The electronic device which computes the coordinates of the photo-events produces an artifact which affects the photometry of the images. A ``hole'' can be seen in the center of the mean auto-correlation of images obtained with the CP40. This hole becomes larger when the photon flux increases.
When two photo-events are very close in the image, they merge into a single spot. The photon centering device is unable to identify it properly and discards such an event. This is the origin of the depletion of high spatial frequencies in the image power spectrum. This problem also affects the photometry since many photons are not recorded in the high intensity regions of the image.
To reduce this effect during our observations, the photon flux had to be limited to around 10,000 photons/sec and a high magnification is used to over-sample by a factor of 3.