Input files
SExtractor accepts images stored in FITS [1]. Both "Basic FITS" (one single header and one single body) and MEF files are recognized. Binary SExtractor catalogs produced from MEF images are MEF files themselves. If the catalog output format is set to ASCII, all catalogs from the individual extensions are concatenated in one big file; the EXT_NUMBER catalog parameter can be used to tell which extension the detection belongs to.
Important
Contrary to most other astronomy packages, SExtractor does not rely on the FITSIO library and instead uses its own library for managing FITS files. As a consequence, some features of FITS such as image compression/tiling are not supported at this time.
For images with \({\rm NAXIS} > 2\), only the first data-plane is loaded. In SExtractor, as in all similar programs, FITS axis #1 is traditionally referred to as the x axis, and FITS axis #2 as the y axis.
Double image mode
When the pixel data for a given field are available using the same pixel grid in several photometric channels, it is often best to measure object characteristics, like magnitudes, exactly at the same positions and through the same apertures for the different channels. This to derive precise color indices for example. SExtractor makes this possible by providing a special mode dubbed "double image mode" where detections are made on one image while measurements are carried out on another. Both images must have the exact same dimensions, obviously. By repeatedly running SExtractor with various "measurement images" while keeping the same "detection image", one ends up with a set of catalogs having the same sources measured through different channels. The detection image will generally be chosen in the band where the data are the deepest. Alternatively, using a "\(\chi2\) image" [2] [1] as a detection image, will allow most of the sources present in at least one channel to be detected and measured.
Double image mode is automatically engaged when providing SExtractor with two images instead of one:
$ sex detection.fits,measurement.fits
A space may be used instead of a coma. In the example above, detection.fits is used as a detection image, while measurements are carried out on measurement.fits.