public documents.sextractor_doc

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