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The first step in the reduction of galaxy surface photometry, the
Flat-Fielding is the correction of errors introduced by differences in
sensitivity between different regions of the focal plane.
These are essentially due to two effects, namely differences in the intrinsic
sensitivity of the detector elements, e.g. the pixels of a CCD, and to
dimming of images towards the edge of the field of view.
The errors arising from this correction usually dominate the final error
budget, e.g. in HST observations, but in the case of GAIA they are expected
to be negligible, at least as far as the mission average measurements are
concerned. This is because the superposition of measurements obtained from
a large number of scans at different positions and position angles will
effectively average over any non-uniformities, thus reducing the flat-fielding
errors to negligible levels.
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Mattia Vaccari
2000-12-05