+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | ACS+WFPC2 | 11 April 2005 +----------------------------------------------------------+ CONTENTS: 1. ACS Photometric Calibration and Performance Paper 2. Calibration Workshop later this year 3. ACS Data Handbook Version 4.0 4. HST Cycle 14 Phase II notes: Deadline: May 13, 2005 5. New ACS Aperture Definitions 6. Results From 2-gyro Testing 7. Pipeline MultiDrizzle Image Orientation 8. Whirlpool Galaxy (M51) Project Observations and Data Release 9. Paper Products for ACS 10. WFPC2 Bug in Nov 2004 MultiDrizzle Release 11. Recent Instrument Science Reports 12. Recent Publications 13. Recent Press Releases +----------------------------------------------------------+ 1. ACS Photometric Calibration and Performance Paper A calibration paper that discusses the ACS photometric performance and photometric transformations is in the final stages of the refereeing process for PASP publication. This paper, "The Photometric Performance and Calibration of the HST Advanced Camera for Surveys" by Sirianni, M., et al. may be found in current form at: http://www.stsci.edu/hst/acs/analysis/transformations This paper provides a thorough discussion of data reduction steps, performance characterizations, and analysis approaches relevant to obtaining accurate photometry in the ACS photometric system. Transformations based on observations of star clusters, and separately with synthetic photometry, are provided to the WFPC2 and UBVRI photometric systems. +----------------------------------------------------------+ 2. Calibration Workshop later this year We plan to host an HST Calibration Workshop at STScI in late 2005. The goal of the workshop is to foster the sharing of information and techniques between observers, instrument developers, and instrument support teams. We invite the ACS/WFPC2 community to provide suggestions for topics to discuss at the calibration workshop. If you have developed a specific calibration technique, you have suggestions for improving the scientific return of ACS data or you would like to discuss the challenges of pushing ACS/WFPC2 instruments to the limit please send a email to cal05@stsci.edu (address is no longer active.) Also, please be reminded that if you have questions about the ACS instrument, issues relating to your ACS data or suggestions to improve the ACS calibration you can always contact help@stsci.edu. +----------------------------------------------------------+ 3. ACS Data Handbook Version 4.0 Version 4.0 of the ACS Data Handbook has been released. It has been completely revised to include discussions on how to run MultiDrizzle on ACS data, as well as describing the MultiDrizzle processing of ACS associations that takes place automatically when the data are requested from the archive. A number of other updates are also described, including bad pixels and data quality flags. +----------------------------------------------------------+ 4. HST Cycle 14 Phase II notes: Deadline: May 13, 2005 See the HST page providing information on the Phase II preparation of accepted programs for Cycle 14 at: http://www.stsci.edu/hst/programs For ACS a change in the default GAIN with the WFC has been made from the old value of 1 electron/DN, to 2 e-/DN. The tradeoffs between these two gain values are discussed in section 8.4.1 -- WFC Gain -- in the current ACS Instrument Handbook. Observers wishing to stay with GAIN = 1 may easily do so by specifying such in their Phase II. +----------------------------------------------------------+ 5. New ACS Aperture Definitions In recent months, several ACS aperture reference positions have been redefined. First, the prism apertures used with HRC and SBC were repositioned. Both apertures show some vignetting in the x-direction when the prism is in place and so the center of the exposed area is not at the full aperture center. This was known and allowed for before launch, but examination of in-flight images showed that positions could be better chosen. The HRC reference point was moved from [x,y] = [741,512] to [671,512] while the SBC was moved from [432, 512] to [425,400]. The additional shift in the y-direction for the SBC was undertaken because there is a broken anode which causes rows 599 to 605 to be disabled. We have moved the reference point for the prism observations away from this area. These changes took effect on November 8th 2004. Three reference points for the Wide Field Camera have also been moved. The former position of nearly all reference points lay on the half-way mark in the x-direction, that