# public documents.sextractor_doc

## [/] [faq.tex] - Rev 19

\section{FAQ (Frequently Asked Questions)}
Fairly often, I am asked by users about the reason for some limitations or
choices in the way things are done in {\sc SExtractor}.  In this section, I try to justify them.

Q: {\bf {\sc SExtractor} supports WCS. So why isn't it possible to have the {\tt ASSOC} cross-identification
working in $\alpha,\delta$ (or any other world-coordinates)?}

A: The {\tt ASSOC} list which is used for cross-identification can be very long (100,000 objects or more).
Performing an exhaustive cross-id in real-time can therefore be extremely slow, unless the {\tt ASSOC}
coordinates are sorted in some way beforehand. In pixel coordinates, such a sorting is simple and very
efficient, as {\sc SExtractor} works line-by-line; but it would be much more difficult in the general WCS context.
This is why this hasn't been implemented, considering it as beyond the scope of {\sc SExtractor}.

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

A: There are two reasons for this. First, it makes the threshold independent of the choice of a {\tt FILTER},
which is a good thing. Second, having $\sigma$ measured on the {\tt FILTER}ed 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.

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