In the following we give a brief decription of the MECS PSF model and of its limits of applicability, for a more detailed description see Boella et al. 97 A&A in press.
The MECS PSF model currently available has been derived from on axis measuraments and can be safely applied to sources with off-axis angles < 4 arcmin. The model is radially symmetric.
The MECS PSF is the convolution of 2 components: the optics PSF and the detector PSF. For a sake of simplicity it has been modeled as the sum of 2 components: a Gaussian describing mainly the detector PSF and a King Profile describing mainly the optics PSF. The parameters of the two components and their relative normalizations have first been derived at the 9 energies for which ground calibration data was available; subsequently the dependency of the above parameters on the Energy has been fitted with algebraic functions. The final result is a fully analytic differential PSF which can be analytically integrated.
The ground derived PSF model has been tested using the Cyg X-1 on-axis observation carried out in June 1996.