Using SExtractor to find stars in FITS images

imwcs and immatch can accept catalogs of pixel positions and instrument magnitudes of stars in an image in the daofind output format

[x pixel center]   [y pixel center]   [instrument magnitude]

Setting up SExtractor

To use SExtractor, Emmanuel Bertin's neural net program which extracts and classifies objects in an image, modify the default.sex file to daofind.sex:

CATALOG_NAME    image.sex       # name of the output catalog
CATALOG_TYPE    ASCII_HEAD      # "ASCII_HEAD","ASCII","FITS_1.0" or "FITS_LDAC"

PARAMETERS_NAME daofind.param   # name of the file containing catalog contents

Other parameters for photometry or background may be modified as necessary.

Then set the parameters you need to emulate the daofind output format in the file daofind.param, which contains simply

X_IMAGE
Y_IMAGE
MAG_BEST

The default.nnw and default.conv files do not need to be changed.

Using SExtractor

Run SExtractor on the image, sort the result by magnitude, and keep the brightest objects. This 1024x1024 image was processed in less than 10 seconds on a SPARCstation 4.

$ sex -c daofind.sex test.fits
Frame: "16139p3131R" / 1024 x 1024 / 16 bits INTEGER data
Background: -0.17547   RMS: 9.83951    / Threshold: 14.7593    
Objects: detected 504      / sextracted 503     
> All done
$ sort -n +2.0 image.sex | head -100 > test.sex

Then run imwcs as in the example:

$ imwcs -vd test.sex -c uac -h 50 test.fits

A shell script, imsex, which requires the file default.sex, has been written to run SExtractor without having to establish local parameter files. It can also automatically extract image extensions from multi-extension FITS files before running SExtractor on them. Returned stars are sorted from brightest to faintest, and the brightest 100 are written to a file named as the input file, with .sex instead of the input .fits (or whatever). If an image extension is extracted, the output files (FITS and DAO-format) have .extension name inserted before the suffix. For example, if the file multitest.fits has four image extensions with EXTNAME set in each one,

	imsex -m -n 200

creates the image files multitest_IM1.fits, multitest_IM2.fits, multitest_IM3.fits, and multitest_IM4.fits, and the SExtracor output files multitest_IM1.sex, multitest_IM2.sex, multitest_IM3.sex, and multitest_IM4.sex, with up to 200 stars each. As with all WCSTools programs, typing imsex with no arguments lists the possible arguments.

Getting SExtractor

ftp from France

References

SExtractor Home Page

SExtractor Users Guide [PDF] [PostScript]

SExtractor Forum

SExtractor: Software for source extraction

Bertin, E. and Arnouts, S. (1996), A&A Supp. v.117 p. 393-404.

SExtractor for Dummies

TN 0034: Some Notes on Using the New Version of Sextractor to Generate Star Lists From TASS Images

Glenn Gombert

Last updated 24 August 2006 by Doug Mink dmink@cfa.harvard.edu

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