public documents.sextractor_doc

\chapter{Measurements}
Once sources have been detected and deblended, they enter the measurement phase.
{\sc SExtractor} performs two categories of measurements. Measurements from the first
category are made on the isophotal object profiles. Only pixels above
the detection \index{threshold} threshold are considered. Many of these isophotal measurements
(like {\tt X\_IMAGE}, {\tt Y\_IMAGE}, etc.) are necessary for the internal
operations of {\sc SExtractor} and are therefore executed even if they are not
requested. Measurements from the second category have access to all pixels of the \index{image} image.
These measurements are generally more sophisticated and are done at a later stage
of the processing (after {\tt CLEAN}ing and {\tt MASK}ing).