+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | for the Advanced Camera for Surveys (ACS) | ACS STAN #1, 28 June 2002 +----------------------------------------------------------+ CONTENTS: 1. The first ACS STAN -- subscribe now for future issues! 2. Preliminary filter sensitivities 3. Flat field calibrations 4. Image quality 5. Image tracking stability 6. Reference file status chart 7. Calibration status of ACS data from the archive 8. Geometric distortion correction 9. Updated aperture locations 10. Notes on observing modes: coronograph, grisms 11. HST Calibration Workshop, 17-18 Oct 2002 +----------------------------------------------------------+ 1. The first ACS STAN This is the first "STAN" for the Advanced Camera for Surveys (ACS). ACS was installed onboard HST in March 2002, and the orbital verification (SMOV) program is nearing completion. Keep up with the latest ACS news by subscribing to receive future issues of this STAN -- see the appended instructions (Cycle 11 ACS GOs have been automatically subscribed). Every issue of the ACS STAN will be archived at: www.stsci.edu/hst/acs/documents/newsletters Explore the ACS website for more information, including "New in the last 40 days" items -- currently featuring preliminary reports on hot pixels and cosmic rays on ACS: www.stsci.edu/cgi-bin/acs +----------------------------------------------------------+ 2. Preliminary filter sensitivities The following chart shows a comparison of the preliminary filter sensitivity measurements to the SYNPHOT models for the ACS CCD imaging modes: www.stsci.edu/hst/acs/performance/delta_synphot.pdf +----------------------------------------------------------+ 3. Flat field calibrations The current "ground" flats are LP-flats for which our SMOV photometry is expected to be good to +-5% for the WFC. Using on-orbit SMOV data, images of 47 Tuc are currently being analyzed to create new L-flat images which will improve the flatfielding to ~1%. We are working to deliver these new reference files by August 2002. Any data which is taken before these flats are delivered will need to be recalibrated. +----------------------------------------------------------+ 4. Image quality The current synphot PSF curves are based on ground calibration data and will be updated once on-orbit data has been further analyzed. A first look shows some evidence of internal scattering of infrared light. This is just a few percent of the total stellar flux in the z band, but becomes more serious at longer wavelengths. The scattering manifests itself in HRC images as prominent PSF wings in the long wavelength filters and as spikes in WFC images. Further work will focus on characterizing the wavelength dependence of the PSF. +----------------------------------------------------------+ 5. Image tracking stability A recent SMOV test involved tracking the position of stars on the CCD over several orbits during which time HST was repositioned to attitudes leading to orbital heat input differences near the upper range of those normally encountered. The results from this provide confirmation that guiding with HST is excellent, but not perfect. The tracking accuracy while on the same guide stars showed offsets of up to about 40 milliarcsecond accumulating over a period of seven hours. This behavior has been seen before with other instruments and has been correctly described in past Call for Proposal documentation: "A drift of up to 0.05" may occur over a timescale of 12 hours..." With the very sharp point spread function supplied by HST, offsets between successive exposures much less than this (e.g. 0.005" is still 10% of a WFC pixel scale) can complicate the rejection of cosmic rays, especially in images dominated by unresolved stellar sources. The scheduling system allows for two components of a CR-SPLIT image to be separated by times approaching an hour, this can lead to offsets at the 10th pixel level and may require special care in analysis to avoid rejection of valid data from cosmic ray detection algorithms. Consideration of such issues should be an integral part of observation planning and post-observation data analysis procedures. +----------------------------------------------------------+ 6. Status of ACS calibration reference files For a brief overview of the current status (availability, pedigree, plans, etc) of ACS reference files, see: www.stsci.edu/hst/acs/analysis/reference_files/ref_status.pdf +----------------------------------------------------------+ 7. Calibration status of ACS data from the archive All ACS data retrieved from the archive through OTFR get processed using the standard calibration pipeline software, CALACS. As of mid-August, CALACS Version 4.0a should be in place in the archive for use on data to be retrieved. This version of CALACS incorporates many of the lessons learned from the initial calibration period following installation of