As mentioned above and shown in Fig. 67, photons entering the
OM detector hit a photo-cathode, which is located at the backside of the
detector entrance window. Electrons emanating from the photo-cathode are
amplified by the MCPs, creating photon splashes at the location of the OM CCD. The detection of a photon entering the detector is performed by reading
out the CCD and determining the photon splash's centre position using a
centroiding algorithm, which is part of the onboard software.
In the process of centroiding a grid of 
``in-memory'' pixels is
defined, leading to an array of
in-memory pixels with a
size of 0.5'' on the sky. In the resulting images at some level there
is always a pattern repeating on an
grid. This can be removed
by subsequent processing on ground.
As with all photon-counting detectors, there is a limit to the maximum count rate achievable before saturation sets in. The frame rate of the OM detectors is 10 ms at slowest, so a dead time correction must be applied in the offline data processing for count rates above ca. 10 counts/s for point sources. In addition, sources which are too bright can depress the local sensitivity of the photocathode: this is a cumulative effect, so that fainter sources observed for long times have the same effect as brighter sources observed for shorter periods. This places some operational constraints on the instrument.
Cosmetically, the OM detectors are good, with few hot or dead pixels,
and little global variation in quantum efficiency. There is a small-scale
granularity on the photocathode, which can be removed by flat-fielding
in the ground processing. To some extent, the granularity (and the
pattern mentioned above) is smoothed by the spacecraft
drift.