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In most of the techniques currently proposed to overcome the well known limitation of the laser Guide Star of not providing a useful tip-tilt signal, the size of the isokinetic patch plays a major role. Isokinetic patch size, defined as the distance at which the tilt correlation falls below a given threshold, can be measured, in principle, using a star cluster, as the few available measurements in the literature report. We have used the edge of the moon as a reference for the measurement of the tilt correlation over angles spanning from a few arcsec to several arcminutes. We constructed a seeing monitor using a 20-cm portable telescope coupled, through a pair of commercial objectives, with an anamorphic optical relay and a small commercially available 12-bit CCD. The image of the edge of the moon (relayed using the anamorphic relay) is frame-shifted in groups of six or eight in a fast manner, and it is read out later. In this way there is no need for a fast analog-to-digital converter, and only a minor modification to the original commercially available devices is needed. A three-day run at the Asiago Observatory during January 1997 has been performed. The collected data have been extensively analyzed in order to assure their reliability. A comparison with seeing measurements done simultaneously at the focus of the main observatory is presented, showing good agreement. Dispersion of the edge tilts consistent with Poisson statistics is pointed out. Scaling to the planned run at the Canary Island in order to pursue the Adaptive Optics module for the TNG telescope is briefly discussed together with the implication of the results for the various LGS tilt-recovery techniques.

Switching-on an adaptive optics loop, the system usually rapidly locks the target in the correction mode. When the sky background contribution is not negligible, the SNR may depend upon the correction status. While in an already closed loop, the SNR could be high enough to maintain the situation of good correction by the adopt system, during the switching-on phase, this condition could be far from true, because it is possible that the SNR isn't enough to lock the loop, starting from the open loop situation. We discuss this problem (called the bootstrap problem for adaptive optics) with particular attention on some filtering system working in the described situation and tuned to solve the problem.

The status report of the adaptive optics module for the 3.5 m TNG telescope is briefly given together with the description of three important subsystems: the off-axis tracking capability of the wavefront sensor; the CCD to be used for the wavefront sensor and the modal filtering approach to be implemented in the module.

ThermoTrex Corporation has designed and built a prototype of the fast steering mirror to be used for image motion control in the TNG adaptive optics system. The principal characteristic of this mirror is the use of voice coil actuators whose positions are controlled with closed loops based on capacitive sensors. Here we report the main features of the mirror assembly and laboratory measurements done to characterize the mirror behavior. The Bode diagram of the mirror is reported and discussed.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.