Multi-conjugate adaptive optics

In order to compensate image blurring caused by turbulence in Earth's atmosphere, present ground-based solar telescopes are equipped with a technology called Adaptive Optics (AO): It uses a single deformable mirror which can change its shape very rapidly to compensate the image blurring in realtime.

A series of sunspot and granulation images have been taken at a wavelength of 709nm, both with classical Adaptive Optics and the Clear Multi-conjugate Adaptive Optics. The image (overall field of view of 35 arcseconds, whereas in the classical AO case, the image quality drops rapidly outside the image center.

Both at GREGOR and the VTT Adaptive Optics has revolutionized observations by providing a much better image quality over a longer period of time. However, up to now, only a very small part of the image (a few arcseconds) could be corrected well.

A collaboration between the National Solar Observatory, the New Jersey Institute of Technology's Big Bear Solar Observatory, and the Kiepenheuer Institute has now developed the next generation of Adaptive Optics: A Multi-Conjugate Adaptive Optics (MCAO) system called Clear which provides a much larger corrected field of view. This is achieved by using three deformable mirrors which consecutively correct atmospheric turbulence at different heights:

One mirror focuses on aberrations near the ground, another mirror on turbulence at approximately three kilometers, and the third mirror focuses on turbulence introduced at approximately eight kilometers above the ground.

During an observing campaign in July 2016 at the 1.6m New Solar Telescope at Big Bear (USA), for the first time ever, three deformable mirrors were successfully combined to increase the corrected field of view from a few to 35 arcseconds.

Clear acts as a pathfinder for the future 4m DKIST and EST solar telescopes which will both be equipped with a powerful MCAO system.