Phasing ELTs for Adaptive Optics: Preliminary results of a Comparison of techniques

摘要

There is no doubt that the future telescopes of diameter more than 10m, dubbed 'ELTs', will employ segmented mirrors. The performance of these telescopes may be severely limited by discontinuities in the wavefront, caused by segmentation errors such as segment piston, tip-tilt, figure errors and edge effects. These will particularly affect AO performance, and it is therefore of interest to know what techniques are available to reduce these errors, and what performance can be expected of them. In the framework of a Research and Training Network on Adaptive Optics for ELTs, funded by the European Commission, we are in the process of evaluating the performance of various piston measurement techniques which are either in use on current segmented telescopes, or which have been proposed. In this article we present an overview of these phasing techniques and the performance criteria we will use to compare them. We give detailed results on our study of one particular technique, the Chanan narrowband phasing technique, which is employed on the 10m Keck telescopes and will also be employed in a modified form on the 10m Gran Telescopio Canarias (GTC). We have developed a new algorithm for extracting the phase information and show that this leads to improved performance. We show that the technique can work with sufficient precision on telescopes as large as 100m.