S T A N / W F P C 2 - Number 33, August 1998
- WFPC2 NEWS:
- Long-Term Photometric Stability of WFPC2
- Long vs. Short Exposure Anomaly in WFPC2 Images
- Excessive Noise in the F160BW Flat
- WFPC2 Phase I Proposal Preparation Page on WWW
- RECENT PREPRINTS
- APPENDIX: WFPC2 CONTACTS
Long-Term Photometric Stability of WFPC2:
by Sylvia Baggett and Shireen Gonzaga
We have analyzed WFPC2 photometric monitoring data taken over the last 4 years, and looked for long-term trends in the data. Images of a spectrophotometric standard star are taken twice a month, usually just before and after the monthly decontamination procedures, in the standard suite of broadband filters (F160BW, F170W, F218W, F255W, F336W, F439W, F555W, F675W, and F814W). We have examined these data for long-term trends in the "uncontaminated" throughput, as well as for changes in the rate at which contaminants accumulate between the monthly decontaminations. The preliminary results indicate that:
1) The long-term photometric throughput has been extremely stable. After correction for the monthly decontamination cycles, most filters show peak-to-peak throughput fluctuations of 2% or less over the four years.
2) The far UV throughput in the PC camera is slowly increasing with time. That is, the clean count rates (immediately after the decons) have increased from 1994 to 1998, e.g., in F160BW by ~12%, and in F170W by ~9%. One possible explanation is that some contaminant is very slowly outgassing from the PC.
3) The contamination growth rates between the monthly decontaminations has decreased over the years. This mostly impacts the far-UV filters. Typically the CCD assemblies are decontaminated (warmed to +20C for several hours) once a month to remove contaminants which slowly build on the cold CCD windows. These contaminants slowly re-accumulate after each "decon." For example, in F160BW on PC1 the accumulation rate has slowed from 0.89% throughput loss per day to 0.61% loss per day. The other cameras show similar reductions in their contamination rates.
We note that the effects of CTE loss are minimal in this data set, since the star is very bright. These new results are being summarized in an Instrument Science Report (ISR) which will appear on our WWW site in the near future. The results will also be incorporated into the WFPC2 time-dependent photometric calibration within SYNPHOT. We note that the standard star monitoring results (measured countrates) are posted and regularly updated on the WFPC2 Docmentation page, under the Calibration and Monitoring Memos section (see link below). This new analysis extends previous work of Whitmore et al. (ISR 96-04) and Baggett et al. (ISR 96-02).
Long vs. Short Exposure Anomaly in WFPC2 Images:
by Stefano Casertano and Max Mutchler
The WFPC2 CCDs exhibit a photometric anomaly where targets appear unexpectedly faint in short exposure images as compared to long exposures. This effect appears to be different from the well-known Charge Transfer Efficiency (or CTE) problem. We have recently concluded a study of of the long vs. short anomaly using images of a star cluster taken with exposures ranging from 10 to 1000 sec (proposal 7630). The main results can be summarized as follows:
1) The "long vs. short" anomaly is a non-linearity of the detector+signal chain that is primarily a function of the total number of source counts. It can be represented as the loss of a fraction of the signal; the fraction increases for fainter signal.
2) In the worst-case scenario of images with no background light, the size of the effect ranges from < 0.02 magnitude for star images containing > 300DN, to as much as ~0.4 magnitude for star images containing only 10 DN.
3) Unlike the well-known CTE error, the signal loss is not a function of position on the chip.
4) The signal loss decreases slightly with increasing background; however, preflash is not a viable method to observe faint sources, as the increase in photon noise overwhelms any other consideration.
5) A simple formula can be used to restore the signal lost and effectively correct the photometry to an accuracy of a few percent for faint sources.
These results are being summarized in an Instrument Science Report and will be posted soon on the WFPC2 web site.
Excessive Noise in the F160BW Flat:
by John Biretta and Sylvia Baggett
We have recently examined the noise characteristics of the far-UV flats, and found that the F160BW flat in particular, is very noisy. The RMS noise near the center of the PC is ~20%, with a few individual pixels showing errors as large as a factor of 6. Observers requiring accurate photometry in F160BW on the PC are advised to use the F255W flat instead, which has much better noise characteristics. We expect that any errors due to wavelength effects will be small compared to the noise in F160BW. The F255W flat is preferred over other flats at nearby wavelengths, since, e.g., the F170W filter has a large redleak, and the F218W flat has larger noise than F255W.
