S T A N / W F P C 2 - Number 43, September 2000
- WFPC2 NEWS:
- WFPC2 Shutter Anomaly and Occasional Blank Images
- WFPC2 Handbook now available in HTML
- Telescope Safing Event Aug. 5, 2000
- Data taken between Aug. 7 and Aug. 10, 2000 are affected
- How CTE Affects Extended Sources
- Update on Charge Trapping and CTE Residual Images in WFPC2
WFPC2 Shutter Anomaly and Occasional Blank Images:
by J. Biretta
We have begun to see anomalies in the WFPC2 shutter operation which result in occassional blank images. So far the error rate is very low, with the loss of three science images during the last 6 weeks (approx. 0.2% failure rate). We believe the mechanics of the shutter and the solenoids which operate it are healthy and fine. The main problem appears to be in an LED/phototransistor which senses the position of the "A" shutter blade. This device is used by the flight software to check the position of the blades prior to operating the shutter. Blank images are produced when the sensor gives an anomalous report, and the current flight software (as a safety measure) then disables the shutter until the status is cleared by external commands. We are currently looking at several work-arounds, including modifying the way the sensor is operated, software patches, and operating only the redundant "B" shutter. Unfortunately we expect it will take some weeks to fully study the problem, and design and test a suitable work-around.
Meanwhile, we plan to continue normal operations, and expect that occasional images will be lost until a long-term solution is implemented. While we plan to notify observers of future problems and lost images, it would nonetheless be prudent for observers to be especially watchful for anomalous or blank images. If blank images are found, they can be reported in the usual way by filing a Hubble Observation Problem Report (HOPR) and re-observation request at the STScI WWW site.
An immediate impact of the anomaly is that we decided to suspended WFPC2 observations between Oct. 2-6 while we conducted an initial investigation of the problem. WFPC2 observations were suspended at 11:47 GMT on 2 Oct. 2000 and were resumed at 19:02 GMT on 6 Oct. 2000. This was necessary while we determined that it was safe to continue operating the camera. Observers will soon be notified if their images were lost during this time.
WFPC2 Handbook now available in HTML:
by J. Biretta, I. Heyer, J. Mack, and the WFPC2 GroupA complete revision of the WFPC2 Handbook was released in June 2000. At that time, the Handbook was made available in PDF format, for downloading and printing. The Handbook is now also available in HTML format, including clickable Table of Contents and Index. Both format versions can be found at the WFPC2 Website.
The new version 5.0 incorporates material from previous updates, as well as new material on CTE, comparisons to expected ACS performance, updated focus history, aperture corrections as a function of field position, the F1042M PSF anomaly, new geometric corrections, and more. The last major revision of the Handbook was prior to Cycle 6 (version 4.0). Since that time we have provided only short update documents.
Telescope Safing Event Aug. 5, 2000:
by Stefano Casertano
On August 5, 2000, WFPC2 suffered an unplanned warm-up as a consequence of a telescope safing. This warm-up may have minor impacts on UV data taken between 7 and 10 August 2000, or data taken between those dates requiring accurate subtraction of hotpixels. During this warm-up the WFPC2 detectors remained for about a day at a temperature of -40 C, compared with the normal operating temperature of -88 C. At the time of the safing, more than three weeks had elapsed since the previous WFPC2 decontamination, thus the UV throughput had been reduced as usual from the nominal (post-decontamination) values.
Photometric verification data taken on Monday, August 7, indicate that the warm-up event acted exactly as a decontamination on the UV throughput of the camera; in other words, the throughput on August 7 was indistinguishable from the throughout the day after a decontamination. Therefore we recommend that all data taken between August 7 and August 10 - the day of the scheduled decontamination, which executed as planned - be corrected for the UV throughput assuming a decontamination event on August 6, at MJD 51762.9049. In most cases, the contamination accumulated over a four-day interval is negligible, and thus contamination corrections can be safely ignored for these data.
