+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | for the Advanced Camera for Surveys (ACS) | ACS STAN #3, 2 January 2003 +----------------------------------------------------------+ CONTENTS: 1. ACS coronagraph update for Cycle 12 proposers 2. Cycle 12 calibration outsourcing proposals 3. ACS sensitivity for red targets in F850LP 4. Impact of the FGS realignment 5. The growth of hot pixels 6. Cosmic ray rejection 7. ACS pipeline-calibrated products 8. Should I recalibrate my ACS data? 9. Extracting spectra from WFC G800L slitless grism data 10. Recent ACS-related publications +----------------------------------------------------------+ 1. ACS coronagraph update for Cycle 12 Proposers Instrument Science Report ACS 2002-11 by John Krist http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf Abstract: Small instabilities in the ACS coronagraph have led to revisions of the coronagraph commanding procedures and to the suggested methods for optimizing coronagraphic observations. An example optimized sequence is given. Due to the instabilities, the flat fields currently used by the pipeline are not accurate and should not be used; new flats will be generated. Also, clarification is provided on the use of the coronagraphic surface brightness profiles in the Cycle 12 ACS Instrument Handbook. +----------------------------------------------------------+ 2. Cycle 12 calibration outsourcing proposals We would like to remind prospective proposers that the Cycle 12 Call for Proposals (see Section 3.6) solicits programs in the category of "Calibration Proposals". These are intended to allow more complete calibration of the instruments, than would be provided by the STScI calibrations that are aimed at covering primary and often used configurations. Those contemplating such a program should carefully read Chapters 12 and 13 of the ACS Instrument Handbook, and consult with an ACS Instrument Scientist well before the deadline. A successful calibration program would be expected to provide a general service to the community. Internal reviews at STScI will provide input to the TAC on feasibility, complementarity to existing calibration plans, and the type of science to be impacted. +----------------------------------------------------------+ 3. ACS sensitivity for red targets in F850LP The sensitivity calibration for ACS will soon be reworked with implied changes for most filters no larger than 1-2% (and therefore not relevant for Phase I plans). An exception to this involves the z-band filter, F850LP with the WFC for which changes for particular target characteristics could be 10s of percent, and therefore relevant for Phase I planning. Over the passband of the F850LP filter the currently assumed quantum efficiency is too high by ~2% at the short wavelengths (<8400A) sampled, with a larger drop in the far-red, e.g. ~20% at 10,000A. A yet more significant effect is that the ACS point spread function develops a large halo at these longest wavelengths, e.g. the encircled energy in a 3x3 pixel aperture for an L-dwarf observed in F850LP is 74% of that for an early type star. The ETCs do not yet take into account the underlying spectral energy distribution in setting the encircled energy, and have not yet been updated for the new wavelength dependent QE. A more complete discussion was provided at the 2002 HST Calibration Workshop, see: http://www.stsci.edu/hst/acs/documents/calworkshop/sensitivity_red.ps +----------------------------------------------------------+ 4. Impact of the FGS realignment On October 21, 2002 the matrices describing the positions of the Fine Guidance Sensors in the HST focal plane were updated. Prior to this date, pointing errors of 2 to 3 arcseconds had been observed in WFPC2 and STIS. These errors are large enough to present a risk of missing targets in small apertures. The ACS pointing history had not been studied in detail. We expect that for any observations after the update the pointing accuracy with ACS (and the accuracy of the absolute coordinates in pipeline reduced products) will be better than one arcsecond. However, this may not be true for earlier observations. We are investigating a couple of earlier observations that had larger errors, one with a 3 arcsecond discrepancy and one with as much as 6 arcseconds. We will report on the cause and frequency of these aberrant results once they are understood. +----------------------------------------------------------+ 5. The growth of hot pixels Instrument Science Report ACS 2002-09 by Adam Riess http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf Abstract: The anneal rate of hot pixels on the ACS WFC is 60%-65%, significantly lower than the characteristic anneal rate of 80-85% seen for other CCD's flown on HST (i.e., WFPC2, STIS, and