+--------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | ACS+WFPC2 | 30 November 2006 +--------------------------------------------------------+ CONTENTS: 1. New ACS Bulletin Board Online 2. HRC Relay Anomaly 3. SBC Operations 4. ACS Calibration Status 5. Calibration Outsourcing 6. Optical Redleak in the ACS SBC Detector 7. New MultiDrizzle Release and Improved Distortion Solutions 8. WFPC2 WF4 CCD Anomaly 9. WFPC2 Closeout Plans 10. Recent Instrument Science Reports 11. Recent Refereed Publications 12. Recent Press Releases +--------------------------------------------------------+ 1. New ACS Bulletin Board Online STScI has established an online ACS bulletin board to allow staff and HST observers to correspond with each other on ACS-related topics. Feel free to peruse ongoing discussions, post a reply to an existing thread, or start a new message of your own. Your participation is needed to establish a useful dialogue, and the postings will be moderated to ensure maximum utility for users and to prevent misuse. To be able to post messages, you must first register with the bulletin board moderators. This is a quick one-time process - simply click on the "Register" link on the bulletin board webpage and follow the instructions. No registration is required to view the ongoing discussions. The bulletin board gives the STScI ACS Team a more efficient method to disseminate late-breaking news, and to respond in a more widely visible way to common questions often handled via the e-mail Help Desk. We look forward to communicating with you. http://forums.stsci.edu/ +--------------------------------------------------------+ 2. HRC Relay Anomaly The ACS suspended on September 23, 2006 because of a problem with one of the relays on a signal processing chain board in the High Resolution Channel [HRC] CCD detector electronics. Analysis of the condition by an ACS Anomaly Review Board concluded that the most likely cause of the problem was a bit of non-conductive debris between two relay contacts that must meet to make a proper electrical connection. The missing voltage resulting from this problem did not affect the ACS Wide Field Channel [WFC], which was brought back into service on October 2, or the Solar Blind Channel [SBC], which will be brought back into service on November 27. After a thorough assessment of the risk to the HRC and other ACS hardware, the Anomaly Review Board recommended that the problematic relay be toggled open to clear the debris and then closed to reestablish the electrical connection. The relay was toggled open and closed on October 9. This procedure evidently cleared the debris because the missing connection was restored when the low voltage power supply for the the CCDs was turned on again. The HRC was configured for science operations and resumed service on October 16. The Anomaly Review Board recommended that the problematic HRC relay be left in its present (working) position. The WFC and HRC will be left powered during routine operations, except when monthly anneals are performed. Presently, all three ACS channels are operating nominally and may be requested for use in Cycle 16 observing proposals. +--------------------------------------------------------+ 3. SBC Operations The Anomaly Review Board's recommendation to leave the HRC relay in its present position required changes to the instrument commanding used to operate the SBC. Previously, SBC use has been tied to a set of commands used to toggle the HRC relay so that the HRC and SBC were not on simultaneously. After studying the electrical current, power, and thermal implications of having all three ACS channels powered simultaneously during SBC operations, approval was given to implement SBC operations with both CCDs powered. The SBC will be powered only when it is being used for science or calibration observations. As before, the SBC cannot be used in parallel with the other ACS channels. For all practical purposes, the changes to the commanding of the SBC are transparent to observers. The SBC observing capabilities now are the same as those that have been available in the past. +--------------------------------------------------------+ 4. ACS Calibration Status Details for the Cycle 15 calibration plans may be found at: http://www.stsci.edu/hst/acs/analysis/calib_plan/index_html#Section7 These programs support the annual science cycle. Emphasis this cycle includes maintaining up-to-date standard reference files, responding to any changes after the switch to Side 2 electronics and 4 degree C cooldown of WFC last July, and providing additional SBC specific calibrations. The WFC temperature decrease, and Side 2 switch in July led to minor calibration changes. New biases and darks were put in place in July with standard releases of these since. The WFC flat fields have changes of ~0.6% in the blue to ~0.15% at F814W, new flat fields for filters with wavelengths between those corresponding to F435W and F814W were delivered November 2, 2006 -- see ACS/ISR 2006-06. The WFC detector sensitivity table will be updated to account for the -2.5% at F435W to about 1% drop for redder filters as a result of the lower operating temperature. We will provide documentation quantifying limits to existing performance changes for the HRC and SBC for which updates are not expected to be necessary. +--------------------------------------------------------+ 5. Calibration Outsourcing The Call for Proposals for Cycle 16 specifies categories of "Calibration GO and AR Programs" in Sections 3.2.4 and 3.4.3 respectively defining how community members may apply for support of activities, observations, and/or archival research to advance instrument calibration. We urge careful consideration of this prospect. In addition to general areas mentioned in the CP, a candidate for such work includes polarimetry calibration for ACS/WFC for which significant calibration data exists that could be analyzed by an interested GO. +--------------------------------------------------------+ 6. Optical Redleak in the ACS SBC Detector Recent observations of cool stars using the ACS SBC PR110L prism have revealed that the sensitivity of the ACS Solar Blind Channel to optical and near-UV light is apparently much larger than was measured during ground testing. The throughput given in the standard reference files for the SBC detector at 3500 angstroms is about 9.5e-9. Preliminary estimates suggest that the real SBC detector throughput at this wavelength is approximately 3.4e-6, or about 2.5 orders of magnitude larger than the existing estimate. For a solar-type spectrum, this means that about 1/2 of the total counts detected are due to optical and near-UV photons rather than from the