The counts for a source are extracted within the aperture radius and the background subtracted. If necessary, the background-subtracted counts are extrapolated to the coincidence-loss area corresponding to a circle of 12 unbinned pixels, using the point-spread function (this correction is tabulated as psf1 corr). A correction is then made for coincidence-losses (this correction is tabulated as coinc corr). For UV data, a further correction is made to extrapolate the counts to a radius of 35 pixels using the point-spread function (this correction is tabulated as psf2 corr). The count-rate is then corrected for the CCD dead-fraction and an instrumental magnitude derived from it. The user can change the aperture radius to exceed the coincidence-loss radius, in which case the photometry will be computed as for an extended source.
The aperture radius can be changed between 6/bin and 12/bin pixels, where bin is the binning factor (1 for unbinned data, 2 otherwise). The upper limit corresponds to the cal calibration radius. Using a smaller radius than the upper limit will result in the source counts being scaled to the calibration radius using the point-spread function (this correction is the tabulated psf1 corr value).