The CSPEC option constructs a psf which is a sum of the named psf evaluated at different energies, weighted by the counts in a user supplied spectrum. The syntax is, CSPEC(psf/model,spec[,nbin]).
The first argument may be any energy dependent psf, eg. XRT_PSPC or XRT_HRI or a POLAR/RECT model using one of these. The second argument is a spectrum in the same data units as the dataset being processed. When using PSS in Cash statistic mode this means a raw counts spectrum should be used.
The third, optional argument is the number of spectral bins into
which the spectrum is divided - the default is one. The psf system
defines those bounds in energy space such that the 
of the
counts fall into each bin. When nbin is unity, the system finds
the mean photon energy for the spectrum supplied.