within a single quadrant (down to nearly zero in one edge), which can hardly be corrected by a flat field map and causes a big non-uniformity of the signal to noise ratio within the elementary frames. The photometry of the image restoration process is also badly affected for intrinsically big objects that spread on the whole image.

10000 The electronic device which computes the coordinates of the photo-events produces an artifact which affects the photometry of the images.

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 causes a depletion of high spatial frequencies in the power spectrum. A “hole” can be seen in the center of the mean auto-correlation, which becomes larger when the photon flux increases. 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 was used to over-sample by a factor of 3.

[*4.2. The Ranicon*]{}

10000 The Ranicon (“Resistive Anode camera”, 