ACS in HST. Prior to this, errors in the error (ERR) and data quality (DQ) arrays were present and would affect the final distortion-corrected product. The latest version of CALACS will also be made available through the public release of the next version of STSDAS, due out by the end of July. The STSDAS WWW pages at stsdas.stsci.edu can be checked for news on when the new release will be available, and for downloading the software. This would provide the capability for re-calibrating your ACS data with the latest software and calibration files. +----------------------------------------------------------+ 8. Geometric distortion correction Standard calibration of ACS data applies a flat-field correction using a sky flat. This results in an output image whose extended source photometry remains consistent, but whose pixels cover different areas of the sky, seriously impacting point-source photometry. Also, the off-axis position of the ACS field-of-view results in significant optical distortion. Both of these affects are corrected during standard pipeline calibration through the use of PyDrizzle, a Python task running under PyRAF which controls the operation of the IRAF image combination task 'drizzle'. This step not only corrects the distortion in single images, but also combines the separate WFC chips images into a single distortion-corrected image. In addition, ACS data taken as a dither pattern as specified in the proposal with the use of a pattern parameter will also be automatically combined in the pipeline using PyDrizzle. All these distortion corrected, and possibly dither-combined, products will have the suffix '_drz.fits' when retrieved from the archive. However, many lessons were learned during the initial calibration program undertaken after ACS was installed that revealed inaccuracies in how PyDrizzle was applying the corrections to the data. These lessons were used to update PyDrizzle to provide a much more reliable and accurate calibrated product. Unfortunately, this new version, Version 3.0, will not be available for use in the calibration pipeline and the archive until mid-August. The trailer file for the calibrated product will contain messages detailing which version of PyDrizzle was used. This can be used to verify that PyDrizzle Version 3.0 was used to calibrate your data. As with all other calibration software, PyDrizzle can also be run offline on your own system to allow for customized parameters for the output product to better match the science in the observations. PyDrizzle can be obtained as part of the next release of STSDAS, which should be available by the end of July. More information about PyDrizzle can be obtained online from the ACS Data Handbook, and from the ACS Drizzling WWW page at: www.stsci.edu/instruments/acs/ +----------------------------------------------------------+ 9. Updated aperture locations Aperture locations have been updated as a result of ACS-FGS alignment measurements. They differ by a few arcseconds from those in the ACS Handbook. The results are referenced on the ACS home page and have been incorporated in flight software and the VTT. The WFCENTER aperture is now fully supported. A further refinement using the results of the distortion analysis is in progress and will cause changes of, at most, a few tenths of an arcsecond. An exception to this may be the SBC which has not been directly measured and had been assumed to be at the same position as the HRC. The full update will be installed at the end of July. +----------------------------------------------------------+ 10. Notes on observing modes: coronograph, grisms The coronographic mode is not yet ready for operation. The optimal aperture location is still being tuned and calibrated against pre-launch models. The ACS group at the ST-ECF reports their initial results from the "first light" grism data: www.stecf.org/instruments/acs/SMOV/WFC/index.html +----------------------------------------------------------+ 11. The 2002 HST Calibration Workshop www.stsci.edu/stsci/meetings/cal02 The 2002 HST Calibration Workshop will be held on October 17 and 18 at the Space Telescope Science Institute. All observers who have new ideas to contribute regarding the calibration of HST instruments are encouraged to present their work. Abstracts for all presentations are due by Aug 17, 2002. The registration deadline is Sep 17, 2002. Also note that the HST Calibration Workshop immediately follows the Astronomical Data Analysis Software & Systems Conference (ADASS XII) that will be held in downtown Baltimore between October 13 and 16. +----------------------------------------------------------+ | Need help? www.stsci.edu/hst/acs/help.html +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to majordomo@stsci.edu with a blank subject line and | the following in the body: [un]subscribe acs_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
