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WFPC2 STAN

S T A N / W F P C 2 - Number 40, April 2000

CONTENTS:

On-The-Fly Calibration: Cessation of Archiving Calibrated WFPC2 Data Products:

As many observers are already aware, the On-The-Fly Calibration (OTFC) system is now available when retrieving WFPC2 (and STIS) images from the archive. OTFC can be accessed through Starview as well as the WWW Archive interface.

Plans are currently underway to proceed with the next step in OTFC development and halt the archiving of calibrated WFPC2 data products. On or about May 15, 2000, calibrated WFPC2 data will be available only via OTFC; the original version of the calibrated data will no longer be archived. The impact to users should be minimal: currently, the vast majority of WFPC2 data retrieval requests are already for OTFC products, not for the original calibrated products. The OTFC system is the recommended source for calibration data, since it provides the best possible results, using the latest software, reference files, and any necessary header keyword fixes. Any observers anticipating requiring access after May 15 to original calibrated data products are asked to contact the helpdesk as soon as possible at help@stsci.edu.

Many 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.

New Tiny Tim Version: includes field dependent aberrations and F1042M PSF halo

by J. Krist

This version incorporates a number of enhancements; of particular interest to WFPC2 observers will be the addition of field-dependent WFPC2 aberrations and an approximation of the F1042M halo. The aberrations (astigmatism, coma, and focus) are adjusted depending upon the specified position of the PSF on the detector; the aberration/position relations are courtesy of Jon Holtzman (NMSU). The F1042M halo, which contains about 20% of the total flux, is seen only in F1042M images, it is NOT seen in any other filters. Its cause is likely due to the fact that the CCD becomes essentially transparent at such long wavelengths and light is scattered within the CCD backing.

AREA Mode Images: Minor Bug in CALWP2:

by J.C. Hsu, M. S. Wiggs, and S. Baggett

There is a minor bug in the way calwp2 performs the AREA mode bias level calculation. In AREA mode (2x2 pixel summation), the bias is determined not from the overscan, as for the FULL mode, but from the video biases stored at the first word (a two-byte unsigned integer) in the rows of the .x0h files. Furthermore, only odd-number rows from 3 to 399 have this information, so there are total of 199 rows or 398 bytes to be used. Each byte contains one bias number (e.g. 157). In the WFPC2 pipeline (and WFPC-1), the bias value was calculated from only the first 199 BYTES, instead of from all 398 bytes. Typically, this can result in a small error (~0.01 DN) in the bias values, though for a very small number of images (~20, those with large numbers of saturated pixels), the error can be ~1DN or larger. There are very few affected exposures: out of a total of nearly 90,000 WFPC2 images in the archive, only ~150 are AREA modes. All Cycle 7 and Cycle 8 GOs using AREA mode are being notified individually and CALWP2 will of course be updated as soon as possible. In the meantime, for assistance with any AREA mode images, please contact help@stsci.edu.

MEETING NEWS:

Detector Workshop June 26-30, 2000 Space Astrophysics Detectors and Detector Technologies:

Advances in detectors play a key role in the development of observational astrophysics. Looking towards the next decade of space missions, NASA's Office of Space Science wishes to bring together scientists and engineers working in areas ranging from gamma-rays to radio wavelengths to present the state-of-the-art in current and emerging detector technologies. The meeting will be of interest to detector developers and technologists as well as to scientists proposing new science missions.

CTE Workshop Proceedings Available Online:

The CTE (Charge Transfer Efficiency) Workshop was held at STScI from Jan 31,2000 to Feb 1,2000. Numerous topics were addressed, including 1) links between the physics of CCDs, their construction and aging in a high radiation environment, and what is seen in practice, 2) summaries of the latest measurements for both WFPC2 and STIS showing the effect of non-perfect CTE as a function of intensity level, sky background, position on detector, gain, time, etc., 3) presentation of attempts to understand/resolve differences in results for both WFPC2 and STIS analyses and discuss the expected level of CTE loss in ACS/WFC3, 4) documentation of steps that can be taken to correct for CTE loss in data, and discussing the inherent limitations of current procedures, and 5) discussion of physics based approaches to modeling CTE losses, and attempting to couple these with empirical calibrations of CTE on existing CCDs.

Preview of June 2000 AAS Poster Abstracts:

Two WFPC2 calibration posters are planned for the AAS meeting in Rochester; please stop by and see us!

