ABSTRACT
Standard adaptive optics systems measure and correct the wavefront error along the path to the reference star. Because the measured volume is very narrow, and because the correction is limited to the region of the measured path, the attained field of view is rather small-a few arc-seconds. The problem is even more severe when the reference star is not the measured star: either it is another natural star which could be far away or an artificial guide star at tens of kilometers above the telescope. In both cases the measured volume of atmosphere (Fig. 1) does not overlap with the observation path to the star and its vicinity, namely a truncated cone whose bases are the telescope and the star field. Even for bright objects, where the measurement is accurate, the corrected isoplanatic angle cannot extend beyond the few arc-seconds limit. One would like to have wider fields, of arc-minutes and more, over which turbulence can be corrected.
