Design and Construction of a Multiple Beam Laser Projector and Dynamically Refocused Wavefront Sensor

Abstract

Adaptive optics using natural guide stars can produce images of amazing quality, but is limited to a small fraction of the sky due to the need for a relatively bright guidestar. Adaptive optics systems using a laser generated artifical reference can be used over a majority of the sky, but these systems have some attendant problems. These problems can be reduced by increasing the altitude of the laser return, and indeed a simple, single laser source focused at an altitude of 95 km on a layer of atmospheric sodium performs well for the current generation of 8-10 m telescopes. For future giant telescopes in the 20-30 m class, however, the errors due to incorrect atmospheric sampling and spot elongation will prohibit such a simple system from working.The system presented in this dissertation provides a solution to these problems. Not only does it provide the 6.5m MMT with a relatively inexpensive laser guide star system with unique capabilities, it allows research into solving many of the problems faced by laser guide star systems on future giant telescopes.The MMT laser guidestar system projects a constellation of five doubled Nd:YAG laser beams focused at a mean height of 25 km, with a dynamic refocus system that corrects for spot elongation and allows integrating the return from a 10 km long range gate. It has produced seeing limited spot sizes in ~1 arcsecond seeing conditions, and has enabled the first on-sky results of Ground Layer Adaptive Optics (GLAO).