SIMBAD references

A new observing procedure and method of analysis of CCD observations are presented. The observing procedure maximizes the on-source time without the need for observing separate blank sky fields. It is shown that a substantial improvement in the correction of sky and detector systematics can be achieved by explicitly separating the sky subtraction from the calibration of the spatial gain variations of the CCD, or flat-fielding. When we perform the sky subtraction before flat-fielding, the requirements on the latter are not as stringent. We test this method with deep I-band images obtained under extremely challenging observing conditions. Because of the presence of illumination gradients due to moonlight, a flat field with an accuracy of better than 10% could not be obtained. Nevertheless, a sky subtraction with an accuracy of approximately 0.1% of the sky brightness was achieved over the whole mosaic.