Laser Guide Star Only Adaptive Optics: The Development of Tools and Algorithms for the Determination of Laser Guide Star Tip-Tilt

Abstract

Adaptive Optics (AO) is a technology which corrects for the effects of the atmosphere and so improves the optical quality of ground based astronomical observations. The bright “guide stars” required for correction are not available across the entire sky, so Laser Guide Stars (LGSs) are created. A Natural Guide Star (NGS) is still required to correct for tip-tilt as the LGS encounters turbulence on the uplink path resulting in unpredictable “jitter”, hence limiting corrected sky coverage. In this thesis an original method is proposed and investigated that promises to improve the correction performance for tomographic AO systems using only LGSs, and no NGS, by retrieving the LGS uplink tip-tilt.
To investigate the viability of this method, two unique tools have been developed. A new AO simulation has been written in the Python programming language which has been designed to facilitate the rapid development of new AO concepts. It features realistic LGS simulation, ideal to test the method of LGS uplink tip-tilt retrieval. The Durham Real-Time Adaptive Optics Generalised Optical Nexus (DRAGON) is a laboratory AO test bench nearing completion, which features multiple LGS and NGS Wavefront Sensors (WFSs) intended to further improve tomographic AO. A novel method of LGS emulation has been designed, which re-creates focus anisoplanatism, elongation and uplink turbulence. Once complete, DRAGON will be the ideal test bench for further development of LGS uplink tip-tilt retrieval.
Performance estimates from simulation of the LGS uplink tip-tilt retrieval method are presented. Performance is improved over tomographic LGS AO systems which do not correct for tip-tilt, giving a modest improvement in image quality over the entire night sky. Correction performance is found to be dependent on the atmospheric turbulence profile. If combined with ground layer adaptive optics, higher correction performance with a very high sky coverage may be achieved.