Test for Extension

A source may be tested for extension using the following procedure:

        icl> idisplay
        icl> icentroid            ! Find the centre of the source 
        icl> imode key            ! Switch to keyboard mode 
        icl> iradial              ! Produce a radial profile 
        icl> ipsf                 ! Compare with the expected PSF

ipsf allows several PSF functions to be compared against the radial profile. The PSPC options may be accessed by selecting XRT_PSPC. The best options at present are, ONAXIS_3 if the source is on axis or VARPROFILE if it is off axis. The routine gives a $\chi^{2}$ value for the quality of the fit.

NB: if a source appears to be extended, the image should be sorted over times when the aspect error value was low, to minimise possible aspect errors, and the above procedure repeated.