Pyramid wavefront sensing using Laser Guide Star for 8m and ELT class telescopes

Laser Guide Stars (LGS) are mandatory to ensure large sky coverage of astronomical Adaptive Optics (AO) systems developed for 8m telescopes and Extremely Large Telescopes (ELT). However, the finite distance of the LGS spot from the telescope makes LGS wavefront sensing not easily scalable from an 8m to an ELT. The use of a Shack-Hartmann (SH) sensor with a Field-of-View (FoV) of about 10-20 arcsec requires fast (1kHz) very large detectors (more than 1000x1000 pixel for M4 5600 actuator) currently unavailable. In the paper, we present numerical simulations to study the behavior of a Pyramid wavefront sensor (PWFS) working with laser generated reference star. As detailed below, such a sensor can be implemented with existing CCD220. Achieved results are encouraging for both 8m and ELT class telescopes. In the 8m case, we studied a 40x40 sub-aperture configuration controlling about 800 modes and we achieved the same behavior as a SH sensor. For the 40m telescope, we considered a PWFS with 80x80 sub-aperture and we computed noise propagation coefficients up to mode 3000, showing an overall noise propagation residual of 54nm with 600 photons per sub-aperture. The simulated PWFS requires a small CCD with 176x176 pixel. We also run an end-to-end simulation: a SR of 70% at H band was achieved with a correction of 2100 modes. These results provide a first evidence that the PWFS can be used in the LGS based AO systems currently in design phase for 8m or 40m telescopes.