Flat field is a type of correction made to the digital image obtained by an astronomical CCD camera, which consists of capturing an image of an object with uniform illumination (the dome of the telescope, the sky in the twilight or a special box equipped with a matt white screen) and divide the original image between this flat field image once normalized. This corrects the image of the defects introduced by variations in the pixel sensitivity to pixel of the CCD, vignetting of the optical system, dirt on the detector, etc.
In practice, astronomers do not take a single image, but use the average of a large number of them (twenty to thirty or more), so that the reading noise and other defects of the chip is greatly reduced. / p>
Flat field correction is usually preceded by a dark field correction. Source
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