public documents.sextractor_doc

[/] [faq.tex] - Rev 39

\chapter{FAQs (Frequently Asked Questions)}
\label{chap:faq}

{\bf Q.:} I heard that {\sc SExtractor} does not do as good a job at
extracting and measuring sources in \index{image} image {\it YYY} as does
package {\it XXX}.

{\bf A.}: The purpose of {\sc SExtractor} is to find a compromise between
refinement in detection/measurement accuracy and computational speed. Although
efforts are continuously being made in implementing more sophisticated
algorithms as computer speed increases, {\sc SExtractor} remains a general
purpose program.

{\bf Q.}: Can {\sc SExtractor} work on X-ray data?

{\bf A.}: X-ray data tends to be low count and source photon noise
limited. In contrast with optical \index{image} images,  the low counts in
X-ray \index{image} images make the Poisson background noise highly
skewed. Since many features in the current version of {\sc SExtractor} assume
that noise distribution is symmetrical around its \index{mean} mean or rely on
$\chi^2$ minimisation, using {\sc SExtractor} to analyze X-ray data
is not recommended at this time.

{\bf Q}: {\bf Why isn't the detection \index{threshold} threshold expressed in
units of the background noise \index{standard deviation} standard deviation
in the {\tt FILTER}{\em ed} \index{image} image ?}

{\bf A}: There are two reasons for this. First, it makes the \index{threshold}
threshold independent of the choice of a {\tt FILTER},
which is a good thing. Second, having $\sigma$ measured on the {\tt FILTER}ed
\index{image} image may have given un-informed users the wrong impression that
increasing filtering systematically improves the detectability of any source,
whereas it depends on scale.

{\bf Q}: {\bf Can SExtractor compute \index{asymmetry} asymmetry and
\index{concentration} concentration parameters?}