Use of Laser Guide Star with Pyramid Wavefront Sensor

Abstract

Laser Guide Star (LGS) reference sources, articially generated at an altitude of 90 km at the atmospheric sodiumlayer, are mandatory to ensure large sky coverage of astronomical Adaptive Optics (AO) systems developed for8m and Extremely Large Telescope (ELT) class telescopes. As a result of the projection eect of an object locatedat a nite distance from the telescope, the AO wavefront sensor (WFS) perceives the LGS as elongated. Thiselongation is a few arcseconds for the 8m class telescopes and can be more than 10 arcseconds for the ELTs. Thiscan pose several challenges when using a Shack-Hartmann wavefront sensor (SHS) such as truncation eects,the requirement for large detectors, and/or the requirement for detectors with geometry that corresponds to thesource shape. In this work, we report the results of numerical simulations focused on the pyramid wavefrontsensor (PWFS) sensitivity when used with a LGS, in this case, a 2D extended object. In addition, the closed-loopperformance of both PWFS and SHS are estimated.