Measuring turbuence in an extended field of view

摘要

Adaptive optics is limited today to correction of turbulence inside a cone extending from the reference source to the telescope aperture. Even when the reference source is a natural star, the measured - and corrected - cylinder does not allow observation of most extended astronomical objects. Hence the search is on for a method to measure turbulence in a conical volume which opens up from the telescope upwards. Various schemes were proposed to widen the field of view by using more artificial or natural guide stars, and by processing the measured data in different ways. It has been shown experiemntally that the existence of tree natural stars around the rim of the required cone is sufficient. Using multiple laser guide stars, schemes varying from separation of measured volumes and stitching of their edges, to integrated methods were suggested. It was also proposed to infer the turbuelnce from the shape of the beams as they propagate up in the atmosphere. Structured light above the turbuelnce is another option that was raised. Such a grid is created by interference of laser beams or by interference of powerful radio beams that break down the air into visibile plasma. It can be shown that these fringes, either from a laser or from radio, can be analyzed optically, reducing the power requirements significantly. This field of atmospheric tomography is likely to produce soon corrected images of extended astronomical objects. In addition, being able to separate the contribution of the atmospheric layers, we will acquire better knowledge of atmospheric turbulence.