ACS HRC). The ACS WFC is annealed in the same way as the other HST CCD's and there is no firm understanding at this time of the source of the difference. After 7-8 successive anneals, the cumulative fraction of annealed pixels reaches an apparent plateau at 70%. Additional anneals would not be expected to help as the annealing likelihood for persistent hot pixels reaches an apparent plateau after 7 to 8 anneals. Approximately 2 years after launch the coverage by hot pixels is expected to exceed that by cosmic rays in a 1000 sec exposure. At the nominal end of the HST mission (2010) the coverage by hot pixels would be 6%, i.e., one out of every 16 pixels. Because hot pixels are readily flagged and corrected or discarded they do not pose a serious threat to science observations, but their growing presence require careful dithering and consideration. +----------------------------------------------------------+ 6. Cosmic ray rejection Elimination of cosmic rays in CR-SPLIT images, particularly in undersampled data, can be tricky in the presence of even small registration differences between multiple exposures. The primary test for the presence of a cosmic ray in one frame is to compare the pixel value with an expected value (e.g. minimum of two exposures, or median of several) and flag as a cosmic ray if more than a specified sigma-level above that. Normal components of the noise model include readout noise and Poisson noise from the object and sky, but even a small fraction of a pixel offset between two images can lead to apparent changes near a bright target that swamp expected Poisson differences -- to account for this an extra noise term "scalenoise" is added as a simple multiple of the intensity. With undersampled data, the scalenoise needs to be roughly equal to the offset in fractions of a pixel to avoid falsely flagging cosmic rays near the core of well exposed stars. See discussion in section 5.1 of ISR ACS 2002-8. This ISR also reports finding that offsets up to 0.3 pixels are not uncommon between ACS cr-split exposures and thus since 18 October 2002 the pipeline has used SCALENSE=30.0 (percent). Before that time values of 3% resulted in many valid pixels being falsely eliminated for cases with registration differences above a few 0.01 pixels. At the new, larger value elimination of cosmic rays on extended objects will be inappropriately biased against, but under the philosophy of "first do no harm" we have adopted the conservative approach that only rarely drops valid data. +----------------------------------------------------------+ 7. ACS pipeline-calibrated products Many ACS observations take advantage of dithering to increase the effective field-of-view, reduce resolution loss due to undersampling, or allow more effective cosmic-ray and hot pixel rejection. However, the current standard calibration processing performed on dithered ACS data produces products which may not meet the expectations of observers in some important ways. We would like to point out these features and present brief details of the solutions which are currently being made available. PyDrizzle is used in the pipeline to perform distortion correction and combination of all ACS dithered data. It invokes the 'drizzle' task to remove geometrical distortion but does NOT have the capability to detect cosmic-rays. As a result the combined dithered data sets, although aligned and geometrically corrected, currently still contain cosmic-rays. CRs are only removed by the pipeline if the images were acquired using the CR-SPLIT mode. A new script called MultiDrizzle has been written by Anton Koekemoer (Koekemoer, Fruchter, Hook & Hack 2002, in the forthcoming proceedings of the 2002 HST Calibration Workshop) to perform cosmic-ray detection and removal using PyDrizzle and the tasks in the STSDAS dither package. This can be viewed as a Python implementation of the scheme for CR detection and flagging described in Fruchter & Hook, 2002, PASP, 114, 144. MultiDrizzle will work best when provided with flat-fielded calibrated products which have not been geometrically corrected. These normally have either '_flt.fits' ' or '_crj.fits' extensions. Unfortunately, these products are not currently available from the STScI archive for dithered ACS data. However, they will become available from February 2003 when the keyword EXPSCORR will be changed to PERFORM. The required files can also be created by individual users at their home institution. To this end, set the keyword EXPSCORR to PERFORM, and rerun the CALACS pipeline. The MultiDrizzle task is available in the STSDAS package. This new task can be used on any calibrated ACS or WFPC2 data on any system which has PyRAF 1.0, STSDAS 3.0 and PyDrizzle 3.3f. If all the default values are used the