+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | for the Advanced Camera for Surveys (ACS) | ACS STAN #4, 3 April 2003 +----------------------------------------------------------+ CONTENTS: 1. Cycle 12 and ACS 2. Dealing with hot pixels, and dithering vs CR-SPLIT 3. A dither pattern for the HRC occulting finger 4. Charge Transfer Efficiency (CTE) 5. Exposure time estimates for red targets in F850LP 6. Reminder on WFC buffer dump overheads 7. Subarrays for WFC and HRC 8. CCD Bias calibration and Gains in Cycle 12 9. ACS Coronagraphy 10. New version of Tiny Tim HST PSF simulation software 11. Pipeline automatically providing flat-fielded images 12. Calibration Workshop Proceedings 13. Recent ACS publications +----------------------------------------------------------+ 1. Cycle 12 and ACS This edition of the STAN provides information that may be of particular relevance to proposers about to prepare Cycle 12 Phase II observing programs. The information collected here covers developments since the Cycle 12 ACS Instrument Handbook release in October 2002, with an emphasis on items relevant for proposal preparation. For assistance with any of the issues discussed here, please send email to help@stsci.edu . +----------------------------------------------------------+ 2. Dealing with hot pixels, and dithering vs CR-SPLIT The hot pixel annealing rate for the HRC has been as expected (~80%), but for the WFC it has been significantly lower than expected (~60%). The growth of unannealed WFC hot pixels is progressing at a rate such that by the middle of Cycle 12 (January 2004) they will contaminate as many pixels as do cosmic rays in a typical (~1000 sec) exposure. For more details on this, see ACS Instrument Science Report 02-09: http://www.stsci.edu/hst/acs/documents/isrs/isr0209.pdf While the standard CR-SPLIT approach allows for cosmic-ray subtraction, without additional dithering it will not eliminate hot pixels in post-observation processing. Hence, we recommend that observers who would have otherwise used a simple CR-SPLIT now use some form of dithering instead. For example, a simple ACS-WFC-DITHER-LINE pattern has been developed, based on integer pixel offsets, which shifts the image by 2 pixels in X and 2 in Y along the direction that minimizes the effects of scale variation across the detector. The specific parameter values for this pattern are given in Section 8.4.3 of the Phase II Proposal Instructions at http://www.stsci.edu/public/p2pi.html (This link is no longer available.) However, any form of dithering providing a displacement of at least a few pixels can be used to simultaneously remove the effects of cosmic ray hits and hot pixels in post-observation processing. The recent development of PyDrizzle and Multidrizzle in the IRAF/STSDAS environment now makes this task easy. There are many resources available to guide users in the reduction of dithered data, including cosmic ray rejection and hot pixel elimination. See for instance: http://www.stsci.edu/hst/acs/analysis/drizzle +----------------------------------------------------------+ 3. A dither pattern for the HRC occulting finger Some HRC users have been surprised to see a substantial region masked out by the occulting finger in their data. The occulting finger is not retractable - it will be in every HRC exposure. However, as with any other detector feature or artifact, the "lost" data can be recovered by combining exposures which were suitably shifted with respect to each other. The following link provides more information on the HRC occulting finger, and a specific dither pattern to allow recovery of data lost behind the finger: http://www.stsci.edu/hst/acs/faqs/finger.html +----------------------------------------------------------+ 4. Charge Transfer Efficiency (CTE) The first external calibration data taken on ACS to measure the effects of CTE degradation on stellar photometry are currently being analyzed. Preliminary results match expectations. For typical observing backgrounds and stellar fluxes, CTE