Multiconjugate AO system for 8-m class telescopes

Abstract

Adaptive optics systems for the current generation of astronomical telescopes are being designed and developed at the present time with different goals and related complexities. In order to make the best use of such systems more and more projects are considering introducing at least one artificial reference star. It is a fact that for 8 m class telescopes a single artificial star will only give benefit in the last part of the NIR region of the spectrum (essentially H and K bands). To push the performances of such systems to shorter wavelengths and/or worse seeing it is required to have more than one reference star. It is known that the multi-reference, multi-conjugate adaptive optics systems (MCAO) could provide great advantages in mitigating the effect of the limited range of the artificial reference stars (the so-called cone effect) while in the mean time enlarging the corrected field of view of an AO system. We are currently approaching the problem of designing such a system from both practical and theoretical point of view and carefully comparing several possible solutions: multiple sodium laser guide stars, multiple Rayleigh back-scattering laser guide stars, hybrid system using both types of stars. The first type of stars has the advantage of having the highest range achievable today but currently the lasers required to produce this type of stars have a limited available power, high costs and low reliability. On the other hand the lasers required for Rayleigh guide stars do not suffer from these limitations but the ranges achieved are lower. Given these limitations we think that a system that would use either a constellation of Rayleigh stars or a hybrid with a single sodium star could be a good compromise between the various choices. In this paper we present some preliminary evaluation results of an MCAO system that would use a constellation of off-axis Rayleigh stars plus an on-axis Rayleigh or sodium star.