S T A N / W F P C 2 - Number 29, January 1998
CONTENTS:
- WFPC2 NEWS:
- Time Dependence of the Charge Transfer Efficiency (CTE) on the WFPC2
- Preliminary version of the Proceedings of the 1997 HST Calibration Workshop available online
- Polarization Calibration Update
- Recent WFPC2 Documentation:
- Instrument Science Report: "WFPC2 Polarization Calibration" (WFPC2 ISR 97-11)
- RECENT PREPRINTS
- APPENDIX: WFPC2 CONTACTS
Time Dependence of the Charge Transfer Efficiency (CTE) on the WFPC2:
by Brad Whitmore
An analysis of WFPC2 observations of the globular cluster Omega Cen (NGC 5139) indicates that the CTE problem on the WFPC2 has increased with time. The observational consequence of CTE loss is that a point source at the top of the chip appears to be fainter than if observed at the bottom of the chip, due to the loss of electrons as the star is read out down the chip. There also appears to be a similar but weaker tendency for stars on the right side of the chip to be fainter (called X-CTE). The effects also depend on the brightness of the star and the background level. A recent Instrument Science Report (WFPC2 ISR 97-08, "New Results on Charge Transfer Efficiency and Constraints on Flat-Field Accuracy" by Whitmore and Heyer); develops a set of formulae that provides a method to correct for CTE when doing aperture photometry.
As a continuation of this project we recently analyzed a set of 7 observations of Omega Cen to determine whether CTE loss is time dependent. The datasets cover the time range from April 28, 1994 (shortly after the cooldown) to June 26, 1997. The F814W filter was used since it provides the highest S/N measurements of the CTE for this particular field. For bright stars (i.e., brighter than 200 DN when using gain = 15; equivalent to 400 DN for gain = 7) the news is good, with no increase in the amount of CTE loss as a function of time. However, for faint stars CTE loss has increased. For example, for very faint stars (i.e., 20 - 50 DN at a gain of 15) the CTE loss has increased from 3% to 22% for a star at the top of the chip. There is no obvious change in the value of X-CTE.
It should be noted that these results are based on very short (14 second) exposures. In general, typical WFPC2 exposures are much longer than these short calibration images, resulting in higher background levels which significantly reduces the CTE loss and minimizes the CTE problem for most science observations.
Preliminary version of the Proceedings of the 1997 HST Calibration Workshop available online:
by Stefano Casertano
A preliminary PostScript version of the contributed papers for the Proceedings of the 1997 Calibration Workshop can now be found on the STScI web site. An Adobe PDF version is also available (linked to the PostScript page); this version results in much smaller file sizes but requires the Acrobat Reader for viewing.
Polarization Calibration Update:
by John Biretta and Matt McMaster
An instrument science report ("WFPC2 Polarization Calibration," WFPC2 ISR 97-11) has been released which details the current state of the WFPC2 polarization calibration. The calibration described therein appears accurate to about 1.5%. The calculation and application of the polarizer flats are described. A physical model for the polarization effects of the WFPC2 optics is then created using Mueller matrices. The model includes corrections for the instrumental polarization (diattenuation and phase retardance) of the pick-off mirror, as well as the high cross- polarization transmission of the polarizer filter. We compare this model against on-orbit observations of polarization calibrators, and show it predicts relative counts in the different polarizer / aperture settings to 1.5% RMS accuracy.
The report also gives detailed examples showing how to calibrate GO data using the on-line WWW polarization tools. These tools have recently been upgraded to include effects related to the MgF2 coating on the pick-off mirror. We have also changed to more accurate matrices for the cross-polarization leakage in the polarizer filter. Differences between the old and new versions of the tools are typically around 1% in fractional polarization.
Recent WFPC2 Documentation:
- Instrument Science Report WFPC2 97-11: "WFPC2 Polarization Calibration"
by J. Biretta and M. McMaster
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.
CALDWELL, N.; ARMANDROFF, T.E.; DA COSTA, G.S.; SEITZER, P. "Dwarf elliptical galaxies in the M81 group: the structure and stellar populations of BK5N and F8D1" KAARESEN, K.F. "Evaluation and applications of the iterated window maximization method for sparse deconvolution" IEEE Trans. Signal Proc. KENNICUTT, R.C. JR.; STETSON, P.B.; SAHA, A.; KELSON, D.; RAWSON, D.M.; SAKAI, S.; MADORE, B.F.; MOULD, J.R.; FREEDMAN, W.K.; BRESOLIN, F.; FERRARESE, L.; FORD, H.; GIBSON, B.K.; GRAHAM, J.A.; HAN, M.; HARDING, P.; HOESSEL, J.G.; HUCHRA, J.P.; HUGHES, S.M.G.; ILLINGWORTH, G.D.; MACRI, L.M.; PHELPS, R.; SILBERMANN, N.A.; TURNER, A.M.; WOOD, P.R. "The HST key project on the extragalactic distance scale. XIII. The metallicity dependence of the Cepheid distance scale" 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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