There is also evidence for a moderate annealing of warm pixels during this event. Therefore some (about 30%) of the hot pixels newly formed over the previous month will no longer be hot between August 6 and August 10. The hot pixel lists used by the task warmpix include this information, and thus the annealed hot pixels should be properly identified. However, if your results depend critically on hot pixel identification, we recommend verifying hot pixels via the special darks taken on a daily basis for this purpose, as part of calibration proposal 8826. Because of resource limitations, we cannot directly support the analysis of these darks.
If you have any specific concerns, please send email to email@example.com.
How CTE Affects Extended Sources:
by Adam Riess
An investigation has been made of the effect of CTE on the shape and structure of extended sources. We subtracted pairs of images of individual galaxies observed near and far from the read-out amplifier. The average profile of the galaxy residuals is asymmetric and indicates that a small amount of charge is lost from the amplifier-side of the galaxy image. The side of the galaxy away from the amplifier suffers little charge loss because charge traps encountered by the image during readout have been filled, and also some release of previously trapped charge occurs into this region. The observed results are reproduced by a simple model of the read-out process. Hence CTE may have some effect on studies of the light profiles of faint galaxies, and there may also small astrometric effects (<0.1 pixel) for faint targets.
Another report on the photometric effects of CTE on extended targets is currently in the works.
Update on Charge Trapping and CTE Residual Images in WFPC2:
by S. Baggett, J. Biretta, and J.C. Hsu
We provide an update on the status of charge trapping in the WFPC2 CCDs, as measured from the strength of residual images in dark calibration frames taken after external science images. The new images support the original finding that the amount of charge trapped appears correlated with the maximum intensity clocked through the pixel during readout (Biretta & Mutchler, 1998). The charge in the residual image is found to be given roughly by ln(residual)~0.36ln(Imax), where Imax is the geometric mean of the maximum pixels in each residual image column. Furthermore, the amount of charge seen in the residual images appears to have been stable over the six years. This stability is in marked contrast to the evolving CTI (charge transfer inefficiency) found via photometry of external stellar images (e.g., Whitmore et al., 1999) and analysis of cosmic ray tails in dark frames (Riess et al., 1999). Finally, there is evidence that these residuals can be relatively long-lived: some residuals appear in darks started more than 20 minutes after the external image was read out.
Cycle 9 Status:
by A. Schultz
The official start of Cycle 9 was Monday July 3, 2000. There were 166 GO proposals accepted for Cycle 9, requesting 5,066 orbits (GO, SNAP). Of the non-SNAP orbits, 1,405 orbits were allocated for WFPC2. Some allocated orbits were dropped due to duplication issues, leaving 1370 Cycle 9 WFPC2 orbits for scheduling. We are 1/4 of the way through the cycle, and 25.5% of the WFPC2 orbits have executed or are scheduled to execute. The table below summarizes the current status of the WFPC2 Cycle 9 GO programs.
status visits orbits % of total -------------------------------------------------- scheduled/executed 198 350 25.5 with plan windows 362 746 54.4 ready/unschedulable 5 11 0.8 waiting PI/CS review 1 1 0.1 not in LRP 127 262 19.1 -------------------------------------------------- Total 693 1,370
Visits are given plan windows on the Long Range Plan (LRP); these are times during which the observation could be scheduled to run on HST. Visits that are ready/unschedulable typically lack guide stars. Visits not yet in the LRP are usually targets of opportunity, those with special observing constraints requiring special handling, or those awaiting coordinate updates. Some of these visits are place holders (with alternate observing parameters) and will not execute.
by Carl Biagetti
The third servicing mission (SM3) was originally planned to provide numerous engineering and scientific upgrades to HST in the year 2001. However, after several gyro problems put HST's science program at increasing risk, SM3 was divided into two parts: SM3a, an emergency mission to restore HST's pointing control, and SM3b to provide the rest of the originally planned upgrades. In addition to the emergency installation of the new gyros, SM3a, performed in December 1999, afforded the opportunity to perform several of the other engineering enhancements originally planned for SM3. Among them were installation of the refurbished FGS, a replacement of the on-board spacecraft computer, replacement of a failed transmitter, and several lesser engineering upgrades.