losses on WFC are typically 1-2% far from the readout amplifier (parallel read-out) and readily correctable. Only in the worst case scenarios (i.e., faint sources observed with narrow-band filters) do CTE losses reach ~10%-20%. Observers requiring better-than-1% precision should consider mitigating CTE by placing sources near the read-out amplifier and giving careful consideration to backgrounds. No measurable CTE effects are seen for the HRC at this time. +----------------------------------------------------------+ 5. Exposure time estimates for red targets in F850LP At wavelengths greater than 7500 A (HRC) and about 9000 A (WFC) ACS CCD observations are affected by a red halo due to light scattered off the CCD substrate. An increasing fraction of the light as a function of wavelength is scattered from the center of the PSF into the wings. For the very broad z-band, F850LP, filter in particular the encircled energy depends on the underlying spectral energy distribution; such an effect is not yet incorporated in the available (CGI) exposure time calculator (ETC). In order to obtain correct estimated count rates for red targets observers are advised to use the Synphot package in IRAF/STSDAS for which a proper integration over wavelength has now been incorporated for encircled energy. To quantify this new synphot capability, we compare ETC results with synphot for a set of different spectral energy distributions and the observation mode WFC,F850LP. In the following table, the spectral type is listed in the first column. The fraction of light with respect to the total integrated to infinity is listed in the other two columns, for the ETC and synphot calculations respectively. These values are derived for a 7x7 pixel box for the ETC calculations and for a 0.2 arcsec aperture for synphot: Sp. Type ETC Synphot O 0.76 0.74 M 0.76 0.7 L 0.76 0.68 T 0.76 0.6 The ETC results are off by 3% (O star), 9% (M star), 12% (L star), and 27% (T star). If this small effect is relevant to particular observations, then the Synphot software package can be used. A new keyword has been implemented to call for the encircled energy tables. The keyword is "aper". The user is allowed to select an aperture (radius in arcsec) and indicate this value by typing "aper#value". Currently, the following apertures are supported: every tenth of arcsec between 0. and .6 arcsec, 0.8, 1., 1.5, 2. and 4. arcsec. When calling "aper#0." the user will obtain the number of counts in the brightest pixel, i.e. the peak counts of the source centered at that pixel. Arbitrary aperture sizes are also permitted but are not recommended, because Synphot provides only a linear interpolation between supported apertures, which is a poor approximation, especially at small apertures. A typical obsmode would now read like: acs,wfc1,aper#0.2,f850lp. From the command line such an obsmode should be entered within quotes, or synphot complains: cl> calcphot "acs,wfc1,aper#0.2,f850lp" GOs can always "epar" the program they want to run, or use Pyraf instead of cl. Users can also provide their own input spectrum, which has to be in the same format as the user's provided spectrum for the ETC. Please refer to the ETC help pages for more specific information about this format. The file should be placed in the directory synphot is run from. +----------------------------------------------------------+ 6. Reminder on WFC buffer dump overheads Users of the WFC should be reminded that it takes a long time (nearly six minutes) to read out and transfer ACS/WFC data for each exposure. For WFC exposures 338 seconds is a magic number; for longer exposure times the next readout and data buffer dump will be hidden in the next exposure; for shorter exposure times the telescope and ACS will sit idle for an extra six minutes after the exposure to deal with the data. This is discussed well in the ACS Handbook (Chapter 9), but thought worth pointing out here. Somewhat new for Cycle 12 is improved support of subarrays which can further help ameliorate