First light on an adaptive optics system using a non-modulation pyramid wavefront sensor for a 1.8 m telescope

Abstract

Our adaptive optics system based on a non-modulation pyramid wavefront sensor is integrated into a 1.8 m astronomical telescope installed at the Yunnan Observatory in LiJiang, and the first light with high-resolution imaging of an astronomical star is successfully achieved. In this Letter, the structure and performance of this system are introduced briefly, and then the observation results of star imaging are reported to show that the angular resolution of an adaptive optics system using a non-modulation pyramid wavefront sensor can approach the diffraction limit quality of a 1.8 m telescope.OCIS codes: 010.1080, 010.7350. doi: 10.3788/COL201614.100101.Optical images of astronomical objects viewed through ground-based telescopes are blurred by the atmosphere [1] . Adaptive optics (AO) attempts to correct, in real time, the distortion induced by the turbulence in the incoming wavefront from the astronomical object of interest, thereby enabling the telescope to reach a diffractionlimited image quality [2] . The AO system uses a wavefront sensor to make measurements of the perturbations introduced by the atmosphere [3] , so the wavefront sensor is a key element of the AO system, and its performance has a large influence on the effectiveness of wavefront correction of the AO system [4] . The pyramid wavefront sensor (PWFS) was first proposed by Ragazzoni in 1996 for astronomical AO [5] , and it has been widely investigated for applications due to its superiority in adjustable gain and flexible sampling sub-apertures, which makes the PWFS increase the number of accessible scientific targets by more efficiently using guiding star photons. Thus, the PWFS is an attractive option for the next large telescope AO system compared with the Shack-Hartmann wavefront sensor [6,7] . The PWFS has been successfully used in the Telescopio Nazionale Galileo telescope and the Large Binocular Telescope, and encouraging on-sky results have been achieved [8,9] . This sensor is also investigated for implementation on the giant Magellan telescope and the Very Large Telescope Optics Facility [10,11] . At present, there are no relevant reports of the PWFS being used in large telescopes in China. In 2014, the PWFS was successfully applied to the 1.8 m telescope at the Yunnan Observatory, and first light on the AO system based on this sensor is reported in this Letter. The 1.8 m telescope at the Yunnan Observatory is a Cassegrain optical structure, and light from a star is reflected by the primary mirror, the secondary mirror, the third mirror, and the other five reflective mirrors in the Coude room. The 127-element AO system installed on the 1.8 m telescope works based on the a ShackHartmann wavefront sensor with a 13 × 13 sub-aperture microlens array, and the first light on high-resolution imaging for stars was achieved in 2009 [12] . In 2014, the new AO system, based on the PWFS and installed on the telescope, provided an alternate wavefront sensor for the existing AO system with a Shack-Hartmann wavefront sensor. The AO system based on th...