S T A N / W F P C 2 - Number 22, December 1996
- WFPC2 NEWS:
- Status Report for Cycle 7 Observers
- RECENT PREPRINTS
- APPENDIX: WFPC2 CONTACTS
Status Report for Cycle 7 Observers:
by C. O'Dea
The WFPC2 is performing normally and as described in the Instrument Handbook. A WWW page containing updated information on topics of interest to Phase II proposers (including observing strategies) is available from the WFPC2 group WWW page.
The updates include information on PSF subtraction, polarization observations, dithering strategy, contamination, new software tools, linear ramp filters, scattered light, etc. Observing Strategies are also discussed in Chapter 7 of the WFPC2 Handbook. Some of these issues are summarized briefly below.
PSF subtraction (to search for faint objects near bright ones) is complicated by the large dynamic range required, the spatially dependent PSF, ghosts, and the undersampled PSF. Observers may want to consider various observation strategies including (1) taking multiple exposures with different exposure times (e.g., 1, 10, 100 sec), (2) observing a star in the same position as the object (also using multiple exposures), (3) taking 2 or more images with sub pixel dithering, and (4) obtaining observations at different roll angles. A new library of observed PSFs is being developed and will be available via the WFPC2 WWW page.
Observers planning WFPC2 polarization observations should be aware that the WFPC2 polarizers are not designed for high precision work. Fractional polarization values measured with WFPC2 may have systematic errors of 0.03. These are due largely to the instrumental polarization induced by the pick-off mirror, and by uncertainties in the polarizer flat fields. Also, the polarizers become ineffective outside the wavelength range spanning filters F255W to F675W. Tradeoffs between ease of scheduling, obtaining optimum polarizer position angles, inter-CCD flat-fielding, and use of the PC1 CCD must be considered. See ISR 95-01: "WFPC2 Polarization Observations: Strategies, Apertures, and Calibration Plans" available on the WFPC2 WWW pages.
Dithering can provide greater image resolution and superior removal of chip defects, but requires a substantial amount of extra work and (occasionally) overhead. Users interested in dithering should read the section in this document on General Advice on Dithering HST Observations. RPS2 now support use of CR-SPLITs with the canned dithering routines specified with optional parameter Dither-Type = Line or Box. Note that (in RPS2 release 7.0) the numbering of exposures on the RPS2 exposure summary page is incorrect for some combinations of the canned dither routine with use of CR-SPLIT and Number of Iterations. However, the total number of exposures on the summary page, as well as the graphical orbit structure diagrams, will be correct. This problem is documented on the RPS2 Advisory Page.
Contact your Program Coordinator regarding questions on RPS2.
Currently 2-5% photometric accuracy is routinely attainable at visual and red wavelengths. UV performance is poorer, and is limited by the continuously changing contamination (see below). See the section on photometry under the WWW WFPC2 page for further information. One of the problems for high-precision photometry is the imperfect CCD Charge Transfer Efficiency (CTE). Experiments are underway to find observation strategies which can reduce this effect (e.g., preflashing, contact your Contact Scientist if interested).
Far UV photometry is affected by contamination building up on the detector windows, and by monthly decontamination procedures to remove contaminants. For example, the throughput in F160BW can degrade by roughly 40% in a month, while the throughput changes at F336W are 5%. Special scheduling of UV observations (e.g. UV campaigns) are currently not supported. Results of routine photometric monitoring can be used to correct UV photometry at the few % level.
In preparing Phase 2 proposals, the observer may find the WFPC2 exposure time calculator (ETC) available on the WWW useful. ETC enables one to quickly estimate exposure times needed to reach a specified signal-to-noise ratio, for both point sources and extended sources. This tool may be easily found from the WFPC2 WWW page.
The WFPC2 linear ramp filters provide a narrowband imaging capability at a central wavelength which is continuously adjustable from 3710 to 9762 Angstroms (with no wavelength gaps). The usable field of view is limited to a ~10 arcsecond diameter, and the filter passband FWHM is ~1.3% of the central wavelength. To use these filters, proposers should specify filter and aperture "LRF" on the phase 2 proposal, along with the desired observation wavelength. A software tool which may be found on the WFPC2 WWW page can help observers determine which CCD (WF or PC) will be automatically assigned for their observation, if this is an important consideration.
Observers using UV filters should recall that several of them have significant red leaks (e.g. F336W, F300W, F170W, ...). See section 3.8 of the WFPC2 Instrument Handbook and the on-line plots of filter transmission.
The serial transfer registers of the CCDs can be kept running during an observation (CLOCKS=YES). However, this mode is rarely used and is not well characterized. Thus we recommend its use only if star images will be so saturated (greater than 1E8 electrons) that significant bleeding off the chip is expected (see section 2.6 of the Instrument Handbook).
Bright stars near the WFPC2 field of view (typically <14th mag.) can cause various refection patterns on the CCDs. The largest effects occur for stars just outside the PC1 field. See the memo on WFPC2 ISR WWW page for further details, and also the ISR "Field Guide to WFPC2 Image Anomalies," available on the WWW.
Occasionally observations in broad visual and red filters are impacted by scattered Earth light in the OTA. This occurs most frequently for long (>600 sec.) exposures in the Continuous Viewing Zone (CVZ), but, in principle, can occur for any observation. In the most serious (and rarest) cases the background increases by ~10 DN/pixel and is modulated by strong diagonal bars across the CCD centers. If you have a deep CVZ imaging program which utilizes multiple filters and which would strongly benefit scientifically from targeted scheduling of the broad band exposures in low sky time, you may wish to discuss the available options with your Contact Scientist. See also the ISR "A Field Guide to WFPC2 Image Anomalies," available on WWW.
Three guiding modes are available, (1) Gyro Hold, (2) Coarse Track, and (3) Fine Lock. Fine Lock is used by default since use of Coarse Track may be harmful to the Fine Guidance Sensors. Use of Gyros only is not generally recommended (even for SNAPs) since (1) The pointing accuracy is only 14" and the target may be missed on the PC, and (2) The drift rate is 0.0014" per sec and moderate exposures (> 100 s) will result in smearing of the image (see section 7.1.1 of the Phase 2 Instructions).
The performance of the Woods Filters is being monitored because of concerns that they may have a limited useful lifetime. Please discuss the use of the Woods filters with your Contact Scientist.
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.
BLAIR, W.P.; DAVIDSON, K.; FESEN, R.A.; UOMOTO, A.; MACALPINE, G.M.; HENRY, R.B.C. "HST/WFPC2 imaging of the Crab nebula. I. Observational overview" ApJS accepted MEECH, K.J.; BUIE, M.W.; SAMARASINHA, N.H.; MUELLER, B.E.A.; BELTON, M.J.S. "Observations of the structures in the inner coma of Chiron with the HST Planetary Camera" AJ accepted FALCO, E.E.; LEHAR, J.; SHAPIRO, I.I. "HST observations and models of the gravitational lens system MG 0414+0434" AJ accepted FORBES, D.A.; BRODIE, J.P.; HUCHRA, J. "Hubble Space Telescope imaging of the globular cluster system around NGC 5846"
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 http://www.stsci.edu/public/propinfo.html.
Analysis, STSDAS or any other questions can also be addressed to firstname.lastname@example.org.
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