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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.

A new concept for a device able to simulate evolving atmospherically distorted wavefronts for the adaptive optics system of the Italian facility Telescopio Nazionale Galileo (TNG) is presented. The goodness of performances of an adaptive optics system depends upon the testing accuracy of its components made under different seeing conditions. Moreover, in order to not loose good seeing nights, the possibility to perform tests during day-light can become strongly important. Introducing a simulator able to generate the image of an astronomical object deformed by atmosphere can help to solve many such problems. In order to create the wanted distortion we use phase changing plate (PCP) screens. A system of moving screens and adjustable zooms provides to change, over a wide range, some fundamental parameters of the atmosphere turbulence like fired parameter ro, Greenwood frequency fG and the isoplanatic angle (theta) o.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.