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On The Problem Of Standard Stars, Extinction, And The Photometric Zero-Point Of Uncalibrated Photographic Plates

Stars of known brightness can be used to estimate the surface brightness of extended objects to a given isophote set by the background sky density on photographic plates. The method requires the digitization of images by the measurement of specular photographic densities. A versatile theory has been proposed by Zou, Chen, and Peterson for which star trails can have arbitrary shape. The method frees observers from the need for spot sensitometry if standard stars of sufficient number and brightness range are in the field. The present formulation is an elaboration which allows for atmospheric extinction and variable background sky density. It permits a solution in principle for the photometric zero-point. A simultaneous solution for the extinction coefficients and the polynomial expansion coefficients can be found by best-fit methods. The photometric zero-point of the standard stars permits calibration of the H-D curve. Energy per pixel area is converted to intensity knowing the exposure time, and the aperture and focal length of the telescope. The incorporation of extinction corrections and variable background should permit photometry of extended objects across wide fields such as those accessible to cameras of the Schmidt design.