head 1.1; access; symbols; locks http:1.1; strict; comment @# @; expand @b@; 1.1 date 2003.12.19.11.45.59; author MarkAllen; state Exp; branches; next ; desc @none @ 1.1 log @Summary of ACS photometry @ text @ Summary of GOODS ACS imaging zero points for data being used in the AVO science demonstration January 2004. Mark Allen, Dec 19, 2003. ------------------------ This summary is intended to be useful for setting the input photometry parameters to the ACE SExtractor service being used in the demo. This summary includes relevant excerpts from the GOODS versions 0.5 and 1.0 release information, and a brief note on ACE configuration files (provided separately). Contents I. GOODS version 0.5 release II. GOODS version 1.0 release III. Comparison between GOODS versions 0.5 and 1.0 zeropoints IV. ACE configuration files I. GOODS version 0.5 release ============================ Excerpt from ftp://archive.stsci.edu/pub/hlsp/goods/v05/h_goods_v05_rdm.txt 7) ZEROPOINTS The magnitude zero point values listed in the header of the images are relative to the HST f-lambda system. We have also calculated the magnitude zero points for the AB system, which were initially: FILTER AB_ZEROPOINT F435W 25.662 F606W 26.505 F775W 25.656 F850LP 24.916 for our 4 passbands. Between HDF-N Search N.1 (4 Jan 2003) and CDF-S N.4 (5 February 2003) The ACS i,v and z-band photometric zeropoints changed. The new zeropoints are: F606W 26.493 F775W 25.641 F850LP 24.843 These zero points transform the count rate in units of counts per second into AB magnitudes according to the formula AB = -2.5*log10(Count_rate) + Zero_Point. Since the images have been normalized in units of counts per second, the above zero points can directly be used in photometry catalogs prepared using the released data. II. GOODS version 1.0 release ============================= Excerpts from ftp://archive.stsci.edu/pub/hlsp/goods/v1/h_goods_v1.0_rdm.txt 2.3 The Science Images The pixel values of the science images report the flux count rate calibrated in electron/second. The zero points to convert the count rate into an AB magnitude for the four GOODS passbands are the following: Z0_F435W = 25.65288 Z0_F606W = 26.49341 Z0_F775W = 25.64053 Z0_F850LP = 24.84315 For the details of the photometric calibration of the ACS camera we refer to the Instrument Handbook, which is available on line at the URL http://www.stsci.edu/hst/acs/documents/handbooks/cycle12/cover.html Also useful is the Instrument Data Handbook, also available on line at http://www.stsci.edu/hst/acs/documents/handbooks/DataHandbookv1/ 6.4 Photometry One known problem is that the photometry of a very small number of bright sources in the mosaics is systematically biased toward low fluxes because of small astrometrical errors, as mentioned above. Another known problem is that the photometry of bright stars in the multi-epoch stacked mosaic is systematically off typically by ~1 percent and in some case by up to ~2 percent, the sense being that the magnitude appears fainter. No such effect is observed in single-epoch stacked images. We discovered that this effect is due to the rejection of the "rotated" diffraction spikes by the cosmic-ray rejection algorithm. This problem seems to affect only bright (z<~20) unresolved sources, namely those for which the energy fraction contained in the diffraction spikes is the highest. It effect becomes progressively less noticeable at fainter flux, because fluctuations in the background become comparable to the spikes. It also becomes less significant if small photometric apertures are used (e.g. less than ~1% for 0.5 arcsec diameter apertures). This is a limitation, albeit a minor one, of the "multi-epoch" approach to the observations. III. Comparison between GOODS versions 0.5 and 1.0 zeropoints ============================================================== FILTER Version 0.5 * Version 1.0 ------- ---------------------- ----------- F435W 25.662 25.65288 F606W 26.505 26.493 26.49341 F775W 25.656 25.641 25.64053 F850LP 24.916 24.843 24.84315 * - Between HDF-N Search N.1 (4 Jan 2003) and CDF-S N.4 (5 February 2003) The ACS i,v and z-band photometric zeropoints changed IV. ACE configuration files =========================== Input parameters for ACE may be saved in a configuration file. For convenience four configuration files have been made for the four ACS filters. The config files are based on the original 'default.ace' file (http://wiki.astrogrid.org/bin/view/Astrogrid/AceClientInstallation) Each config file contains the relevant GOODS version 1.0 zero point. ACE also allows specification of the photometry UCDs in the output VOTable. It does this by allowing a user to specify the passband being used. The config files have "Generic Passband" and "Generic Mag" selected, with the passbands as follows. FILTER Generic Passband ------- ---------------- F435W B F606W V F775W R F850LP I Note that no other parameters have been changed from the original default.ace file. Other SExtractor parameters are very likely in need of tweaking in order to get robust scientific results out of ACE. Using the above config files ensures that the output magnitudes from ACE (on ACS images) are approximately correct (this was sufficient for the previous demos) but no serious checking of the results has been done. The ACE config files referred to here are F435W-v1-config.ace F606W-v1-config.ace F775W-v1-config.ace F850LP-v1-config.ace which are available at: http://www.euro-vo.org/twiki/bin/view/Avo/AcsPhotometry ------------------------------------------------------------------------ @