+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | for the Advanced Camera for Surveys (ACS) | ACS STAN #2, 9 August 2002 +----------------------------------------------------------+ CONTENTS: 1. New WFC flat field reference files available (Aug 6) 2. Sensitivity table updates for SYNPHOT (Aug 8) 3. New version of PyDrizzle (Aug 7, in OTFR by Aug 21) 4. Reprocessing of fast-access ERO and GOODS data (Aug 28) 5. HST Calibration Workshop (abstracts due August 17) +----------------------------------------------------------+ 1. New WFC flat field reference files available The original laboratory flat fields obtained pre-launch resulted in photometric errors of +-5% to +-9% from corner-to-corner for the WFC. As of August 6, new flat fields for 13 WFC filters are being used in the pipeline, and for the calibrated data coming from the HST archive. Any data which was taken prior to August 6 will need to be recalibrated. Initial results suggest that the photometric response for a given star is the same to one sigma <~ +/- 1% for any position in the WFC field of view. The rootnames of the new flat field reference files are: m820832ej_pfl.fits (F435W) m820832fj_pfl.fits (F475W) m820832gj_pfl.fits (F502N) m820832hj_pfl.fits (F550M) m820832ij_pfl.fits (F555W) m820832jj_pfl.fits (F606W) m820832kj_pfl.fits (F625W) m820832lj_pfl.fits (F658N) m820832mj_pfl.fits (F660N) m820832nj_pfl.fits (F775W) m820832oj_pfl.fits (F814W) m820832pj_pfl.fits (F850LP) m820832qj_pfl.fits (F892N) The following chart provides an overview of the current status of each type of ACS calibration reference file: www.stsci.edu/hst/acs/analysis/reference_files/ref_status.pdf +----------------------------------------------------------+ 2. Sensitivity table updates for SYNPHOT With the flats in place, standard stars observed at the centers of the WFC1 and WFC2 CCDs give internally consistent results (which had not been the case for results mentioned in the previous STAN). In the interim, observations of a second standard star have also been obtained, with results generally consistent both for the two stars and the two CCDs. The following table shows the pivot wavelength (nm) for each WFC broadband filter and the mean count rate in the standard star observations compared to predictions using SYNPHOT sensitivity tables based on ground-test results: filter lambda ratio (observed/predicted) F435W 432.0 1.210 F475W 475.3 1.189 F555W 536.9 1.145 F606W 592.5 1.116 F625W 630.7 1.115 F775W 770.4 1.094 F814W 809.2 1.075 F850LP 913.4 0.996 Given that large errors relative to expected sensitivities exist, we are at least in the pleasant domain of having higher than expected response over nearly the full wavelength domain. Although filter-to-filter differences could contribute at the level of a few percent, the monotonic trend with wavelength reflects uncertainties in the pre-launch calibrations used to populate the component tables, the most likely is the CCD quantum efficiency table (same values were and will be used for WFC1 and WFC2 for this). We have therefore generated a new CCD quantum efficiency table that factors in higher response as a smooth spline fit in wavelength to the above values. This will in turn change the pivot wavelengths by small amounts since this results in spectral dependent changes across some of the filters. With additional absolute sensitivity changes in hand later this month we will: (1) assess whether there is any temporal component to the sensitivity, (2) possibly tweak (order of magnitude smaller changes if any) the QE table further, and (3) absorb residual discrepancies filter-by-filter by scaling the filter response curves. It is important to recognize that the above approach poorly constrains updates to the CCD sensitivity table outside the wavelengths enumerated. This has been dealt with by adopting a constant ratio of 1.21 below 432 nm, and continuing the linear trend over ~800-913 nm out to the assumed cutoff at 1100 nm. Since much of the ACS science program involves use of F850LP (z band), we are dedicating additional resources to refine the response over 850-1100 nm using independent information from grism and ramp filter observations in this domain as well as assessing the magnitude error for stars over a wide color range observed in the z band. If you would like to do more detailed calculations with the new CCD QE tables in synphot, you can create total-system-throughput tables (instrument plus OTA) using the synphot calcband task. Calcband takes any valid obsmode command string as input and produces an STSDAS table with two columns of data called "wavelength" and "throughput" as its output. For example, to evaluate the throughput for the F475W filter and the WFC detector, chip 1, you would use the command (the resulting throughput table is stored in sdssg_thpt): > calcband acs,wfc1,f475w sdssg_thpt +----------------------------------------------------------+ 3. New version of PyDrizzle A new version of PyDrizzle addresses several issues identified during SMOV calibration. It uses the 4th-order geometric distortion polynomials which are now in the IDCTAB reference file, solves a registration problem with dithered data, correctly processes subarray data (distortion and offsets), and addresses faulty input mask files. The standalone version also has modifications for registration, blot, and masking, to eventually support cosmic-ray rejection with PyDrizzle (not currently in the pipeline). This new version of PyDrizzle will be incorporated into the OTFR calibration pipeline by August 21. All data retrieved from OTFR prior to this date should be reprocessed. In addition, this version of PyDrizzle has been included in the latest release of STSDAS, which is already available via: stsdas.stsci.edu Note that PyDrizzle requires the installation Python and PyRAF, available from: pyraf.stsci.edu +----------------------------------------------------------+ 4. Reprocessing of fast-access ERO and GOODS data The ERO and GOODS data available via ftp will be reprocessed using the new reference files and revised pipeline software described above. The reprocessing will be completed by August 28. Watch this webpage for details: archive.stsci.edu/hst/fastaccess.html +----------------------------------------------------------+ 5. 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? Need help? Please contact us at the helpdesk. +----------------------------------------------------------+ | 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 | Previous STANs are archived at: | www.stsci.edu/hst/acs/documents/newsletters +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
