Correlation Wavefront Sensing and Turbulence Profiling for Solar Adaptive Optics

Abstract

Ground based telescopes suffer from degraded resolution due to aberrations induced by the atmosphere which prevent them from reaching the diffraction limit. Adaptive Optics (AO) is a technology which corrects for this effect in real-time, restoring the resolution of a telescope. However, it only corrects for a very narrow field of view (FOV) around the guide source. Tomographic AO uses multiple guide sources to increase the size of the corrected FOV, however, these forms of AO are affected by the vertical distribution of turbulence in the atmosphere (turbulence profile). This thesis presents work to develop turbulence profiling instruments for daytime astronomy and improve centroiding techniques for correlating wavefront sensors (WFS) which are used in slope based turbulence profiling instruments.

The development of centroiding techniques for use on extended objects is based on cross-correlation techniques. Two methods are presented, one for optimising centroiding parameters on cross-correlation images and another for improving the signal to noise in cross-correlation images created from images with large relative shifts by using supersized reference images. Choosing optimal centroiding parameters for correlating WFSs is demonstrated in simulation, optimising a windowed, thresholded center of mass. The creation and use of supersized reference images is also demonstrated in simulation, where they are created from WFS data and shown to drastically improve the accuracy of centroiding for centroiding extended objects which have continuous structure across the whole field.

So-SLODAR (solar-slope detection and ranging) was developed as a slope based instrument for measuring the turbulence profile on the Swedish Solar Telescope (SST), La Palma. The technique is based on SLODAR, with developments to take advantage of the continuous structure of the solar surface offering multiple guide sources. A full description of the technique and its data reduction is presented, along with the first results from on-sky tests on the SST.