Atmospheric turbulence profiling with a Shack-Hartmann wavefront sensor
ABSTRACT
In order to measure the altitude profile of the atmospheric turbulence in real-time, we are applying a MASSDIMM method (Multi Aperture Scintillation Sensor and Differential Image Motion Monitor) to the ShackHartmann wavefront sensor data. Tomographic estimation of the atmospheric turbulence is a key technique in new generation of adaptive optics systems with multiple guide stars, and the real-time turbulence profiling provide a useful prior information for the tomography, which is an ill-posed inverse problem. By using the data of a Shack-Hartmann sensor, a turbulence profile in the same direction as the AO correction can be acquired. Moreover, since more information can be used compared with the traditional MASS-DIMM, the resolution in the height direction can be increased. This time, the data of the Shack-Hartmann sensor attached to Tohoku University 50cm telescope was analyzed, and the estimation of the turbulence profile was obtained. Similar profiles were obtained while the elevation of the star and the apparent distance to the turbulence changed in one hour monitoring measurements. The results supported the validity of the method.
1. INTRODUCTION
In the last decade, adaptive optics (AO) systems using multiple laser guide stars (LGSs) and wavefront sensors (WFSs) have been demonstrated or developed for 8m-class telescopes.1–5 These systems measure the wavefront distortion in several lines of sight and reconstruct the distortions optimized in the direction of science objects using tomographic estimation. The tomographic estimation of the three-dimensional turbulence structure requires prior information of the turbulence strength as a function of altitude, which is called atmospheric turbulence profile. Because atmospheric condition varies with time, turbulence profile should be updated in a timescale of tens of minutes, which is the typical time scale of the profile time evolution.6, 7
A number of methods to obtain real-time atmospheric turbulence profile based on optical triangulation have been developed.8–10 However, these triangulation-based methods do not have any sensitivity to turbulence at high altitudes since spatial correlation length created by high turbulence layer is larger than the size of the pupil. Multi aperture scintillation sensor and differential image motion monitor (MASS-DIMM11) is one of the most common profilers, which uses a single star and has lower altitude resolution compared to the triangulation-based methods.
In this article, we propose a new turbulence profiling method, which carries out scintillation measurement similar to MASS using spot brightness fluctuation data of Shack-Hartmann wavefront sensor (SH-WFS). This new method, SH-MASS can measure the scintillation with more spacial patterns than the traditional MASS instrument, and makes it possible to profile the atmospheric turbulence with high altitude resolution by the observation of scintillation of a single star.
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