The size and shape of the aperture can be specified from the keyboard by entering values for the semi-major axis (in pixels), the eccentricity and the orientation of the ellipse defining the aperture. When using optimal extraction this command is disabled, the clipping radius can be changed using the V menu option.
An eccentricity of 0 gives a circular aperture with a radius equal to the semi-major axis. The orientation of the ellipse is given in degrees and specifies the rotation of the semi-major axis of the ellipse anti-clockwise with respect to the vertical axis of the pixel array.
PHOTOM --- A Photometry Package