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R.P. Boyle (Vatican Obs.), F. Smriglio, A. Dasgupta (U. Roma), A. Kaslauskas, V. Straizys (ITPA, Vilnius), A.G.D. Philip (ISO/Union College)
Accurate stellar photometric classification requires one percent photometry. We are using the Strömvil photometric system on the Vatican Advanced Technology Telescope (VATT) to classify field and cluster stars in temperatures and surface gravities. Flat-fielding the CCD stellar fields should be controlled so as not to deteriorate the precision inherent in the stellar measurement over the field of view. Measuring the relative sensitivity on the pixel-to-pixel scale is easy; keeping the magnitude zero-point scale acceptably constant to one percent over the field-of-view is the challenge. Baffling stray light out of the focal plane is required to make acceptable flats. We will describe some tests for knowing the error of the flat and some means of decreasing that error.