\chapter{Handling of \index{image} image data}
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\chapter{Handling of \index{image} image data}
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\section{Image format}
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\section{Image format}
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{\sc SExtractor} accepts \index{image} images stored in FITS\footnote{\it Flexible Image Transport System}
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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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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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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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\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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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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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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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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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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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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{\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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header, it is automatically used by {\sc SExtractor} to compute astrometric parameters. Other
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astrometric descriptions
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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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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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\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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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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{\tt X} axis, and FITS axis ``2'' as the {\tt Y} axis.
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\section{Double image mode}
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\section{Double image mode}
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If data are available for the same field in several photometric bands,
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If data are available for the same field in several photometric bands,
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it is usually desirable to measure object characteristics such as
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it is usually desirable to measure object characteristics such as
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magnitudes exactly at the same positions and through the same apertures
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magnitudes exactly at the same positions and through the same apertures
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for the different bands, to derive precise colour indices for example.
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for the different bands, to derive precise colour indices for example.
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{\sc SExtractor} makes this possible by providing a special mode dubbed
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{\sc SExtractor} makes this possible by providing a special mode dubbed
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``double image mode'' where detections are made on one image while
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``double image mode'' where detections are made on one image while
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measurements are carried out on another (both images must have exactly the
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measurements are carried out on another (both images must have exactly the
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same number of pixels). By repeatedly running {\sc SExtractor}
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same number of pixels). By repeatedly running {\sc SExtractor}
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with various ``measurement images'' while keeping the same ``detection image'',
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with various ``measurement images'' while keeping the same ``detection image'',
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one ends up with a set of catalogues having the same sources measured through
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one ends up with a set of catalogues having the same sources measured through
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different channels.
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different channels. The detection image will generally be chosen in the band
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where the data are the deepest. Alternatively, using a ``$\chi2$ image''
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In this case, Sextractor should be run first in single image mode on
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\citep{szalay:al:1999} (produced e.g., by
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the image with the ``best'' quality, usually the band where the data
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\href{http://astromatic.net/software/swarp}{\sc SWarp}) as a detection image,
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are the deepest and/or where the seeing is the best (hereafter image
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will allow most of the sources present in at least one channel to be detected
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1).
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and measured.
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Objects will be detected and measured in the deepest image.
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Double image mode is automatically engaged by providing {\sc SExtractor} with
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two images instead of one:
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As usual before running SExtractor, make sure that you have defined
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the parameters you want to measure on the image by editing the
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\% {\tt sex detection.fits,measurement.fits}\\
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default.param file. You must also edit the default1.sex file (adapted
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to the characteristics of image 1).
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A space may be used instead of a coma. In the example above,
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{\tt sex detection.fits} is used as a detection image, while measurements are
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You can then run SExtractor on image 1 by typing something like:
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carried out on {\tt measurement.fits}.
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{\tt sex image1.fits -c default1.sex}
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In order to run SExtractor on a second image (image 2) using the
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detections made on image 1, it is necessary to edit a default2.sex
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file (adapted to the characteristics of image 2).
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You can then run SExtractor on image 2 in double image mode by typing
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something like:
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{\tt sex image1.fits,image2.fits -c default2.sex}
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You then obtain two different catalogues for the two images, which
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You then obtain two different catalogues for the two images, which
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have exactly the same numbers of lines and can be put together.
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have exactly the same numbers of lines and can be put together.
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If you have images in other bands covering the same regions, you can
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If you have images in other bands covering the same regions, you can
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measure them in double image mode as well, always taking as a
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measure them in double image mode as well, always taking as a
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reference image 1, and then combine all the catalogues obtained.
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reference image 1, and then combine all the catalogues obtained.
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