  The electronic response of a telescope under direct illumination by a
point-like light source is based on photon counting. With the data obtained
using the SNDICE light source and the Megacam camera on the CFHT telescope, we
show that the ultimate precision is only limited by the photon statistical
fluctuation, which is below 1 ppm. A key feature of the analysis is the
incorporation of diffuse light that interferes with specularly reflected light
in the transmission model to explain the observed diffraction patterns. The
effect of diffuse light, usually hidden conveniently in the Strehl ratio for an
object at infinity, is characterized with a precision of 10 ppm. In particular,
the spatial frequency representation provides some strong physical constraints
and a practical monitoring of the roughness of various optical surfaces.
