+--------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | ACS+WFPC2 | 4 January 2006 +--------------------------------------------------------+ CONTENTS: 1. HST Calibration Workshop 2. Two-gyro Observations and Scheduling 3. ACS Instrument Handbook for Cycle 15 4. Calibration Outsourcing 5. ACS SBC Flat Fields and Zero Point Checks 6. Multidrizzle Developments, Expanded Associations 7. New Version of the aXe Spectral Extraction Package 8. New Apertures for Ramp Filters 9. Update on WFPC2 - WF4 Bias Anomaly 10. Recent Instrument Science Reports 11. Recent Publications 12. Recent Press Releases +--------------------------------------------------------+ 1. HST Calibration Workshop The 2005 HST Calibration Workshop was held at STScI on October 26 to 28. Video archives and supporting materials for oral presentations, as well as electronic versions of posters have been made available on the Calibration Workshop website at http://www.stsci.edu/institute/conference/cal05 Writeups of talks and posters received by 2005-Dec-02 will be included in the Workshop Proceedings. The planned distribution date of the Proceedings is 2006-Mar-31. +--------------------------------------------------------+ 2. Two-gyro Observations and Scheduling. Since transitioning to two-gyro mode for observations on August 28, 2005 the data quality for ACS and WFPC2 imaging has remained unchanged with respect to earlier three gyro data. The failure rate for guide star acquisitions is only slightly higher than in three-gyro mode. Full trending of the failure rate will require a longer time baseline. Recent adjustments to the two-gyro mode and parameters related to guide star selection should improve performance even further. The Institutes website provides additional details: http://www.stsci.edu/hst/HST_overview/TwoGyroMode +--------------------------------------------------------+ 3. ACS Instrument Handbook for Cycle 15 Version 6.0, October 2005, of the ACS Instrument Handbook was made available to support observations with HST in Cycle 15. Recent use found that the PDF version on our web page was out-of-date; the HTML version was correct. Although the differences are expected to be minor, we advise anyone having downloaded the PDF version before December 21 to download a new version. When downloading the current version of the PDF file, please make sure to clear your memory/disk caches in your browser first so that the browser retrieves the current version instead of a previously cached version. +--------------------------------------------------------+ 4. Calibration Outsourcing The Call for Proposals for Cycle 15 specifies a category of "Calibration GO Programs" in Section 3.2.3 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, candidates for such work include development of polarimetry calibration for ACS/WFC, and analysis of ramp-filter flat field data. Significant polarimetry and ramp filter calibration data exist or are planned in Cycle 14 that could be analyzed by an interested GO. +--------------------------------------------------------+ 5. ACS SBC Flat Fields and Zero Point Checks Improved flatfields have been derived for the SBC long-pass imaging filters and were delivered to the calibration pipeline on November 16. Using dithered observations of a UV-bright field, low-frequency variations in detector response of ±8% to ±20% have been measured. The new pipeline flats are expected to be accurate to ~1% for F115LP, F125LP, and F140LP and to ~2% for F150LP, F165LP, and F122M. Small drifts of quantum efficiency in time (as seen earlier with STIS MAMAs) were also derived. For more information, see ACS ISR 05-13: http://www.stsci.edu/hst/acs/documents/isrs/isr0513.pdf The SBC absolute zeropoints were derived just after launch using 10 spectroscopic standards in NGC6681. With the application of the new positionally dependent flatfield corrections, these zeropoints may have changed significantly. Using the same set of observations, we have applied the new flats and measured the mean change in stellar magnitude. The differences with respect to the original calibration are less than 3% for all 6 filters, i.e. less than the uncertainty in the absolute calibration of the UV standards. No changes to the existing SBC throughput curve will be made at this time. Improved calibrations for the SBC are in progress, including an updated geometric distortion solution, aperture corrections, and photometric zeropoints using new observations obtained in Cycle 14. Once these data have been analyzed, new SBC reference files will be delivered for use in the ACS calibration pipeline. +--------------------------------------------------------+ 6. Multidrizzle Developments, Expanded Associations MultiDrizzle improvements since Spring 2005 include improved geometric distortion calculations, as well as more robust treatment of associations with mixed apertures. Work is currently underway to improve the combination of datasets containing a mixture of very short and long exposures, where the short exposures can provide additional information on bright objects that may be saturated in the long exposures. The initial prototype version of Tweakshifts has been publicly released in Pyraf/STSDAS version 3.4 (Nov. 2005). This script can be used to refine the shifts of a set of exposures, using catalog matching or cross-correlation, with parameters available to control its behaviour. The resulting shifts are written to a file that can be directly given to MultiDrizzle, improving