ACS/WFC: Differential CTE Corrections for Photometry and Astronomy from Non-Drizzled Images

Details ACS/WFC: Differential CTE Corrections for Photometry and Astronomy from Non-Drizzled Images ACS/WFC: Differential CTE Corrections for Photometry and Astronomy from Non-Drizzled Images

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007acs..rept....4k&link_type=abstract

Instrument Science Report ACS 2007-04, 17 pages

Astronomy and Astrophysics

Astronomy

Hubble Space Telescope, Hst, Space Telescope Science Institute, Acs, Advanced Camera For Surveys, Charge Transfer Efficiency, Cte, Photometry

Scientific paper

We present an analysis of the Charge Transfer Efficiency correction for a science project using high-precision photometry in a rather crowded field derived with the effective PSF" (ePSF) method of Anderson & King (2006) on ACS/WFC non-drizzled (* flt.fits) images. The analysis shows that the CTE effect induces not only a loss of ux but also a centroid shift, both of which have an amplitude that depends on the star's signal level and position on the chip. Centroid shifts are typically of order < 0:1 pixels, which is significant for high-precision astrometry. We also show that the detailed dependency of ACS/WFC CTE loss on signal level (and/or sky background level) is different for ePSF-fitting photometry than that implied by the Riess & Mack (2004) algorithm which was derived from aperture photometry on drizzled images. We present a CTE correction technique which can be used for data sets where images of a given field with different exposure times are compared or combined. We show that this technique extends the photometry and astrometry accuracies advertized for ePSF fitting of well-exposed stars (i.e., 1% and 0.01 pixel rms, respectively) down to very faint levels. The CTE-induced photometric losses and centroid shifts are parameterized in terms of the location of the source on the ACS/WFC CCD chips and the magnitudes of sources.

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