desired output name for the final product is the only input. It was primarily designed for data taken during the same visit using the same guide stars to minimize any potential pointing offsets errors between observations. Ultimately, this task will be generalized to correct for pointing offsets automatically and be incorporated into the standard pipeline processing within the next year. Until then, this initial release should provide cosmic-ray cleaned, dither-combined ACS data useful for analysis. +----------------------------------------------------------+ 8. Should I recalibrate my ACS data? Our calibration and characterization of the ACS continues to improve with time. As new reference files become available, you may want to recalibrate your data. To decide whether this is worthwhile, please regularly check the web page:http://www.stsci.edu/hst/acs/analysis/reference_files/ref_updates.html This page gives a detailed history of reference file updates and explains the type and magnitude of the changes in calibration. UPDATE:
The previous link is no longer available. See the following pages for information on reference file updates: ACS Reference File Webpage
ACS Reference File Logsheet +----------------------------------------------------------+ 9. Extracting spectra from WFC G800L slitless grism data The wavelength solution for the G800L grism with the WFC has been derived from in-orbit spectra of two Galactic Wolf-Rayet stars from SMOV and Cycle 11 calibration data. The mean dispersion is 39.2 A/pix in first order, 20.5 A/pix in second and -42.5 A/pix in negative first order. The dispersion solution is strongly field-dependent, with an amplitude of variation of about 11% from the center to the corners. A paper describing these results will appear in the proceedings of the 2002 HST Calibration Workshop, and is currently available at: http://xxx.lanl.gov/abs/astro-ph/0211651 The configuration file for the ACS spectral extraction software, aXe, incorporating these dispersion solutions, is available on the aXe calibration page: http://www.stecf.org/instruments/acs/calib/Cycle11/. (This link is no longer available. Please refer to the aXe web page.) The flat field for slitless spectroscopy data is both field and wavelength dependent. The effect of the wavelength dependent flat field for the WFC G800L grism mode has been investigated from observations of a flux calibrator at different positions in the field. A flat field cube has been derived by fitting the in-orbit broad-band filter flats as a function of wavelength and pixel and provides good correction (<2% residuals) across the WFC field. A description will appear in the proceedings of the 2002 Calibration Workshop, and is currently available at: http://xxx.lanl.gov/abs/astro-ph/0212021 The flat field cube is available for downloading at: http://www.stecf.org/instruments/acs/calib/Cycle11/ (This link is no longer available. Please refer to the aXe web page.) The flux standard also provides the (single, field independent) absolute flux calibration for the WFC G800L spectra. The first order sensitivity file, which must be used in aXe together with the flat field cube is also available on the same Web page. +----------------------------------------------------------+ 10. Recent ACS-related publications This listing was generated with STEPsheet: http://stepsheet.stsci.edu/ BOND, H.E.; PANAGIA, N.; SPARKS, W.B.; STARRFIELD, S.G.; WAGNER, R.M. "V838 Monocerotis" IAUC 7892: 1, 2002 IAUC 7892 BOND, H.E.; PANAGIA, N.; SPARKS, W.B.; STARRFIELD, S.G.; WAGNER, R.M.; HENDEN, A. "V838 Monocerotis" IAUC 7943: 1, 2002 IAUC 7943 EDMONDS, P.D.; GILLILAND, R.L.; CAMILO, F.; HEINKE, C.O.; GRINDLAY, J.E. "A Millisecond Pulsar Optical Counterpart with Large-Amplitude Variability in the Globular Cluster 47 Tucanae" ApJ 579: 741-751, 2002 ApJ 579: 741-751 GO-08267, GO-07503, GO-08219 GIAVALISCO, M.; RIESS, A.; Casertano; Dahlen; Dickinson; Ferguson; Hook; Idzi; Koekemoer; Mobasher; Moustakas; Ravindranath; Strolger; Tonry; Challis; "Supernovae 2002ez, 2002fv, 2002fw, 2002fx, 2002fy, 2002fz, 2002ga" IAUC 7981: 1, 2002 IAUC 7981 MAGEE, D.; BOUWENS, R.; ILLINGWORTH, G.; FORD, H.; BENITEZ, N.; BLAKESLEE, J.; CROSS, N.; GRONWALL, C.; TSVETANOV, Z.; CLAMPIN, M.; HARTIG, G. "Supernova 2002dc" IAUC 7908: 1, 2002 IAUC 7908 RIESS, A.; GROGIN, N.; HORNSCHEMEIER, A.; LUCAS, R.; RICHARDSON, M. "Supernovae 2002hp, 2002 hq, 2002hr, 2002hs, 2002ht, 2002hu, 2002hv" IAUC 8012: 1, 2002 IAUC 8012 TSVETANOV, Z.; BLAKESLEE, J.; FORD, H.; MAGEE, D.; ILLINGWORTH, G.; RIESS, A. "Supernova 2002dd" IAUC 7912: 1, 2002 IAUC 7912 +----------------------------------------------------------+ | 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. +----------------------------------------------------------+
