Maximum separation between guide stars in atmospheric tomography

摘要

The quality of the tomographic wave-front reconstruction at large telescopes is limited by the vertical turbulence profile and by the configuration of guide stars (GSs). The tomographic error (in square radians) scales as (Θ_(r_0)/δ)~(5/3), where ? is the size of GS constellation, ro is the Fried radius, and 5 is the "effective thickness" of a given profile. Compared to the classical AO, the gain of the field-of-view size is proportional to the ratio of mean turbulence altitude h to δ, some ～ 10 times for real profiles with 3 GSs. It is shown that the residual anisoplanatism in a layer-oriented multi-conjugate adaptive optics system obeys a similar scaling law and that this approach offers a practically interesting sub-optimal solution of the tomographic problem. For a given total number of photons available from GSs, the photon noise in the compensated wave-front is proportional to the square of the number of layers. It means that even by increasing the number of layers it will not be possible to extend the correction into the visible range using only natural GSs, but laser GSs will be needed.