Adaptive Aperture Photometry (T. Jarrett)

A circular or ellipse aperture is used to capture all of the flux of an object not lost in the background noise. For spiral galaxies, this flux should represent the total flux, but for elliptical galaxies, at least 10% of the flux for the larger galaxies (K < 11) is buried in the noise because the flux from elliptical galaxies falls off very slowly with radius.

The basic method is to increase the aperture size until the change in the integrated flux is negligible. In practice, however, we want to minimize the affect of sky noise and contamination from nearby stars and other structures not associated with galaxy. This is accomplished by looking for unexpected (i.e., reverse slope changes) in the mean surface brightness corresponding to the outer annular area of the aperture.

To summarize the basic algorithm:

The aperture radius (j) is grown until either of two criteria are met:
1. the change in total flux between adjacent apertures (j vs. j-1) is smaller than some limit

or

2. the change in average surface brightess between adjacent aperture annuli is less than some limit

Current Settings for criteria limits:

1. change in total flux: 0.005 mag
2. % change in surface brightness: -2 * sig(back) / SQRT[area (R=10)]
(that is to say, the change in surface brightness greater than 2 * sigma (aperture), assuming the aperture is of size R=10 pixels)