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\chapter{What is {\sc SExtractor}?}
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{\sc SExtractor} ({\em Source-Extractor}) is a program that builds a catalogue of objects from an
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astronomical image. It is particularly oriented towards the reduction of large scale galaxy-survey
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data, but it also performs well on moderately crowded star fields.
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Its main features are:
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\begin{itemize}
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\item Support for multi-extension FITS.
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\item Speed: typically 1--10 Mpixel/s or 10--1000 sources/s with a 2 GHz processor.
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\item Ability to work with very large images (up to $65{\rm k}\times65{\rm k}$
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pixels on 32 bit machines, or $2{\rm G}\times2{\rm G}$ pixels on 64 bit machines),
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thanks to buffered image access.
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\item Real-time match-filtering of images to improve detectability. \gam{This
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jargon might not be understood until reading the entire documentation.}
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\item Robust deblending of overlapping extended objects.
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\item Flexible catalogue output of desired parameters only.
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\item Pixel-to-pixel photometry in dual-image mode.
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\item Fast Point-Spread-Function and galaxy model fitting.
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\item Handling of weight maps and flag maps.
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\item Optimum handling of images with variable S/N.
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\item Built-in catalogue cross-identification.
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\item Special mode for photographic scans.
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\item XML VOTable-compliant catalogue output.
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\end{itemize}
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Back in the early nineties, the purpose of {\sc SExtractor} was to find a compromise
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between refinement in both detection and measurements, and computational speed. By today's
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standards, {\sc SExtractor} would be more accurately described as a ``quick-and-dirty'' tool.
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