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As said in the previous section, avalanche photodiodes used in Geiger counter mode to detect individual photons have great interest for astronomy.
Two years ago together with the Department of Electronics, the Politecnico di Milano, the Department of Information Engineering, University of Pisa and the ESO European Southern Observatory our group started a project (that received funds from MURST) to design and realize a complete system based on a SPAD array detector named "SPADA'' to be used in two fields:

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 $18 mm
\times 18 mm$.
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.

Figure 1.30: Architecture of the SPADA system. The chip is assembled on a ceramic holder and on a Peltier stage (right) and is driven by the detection board (on right the schematic block diagram). The cross-section of the mechanical mounting is shown o the down part.

next up previous contents index
Next: Telescopio Nazionale Galileo Up: Detectors Previous: SPAD detector   Contents   Index
Innocenza Busa' 2005-11-14