THE FLAT SKY: CALIBRATION AND BACKGROUND UNIFORMITY IN WIDE FIELD ASTRONOMICAL IMAGES

Gradients in the surface brightness of the twilight and night sky require careful consideration both in the flat-field calibration and in the subsequent reduction of images that cover a large angular field. We present observations of BVRI surface brightness gradients in the twilight sky, and discuss some strategies for the treatment of gradients in the moonlit and dark sky.