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WFPC2 STAN

S T A N / W F P C 2 - Number 28, October 1997

CONTENTS:

The 1997 Calibration Workshop:

by Stefano Casertano

The 1997 HST Calibration Workshop was held on September 22-24. Talks and posters relevant to WFPC2 users are listed below. The proceedings will be published by STScI, and are expected to be available in mid-December. In addition to the formal presentations listed below, there was also a WFPC2 splinter session which allowed for more detailed discussion of three hot topics: relative astrometry between WFPC2 chips, which appear to have experienced a relative drift of about 0.1" since launch (see poster by Ratnatunga); photometric anomalies, especially the CTE effect (discussed in ISR WFPC2 97-08, as well as in the talks by Whitmore, Trauger and Casertano), and PSF subtraction (see posters by Surdej and collaborators and references therein). The WFPC2 group described some details of the Cycle 7 calibration plan, especially in relation to the calibration of the long vs. short photometric anomaly (see ISR WFPC2 97-06). An interesting addition to the poster session was a live demonstration by Max Mutchler of the use of Drizzle to remove cosmic rays from dithered images. The tasks he used are in development: some will be released in November, with the remainder expected to be ready for general release in the Spring of 1998.

WFPC2-related talks and posters at the Calibration Workshop:

   Brad Whitmore:  WFPC2 Status and Overview
   John Trauger: CCD Nonlinearities
   Stefano Casertano: Photometric Anomalies in WFPC2 Data
   Andy Fruchter: The "Dither" Package and the Reconstruction of Undersampled Images
   G. Dudziak and J. R. Walsh: WFPC2 Calibration for Emission Line Images
   P. Linde, A. Ardeberg, T. Bensby, and R. Snel: HST, uvby Photometry and System Calibration
   Alberto Micol, P. Bristow and B. Pirenne: The WFPC2 Associations Project
   Max Mutchler and Andy Fruchter: Drizzling Dithered WFPC2 Images - a Demonstration
   Kavan U. Ratnatunga, E. J. Ostrander and R. E. Griffiths: Calibrating the WFPC2 Pointing
   M. Remy, J.-F. Claeskens, and J. Surdej: HST Observations of the Gravitational Lens Systems HE1104-1805 and J03.13
   M. Remy, J. Surdej, S. Baggett, M. S. Wiggs: WFPC2 Photometry via Subtraction of TinyTim PSFs
   A. Suchkov and S. Casertano: Aperture Corrections for WFPC2 Stellar Photometry
   J. Surdej, S. Baggett, M. Remy, and M. S. Wiggs: WFPC2 Photometry from Subtraction of Observed PSFs
   M. S. Wiggs, S. Baggett, J. Surdej, and C. Tullos: The WFPC2 PSF Library
   M. S. Wiggs, B. Whitmore, and I. Heyer: The WFPC2 Clearinghouse
   Nailong Wu and John Caldwell: Deconvolution of Sub-stepped 1-D and 2-D HST Data

Small Discontinuity in the Photometric Monitoring Data:

by Brad Whitmore

Photometric monitoring observations of the star GRW+70D5824 are taken with the WFPC2 ever month. Based on these data, the possibility of a small increase (1 - 2 %) in the throughput of the F814W filter was noticed last year, and mentioned in the Instrument Science Report WFPC2 97-01. A longer baseline makes it clear that other filters show a similar effect, and the cause of this apparent anomaly is a discontinuity in February 1995, rather than a steady increase. This is when the exposure times for the photometric monitoring observations were increased (by a factor of 1.4 to 4 for the different filters). Hence, it is likely that the discontinuity is related to the long vs. short anomaly and/or CTE (i.e., CTE is reduced both when the background is increased and when the target is brighter, as would be the case for longer exposure times). The new equations for CTE in ISR 97-08 suggests discontinuities of 1 - 2 % might be expected in these cases, but the equations do not appear to explain the relative numbers for the different filters and chips very well, so this may not be the entire answer.

In any case, it is important to note that it is not the throughput that is changing (i.e., the throughput before and after Feb. 1995 is constant to better than 1 percent), but the discontinuity does highlight the fact that zeropoint determinations are most appropriate for observations which are similar to those used in the calibration observations (i.e., similar background and target brightness). An extensive set of new observations will be made in November to better quantify the long vs. short and CTE effects for the WFPC2.

Recent WFPC2 Documentation:

  • Instrument Science Report WFPC2 97-09: "Results of the WFPC2 post-Servicing Mission-2 Calibration Program"
  • by J. Biretta and the WFPC2 Group

    A summary of the seven TIRs based on the 1997 HST servicing mission. The servicing mission appears to have caused essentially no change in the performance and calibration of the WFPC2. The only discernable effect was a small and temporary increase in the rate of contamination as evidenced by slightly decreased UV throughput.

  • Instrument Science Report WFPC2 97-10: "WFPC2 Synphot Update"
  • by S. Baggett, S. Casertano, S. Gonzaga, C. Ritchie

    The SYNPHOT tables used to provide photometric calibration information in the image headers have been updated. Most filters were only changed by 1-2%, but a few changes were larger (e.g., F170W, F218: ~5% and F160BW, F375N: ~10% or more).

    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.

