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WFPC2 STAN

S T A N / W F P C 2 - Number 39, January 2000

CONTENTS:

On-The-Fly Calibration Now Available:

The On-The-Fly Calibration (OTFC) system is now an option available when retrieving WFPC2 (and STIS) images from the archive. OTFC recalibrates the images as they are requested by users, using the latest calibration software, calibration files, and up-to-date keyword values. OTFC can be accessed through Starview as well as the WWW archive interface.

Several groups within STScI participated in the OTFC project: the Archive Branch, the Archive Team, Computer and Network Services, the Data Processing Team, OPUS operations, the Software Support Group, Tools and Technology, and the STIS and WFPC2 instrument groups.

Phase II Deadline is Approaching!:

The Cycle 9 Phase II GO deadline is March 3, 2000. There are updates for WFPC2 observers available on the WFPC2 Website.

The updates include information on the syntax for patterns (e.g., to be used for dithering) and locating a target at low values of x,y on the chip to reduce the effects of CTE loss. If you have questions about Phase II issues or feel you need to have a Contact Scientist assigned please send email to help@stsci.edu.

WFPC2 is Back to Normal Operations: Preliminary Calibration Results from Servicing Mission 3a:

by Stefano Casertano and the WFPC2 group

As shown dramatically by the recently released ERO observations of the Eskimo Nebula and of Abell 2218, WFPC2 is now back to normal operations and is executing an intensive program of science observations.

A careful program of contamination monitoring and control has been followed to protect WFPC2 from dangerous levels of UV contamination during the first two weeks after the servicing mission. Cool down has followed a two-step procedure, with measurements at an intermediate temperature to verify the safety of the camera. Numerous observations have tracked the faster-than-normal buildup of contaminants that typically follows a servicing mission, and more frequent decontamination procedures will be carried out for the first month of operations to ensure that the overall contamination does not exceed normal values.

Preliminary verifications of the WFPC2 characteristics confirm thus far that there have been no major changes in the calibration of WFPC2. To the precision of these observations (typically a few percent), the photometric throughput appears normal, and the first decontamination has restored the UV sensitivity to pre-servicing mission values. The image quality remains unchanged, and the camera electronics are normal, with a possible increase in the dark current that continues an established trend in WFPC2.

We are currently carrying out a program of calibration observations that will enable us to re-establish the WFPC2 calibration to the accuracy achieved in normal WFPC2 operations. These observations include a photometric sweep with the most frequently used filters, a test of the throughput in the far UV, a new flat field and PSF calibration, and new measurements of camera internals. The results of these calibrations will be available by the end of February, and will be applicable to all post-servicing mission observations.

Time Dependence of CTE from Cosmic Ray Trails:

by A. Riess, J. Biretta, and S. Casertano

We have developed a method to measure the counts in cosmic ray trails (CRTs), which result from imperfect charge transfer efficiency (CTE). Like previous measures of CTE, the counts in the CRTs are a strong function of the number of serial and parallel transfers, background, source counts, and lifetime of the instrument. Analysis of the CRTs reveals that some charge is trapped and released on short timescales (~seconds or less), although the charge in CRTs does not account for all observed CTE losses. The time dependence of Y-CTE and X-CTE are shown for WFPC2 and STIS using the STScI archive of dark calibration frames. All show a steady growth with time since the installation of the respective instruments. There is also evidence for a mild increase in the growth rate (i.e. acceleration), of the WFPC2 Y-CTE. The results indicate that this method may provide a precise way to monitor CTE with greater time sampling than is currently feasible and without the cost of additional pointed observations.

