Improving adaptive optical systems by the use of multiple laser beacon configurations

Abstract

The field of adaptive optics (AO) and laser-beacon AO has been successfully implemented in the last part of the 20th century. Adaptive optics greatly improves the resolution capabilities of ground-based telescopes by correcting for atmospheric turbulence. The initial implementation of laser-beacon AO was done on relatively small telescopes, on the order of 1.5 m. However, with larger aperture telescopes being built, such as the 8-m class Gemini telescopes, there is much room for improvement. Errors resulting from laser-beacon AO, such as focus anisoplanatism, become worse with an increase in aperture diameter. Tilt anisoplanatism is also a problem, regardless of the size of telescope, and also needs to be reduced to enhance the resolution of the objects being observed. This dissertation investigates alternate laser-beacon AO configurations, to reduce the effects of focus and tilt anisoplanatism for larger aperture telescopes. The configurations investigated include single and multiple laser beacons at single altitudes and single and multiple laser beacons at multiple altitudes. These second configurations are referred to as hybrid beacon systems and consist of Rayleigh beacons at altitudes of 10 to 20 km and sodium beacons at about 90 km, the location of the sodium layer. Hybrid systems are shown to reduce both focus and tilt anisoplanatism as opposed to the first configurations which only aid in reducing focus anisoplanatism. An addition to the hybrid systems with multiple beacons, the use of multiple deformable mirrors (DM's) is investigated. These additional DM's are placed conjugate to atmospheric altitudes with predominant turbulence, beyond the traditional conjugate location of the primary mirror. They correct for turbulence at these atmospheric layers and are referred to as multi-conjugate adaptive optical (MCAO) systems. The purpose of MCAO configurations is to increase the corrected field of view. For the types of systems investigated in this dissertation, radiometric characteristics are calculated. These include the optimum range-gate at the minimum noise equivalent angle (NEA) for several laser pulse energies, wavefront sensor sampling rate as a function of bandwidth, and the optimum system bandwidth for several laser powers. These characteristics can be used to further define a practical AO system that will enhance performance.