6 Implementation notes

EXTRACTOR, will read data in floating point format. It will also read NDFs that contain BAD pixels. These will appear as “missing” regions, so any objects that are intersected will be split into parts.

Since EXTRACTOR uses the AST (SUN/211) library to perform astrometry, it can be used with images that contain DSS calibrations, as well as FITS-WCS and AST native ones (native SExtractor will only use images that have FITS-WCS calibrations). The GAIA display tool is a convenient way to add such astrometrical calibrations.

Two current limitations are that no support is provided for the use of NDF variance arrays (although these would probably map onto the “weight” image concept of SExtractor) and that the propagation of the input NDF to the check image is not done (so WCS calibration, for instance, is lost).

NDF pixel coordinates (i.e. ones that include the NDF origin) can be obtained using the X_PIXEL and Y_PIXEL parameters.

The SExtractor program has a very large range of processing options. Consequently many of these options have not been tried extensively in the EXTRACTOR incarnation, so some problems may arise, when using non-default configurations.

A SExtractor facility that has not been implemented is the piping of catalogues to standard output. The use of weight and flag images are not supported.

Finally as noted earlier the galaxy fitting parameters are not available.

If you come across any problems with EXTRACTOR please notify Starlink Software Support (starlink@jiscmail.ac.uk).