The procedure involves fitting the point-spread function to one or more sources by a least-squares fit. The background can either be determined in the fit, or a fixed background value. Additionally, the x and y coordinates can be determined using a simplex minimization routine- the user should be warned that this method increases the CPU time significantly. For more details of the PSF fitting method please refer to the documentation on the SAS task omphotom.
The weighting scheme used in the fitting can be either 1/pixel variance or 1 (equal weights).
For information about how the latter is done, please refer to the documentation for the SAS task omphotom.