SM3b, now scheduled for November 2001, will provide major scientific upgrades as well as significant engineering refurbishments to the Observatory. First, the mission's highest priority will be the installation of the Advanced Camera for Surveys (ACS). This third generation HST instrument will be installed in the place of the Faint Object Camera (FOC), an original HST instrument, which will be removed and returned to earth. The second scientific upgrade will result from the installation of the NICMOS Cryo Cooler (NCS). This device will enable, after the mission, the recooling of NICMOS to operational temperature and restore HST's infrared science capability. The engineering upgrades include replacement of the existing solar panels (SA-II) with new, rigid arrays (SA-III). These arrays are also slightly smaller, thereby reducing drag effects, and more efficient than SA-II. The mission will also include a swap-out of the Power Control Unit (PCU), a critical item which distributes electrical power from the solar arrays and batteries to the spacecraft subsystems. None of these tasks are simple, the SA-III installation requiring two EVAs by the astronauts, and the PCU requiring a complete spacecraft power-down for several hours. The mission now includes five EVA days as well as an orbital reboost. Four members of the crew have been named and have begun the training program for the mission.
StarView 6.0 is here, Xstarview to be disabled Nov. 1, 2000
by Megan Donahue for the StarView team including Sara Anderson, Scott Binegar, Molly Brandt, Niall Gaffney, and Ed Hopkins
StarView 6.0 is a Java-based astronomical data base browser and data retrieval tool. Its database interaction is generalizable to any Sybase database. Its main function today is to provide access to the HST Data Archive and Distribution System (DADS), allowing searches for scientific data, examination of calibration steps to be used by On-the-fly calibration for any dataset, and examination of proposal information, including instrument configurations and setup for all HST observations. StarView allows fully-customizable searches of all of the catalogs available for HST. We also have a library of standard StarView forms such as the General Search and instrument data searches.
The older version of StarView (version 5.4a, invoked by the name xstarview) will be phased out after the HST Cycle 10 proposal deadline. It requires a special server which we are decomissioning.
StarView is available for Unix and Windows systems with Java 2 Runtime Engines (or Java 1.2). Most such systems are running Java 1.2 already.
StarView will update itself, with permission from the user. System managers for centralized installations should contact us at: firstname.lastname@example.org. Our next release will contain system management features, and we would like to coordinate that release with system managers already maintaining a central installation of StarView.
Future plans include expanding StarView to allow searches of any MAST catalog, and to enable retrievals from any other mission, including FUSE, IUE, and others. We will still maintain the basic MAST web resources for all of these projects as we expand the capabilities of StarView as a generalizable and modular research tool.
Questions, Comments, and Suggestions: email email@example.com. Please forward this message to anyone who may be interested.
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.
CHABOYER, B.; SARAJEDINI, A.; ARMANDROFF, T.E. "The Age of the Inner Halo Globular Cluster NGC 6652" AJ 12-00 HUNTER, D.A.; O'CONNELL, R.W.; GALLAGHER, J.S.; SMECKER-HANE, T.A. "The Star Clusters in the Starburst Irregular Galaxy NGC 1569" AJ 11-00 MACKENTY, J.W.; MAIZ-APELLANIZ, J.; PICKENS, C.E.; NORMAN, C.A.; WALBORN, N.R. "HST/WFPC2 and VLA Observations of the Ionized Gas in the Dwarf Starburst Galaxy NGC 4214" AJ 11-00 PUZIA, T.H.; KISSLER-PATIG, M.; BRODIE, J.P.; SCHRODER, L.L. "Globular Clusters in the dE,N Galaxy NGC 3115 DW1: New Insights from Spectroscopy and Hubble Space Telescope Photometry" AJ 120: 777-790, 2000 ALVES, D.R.; BOND, H.E.; LIVIO, M. "Hubble Space Telescope Observations of the Planetary Nebula K 648 in the Globular Cluster M15" AJ 10-00
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 firstname.lastname@example.org.
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