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We have mapped the gradient in the surface brightness of the sky during morning and evening twilight from two different sites. In the BVRI bandpasses relative gradients of up to several percent per degree are present, except near a null point close to the zenith. Although the details of the gradient map are a function of the changing zenith distance of the sun, as well as other variables, we are nevertheless able to suggest a reasonable strategy for use of the twilight sky in the flat field calibration of astronomical CCD cameras.
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