is at x=2048 on the 4096 pixel wide effective detector area. This is right on the boundary of the amplifier readout; the range x=1 to 2048 is read out through A and C amplifiers while 2049 to 4096 is read by the B and D amplifiers. Because of this there is a small discontinuity in the bias image at the boundary. This might slightly complicate the analysis of a point source landing at this position and so we have shifted the normal reference points for WFC, WFC1 and WFC2 to the position x=2124, a change of 74 pixels or about 2.5 arcseconds. This still keeps the point near the center of the 200 arcsecond square aperture and was chosen to avoid known bad columns. For observers who need to repeat observations at the same position as before, the apertures WFC-FIX, WFC1-FIX and WFC2-FIX remain available and, as their names suggest, will never be moved. This change was announced by direct e-mail to all observers who might have been affected to provide an opportunity to change to the FIX apertures where necessary. This change took effect on March 14th, 2005. +----------------------------------------------------------+ 6. Results From 2-gyro Testing For many months work has been underway at STScI and GSFC to develop a two-gyro observing capability for HST. In this mode, information from one of the optical Fine Guidance Sensors is substituted for pointing information which is lost when observing with two gyros instead of the normal three gyros, such as might occur after additional gyro failures. On Feb 21-23 this work culminated in an on-orbit imaging test of the new Two-Gyro Mode. Approximately 400 images were taken with ACS and WFPC2 spanning exposure times from 10s - 500s, guidestars from 11th to 14th magnitude, different HST roll angles, and different targets spread around the sky. In all cases the performance was superb. No significant differences were seen in the PSF size or shape, pointing accuracy or stability, or coronagraphic performance between three-gyro and two-gyro modes. Results are summarized at the Two-Gyro link on the ACS web page: http://www.stsci.edu/hst/acs/performance/twogyro (This link is no longer available.) And for an update to the HST Two Gyro Handbook for Cycle 14: http://www.stsci.edu/hst/HST_overview/TwoGyroMode (This link is no longer available.) Further details will appear in forthcoming ACS instrument reports. We plan to make additional tests if/when HST does enter two gyro mode, but no changes in data quality are expected. This is good news for Cycle 14 and future observers. We still expect to see some change in the HST scheduling rules which observers will need to consider; these will be incorporated in the APT Phase II software for Cycle 14. It is possible that a decision might be made to intentionally enter two-gyro mode early in Cycle 14, so as to conserve the remaining gyros onboard HST. +----------------------------------------------------------+ 7. Pipeline MultiDrizzle Image Orientation The default orientation for pipeline MultiDrizzle ACS images ("*_drz.fits") has been changed to the unrotated image frame, instead of having North toward the top. This leads to smaller images which are easier to transfer and display, and can still readily be oriented with a North-up orientation within most image display programs such as "ds9". More importantly, images delivered in the unrotated frame allow for instrument-specific effects to be much more easily identified and accounted for, including bad columns, stellar diffraction spikes, CTE trails, and PSF effects, and are also more useful for polarimetric, coronagraphic and grism data. The ACS CTE correction formulae can also be more readily applied to unrotated images. If images with north up are still required, these can be readily generated by appropriately setting the "final_rot" parameter when running MultiDrizzle off-line on the calibrated FLT files delivered from the pipeline. +----------------------------------------------------------+ 8. Whirlpool Galaxy (M51) Project Observations and Data Release An ACS mosaic of the Whirlpool Galaxy to be released April 25, see: http://archive.stsci.edu/prepds/m51/ In January 2005, a large 4-color (B, V, I, and H-alpha) ACS mosaic image of the Whirlpool Galaxy (M51), and its companion NGC 5195, was obtained. Cycle 14 HST proposers were encouraged to submit General Observer (GO) and Archival Research (AR) proposals to complement and/or analyze this unique dataset. On April 25, Hubble's 15th anniversary, the data will be released to the community, in conjunction with a press release. In