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.CSPEC(psf/model,spec[,nbin])
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 1/nbin of the counts fall into each bin. When nbin is unity, the system finds the mean photon energy for the spectrum supplied.