FOC Instrument Science Report # 74 "Correcting for the Nonlinear Behavior of Stars" by P. Greenfield Abstract excerpted from the report: A simple method for correcting for moderate amounts of nonlinear behavior of stars or unresolved sources is presented. It appears to work even in moderately crowded fields with a substantial background count rate. The method requires measuring the flux of the point source in an approximately 5.5 pixel radius aperture for 512x512 images and essentially consists of applying the flat field nonlinearity correction to the average flux in that aperture. The appropriate radius for the full format depends on whether the circular aperture is applied to zoomed pixels or dezoomed pixels. The appropriate radius for the former apppears to be 5.7 pixels and for the latter, 8.6 pixels. This method has been tested on selected stars in a number of exposures taken in various formats of the center of M15 and has markedly improved the consistency of the results. This method must be considered tentative until further corroborating tests can be carried out.
