In most cases, the performance of an optical telescope is limited not by the telescope's optical
properties but by the turbulence in the atmosphere above. The temperature fluctuations in the
atmosphere lead to fluctuations of the index of refraction which by themselves lead to degradation
of the image quality.
There basic parameters, Strehl ratio, Freid's parameter r0, and atmospheric time constant \tau decides the
quality of telescope images. Desinging an AO system requires a detailed knowledge of these parameters. The number of subapertures
determines the number of degrees of freedom and therefore the maximum correction which can be achived. Since the
solar granulation is the structure (2arcsec spacial size) the AO has to lock on, the subaperture diameter has to be
at least 63mm. Based on site characterisation results, the proposed AO system for NLST is shown in Figure 1. The actuator
spacing in x direction is 7.8mm and the spacing in y direction is 7.2 mm. 25 subapertures across the telescope
(i.e., 26 actuators across the telescope) corresponds to a subaperture diameter of 80mm.
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