Computer Science
Scientific paper
Feb 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005acs..rept...10w&link_type=abstract
ST-ECF Instrument Science Report ACS 2005-002, 21 pages
Computer Science
Hubble Space Telescope, Hst, Space Telescope Science Institute, Advanced Camera For Surveys, Acs, Flat Field, Slitless Spectroscopy
Scientific paper
Flat-fields are an essential component in creating scientific quality images of uniform sensitivity. For filter imaging, a single flat-field or a combination of a low spatial frequency and a pixel-to-pixel flat suffices. However for slitless spectroscopy data, a flat-field must effectively be provided at each wavelength of the spectral sensitivity, since each pixel can receive any wavelength. The flat-fielding of the ACS WFC and HRC for the G800L grism is described. For slitless spectroscopy data the flat-field can be described as a cube with the spatial dimensions those of the detector and the 3rd dimension that of wavelength. Two options are available for forming the ACS flat-field cube: to use the ground-based flatfields taken with a monochromator or to use the in-orbit filter flats. On account of the vignetting problems affecting the ground-based flats, in-orbit flats have been used. The contruction of this flat-field is described. The efficacy of this flat-field cube has been investigated from observations of a spectrophotometric standard at different positions in the field. By requiring the extracted spectra from different positions over the field to match in flux, a smooth function was derived and applied to the flat-field cube. The single sensitivity curve for the whole detector area is then determined from the mean spectrum at the different positions. Good results are reported with rms deviations in the shape of extracted spectra <2% across the WFC field and about 1% in the HRC, except at the blue and red edges of the wavelength range. These flat-fields and sensitivity curves are supplied for routine spectral extraction of ACS slitless spectral data as part of the aXe package.
Pirzkal Nor
Walsh Jonathan R.
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