Multi-Conjugate Adaptive Optics with laser guide stars: performance in the infrared and visible

In this paper, the performance of a Multi-Conjugate Adaptive Optics (MCAO) system is evaluated, in the case where several (five) laser guide stars (LGSs) are used. The low-order modes are measured from either a single natural guide star (1NGS) with a 3 × 3 Shack-Hartmann sensor, or three tip–tilt stars (3NGS), each measuring only tip and tilt. To widen the corrected field of view, three deformable mirrors are used. These simulations are performed in closed loop, using a Maximum A Posteriori (MAP) control algorithm. It is shown that both systems significantly increase the corrected field of view (FOV). Residual anisoplanatism is seen over the 30-arcsec (diameter) corrected FOV, and it is due to errors in measuring low-order modes. In the visible part of the spectrum, the modelled infrared-optimized system still provides high angular resolution (22 mas – not strictly diffraction limited) over a field of view 10 times larger than the isoplanatic angle. This has a significant impact on the astrophysical instrumentation to be used. Both 1NGS and 3NGS methods provide good correction quality, the second one providing better stability of correction quality within the FOV. The 3NGS system allows the use of guide stars ∼2 mag fainter than the 1NGS approach.