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7.3 Geometric Correction
The current geometric correction algorithm is good at correcting the gross characteristics of the FOC's geometric distortion, rectifying it to 0.5 pixels rms over most of the imaging format. However, the plate scales and orientations of FOC images are known to be time-dependent. The maximum change in scale from just after switch-on until the FOC has stabilized fully was measured during the initial orbital verification to be approximately 0.3%. A systematic study of the time dependence of the plate scale has not been done since, but repeated observations in the crowded-field analysis of fine-scale distortions (see page 6-6) did show plate scale differences of 0.1-0.2% even after the FOC had been warmed up for a long time. Angular rotations on the order of 0.1% from exposure to exposure can also occur. The pipeline does not attempt to correct for time-dependent aspects of the geometric distortion, and this deficiency can lead to astrometric errors between images taken at different times.
Geometrically corrected images displayed with high contrast close to the background, often show relatively low-frequency fringes with scale lengths of between 40 and 100 pixels (see Figure 4.5). This effect, a product of the geometric correction procedure caused by the algorithms used in re-binning the data, is merely a modulation of the noise characteristics of the data. The mean intensities in the image are not affected.
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