APERSTAR estimates the brightness of each star on a photometry file using aperture photometry. The brightness is the sum of the counts within a specified radius around each star, with the sky estimated by the mean level of counts in an annulus around the stars. APERSTAR performs the same task as the DAOPHOT PHOTOMETRY routine, but the latter probably does a better job and is recommended for most occasions.
There are three steps to measuring stellar brightnesses using APERSTAR. First locate the stars you want to be measured using AUTOMARK or MARKSTAR. Then measure the brightness using APERSTAR. Finally, save the results to disk using SAVE PHOT or print then using the command PRINT PHOT.
You MUST specify the radius of a stellar aperture, and the radii of an annulus used to estimate the sky. The inner radius of the sky annulus must be larger than the radius of the star aperture. The sky radii must differ by at least 2. Exception: if you give the word SKY=NONE, the sky value is assumed to be 0.0. This allows you to determine the sky value globally for the image (using the SKY program), then having all stars measured with the same sky level. If SKY=NONE is not given, the sky level is estimated for each star.
The algorithm for determining the sky level works like this: The MEAN and standard deviation of the intensities is measured in a first pass. Those pixels which differ from the mean by four times the standard deviation are then excluded from the list, and the mean level computed again. This mean is taken as the sky brightness. The brightness of the star is then the sum of (pixel - sky) for all pixels in the star's aperture. The rejection level can be changed from 4-sigma to a user specified value using REJECT=sig, which gives the number of sigma objects to reject.
A similar program is APER, which measures the brightness of an single object using a series of apertures. Also see PROFILE or PRAD, which measure the radial profile of objects on a frame.