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 interfers 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 .