How to use SExtractor to calculate Petrosian Radius?

Question

I'm using Sextractor to run some benchmarks on the code I'm working on. I'm mainly focused on Petrosial Radius (PETRO_RADIUS).

I have two main questions that I could not find in the documentation:

1. Usually, Petrosian Radius is calculated on a fixed fraction of the total flux of the galaxy given by ETA function. The most common value of the fraction is 0.2. It seems that it is not possible to set this value as the input parameter. What value is used internally in Sextractor? Is it possible to influence it with some input parameters?
2. Regarding units of PETRO_RADIUS. The documentation states that "Petrosian apertures in units of A or B". Im a bit confused in a sense, how can I convert it to pixels? Do I multiply by A or B? Or sqrt(A*B)?

Thanks ahead for the help.

Answer 1

Response from Emanuelle Bertin:

1. Yes the fraction is 0.2 and it is hard-coded.
2. In SExtractor automated aperture measurements, including Petrosian magnitudes, are done in an ellipse which is a scaled version of the ellipse derived from the 2nd order moments of the isophotal light distribution. So the area of the photometric aperture would be pi * A_IMAGE * B_IMAGE * PETRO_RADIUS^2 pixels.