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\section{FAQ (Frequently Asked Questions)}
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\section{FAQ (Frequently Asked Questions)}
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Fairly often, I am asked by users about the reason for some limitations or
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Fairly often, I am asked by users about the reason for some limitations or
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choices in the way things are done in {\sc SExtractor}. In this section, I try to justify them.
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choices in the way things are done in {\sc SExtractor}. In this section, I try to justify them.
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Q: {\bf {\sc SExtractor} supports WCS. So why isn't it possible to have the {\tt ASSOC} cross-identification
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Q: {\bf {\sc SExtractor} supports \index{WCS} WCS. So why isn't it possible to have the {\tt ASSOC} \index{cross-identification} cross-identification
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working in $\alpha,\delta$ (or any other world-coordinates)?}
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working in $\alpha,\delta$ (or any other world-coordinates)?}
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A: The {\tt ASSOC} list which is used for cross-identification can be very long (100,000 objects or more).
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A: The {\tt ASSOC} list which is used for \index{cross-identification} cross-identification can be very long (100,000 objects or more).
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Performing an exhaustive cross-id in real-time can therefore be extremely slow, unless the {\tt ASSOC}
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Performing an exhaustive cross-id in real-time can therefore be extremely slow, unless the {\tt ASSOC}
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coordinates are sorted in some way beforehand. In pixel coordinates, such a sorting is simple and very
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coordinates are sorted in some way beforehand. In pixel coordinates, such a sorting is simple and very
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efficient, as {\sc SExtractor} works line-by-line; but it would be much more difficult in the general WCS context.
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efficient, as {\sc SExtractor} works line-by-line; but it would be much more difficult in the general \index{WCS} WCS context.
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This is why this hasn't been implemented, considering it as beyond the scope of {\sc SExtractor}.
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This is why this hasn't been implemented, considering it as beyond the scope of {\sc SExtractor}.
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Q: {\bf Why isn't the detection threshold expressed in units of the background noise standard deviation
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Q: {\bf Why isn't the detection \index{threshold} threshold expressed in units of the background noise \index{standard deviation} standard deviation
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in the {\tt FILTER}{\em ed} image ?}
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in the {\tt FILTER}{\em ed} \index{image} image ?}
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A: There are two reasons for this. First, it makes the threshold independent of the choice of a {\tt FILTER},
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A: There are two reasons for this. First, it makes the \index{threshold} threshold independent of the choice of a {\tt FILTER},
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which is a good thing. Second, having $\sigma$ measured on the {\tt FILTER}ed image may have given
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which is a good thing. Second, having $\sigma$ measured on the {\tt FILTER}ed \index{image} image may have given
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un-informed users the wrong impression that increasing filtering systematically improves the detectability
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un-informed users the wrong impression that increasing filtering systematically improves the detectability
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of any source, whereas it depends on scale.
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of any source, whereas it depends on scale.
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Q: {\bf Can SExtractor compute asymmetry and concentration parameters?}
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Q: {\bf Can SExtractor compute \index{asymmetry} asymmetry and \index{concentration} concentration parameters?}
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\end{document}
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