    DEUTSCH, E.W.; ANDERSON, S.F.; MARGON, B.; DOWNES, R.A.
    "The probable optical counterpart of the luminous X-ray
    source in NGC 6441"  ApJ 2-1-98
    
    HOLLAND, S.; FAHLMAN, G.G.; RICHER, H.B.  "Deep HST V- and
    I-band observations of two globular clusters in the halo of
    M31"  AJ 10-97
    
    JACOBY, G.H.; MORSE, J.A.; FULLTON, L.K.; KWITTER, K.B.;
    HENRY, R.B.C.  "Planetary nebulae in the globular clusters
    Pal 6 and NGC 6441"  AJ 12-97
    
    LAUER, T.R.; TONRY, J.L.; POSTMAN, M.; AJHAR, E.A.;
    HOLTZMAN, J.A.  "The far field Hubble constant"  ApJ
    
    MIGHELL, K.J.  "WFPC2 observations of the Carina dwarf
    spheroidal galaxy"  AJ accepted
    
    SAHU, K.C.; LIVIO, M.; PETRO, L.; BOND, H.E.; MACCHETTO,
    F.D.; GALAMA, T.J.; GROOT, P.J.; VAN PARADIJS, J.;
    KOUVELIOUTOU, C.  "Observations of GRB 970228 and GRB
    970508, and the neutron-star merger model"  ApJ accepted
    
    MADORE, B.F.; FREEDMAN, W.L.  "Calibration of the
    extragalactic distance scale"
    
    MILLER, B.W.; WHITMORE, B.C.; SCHWEIZER, F.; FALL, S.M.
    "The star cluster system of the merger remnant of NGC 7252"
    AJ accepted
    
    ROSER, H.-J.; MEISENHEIMER, K.; NEUMANN, M.; CONWAY, R.G.;
    AVIS, R.J.; PERLEY, R.A.  "The jet of the quasar 3C 273 at
    high resolution"  Rev. Mod. Astron. 10: 253-261, 1997
    
    BURG, C.A.; WINDHORST, R.A.; ODEWAHN, S.C.; DE JONG, R.S.;
    FROGEL, J.A.  "A systematic UBVR IJHK survey of nearby
    galaxies to classify faint galaxies from deep HST surveys"
    UV Universe at Low and High Redshift and Hubble Deep Field
    
    COLE, A.A.; MOULD, J.R.; GALLAGHER, J.S. III; CLARKE, J.T.;
    TRAUGER, J.; BALLESTER, G.E.; BURROWS, C.J.; CASERTANO, S.;
    CRISP, D.; GRIFFITHS, R.; HESTER, J.J.; HOESSEL, J.G.;
    HOLTZMAN, J.A.; SCOWEN, P.A.; STAPELFELDT, K.R.; WESTPHAL,
    J.A.  "Far-ultraviolet imaging of the Large Magellanic
    Cloud populous cluster NGC 1978 with WFPC2"  AJ 11-97
    
    ODEWAHN, S.C.; BURSTEIN, D.; WINDHORST, R.A.  "The axis
    ratio distributions of local and distant galaxies"  AJ 12-97
    
    SHARA, M.M.; DRISSEN, L.; RICH, R.M.; PARESCE, F.; KING,
    I.R.; MEYLAN, G.  "Supra horizontal branch stars and
    population gradients in the galactic center globular
    cluster NGC 6522"  ApJ 3-98
    
    WINDHORST, R.A.; ODEWAHN, S.; COHEN, S.; BURG, C.; DE JONG,
    R.S.; DRIVER, S.P.; MARZKE, R.O.; TYSON, J.A.;
    DELL'ANTONIO, I.  "The HST/WFPC2 B-band galaxy counts vs.
    type for 19 <~ B <~ 29 mag"  UV Universe at Low and High
    Redshift
    
    DOHM-PALMER, R.C.; SKILLMAN, E.D.; SAHA, A.; TOLSTOY, E.;
    MATEO, M.; GALLAGHER, J.; HOESSEL, J.; CHIOSI, C.; DUFOUR,
    R.J.  "The dwarf irregular galaxy Sextans A. II. Recent
    star formation history"  AJ accepted
    
    DOHM-PALMER, R.C.; SKILLMAN, E.D.; SAHA, A.; TOLSTOY, E.;
    MATEO, M.; GALLAGHER, J.; HOESSEL, J.; CHIOSI, C.; DUFOUR,
    R.J.  "The dwarf irregular galaxy Sextans A. I. HST
    photometry of the resolved stars"  AJ accepted
    
    ELSON, R.A.W.; GRILLMAIR, C.J.; FORBES, D.A.; RABBAN, M.;
    WILLIGER, G.M.; BRODIE, J.P.  "HST imaging of the globular
    clusters in the Fornax cluster: NGC 1379"  MNRAS accepted
    
    FERRARO, F.R.; PALTRINIERI, B.; FUSI PECCI, F.; DORMAN, B.;
    ROOD, R.T.  "Faint ultraviolet objects in the core of M13:
    optical counterparts of the low luminosity X-ray source?"
    MNRAS accepted
    
    MCCARTHY, P.J.; MILEY, G.K.; DE KOFF, S.; BAUM, S.A.;
    SPARKS, W.B.; GOLOMBEK, D.; BIRETTA, J.; MACCHETTO, F.
    "Hubble Space Telescope snapshot survey of 3CR radio
    sources counterparts. I. Radio galaxies with z > 0.5"  ApJS
    112: 415-421, 1997
    

    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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