I'm no expert at any of this but I will take a stab at answering why you might see less field curvature when binning vs not binning. I think CCDIS measure FWHM over the image and derives a curve based on the changes in FWHM. When binning pixels are summed so I would think that sharpness would diminish in general, and perhaps differences in FWHM would also reduce giving a flatter field. In other words, measuring differences in a blurry image would be more difficult than one sharply in focus, and in the blurry image differences would appear diminished.
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