In adaptive optics, the servo control systems need a relatively moderate number of very
sensitive sensors, with frame rates that oscillate between 500 and 5000 Hz. At these frame
rates, the read-out noise of CCDs degrades the performance dramatically. Moreover in the
applications with pulsed laser guide stars, electronic gating is available with about
nanosecond response times.
So far only single-diode solid-state counters are available. Those who have implemented curvature sensor adaptive optics systems have resorted to commercial modules based on PerkinElmer SPCM modules. These fiber-fed individual modules make it for bulky Wavefront Sensor Systems, cabinets of cooled SPCM modules, complicated bundles of fibres and light losses through the fiber coupling. The commercial modules are operated at high voltages (about 400V) and due to the module electronics they have shown a not too long a Mean Time Between Failures (MTBF), which has disappointed several groups.
The SPADA sensor is made of 60 SPAD elements placed at the focus of small sphere lenslets arrays, placed at the field focus of a 60 subaperture segmented lenslets array, as shown in Figure 1.30. The overall SPADA chip is squared, with dimensions of .
Each part constituting the SPADA system has been completed and some characterization tests have been carried-out. Next year we expect to assemble all the mechanical parts and begins the tests at the telescopes.