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\chapter{Handling of \index{image} image data}
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{\sc SExtractor} accepts \index{image} images stored in FITS\footnote{\it Flexible Image Transport System}
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format (Wells \etal 1981, see also {\tt http://fits.gsfc.nasa.gov}).
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Both ``Basic FITS'' (one single header and one single body) and ``Multi-Extension-FITS'' (MEF)
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\index{image} images are recognized. Binary {\sc SExtractor} catalogues produced from MEF \index{image} images are MEF files
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themselves. If catalogue output is in ASCII format, all catalogues from the individual extensions
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are concatenated in one big file; the {\tt EXT\_NUMBER} catalogue parameter must be used to tell
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which extension the source belongs to.
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For \index{image} images with ${\tt NAXIS} > 2$, only the first data-plane is loaded.
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If \index{WCS} WCS\footnote{\it World Coordinate System} information (Greisen \& Calabretta 1995,
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{\tt http://www.cv.nrao.edu/fits/documents/wcs/wcs.all.ps}) is available in the
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header, it is automatically used by {\sc SExtractor} to compute astrometric parameters. Other
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astrometric descriptions
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like AST ({\it Starlink} format) \gam{Provide URL in footnote} or the solution coefficients of the DSS
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\footnote{{\it Digital Sky Survey} \gam{URL in footnote}} plates are not recognized by the software.
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In {\sc SExtractor}, as in all similar programs, FITS axis ``1'' is traditionaly referred as the
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{\tt X} axis, and FITS axis ``2'' as the {\tt Y} axis.
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