# public documents.sextractor_doc

## [/] [handle.tex] - Diff between revs 40 and 42

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double image mode'' where detections are made on one image while
double image mode'' where detections are made on one image while
measurements are carried out on another (both images must have exactly the
measurements are carried out on another (both images must have exactly the
same number of pixels). By repeatedly running {\sc SExtractor}
same number of pixels). By repeatedly running {\sc SExtractor}
with various measurement images'' while keeping the same detection image'',
with various measurement images'' while keeping the same detection image'',
one ends up with a set of catalogues having the same sources measured through
one ends up with a set of catalogues having the same sources measured through
different channels.
different channels. The detection image will generally be chosen in the band
 
where the data are the deepest. Alternatively, using a $\chi2$ image''
In this case, Sextractor should be run first in single image mode on
\citep{szalay:al:1999} (produced e.g., by
the image with the best'' quality, usually the band where the data
\href{http://astromatic.net/software/swarp}{\sc SWarp}) as a detection image,
are the deepest and/or where the seeing is the best (hereafter image
will allow most of the sources present in at least one channel to be detected
1).
and measured.
 
 
Objects will be detected and measured in the deepest image.
Double image mode is automatically engaged by providing {\sc SExtractor} with
 
two images instead of one:
As usual before running SExtractor, make sure that you have defined
 
the parameters you want to measure on the image by editing the
\% {\tt sex detection.fits,measurement.fits}\\
default.param file. You must also edit the default1.sex file (adapted
 
to the characteristics of image 1).
A space may be used instead of a coma. In the example above,
 
{\tt sex detection.fits} is used as a detection image, while measurements are
You can then run SExtractor on image 1 by typing something like:
carried out on {\tt measurement.fits}.
 

{\tt sex image1.fits -c default1.sex}

 

In order to run SExtractor on a second image (image 2) using the

detections made on image 1, it is necessary to edit a default2.sex

file (adapted to the characteristics of image 2).

 

You can then run SExtractor on image 2 in double image mode by typing

something like:

 

{\tt sex image1.fits,image2.fits -c default2.sex}

 
 
You then obtain two different catalogues for the two images, which
You then obtain two different catalogues for the two images, which
have exactly the same numbers of lines and can be put together.
have exactly the same numbers of lines and can be put together.
 
 
If you have images in other bands covering the same regions, you can
If you have images in other bands covering the same regions, you can