# public documents.sextractor_doc

## [/] [faq.tex] - Rev 29

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

A: The {\tt ASSOC} list which is used for \index{cross-identification} 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 \index{WCS} 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 \index{threshold} threshold expressed in units of the background noise \index{standard deviation} standard deviation
in the {\tt FILTER}{\em ed} \index{image} image ?}

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.

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