The SPADA sensor is made of 60 SPAD elements placed at the focus of small sphere lenslet arrays, and each element is arranged to correspond to each sphere as shown in Fig. 2 (left). The overall SPADA chip is squared, with dimensions of 18 mm x18 mm. Each element consists of four SPADs, with diameters of 20 ^m, 35 ^m, 50 ^m, and 75 ^m (see Fig. 2 (right)).
The SPADA chip is assembled on a ceramic holder over a Peltier cooler and then mounted on the central plate of the housing as shown in Fig. 3. The focusing opto-mechanics are aligned on top of the SPADA through alignment marks patterned on the SPADA chip, on the SPADA holder, and on the metal plate.
To reduce dark counts to a few counts per second, it is mandatory to operate the SPAD at temperatures below 0°C. However temperatures lower than -30°C must be avoided because spurious counts increase due to the afterpulsing effect [1,2]. The cooling system based on a Peltier thermoelectric cooler has been designed in collaboration with ESO and finalized via industrial contracts. The housing is filled with dry nitrogen to avoid fogging and condensation.
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