SIMBAD references

When accurate CCD stellar photometry is required for classifying stars, correct flatfielding calibration is most important. The method of reshaping the surface of an initial flatfield by use of differential photometry of offset exposures of a well-populated starfield is valid if done carefully. However, more appealing for its efficiency and success is the method described here of matching new observations to an accurate catalog of many CCD standard stars populating practically the whole field while reshaping the flatfield during the photometric calibration.