These formulae are derived from the description
in . The distribution is computed according to the
inner () and outer () radii of the XRT assemblies, so that
the contribution appears like the projected telescope aperture for
finite defocus distance (which is provided by CAL_getRFCdefocus).
with , which is the result from Eq. (24),
and as in Eq. (21), is the Heaviside-step
function with its derivative , and
Eq. (25) needs to be integrated for each bin
of , and thus the probability for bin is
and . is filled such that the first moment of is 0.