+--------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | ACS+WFPC2 | 25 July 2006 +--------------------------------------------------------+ CONTENTS: 1. Failure of the Side-1 Low Voltage Power Supply 2. Switch to ACS Side-2 Electronics and Science Resumption 3. Change in WFC temperature Setpoint 4. Calibration results for Side 2 5. Cycle 15 calibration plans 6. MultiDrizzle update 7. HRC Quantum Yield Correction Implemented in the ETC 8. Dithering in Cycle 16 9. Recent Instrument Science Reports 10. Recent Refereed Publications 11. Recent Press Releases +--------------------------------------------------------+ 1. Failure of the Side-1 Low Voltage Power Supply A recent hiatus in ACS observing occurred when the ACS suspended operations on June 19, 2006. ACS had been observing without major interruption since installation in HST in March 2002. ACS accounted for ~70% of the observing time with HST in Cycle 14. An Anomaly Review Board consisting of members from the Goddard Space Flight Center, Ball Aerospace, STScI, and corporate contractors was convened to determine the cause of the suspension and to provide a recommendation for restoring ACS to service. The ARB found that the most likely explanation for the suspension was a failure within the +15V circuitry in the Side-1 electronics of the Low Voltage Power Supply (LVPS) or the Main Electronics Box (MEB). This problem affects both the Wide Field Channel (WFC) and High Resolution Channel (HRC) CCDs. It does not affect the Solar Blind Channel (SBC) MAMA detector. The ARB concluded that it would not be possible to restore full observing capabilities on Side-1, but that it would be possible to do so if the CCDs and MAMA were controlled from the redundant set of electronics on Side-2. +--------------------------------------------------------+ 2. Switch to ACS Side-2 Electronics and Science Resumption The Side-2 electronics and the CCD electronics boxes on ACS were powered up on June 30, 2006. After passing checkout, ACS was safed to await normal resumption of activities on July 2. After an initial series of calibration bias, dark and flat field exposures, the first science observations on Side-2 were obtained on the evening of July 4. ACS is presently conducting a full complement of GO science programs and routine calibration observations. A press release describing one of the first science images obtained on Side-2 can be found at http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/36/ A brief moratorium imposed on SBC activities during the Anomaly Review Board's investigation has been lifted, and SBC orbital verification activities are currently in progress. Science observations with the SBC will resume at the next schedulable opportunity. At this time, there are no on-orbit tests of the failed Side-1 electronics planned. ACS operations will rely upon the Side-2 electronics until further notice. The ACS Team at STScI is in the process of providing calibration reference files for new data obtained on Side-2. Additional details about these activities can be found below and in future STANs. +--------------------------------------------------------+ 3. Change in WFC temperature Setpoint On July 4, 2006, the temperature setpoint of the WFC CCDs was lowered from -77 C to -81 C soon after the switch to the Side-2 electronics. This change allows a significant amount of the re-calibration effort associated with the side-2 switch to be consolidated with the calibration needed for a temperature change. Such a temperature change would likely have been needed within the next 12-24 months to mitigate the impact of radiation damage to the CCDs. Setting the temperature of the ACS WFC to -81 C has several beneficial effects: - The dark current rate and the hot pixel contamination are reduced by more than 50%. - The Charge Transfer Efficiency (CTE) tails are fainter. - The fainter CTE tails and the lower hot pixel contamination reduce the image noise. We estimate a gain in sensitivity for deep exposures of ~ 0.1 mag ( or equivalently, ~ 20% in observing time). The decrease in the temperature setpoint has slightly decreased the quantum efficiency (by 1-2%) and may have modified the flat field structure by a smaller amount. Recalibration is in progress. Results will be posted on the ACS web site. +--------------------------------------------------------+ 4. Calibration results for Side 2. New calibrations taken after the Side-2 switch and lowering of the WFC temperature will be used to assess a full range of calibration issues: biases, darks, flats, gains, and sensitivity. New bias and dark reference files are now in the pipeline. The biases reflect only small changes. The darks show the (expected) factor of 2 reduction in hot pixel numbers for the WFC. On the WFC the quantum efficiency is slightly lower (as expected) by ~2.5% at F435W declining to about a 1% drop for F555W and redder filters. The gains (WFC) do not show any change with Side-2 operations. Read noise levels differ only in detail, being comparable on the two sides. An initial QE measurement for the HRC based on repeated GO observations using the G800L grating showed no change (at 0.1% level) across the side switch (none was expected since the operating temperature should be nearly the same). The pointing has been verified to be the same for HRC and WFC as it was on Side-1. PSFs and geometric distortion have shown no detectable change as a result of the side switch or temperature change. Confirmation calibrations will be obtained for the SBC during the first week of August -- no changes are anticipated. +--------------------------------------------------------+ 5. Cycle 15 calibration plans The calibration program to support the annual science cycle is always developed within the few months following ingest of the Phase II science programs in order to allow a detailed match to support science. We are now in advanced phases of discussing the Cycle 15 calibration