buffer management overheads, see the next item for details on new subarrays. +----------------------------------------------------------+ 7. Subarrays for WFC and HRC In cycle 12, selection of general subarrays will be available, and in some cases, supported. In this context, supported means that bias frame calibrations will be supplied by STScI for a predefined set of subarrays. See: http://www.stsci.edu/hst/acs/faqs/subarrays.html +----------------------------------------------------------+ 8. CCD Bias calibration and Gains in Cycle 12 The ACS Instrument Handbook for Cycle 12 included details on readout noise and full well depths for the two CCD instruments (e.g. see Chapter 7). The supported GAINS in Cycle 12 are HRC = 2, 4 electrons/DN; WFC = 1,2. The Cycle 11 calibrations were concentrated for the default GAINS (WFC 1, HRC 2). Experience has shown that calibration reference files are independent of gain selected with the important exception of biases which have unique gain-dependent structure. We will provide equally good calibration reference files at both of the supported GAINs for HRC and WFC in Cycle 12. Users should therefore feel free to choose the best gain for their science independent of calibration program considerations. +----------------------------------------------------------+ 9. ACS Coronagraphy The new developments for this may be found in Instrument Science Report ACS 2002-11, "ACS Coronagraph Update for Cycle 12 Proposers," by John Krist: http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf +----------------------------------------------------------+ 10. New version of Tiny Tim HST PSF simulation software Version 6.1 of Tiny Tim will be released in mid-April 2003. This will be the first update since ACS was put into orbit and includes significant enhancements to its ACS PSF modeling capabilities. On-orbit aberrations are included as well as the most recent geometric distortion coefficients. Field-dependent aberrations have also been updated. Field-dependent CCD charge diffusion is now also included, for both the WFC and HRC (this is the largest factor in the variation of the PSF FWHM over the field of view). Tiny Tim is available at: http://www.stsci.edu/software/tinytim +----------------------------------------------------------+ 11. Pipeline automatically providing flat-fielded images As of 12 March 2003, the ACS calibration pipeline (CALACS) has begun automatically and always providing the individual flat-fielded images (*flt.fits) which were used to produce the combined flat-fielded products, such as the cosmic-ray cleaned (*crj.fits) and drizzled (*drz.fits) images. Previously, these files were only provided for some types of observations, so users who desired them had to set EXPSCORR=PERFORM and re-run CALACS themselves. Since these individual flat-fielded images are the input for MultiDrizzle, they have become essential for the reduction of most ACS observations, regardless of how the exposures were defined and executed. For information on MultiDrizzle, please see the STSDAS Web page. +----------------------------------------------------------+ 12. Calibration Workshop Proceedings Proceedings from the October 2002 HST Calibration Workshop are now available online: http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2002/ Archived webcasts of the presentations are also available for viewing. +----------------------------------------------------------+ 13. Recent ACS publications This listing was generated with STEPsheet: http://stepsheet.stsci.edu/ ANDERSON, J. "Astrometry with the Advanced Camera: PSFs and Distortion in the WFC and HRC" 2002 HST Calibration Wksp. 13-22, 2003 BOND, H.E.; HENDEN, A.; LEVAY, Z.G.; PANAGIA, N.; SPARKS, W.B.; STARRFIELD, S.; WAGNER, R.M.; CORRADI, R.L.M.; MUNARI, U. "An Energetic Stellar Outburst Accompanied by Circumstellar Light Echoes" Nature 422: 405-408, 2003 CASERTANO, S.; GOODS TEAM; LAIDLER, V.; CAPAK, P.; COWIE, L.; HUBBLE HIGHER-Z SN TEAM; HICKEN, M. "Supernovae 2002kh, 2002ki, and 2003N" IAUC 