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R.P. Boyle (Vatican Obs.), R. Janusz (U. College Ignatianum, Krakow), A.G.D. Philip (Union College and ISO), V. Laugalys, A. Kazlauskas (ITPA, Vilnius)
How obtain in the Strömvil system CCD photometry of one percent quality needed for subsequent stellar classification in temperature, surface gravity, reddening and metallicity? At the Vatican 1.8-meter telescope on Mt. Graham we always observe M 67 as a standard field. Laugalys et al. (Baltic Astronomy, 13, 1, 2004) demonstrate the quality of their CCD photometry in M 67 as free of flatfielding error in the published CCD photometry in the Vilnius system. The Strömvil and Vilnius photometric systems are linearly transformable. We now find that we can use that CCD photometry as standards to show and correct for any systematic error of the initial flat used for our observations. We fit instrumental magnitudes per filter as dependent on the CCD standards not only with zero-point and color term but also with terms in x, y over the field. We then extract an illumination correction from these x, y fitted terms and thereby make a corrected flat. Use of the corrected flat shows any systematic error of the initial flat has vanished. Consequently we can use with confidence this corrected flat on new object fields where only a few standards are needed for fixing the zero-point for the color transformation.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.