipeline. This new version was installed in the pipeline on Jan. 3rd, 1997; it will also be released in the next scheduled STSDAS update, planned for late Spring 1997. If you require the new version (calwp2 version 1.3.5.2) before the official release, it may be obtained via anonymous ftp:

ftp  ftp.stsci.edu	(login anonymous, email address is password)
cd   /outside-access/out.going/calwp2/

Installation instructions can be found in the README file in that directory.

Anomalies in Some Far UV PSFs:

by M. Stiavelli, S. Casertano, S. Gonzaga, M. Mutchler

Recent observations with some far UV filters have shown anomalous PSFs, particularly in the PC. The filters and filter combinations showing anomalous PSFs typically use two crossed filters, namely: F122M crossed with F130LP, F160BW crossed with F130LP, and F160BW crossed with F165LP. The problem is also seen in F122M alone. The worst of these PSFs is the one seen in F122M crossed with F130LP. Exposures taken with F555W at the same time as the UV images have normal PSFs. By analyzing archival exposures in the same filters we have been able to establish that: 1) occasionally non-optimal PSFs have occurred in the past and therefore we are not in the presence of a sudden deterioration of WFPC2, 2) the bad PSFs are not affecting the photometric accuracy by more than 5 per cent (and probably much less), 3) very few GO observations have actually used these filters, and mostly to observe well resolved objects like, e.g., planets. The anomaly could be qualitatively described as a combination of defocusing and coma. A preliminary analysis of the problem done in collaboration with WFPC2 IDT team members (C. Burrows, D. Crisp, J. Trauger) suggests that the problem could be due to a combination of focus drift, focus breathing and intrinsic power in the filter. Indeed, F130LP, F165LP, and F122M all contain a weak lens. A more detailed study of the problem is in progress.

Second Update on WFPC2 Polarization Calibration:

by J. Biretta and M. McMaster

A second generation WWW tool for WFPC2 polarization calibration has been released. It allows much easier calibration of point sources, as well as calibration of extended targets. Given observed aperture count rates for a stellar target measured in three different settings of the POLQ filter, the tool will automatically solve the Mueller matrix equations and output the polarization properties of the target. If instead, one has an extended target, the same tool will generate simple recipes for the Stokes parameters which are then used to compute the polarization vector field (e.g. using imarith or imcalc in IRAF). Both calculations include corrections for the WFPC2 instrumental polarization. Examples are given for analysis of both point sources and extended targets, including derivation and plotting of the polarization E-vector field.

New Version of the Data Handbook is Planned:

by M. Voit

A revised version of the HST Data Handbook, including new chapters on NICMOS and STIS, is being planned for this fall. To aid in our planning, we would like your comments on the current version (Version 2, December 1995). How can we improve it? What features do you find helpful or unhelpful? Does the current structure quickly lead you to what you need to know? What issues should be covered in greater depth? Is there any material you have found particularly confusing or unclear?

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.

KRIST, J.E.; BURROWS, C.J.; STAPELFELDT, K.R.; BALLESTER,
G.E.; CLARKE, J.T.; CRISP, D.; EVANS, R.W.; GALLAGHER, J.S.
III; GRIFFITHS, R.E.; HESTER, J.J.; HOLTZMAN, J.A.;
HOESSEL, J.H.; MOULD, J.R.; SCOWEN, P.A.; TRAUGER, J.T.;
WATSON, A.M.; WESTPHAL, J.A.  "HST WFPC2 images of emission
nebulosity near XZ Tauri"  ApJ accepted

ALONSO, M.V.; MINNITI, D.  "Infrared photometry of 487
sources in the inner regions of NGC 5128 (Cen A)"

FERRARO, F.R.; CARRETTA, E.; BRAGAGLIA, A.; RENZINI, A.;
ORTOLANI, S.  "The luminosity function of the globular
cluster NGC 6752 with the Hubble Space Telescope: evidence
of mass segregation"  MNRAS accepted

O'DELL, C.R.; BURKERT, A.  "The nature of the cometary
knots in the helix nebula as determined by HST images"  IAU
Symp. 180

PROVENCAL, J.L.; SHIPMAN, H.L.; WESEMAEL, F.; BERGERON, P.;
BOND, H.E.; LIEBERT, J.; SION, E.M.  "WFPC2 observations of
Procyon B"  European White Dwarf Wksp. 10 - in Aparicio
preprint

SAHA, A.; SANDAGE, A.; LABHARDT, L.; TAMMANN, G.A.;
MACCHETTO, F.D.; PANAGIA, N.  "Cepheid calibration of the
peak brightness of SNe Ia. VIII. SN 1990N in NGC 4639"

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

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

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