ii. Axis Ratio and Small Radii
For small radii, we adjust the axis ratio such that the minimum semi-minor axis is 3 arcsec. This modification is needed to counteract the circularizing effects of the PSF. The half-light "effective" aperture and Kron aperture are particularly affected by this modification, since they are allowed to shrink to radii near the PSF.
An example of a dynamically adjusted axis ratio is shown in Figure 1. The nominal axis ratio is 0.4, and the position angle is -45°. Note the increasingly circular aperture, as the radius approaches the size of the PSF.
[Last Updated: 2002 Jul 15; by T. Jarrett]