Using Unfiltered Images to Perform Standard Filter Band Photometry

Volume 45 number 1 (2017)

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Joe Garlitz

1155 Harford St; Elgin, OR 97827 garlitzj@eoni.com

Abstract

This paper demonstrates that raw instrumental magnitudes of stars measured from a single unfiltered CCD image can be transformed into standard passband magnitudes. Star fields that have good catalogued photometric magnitudes can be used as a reference to transform unfiltered instrumental magnitudes into a standard system. To demonstrate this, the AAVSO (VSP) M-67 catalogued stars are used. It is shown that, within certain constraints, the standard B, V, Rc, and Ic magnitudes can be accurately determined from unfiltered instrumental magnitudes. For well behaved, well calibrated stars, the transformations to standard magnitudes can be done within a standard deviation of better than 0.021 magnitude. The paper further presents a simple spreadsheet tool to automatically derive the “standard” magnitudes from the raw instrumental magnitudes. This greatly simplifies the task of calculating transformation coefficients, and makes it possible to calibrate a CCD imaging system on an image-by-image basis.