> Petrosian magnitudes were designed to help with the difficult task of determining a galaxy's total light. Although these magnitudes (taken here as the flux within 2R_Pet, with the inverted Petrosian index 1/eta(R_Pet)=0.2) can represent most of an object's flux, they do of course miss the light outside of the Petrosian aperture. Under the assumption of similar surface brightness distributions, this missing flux is simply a constant value and therefore easily corrected for. The size of the flux deficit, however, varies monotonically with the shape of a galaxy's light-profile, i.e., its concentration. In the case of a de Vaucouleurs R^{1/4} profile, the deficit is 0.20 mag; for an R^{1/8} profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total magnitudes from Petrosian magnitudes using only the galaxy concentration within the Petrosian aperture. We also show how this concentration can be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the SDSS DR2 Petrosian parameters, yielding excellent agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the NYU-VAGC Sersic fits by Blanton et al. (2005). Although the method described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.
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