The properties of avalanche photodiodes and associated electronics required for photon counting in the Geiger and the sub-Geiger modes are reviewed. When the Geiger mode is used, there are significant improvements reported in overall photon detection efficiencies (approaching 70% at 633 nm), and a timing jitter (under 200 ps) is achieved with passive quenching at high overvoltages (20-30 V). The results obtained by using an active-mode fast quench circuit capable of switching overvoltages as high as 15 V (giving photon detection efficiencies in the 50% range) with a dead time of less than 50 ns are reported. Larger diodes (up to 1 mm in diameter) that are usable in the Geiger mode and that have quantum efficiencies over 80% in the 500-800-nm range are also reported.

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Optimal removal oftip-tilt or image motion is essential for Adaptive Optics. The tilt loop residualjitter considerably affects the performance of the AO, given the high noise propagation coefficients of this term. Therefore we have dedicated efforts for the development of a state-of-the-art servo-loop with optimized sensor and loop-control strategies. The System for Tip-tilt Removal with Avalanche Photodiodes (STRAP) is being tested and will be implemented in a number of VLT instruments. It is compliant with the ESO VLT standards in hardware and software. We report here the rationale and a description of STRAP in its components and subsystems.

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