Charge Transfer Efficiency in the WFPC2 CCD Arrays:

J. Biretta, A. Riess, S. Baggett, B. Whitmore, S. Casertano, I. Heyer, A. Schultz, S Gonzaga, M. S. Wiggs, M. McMaster, C. O'Dea, A. Koekemoer (STScI)

We present an overview of Charge Transfer Efficiency (CTE) issues in the WFPC2 CCDs, including results of recent on-orbit tests, and advice on mitigating CTE effects on photometry. New studies using hotpixels, cosmic rays, and residual images as probes of CTE have revealed at least four distinct components of CTE losses. The largest effect appears related to trapping and release of charge on timescales of hundreds of milliseconds during the readout process. This is manifest as tails on images which extend for dozen of pixels in the Y-direction (parallel register direction) on the CCDs, which have the effect of robbing counts from typical small apertures used for photometry. There is considerable evidence now for a quasi-linear increase in CTE effects with time, due to on-going radiation damage to the CCD arrays in the harsh space environment. We review empirical results on stellar photometry, and preliminary results of new tests on extended targets (faint galaxies). We suggest observational strategies for reducing the impact of CTE, and photometric corrections which can be applied during data analysis.

Wide Field Planetary Camera II Status Update:

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

We review the status of the Wide-Field Planetary Camera II (WFPC2) onboard the Hubble Space Telescope as well as summarize the re-commissioning tests performed after the December 1999 Servicing Mission (SM3a). While WFPC2 itself was not serviced, there is always concern about contamination or other unexpected effects. Although contamination rates increased during the mission (as expected), bi-weekly decontaminations immediately after SM3a, and subsequent monthly decons have successfully restored the UV throughput to their pre-SMOV values; there is no evidence of any permanent contamination. A photometric sweep covering the entire wavelength range of WFPC2 shows that any changes in the photometric response are less than ~1% at visible wavelengths, and less than 2% or 3% in the UV. No changes due to SM3a were seen in the flat field response, read noise, dark current, a-to-d gain ratios, and point spread function tests. In addition to the SM3a results, we also discuss the general status of WFPC2, including improved documentation for dithering, the now-public on-the-fly calibration (OTFC) of the archive, and plans for calibrating WFPC2 in Cycle 9. CTE results are presented in a separate poster.

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.

GALLAGHER, J.S. III; HOMEIER, N.L.; CONSELICE,
C.J.; WFPC-2 IDT "Patterns of Super Star Cluster
Formation in `Clumpy' Starburst Galaxies" Massive
Stellar Clusters

KRIST, J.E.; STAPELFELDT, K.R.; MENARD, F.;
PADGETT, D.L.; BURROWS, C.J. "WFPC2 Images of a
Face-On Disk Surrounding TW Hydrae" ApJ accepted

FERRARO, F.R.; PALTRINIERI, B.; ROOD, R.T.; FUSI
PECCI, F.; BUONANNO, R. "Another Faint UV Object
Associated with a Globular Cluster X-ray Source:
the Case of M92" ApJ accepted

LOPEZ, J.A.; MEABURN, J.; RODRIGUEZ, L.F.; VAZQUEZ,
R.; STEFFEN, W.; BRYCE, M. "The Formation of a
Multiple Planetary Nebula: HST/WFPC2 Observations
of KjPn 8" ApJ accepted

PANAGIA, N.; ROMANIELLO, M.; SCUDERI, S.; KIRSHNER,
R.P. "Young Stellar Populations Around SN 1987A"
ApJ accepted

CHEN, C.-H.R.; CHU, Y.-H.; GRUENDL, R.A.; POINTS,
S.D. "HST WFPC2 Imaging of Shocks in Superbubbles"
AJ 3-00

CONSELICE, C.J.; GALLAGHER, J.S.; CALZETTI, D.;
HOMEIER, N.; KINNEY, A. "Panchromatic Study of
Nearby Ultraviolet-Bright Starburst Galaxies:
Implications for Massive Star Formation and
High-Redshift Galaxies" AJ 119: 79-93, 2000

FALCKE, H.; WILSON, A.S.; HENKEL, C; BRUNTHALER,
A.; BRAATZ, J.A. "HST and VLA Observations of the
H2O Gigamaser Galaxy TXS2226-184" ApJ accepted

GILL, C.D.; O'BRIEN, T.J. "Hubble Space Telescope
Imaging and Ground Based Spectroscopy of Old Nova
Shells - I. FH Ser, V533 Her, BT Mon, DK Lac, V476
Cyg" MNRAS accepted

LEINERT, C.; ALLARD, F.; RICHICHI, A.; HAUSCHILDT,
P.H. "The Multiple System LHS 1070: a Case Study
for the Onset of Dust Formation in the Atmospheres
of Very Low Mass Stars" A&A 353: 691-706, 2000

REDMAN, M.P.; O'CONNOR, J.A.; HOLLOWAY, A.J.;
BRYCE, M.; MEABURN, J. "A 500 km S(-1) Outflow from
the Young Bipolar Planetary Nebula Mz 3" MNRAS
accepted

CECIL, G.; GREENHILL, L.J.; DEPREE, C.G.; NAGAR,
N.; WILSON, A.S.; DOPITA, M.A.; PEREZ-FOURNON, I.;
ARGON, A.L.; MORAN, J.M. "The Active Jet in NGC
4258 and Its Associated Shocks" ApJ 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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