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 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 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).