8052: 1, 2003 CLAMPIN, M.; SIRIANNI, M.; BLAKESLEE, J.P.; GILLILAND, R.L. "Status of the Advanced Camera for Surveys" 2002 HST Calibration Wksp. 3-12, 2003 COX, C.; GILLILAND, R.L. "The Effect of Velocity Aberration on ACS Image Processing" 2002 HST Calibration Wksp. 58-60, 2003 DAHLEN, T. "Supernovae 2003az, 2003ba, 2003bb, 2003bc, 2003bd, 2003be" IAUC 8081: 1, 2003 DE GRIJS, R.; LEE, J.T.; MORA HERRERA, M.C.; FRITZE-V. ALVENSLEBEN, U.; ANDERS, P. "Stellar Populations and Star Cluster Formation in Interactivng Galaxies with the Advanced Camera for Surveys" NewA 8: 155-171, 2003 FAN, X.; STRAUSS, M.A.; SCHNEIDER, D.P.; BECKER, R.H.; WHITE, R.L.; HAIMAN, Z.; GREGG, M.; PENTERICCI, L.; GREBEL, E.K.; NARAYANAN, V.K.; LOH, Y.-S.; RICHARDS, G.T.; GUNN, J.E.; LUPTON, R.H.; KNAPP, G.R.; IVEZIC, Z.; BRANDT, W.N.; COLLINGE, M.; HAO, L.; HARBECK, D.; PRADA, F.; SCHAYE, J.; STRATEVA, I.; ZAKAMSKA, N.; ANDERSON, S.; BRINKMANN, J.; BAHCALL, N.A.; LAMB, D.Q.; OKAMURA, S.; SZALAY, A.; YORK, D.G. "A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey. II. Discovery of Three Additional Quasars at z > 6" AJ 125: 1649-1659, 2003 GILLILAND, R.L.; RIESS, A. "Extreme Red Sensitivity of ACS/WFC" 2002 HST Calibration Wksp. 61-64, 2003 MACK, J.; BOHLIN, R.C.; GILLILAND, R.L.; VAN DER MAREL, R.; DE MARCHI, G.; BLAKESLEE, J.P. "ACS Flat Fields and Low-Order 'L-flat' Corrections from Observations of 47 Tucanae" 2002 HST Calibration Wksp. 23-30, 2003 MEURER, G.R.; LINDLER, D.; BLAKESLEE, J.P.; COX, C.; MARTEL, A.R.; TRAN, H.D.; BOUWENS, R.J.; FORD, H.C.; CLAMPIN, M.; HARTIG, G.F.; SIRIANNI, M.; DE MARCHI, G. "Calibration of Geometric Distortion in the ACS Detectors" 2002 HST Calibration Wksp. 65-69, 2003 MUTCHLER, M.; KOEKEMOER, A.M.; HACK, W. "Drizzling Dithered ACS Images - A Demonstration" 2002 HST Calibration Wksp. 70-73, 2003 PASQUALI, A.; PIRZKAL, N.; WALSH, J.R. "The Wavelength Calibration of the WFC Grism" 2002 HST Calibration Wksp. 38-46, 2003 PIRZKAL, N.; PASQUALI, A.; WALSH, J.R. "Flat-fielding of ACS WFC Grism Data" 2002 HST Calibration Wksp. 74-77, 2003 RATNATUNGA, K.U. "Statistical Analysis of ACS Data without Covariance in Errors" 2002 HST Calibration Wksp. 78-81, 2003 RIESS, A. "Growth of Hot Pixels and Degradation of CTE for ACS" 2002 HST Calibration Wksp. 47-52, 2003 SIRIANNI, M.; DE MARCHI, G.; GILLILAND, R.L.; BOHLIN, R.C.; PAVLOVSKY, C.; MACK, J. "On-Orbit Sensitivity of ACS" 2002 HST Calibration Wksp. 31-37, 2003 SIRIANNI, M.; MARTEL, A.R.; JEE, M.J.; VAN ORSOW, D.; SPARKS, W.B. "Bias Subtraction and Correction of ACS/WFC Frames" 2002 HST Calibration Wksp. 82-85, 2003 SPARKS, W.B. "ACS Calibration Software" 2002 HST Calibration Wksp. 53-57, 2003 TRAN, H.D.; MEURER, G.R.; FORD, H.C.; MARTEL, A.R.; SIRIANNI, M.; BOHLIN, R.C.; CLAMPIN, M.; COX, C.; DE MARCHI, G.; HARTIG, G.; KIMBLE, R.; ARGABRIGHT, V. "On-Orbit Performance of the ACS Solar Blind Channel" 2002 HST Calibration Wksp. 86-89, 2003 TRAN, H.D.; SIRIANNI, M.; FORD, H.C.; ILLINGWORTH, G.D.; CLAMPIN, M.; HARTIG, G.; BECKER, R.H.; WHITE, R.L.; BARTKO, F.; BENITEZ, N.; BLAKESLEE, J.P.; BOUWENS, R.; BROADHURST, T.J.; BROWN, R.; BURROWS, C.; CHENG, E.; CROSS, N.; FELDMAN, P.D.; FRANX, M.; GOLIMOWSKI, D.A.; GRONWALL, C.; INFANTE, L.; KIMBLE, R.A.; KRIST, J.; LESSER, M.; MAGEE, D.; MARTEL, A.R.; MCCANN, W.J.; MEURER, G.R.; MILEY, G.; POSTMAN, M.; ROSATI, P.; SPARKS, W.B.; TSVETANOV, Z. "Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214" ApJ 585: 750-755, 2003 WALSH, J.R.; PIRZKAL, N.; PASQUALI, A. "Modelling the Fringing of the ACS CCD Detectors" 2002 HST Calibration Wksp. 90-93, 2003 YAN, H.; WINDHORST, R.A.; COHEN, S.H. "Searching for z~=6 Objects with the Hubble Space Telescope Advanced Camera for Surveys: Preliminary Analysis of a Deep Parallel Field" ApJ 585: L93-L96, 2003 +----------------------------------------------------------+ | 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: | http://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. +----------------------------------------------------------+