Preliminary Results of the Noiseless Preflash Test (Proposal 8450):

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

We report preliminary results from analysis of the WFPC2 noiseless preflash test data (program ID: 8450). The short 16 sec. exposures were analyzed for this report. (The long exposure observations of 80 sec. and 400 sec. were lost due to guide star reacquisition failure, HOPR 587 was filed against 8450, and a repeat has been approved.) The calibration lamp was used to preflash the wide field CCDs to about 2000 counts, and the preflash was read out before the start of the science observations. In principle, this might fill the CCD traps while introducing no noise in the science image (i.e. noiseless). Stars within the globular cluster Omega Cen (HD116790, NGC 5139) were positioned in the WF4, WF3, and WF2 apertures. Photometry of faint star images showed on average a 3.0 +/- 0.9% enhancement in the stellar counts (at Y=800) in the preflash exposures. This is consistent with the noiseless preflash giving only a partial reduction in CTE. The majority of the CTE effect must be due to traps which release their charge on time scales of less than two minutes.

For a figure illustrating the effect, please see the online copy of the AAS poster from Jan 2000, on the WFPC2 Documentation Webpage.

WFPC2 Cycle 7 Closure Report:

by S. Baggett, J. Biretta, S. Casertano, S. Gonzaga, I. Heyer, M. McMaster, C. O'Dea, A. Schultz, B. Whitmore, M. S. Wiggs

This report describes in detail the WFPC2 observations used to maintain and improve the quality of WFPC2 calibrations during Cycle 7 and their status as of November 1999. Also included are the WFPC2 programs executed during the NIC3 Campaign in 1998 and summaries of the Cycle 6 proposal analyses completed since the writing of the Cycle 6 Closure report.

WFPC2 Status Update (From the January, 2000 AAS in Atlanta):

by I. Heyer, J. Biretta, S. Baggett, S. Casertano, C. O'Dea, A. Schultz, S. Gonzaga, M. McMaster, M. S. Wiggs, A. Koekemoer, A. Riess

We review the status of the Wide-Field Planetary Camera II (WFPC2) onboard the Hubble Space Telescope as well as recent enhancements to calibration and analysis methods. The instrument continues to perform extremely well after ~70000 images*. Low level effects such as CTE and dark current continue to increase with long-term radiation exposure, but still affect only a minority of observations; work continues to characterize and monitor these effects. The long-term photometric stability appears to be excellent, with most filters showing changes of a few percent or less. Recent work shows that aperture photometry for small apertures (1-2 pixel radius) will be affected at the 5% to 10% level by target position in the field-of-view due to small focus variations. A new "on-the-fly calibration" feature of the HST archive allows instant re-calibration of WFPC2 data with the latest reference files; this should be a great benefit to both new observers and archival WFPC2 users. Recent results on polarization calibration, CTE, and other topics are discussed.

*Based on the performance prior to the December 1999 Servicing Mission; post-SM3A recommissioning is proceeding normally.

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.

SIRIANNI, M.; NOTA, A.; LEITHERER, C.; DE MARCHI,
G.; CLAMPIN, M. "The Low End of the Initial Mass
Function in Young LMC Clusters. I. The Case of
R136" ApJ accepted

DEUTSCH, E.W.; MARGON, B.; ANDERSON, S.F.
"Ultracompact X-ray Binaries in Globular Clusters:
Variability of the Optical Counterpart of X1832-330
in NGC 6652" ApJ accepted

POGGE, R.W.; MAOZ, D.; HO, L.C.; ERACLEOUS, M. "The
Narrow-Line Regions of LINERs as Resolved with the
Hubble Space Telescope"

URRY, C.M.; SCARPA, R.; O'DOWD, M.; FALOMO, R.;
PESCE, J.E.; TREVES, A. "The HST Survey of BL
Lacertae Objects. II. Host Galaxies" ApJ accepted

BRANDNER, W.; GREBEL, E.K.; CHU, Y-H.; DOTTORI, H.;
BRANDL, B.; RICHLING, S.; YORKE, H.W.; POINTS,
S.D.; ZINNECKER, H. "HST/WFPC2 and VLT/ISAAC
Observations of PROPLYDS in the Giant HII Region
NGC 3603" AJ 1/00

DA COSTA, G.S.; ARMANDROFF, T.E.; CALDWELL, N.;
SEITZER, P. "The Dwarf Spheroidal Companions to
M31: WFPC2 Observations of Andromeda II" AJ accepted

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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