plans. Further details may be found on the ACS web site at: http://www.stsci.edu/hst/acs/hst/acs/analysis/calib_plan Comments on our calibration plans are always welcome, though the next month would be a particularly opportune time for these since the normal planning is still under way. Comments should be sent to: help@stsci.edu The electronics side switch and lowering of the WFC operating temperature may require additional calibrations in some areas (analogous to those needed just after launch of ACS) to reach standard levels of calibration for ACS (in particular for flat fields, and sensitivity in the far red with WFC). +--------------------------------------------------------+ 6. MultiDrizzle update During recent months we have been rigorously verifying the astrometric precision of MultiDrizzle, aiming to quantify time-dependent changes in the ACS distortion and improve the accuracy of the code. To date, discrepancies up to 0.1 - 0.2 pixels have been reported, occasionally larger when applying external shifts by means of input shiftfiles. Subsequently, significant improvements have been made to the software to ensure that the fourth-order polynomial distortion solutions are now handled to a precision better than 0.001 - 0.01 pixels internal to the code itself, reaching the limits of the distortion model. The improved code will be in the next Pyraf/STSDAS release, around late summer 2006. However, inherent astrometric uncertainties up to ~0.1 pixel may still exist in the observations and can be introduced by differences between commanded offsets versus those that are obtained, as well as long-term thermal changes and other effects that result in evolution of the distortion solutions over time. All ACS data currently make use of a single distortion solution obtained soon after the installation of ACS, and discussions are underway to produce time-dependent solutions that would be more appropriate for more recent data. In the meantime, it is possible to refine shifts iteratively, if necessary, as described in section 4.6 of the ACS Data Handbook. Observers will be notified as soon as more updated time-dependent distortion solutions are available. +--------------------------------------------------------+ 7. HRC Quantum Yield Correction Implemented in the ETC In the ACS/HRC CCD a single UV photon (below ~ 3350 A) may generate more than one photo-electron (Quantum Yield > 1). In order to accurately calculate the S/N ratio of an HRC observation in the UV, the shot noise characteristics of the incident photons cannot be directly inferred from the number of electrons generated. The source signal and the sky level need to be corrected for the quantum yield. Without such a correction, the S/N can be overestimated up to ~18% in the F220W and ~26% at the bluest edge of PR200L. In late July 2006 a revised version of the Exposure Time Calculator (ETC) that includes such correction will be released. http://www.stsci.edu/hst/acs/software/etcs/ETC_page.html +--------------------------------------------------------+ 8. Dithering in Cycle 16 The ACS Team has long recommended that observers dither (or offset) their observations to remove hot pixels, cosmetic defects, and cosmic rays in the combined data. Dithering allows for optimal sampling of the point spread function and provides substantive improvements over the use of CR-SPLITs, which do not remove hot pixels or other permanent cosmetic defects (e.g., bad columns or the WFC interchip gap). Dithering can be achieved in two ways: 1) By using exposure-to- exposure position offsets specified explicitly by POS-TARGs, or 2) By using flexible predefined dither patterns. Both POS-TARGs and dither patterns now result in associations, and dither patterns can also be nested to achieve various pixel sampling strategies for a wide variety of science goals. Currently available dither patterns within APT and their recommended uses are described on the ACS Dither Web page: http://www.stsci.edu/hst/acs/proposing/dither We strongly encourage observers to dither their ACS observations. Beginning in Cycle 16, observers who choose not to dither their ACS WFC and HRC imaging exposures should provide sound scientific justification for not doing so in the Phase I Proposal Description of Observations. The choice of dither pattern is left to the discretion of the observer since it may depend upon the science goals of the program. The ACS Team is ready to assist observers who need help in selecting the appropriate pattern for their observations. +--------------------------------------------------------+ 9. Recent Instrument Science Reports ACS: Policy and Procedure for MAMA Targets Subject to Unpredictable Outbursts(ACS ISR 06-04) http://www.stsci.edu/hst/acs/documents/isrs/isr0604.pdf Wavelength and Flux Calibration of the ACS/HRC PR200L prism (ISR 06-03) http://www.stsci.edu/hst/acs/documents/isrs/isr0603.pdf Wavelength and Flux Calibration of the ACS/SBC PR110L and PR130L prisms(ISR 06-02) http://www.stsci.edu/hst/acs/documents/isrs/isr0602.pdf PSFs, Photometry, and Astrometry for the ACS/WFC (ISR 06-01) http://www.stsci.edu/hst/acs/documents/isrs/isr0601.pdf WFPC2: A list of the recent WFPC2 ISRs can be found at: http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html +----------------------------------------------------------+ 10. Recent Refereed Publications ACS: http://www.stsci.edu/hst/acs/documents/pub_acs_july_2006.pdf WFPC2: http://www.stsci.edu/hst/acs/documents/pub_wfpc2_july_2006.pdf 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 +----------------------------------------------------------+ 11. Recent Press Releases ACS: http://www.stsci.edu/hst/acs/documents/press_acs_july_06.html WFPC2: http://www.stsci.edu/hst/acs/documents/press_wfpc2_july_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/newsdesk/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. +----------------------------------------------------------+
