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Markus,
Basically a simplistic way of analysing your image when using the reducer.
If the star images in the corners look like comets - flaring away from the centre then the spacing of the reducer to CCD is too close and should be increased. It doesn't give a quantifiable outcome, just a basis for some trial and error.
It doesn't replace some of the commercial programs available to measure and quantify field curvature. (e.g. CCDInspector etc.)
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