Open the StarEdit window by clicking 
on the Star Removal Toolbar.
Star Removal Aperture Tool
The StarEdit Tool is used to adjust the shape of the markers used in the Star Removal module.
Open the StarEdit window by clicking on the Star Removal Toolbar.
The Star
Edit Window makes it easy to change the marker parameters.
Doing so changes the way templates and targets are measured and
removed from the image.
The Inner aperture specifies the region where the signal from each star is normalized to equal value. The Middle Aperture defines the region sampled for making the master template and the region actually subtracted from the targets. The Outer Aperture defines the outer limit for pixels used in subtracting the background. The background is computed using pixels between the middle and outer apertures.
The control above the large round dial, changes the Ellipticity of the markers. Ellipticity is defined as E = 1 - b/a, in which b is the semiminor axis and a is the semimajor axis length. All 3 apertures have the same ellipticity, so they also change together. Setting a value of 0 makes the apertures circular.
The Angle dial orients the direction of the major axis of the marker. This is only useful if the Ellipticity is greater than 0, meaning that the apertures are not circular. The angle is always measured so that it increases from positive x through positive y (see Angle Measurement Definition), except that the astronomical 0 angle is taken along the y axis, which is north (up) if the image is a FITS format image displayed in the normal way.
The [Default] button saves the current aperture parameters as default values for future sessions of the Star Removal package.
Star Removal Package, Star Removal Preferences, Aperture Photometry