addition to the standard archival products, carefully drizzle-combined mosaic FITS images will also be made available for scientific analysis, as "High-Level Science Products" via a MAST webpage (URL above). Since we anticipate heavy network traffic on April 25, researchers who wish to avoid downloading these large mosaic images (~2 GB total) can sign up to have the data mailed to them (in time to arrive on April 25) on DVD. See the webpage above for further details. +----------------------------------------------------------+ 9. Paper Products for ACS In the early days of HST, observers were provided with printed material which gave an overview of their observations in a convenient printed and graphical form. Subsequently this feature was turned into a tool within STSDAS (hst_calib.paperprod.pp_dads) which could display similar information in the form of a Postscript file. This tool is now very infrequently used and has been found to not be fully functional for ACS observations. In view of the very low demand we no longer intend to support this facility. +----------------------------------------------------------+ 10. WFPC2 Bug in Nov 2004 MultiDrizzle Release WFPC2 observers who have used, or are contemplating using MultiDrizzle to process their data should be aware of a problem discovered earlier this year. It affected all WFPC2 data processed with the version of MultiDrizzle released with STScI_Python V2.0 and STSDAS 3.3 in Nov 2004. The bug was fixed in the release that went out with STScI_Python V2.1 on 1 April 2005. The bug caused the shifts used in drizzling to be mis-applied to the 'blot' step due to not taking into account any scale changes. As a result, the median image was not blotted back to match the inputs and all pixels were subsequently flagged as bad. However, this was covered up due to the default value of 'fillval' being 'INDEF', leaving the values from the last image as the sole source of flux for the final output. This gave the appearance of having worked, when in fact, anyone looking at the WHT extension could tell everything was masked out. This affected all WFPC2 data, all the time, and any other data when the single-drizzling was done at a pixel scale other than the default for the detector. As stated in the MultiDrizzle release notes, use on WFPC2 data is not fully supported; but we do apologize for any inconvenience which this caused. Please contact help@stsci.edu for questions or additional information. +----------------------------------------------------------+ 11. Recent Instrument Science Reports ACS: Internal monitoring of ACS charge transfer efficiency (ISR 05-03) http://www.stsci.edu/hst/acs/documents/isrs/isr0503.pdf Flat-field and Sensitivity Calibration for ACS G800L Slitless Spectroscopy Modes (ISR05-02) http://www.stsci.edu/hst/acs/documents/isrs/isr0502.pdf Switching ACS from Side 1 to Side 2 electronics (ISR05-01) http://www.stsci.edu/hst/acs/documents/isrs/isr0501.pdf The Photometric Stability of ACS: Revisiting the Hubble Deep Field (ISR04-17) http://www.stsci.edu/hst/acs/documents/isrs/isr0417.pdf For a list of all ACS ISRs please see: http://www.stsci.edu/hst/acs/documents/isrs/ WFPC2: The Accuracy of WFPC2 Photometric Zeropoints (ISR 04-01) http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0401.html For a list of all WFPC2 ISRs please see: http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html +----------------------------------------------------------+ 12. Recent Refereed Publications ACS: http://www.stsci.edu/hst/acs/documents/pub_acs_2005_04.pdf (This document is no longer available.) WFPC2: http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2005_04.pdf (This document is no longer available.) WF/PC: http://www.stsci.edu/hst/acs/documents/pub_wfpc1_2005_04.pdf (This document is no longer available.) The papers listed above are based on data from ACS, WFPC2, and WF/PC. 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/ +----------------------------------------------------------+ 13. Recent Press Releases ACS: http://www.stsci.edu/hst/acs/documents/press_acs_2005_04.html (This web page is no longer available.) WFPC2: http://www.stsci.edu/hst/acs/documents/press_wfpc2_2005_04.html (This web page is no longer available.) 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/hst/wfpc2 |Need help? Please contact us at the helpdesk. +----------------------------------------------------------+ | 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/hst/wfpc2/documents/newsletters +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