the quality of the resulting combined image. (Koekemoer et al. 2005, HST Calibration Workshop, in press). The original definition of associations was expanded to include all exposures from a given visit that were obtained using the same filter and aperture, for a given visit, if the guidestars remain unchanged. This therefore allows POS TARG dithers to be associated and combined in the HST archive pipeline, thereby broadening the number of datasets for which scientifically useful combined drizzled images can be produced automatically by the pipeline. +--------------------------------------------------------+ 7. New Version of the aXe Spectral Extraction Package In November a new version of the spectroscopic data extraction software aXe was released as part of STSDAS 3.4. The aXe package is designed to handle the ACS data from all slitless modes (WFC and HRC with the G800L grism, HRC with the PR200L prism and SBC with the PR110L and PR130L grisms). The new version 1.5 of aXe features substantial improvements in several areas: (a) a new task helps to create the object lists for the spectral extraction; (b) the possibility to specify a fixed extraction width (in pixels) was added; (c) the "quantitative contamination" method estimates, for every spectral element extracted from the data, the contribution of all neighbouring objects to the extracted flux in a quantitative way; (d) optimal extraction using the weighting scheme developed by Horne (1986, PASP 98, 609) was implemented. Details on aXe and the new aXe-1.5 are available on the aXe webpages: http://www.stecf.org/software/aXe Further details on quantitative contamination and optimal extraction are given in Kuemmel, Larsen & Walsh (2005, ST-ECF Newsletter 38, 8)and Walsh, Kuemmel & Larsen (2005, ST-ECF Newsletter 39, 8), respectively. +--------------------------------------------------------+ 8. New Apertures for Ramp Filters Starting in Cycle 15 we will provide users of the ramp filters on ACS with an easy means of obtaining either single quadrant readouts only to allow efficiency gains (when the wavelength of interest is contained with a single quadrant), or full frame images (with serendipitous coverage at other wavelengths). Included will be a capability of using the new aperture specifications with broadband filters as well to enable observations with ramps and standard filters to be obtained at the same detector positions. Full details will be available in the new Phase II instructions next spring. +--------------------------------------------------------+ 9. Update on WFPC2 - WF4 Bias Anomaly The anomaly in the WF4 CCD in WFPC2 continues to worsen. The bias level is unstable, with more and more images either being blank or showing faint streaks in the background. We have also confirmed photometric errors associated with the anomaly -- otherwise normal looking images of faint targets in WF4 can be up to ~70% too faint depending on the bias level and gain setting. Details of the anaomaly as well as techniques for repairing affected images are described in a new report available at http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0502.html We may be able to mitigate the anomaly by modifying temperature set points inside WFPC2; tests of this strategy will be conducted in January. +--------------------------------------------------------+ 10. Recent Instrument Science Reports ACS: SBC L-flat Corrections and Time-Dependent Sensitivity (ISR 05-13) http://www.stsci.edu/hst/acs/documents/isrs/isr0513.pdf Earth Flats (ISR 05-12) http://www.stsci.edu/hst/acs/documents/isrs/isr0512.pdf ACS/HRC Polarimetry Calibration IV. Low-Frequency Flat-Fields for Polarized Filters (ISR 05-10) http://www.stsci.edu/hst/acs/documents/isrs/isr0510.pdf The Internal CCD Flat Fields (ISR 05-09) http://www.stsci.edu/hst/acs/documents/isrs/isr0509.pdf Updated Wavelength Calibration for the WFC/G800L Grism (ISR 05-08) http://www.stsci.edu/hst/acs/documents/isrs/isr0508.pdf Two-Gyro Pointing Stability of HST Measured with ACS (ISR 05-07) http://www.stsci.edu/hst/acs/documents/isrs/isr0507.pdf Demonstration of a Significant Improvement in the Astrometric Accuracy of HST Data (ISR 05-06) http://www.stsci.edu/hst/acs/documents/isrs/isr0506.pdf ACS Coronograph Performance in Two-Gyro Mode (ISR 05-05) http://www.stsci.edu/hst/acs/documents/isrs/isr0505.pdf Flats: SBC Internal Lamp P-Flat (ISR 05-04) http://www.stsci.edu/hst/acs/documents/isrs/isr0504.pdf For a list of all ACS ISRs please see: http://www.stsci.edu/hst/acs/documents/isrs/ WFPC2: Early Assessment of the WF4 Anaomaly (ISR 05-02) http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0502.pdf Hot Pixels as a Probe of WFPC2 CTE Effects (ISR 05-01) http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0501.pdf For a list of all WFPC2 ISRs please see: http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html +----------------------------------------------------------+ 11. Recent Refereed Publications ACS: http://www.stsci.edu/hst/acs/documents/pub_acs_2006_01.pdf WFPC2: http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2006_01.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 +----------------------------------------------------------+ 12. Recent Press Releases ACS: http://www.stsci.edu/hst/acs/documents/press_acs_2006_01.html WFPC2: http://www.stsci.edu/hst/acs/documents/press_wfpc2_2006_01.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. +----------------------------------------------------------+
