The encircled energy function is calculated by integrating the PSF, described as a single King function, out to the input radius by:

Where is the core radius in arcseconds and is the slope.

The parameters of the King function have been measured for the three mirror modules and described in terms of photon energy and off-axis angle by the formulae:

From these formulae the parameters and have been tabulated as a function of energy and and stored in the CCF components XRT?_XPSF. This routine uses spline fitting to interpolate between these tabulated values.

Michael Smith 2011-09-20