AAS 201st Meeting, January, 2003
Session 55. Instrumental Calibration and Data Analysis Techniques
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[55.03] ACS Flat Field Corrections from Observations of 47 Tucanae

J. Mack, R. C. Bohlin, R. L. Gilliland, R. van der Marel (STScI), J. P. Blakeslee (JHU), G. de Marchi (STScI)

The uniformity of the WFC and HRC detector response has been assessed using multiple dithered pointings of 47 Tucanae. By placing the same stars over different portions of the detector and measuring relative changes in brightness, low frequency spatial variations in the response of each detector have been measured. The original WFC and HRC laboratory flat fields produce photometric errors of 5 to 18 percent from corner-to-corner. The required low-order correction (L-flat) has been applied to the lab flats, and new flat fields have been delivered for use in the calibration pipeline. Initial results suggest the photometric response for a given star is now the same to ~1 for any position in the field of view. As a further test, the improved flat fields are compared with observations of the bright earth at UV wavelengths (F330W) and with skyflats from ERO data at long wavelengths (F775W).

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