Source Extension

  In expert SEARCH mode, PSS can test for source extension. The controlling parameter is EXTEN,

   EXTEN - Fit for extension measure /NO/ >

Note that this method is suitable only for moderately extended sources -- the limit is roughly a FWHM more than twice the size of the psf itself. This problem has several causes,

• As the source becomes more extended, it becomes susceptible to removal by background subtraction software.
• As the convolution width becomes large, the significance tends to drop as less of the psf being taken into account. The source can therefore be missed in the first pass. This effect can be be alleviated to a certain extent by two stage thresholding (as decsribed in the tutorial), letting weaker peaks through to fitting and extension testing before applying a more suitable threshold for output.

The solution to these problems is to use the ASTERIX application CREPSF to create a tabular psf. This can be convolved with various functions and used in PSS using the TABULAR option at the PSF prompt. An example of how to do this can be found in Section 6.8.

Selecting extension testing causes two extra fields to be written to the output SSDS, the extension in arcminutes with error, EXTEN, and the significance of the extension, EXTENSIG. The latter value expresses the probability that the best fit extension is different from zero.