expected FUV photons. There is also some evidence that the amplitude of this 'red leak' changes as the SBC detector warms up, increasing by as much as 30% over the course of 5 orbits. It is not yet clear whether this red leak has also been increasing secularly over time. Imaging data taken with the STIS FUV-MAMA detector (for which the ACS SBC was the backup detector) seem to show a similar, although perhaps somewhat smaller effect. For dispersed PR110L and PR130L observations, it is straightforward to identify this extra red light; however, it clearly also affects SBC imaging observations done with the long-pass filters. The ACS team is undertaking efforts to obtain a better calibration of the red leak effect. Until this better characterization is in place, extreme caution should be exercised when interpreting FUV imaging observations of red targets. Observers conducting FUV imaging of red targets with the SBC may wish to consider interleaving observations with two different long-pass filters (e.g., F140LP and F165LP), so that the difference in the count rates can be used to isolate the true FUV flux. +--------------------------------------------------------+ 7. New MultiDrizzle Release and Improved Distortion Solutions We announce the release of updated MultiDrizzle/PyDrizzle software, together with improved polynomial distortion solutions ("IDCTABs") as well as improved distortion residual images ("DXY" files). The new software (MultiDrizzle v.2.7.2, PyDrizzle v5.7.0) is available at http://www.stsci.edu/resources/software_hardware/pyraf and has also been incorporated into the HST archive pipeline. The new distortion solutions were provided by J. Anderson and will be available from the archive, and from the reference file webpage: http://www.stsci.edu/hst/observatory/cdbs/SIfileInfo/reftablequeryindex Together, the distortion files and updated software are capable of providing an astrometric precision of about 0.01 pixels for data obtained during the first few years of ACS operation, thereby correcting pixel shift problems that had been previously reported. We caution that inherent uncertainties up to ~0.1 pixel or more may still exist in the observations. These can be introduced by guide star problems which cause errors in the commanded offsets, as well as long-term thermal changes and other effects resulting in evolution of the distortion solutions over time. The released versions of the software and distortion solutions do not yet contain time-dependent information, which will be available in a subsequent release. Meanwhile, it is possible to iteratively refine shifts, if necessary, as described in section 4.6 of the current ACS Data Handbook. Observers will be notified as soon as more updated time-dependent distortion solutions are available. +--------------------------------------------------------+ 8. WFPC2 WF4 CCD Anomaly The bias level in the WF4 CCD has begun to trend downwards again, mimicking its behavior in late 2004 and early 2005. We have not yet seen any further blank images in WF4, but a few blank images could occur if the trend accelerates during the next two months. We are tentatively planning another WFPC2 temperature adjustment in January or February 2007 to counteract the trend. Observers are reminded that WF4 images with low bias values (below about 300 DN, and above about 5 DN) will have low photometric counts, but the photometry should be correctable. This information will be updated as needed at http://www.stsci.edu/hst/wfpc2. +--------------------------------------------------------+ 9. WFPC2 Closeout Plans A summary of the WFPC2 calibration observations for its remaining time onboard HST has been posted at http://www.stsci.edu/hst/wfpc2/analysis/WFPC2_cyc1516_cal_plan_final.pdf As always, observers are welcomed to suggest additional calibrations, or propose additional calibration observations via Calibration Outsourcing proposals. These Outsource proposals can also be of an archival nature, and utilize existing or planned WFPC2 calibration data. We would be happy to discuss current calibration plans and potential outsource projects with interested parties. +--------------------------------------------------------+ 10. Recent Instrument Science Reports ACS: ACS Post Flash Measurements (ACS ISR 06-07) http://www.stsci.edu/hst/acs/documents/isrs/isr0607.pdf WFC L-flats Post Cooldown (ACS ISR 06-06) http://www.stsci.edu/hst/acs/documents/isrs/isr0606.pdf Relative Astrometry Within ACS Visits (ACS ISR 06-05) http://www.stsci.edu/hst/acs/documents/isrs/isr0605.pdf Related ISR from the Telescopes branch: Modeling Aberrations in the Advanced Camera for Surveys(ISR TEL 06-01) http://www.stsci.edu/institute/org/telescopes/Reports/ISR-TEL-2006-01.pdf WFPC2: A list of the recent WFPC2 ISRs can be found at: http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html +--------------------------------------------------------+ 11. Recent Refereed Publications ACS: http://www.stsci.edu/hst/acs/documents/pub_acs_nov_06.ps WFPC2: http://www.stsci.edu/hst/acs/documents/pub_wfpc2_nov_06.ps The papers listed above are based on data from ACS and WFPC2. The lists include all refereed papers received and identified by the STScI Library during the current year. Please include our Library in your preprint distribution list. The listings were generated with STEPsheet: http://stepsheet.stsci.edu/ A more complete list can be found through the ADS abstract service by using the standard search parameters: http://adsabs.harvard.edu/abstract_service.html +--------------------------------------------------------+ 12. Recent Press Releases ACS: http://www.stsci.edu/hst/acs/documents/press_acs_nov_06.html WFPC2: http://www.stsci.edu/hst/acs/documents/press_wfpc2_nov_06.html The press releases listed above are based on data from WFPC2 and/or ACS. The listing was compiled from the collection of STScI Press Releases available at http://hubblesite.org/newscenter/archive/ +----------------------------------------------------------+ | Visit the ACS and WFPC2 websites for more news: | ACS: http://www.stsci.edu/hst/acs | WFPC2: http://www.stsci.edu/instruments/wfpc2 | Need help? http://www.stsci.edu/hst/acs/help.html | or send your questions to the Help Desk: help@stsci.edu +----------------------------------------------------------+ | To subscribe or unsubscribe to this STAN, send a message | to majordomo@stsci.edu with a blank subject line and | the following in the body: [un]subscribe acs_wfpc2_news"> | Previous ACS and WFPC2 STANs are archived at: | ACS: http://www.stsci.edu/hst/acs/documents/newsletters | WFPC2: http://www.stsci.edu/instruments/wfpc2/wfpc2_stan.html +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