For F160BW data on the WF CCDs, the F255W flat can be used, but the large-scale vignetting of the F160BW filter will require special treatment. For example, one might perform a vignetting correction by computing the ratio (F160BW flat) / (F255W flat), smoothing it with a 20 pixel FWHM Gaussian function, and then dividing this smoothed ratio image into the data already flattened with F255W. We expect to revise the far-UV flats in the coming months.
WFPC2 Phase I Proposal Preparation Page on WWW:
by Michael S. Wiggs and John Biretta
We note that a new Phase I proposal preparation WWW page has been added to our web site, which collects various resources useful for Phase I preparation. We have also updated the WFPC2 Exposure Time Calculator with the most recent throughput data; the ETC is also available on the new Phase I page.
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.
ELMEGREEN, B.G.; ELMEGREEN, D.M.; BRINKS, E.; YUAN, C.; KAUFMAN, M.; KLARIC, M.; MONTENEGRO, L.; STRUCK, C.; THOMASSON, M. "Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207" ApJ RUBIO, M.; BARBA, R.H.; WALBORN, N.R.; PROBST, R.G.; FARCIA, J.; ROTH, M.R. "Infrared Observations of Ongoing Star Formation in the 30 Doradus Nebula and a Comparison with HST/WFPC2 Images" AJ 10-98 BIRETTA, J.A.; PERLMAN, E.; SPARKS, W.B.; MACCHETTO, F. "HST Observations of the M87 Jet" M87 Wksp. COLE, G.H.J.; PEDLAR, A.; MUNDELL, C.G.; GALLIMORE, J.F.; HOLLOWAY, A.J. "Neutral Hydrogen Absorption Observations of the Central Region of NGC 5929" MNRAS MIGHELL, K.J.; SARAJEDINI, A.; FRENCH, R.S. "WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud" AJ accepted VAN DER MAREL, R.; VAN DEN BOSCH, F.C. "Evidence for a 3 X 10(8) M(o) Black Hole in NGC 7052 from HST Observations of the Nuclear Gas Disk" AJ accepted XANTHOPOULOS, E.; BROWNE, I.W.A.; KING, L.J.; KOOPMANS, L.V.E.; JACKSON, N.J.; MARLOW, D.R.; PATNAIK, A.R.; PORCAS, R.W.; WILKINSON, P.N. "The New Gravitational Lens System B1030+074" MNRAS accepted BRODIE, J.P.; SCHRODER, L.L.; HUCHRA, J.P.; PHILLIPS, A.C.; KISSLER-PATIG, M.;FORBES, D.A. "Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275" AJ accepted GONZALEZ, R.A.; ALLEN, R.J.; DIRSCH, B.; FERGUSON, H.C.; CALZETTI, D.; PANAGIA, N. "The Opacity of Nearby Galaxies from Colors and Counts of Background Galaxies: I. The Synthetic Field Method and Its Application to NGC 4536 & NGC 3664" ApJ 10-10-98 LUBIN, L.M.; POSTMAN, M.; OKE, J.B.; RATNATUNGA, K.U.; GUNN, J.E.; HOESSEL, J.G.; SCHNEIDER, D.P. "A Study of Nine High-Redshift Clusters of Galaxies. III. HST Morphology of Clusters 0023+0423 and 1604+4304" AJ accepted OSTLIN, G.; BERGVALL, N.; RONNBACK, J. "Globular Clusters in the Blue Compact Galaxy ESO 338-IG04 (Tol 1924-416), as Tracers of the Star Formation History. Results from HST/WFPC2 Observations" A&A 335: 85-112, 1998 PASCARELLE, S.M.; WINDHORST, R.A.; KEEL, W.C. "Compact Lyman-alpha Emitting Candidates at z ~= 2.4 in Deep Medium-Band HST WFPC2 Images" AJ
APPENDIX: WFPC2 Contacts:
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