Stored as petroMag. For galaxy photometry, measuring flux is more difficult than for stars, because galaxies do not all have the same radial surface
brightness profile, and have no sharp edges. In order to avoid
biases, we wish to measure a constant fraction of the total light,
independent of the position and distance of the object. To satisfy these
requirements, the SDSS has adopted a modified form of the
Petrosian (1976) system, measuring galaxy fluxes within a circular
aperture whose radius is defined by the shape of the azimuthally
averaged light profile.