THE ASTRONOMICAL JOURNAL       VOLUME 105, NUMBER 3, PAGE 1206       MARCH 1993

               AN EXPOSURE GUIDE FOR TAKING TWILIGHT FLATFIELDS
                            WITH LARGE FORMAT CCDS

                                 NEIL D. TYSON

  Princeton University Observatory, Peyton Hall, Princeton, New Jersey 08540

                                  ROY R. GAL

    Department of Astronomy, Columbia University, New York, New York 10027


                                   ABSTRACT

    Driven by the long readout times of today's large format CCDs, we
    present an efficient method to compute an ideal sequence of exposure
    times for twilight skyflats.  The procedure will allow the observer to
    maximize the number of flats that can be obtained in the rapidly
    changing surface brightness of the dusk or dawn sky.