Scan distortion manifests itself as a series of regular, vertical stripes down the right side of every image (see Fig. 1) in every format. It is believed that this effect is caused by the non-uniform motion of the scanning beam as it begins reading each line. If this is indeed the case, then the scan distortion will introduce variations in the geometric distortion and both the detector sensitivity and non-linear response. The scan distortion is present at the right hand edge of all formats and has a peak-to-peak scalelength of 60-70 pixels, about the same as the reseau separation. For this reason it is not removed, or even significantly modified by the geometric correction.
The intensity variation which results from the scan distortion behaves something
like a damped sine wave and extends for at least 3 full oscillations (about
200 pixels), with its amplitude decreasing from about 15%, to 6%, to
2%, as it progresses inward from the right hand edge. Because of the
smoothing which has been applied to the pipeline calibration flat fields (UNI
files), and because the flatfield correction is applied after geometric
correction, the flat fielding process does little to correct the sensitivity
variation induced by the scan. It should also be noted that smaller FOC
formats,
which use only a subset of the 5121024 (zoomed) format, do not include
the scan distortion at all.
Since it is an inherent characteristic of the FOC cameras, there is no reason to expect that scan distortion will be changed in any way by the installation of COSTAR. Having said that, a new method of geometrically correcting the smaller formats (see the next section) may result in a reduction of the effect for these formats.