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Hubble Space Telescope
FOC Instrument Science Report # 74

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.