Over the last few years increasing consideration has been given to the study of laser guide stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near‐infrared (near‐IR) astronomical observations from the ground. A possible method for the generation of a LGS is the excitation of the sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack–Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect, a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the weighted centre of gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture.
Abstract. MAORY is the multi-conjugate adaptive optics module for the European Extremely Large Telescope.A conceptual design study is in progress and is approaching the final review. A description of the current design and of the expected performance is presented.
The paper describes the design of the single conjugate Adaptive Optics system to be installed on the LBT telescope. This system will be located in the Acquisition, Guiding and Wavefront sensor unit (AGW) mounted at the front bent Gregorian focus of LBT. Two innovative key features of this system are the Adaptive Secondary Mirror and the Pyramid Wavefront Sensor. The secondary provides 672 actuators wavefront correction available at the various foci of LBT. Due to the adaptive secondary mirror there is no need to optically conjugate the pupil on the deformable mirror. This allows having a very short sensor optical path made up using small dimension refractive optics. The overall AO system has a transmission of 70 % and fits in a rectangle of about 400x320mm. The pyramid sensor allows having different pupil sampling using on-chip binning of the detector. Main pupil samplings for the LBT system are 30x30, 15x15 and 10x10. Reference star acquisition is obtained moving the wavefront sensor unit in a field of view of 3x2 arcmin. Computer simulations of the overall system performance show the good correction achievable in J, H, and K. In particular, in our configuration, the limiting magnitude of pyramid sensor results more than one magnitude fainter with respect to Shack-Hartmann sensor. This feature directly translates in an increased sky coverage that is, in K band, about doubled with respect to the same AO system using a Shack-Hartmann sensor.
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