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.
Figure 1 Actuator pattern with 496 actuators for a mirror used under 22.5 degrees
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.
