Adaptive optics for the 8-meter Chinese Giant Solar Telescope

Abstract

Solar Extremely Large Telescopes (ELTs) enable diffraction limited imaging of the basic physical structure of the solar atmosphere. Magneto-hydrodynamic considerations limit their size to about 0.03 arcsec. To observe them in the near-infrared 8-meter class telescopes are needed. The Chinese Giant Solar Telescope, or CGST, is such a near-infrared solar ELT. It is a Ring Telescope with an 8-meter outer diameter and a central clear aperture of about 6-meter diameter. At present various options for such a Gregorian type telescope are under study, like a continuous ring made out of segments or a multiple aperture ring made of 6 or 7 off-axis telescopes. The advantages of such a ring telescope is that its MTF covers all spatial frequencies out to those corresponding to its outer diameter, that its circular symmetry makes it polarization neutral, and that its large central hole helps thermal control while it provides ample space for MCAO and Gregorian focus instrumentation. We present the current status of the design of the CGST. Our thinking is guided by the outstanding performance of the 1-meter vacuum solar telescope of the Yunnan Solar Observatory which like the CGST uses both AO and image reconstruction. Using it with a ring-shape aperture mask the imaging techniques for the CGST are being explored. The CGST will have Multi-Conjugate Adaptive Optics (MCAO). The peculiarities of Atmospheric Wavefront Tomography for Ring Telescopes are aided by the ample availability of guide stars on the Sun. IR MCAO-aided diffraction limited imaging offers the advantage of a large FOV, and high solar magnetic field sensitivity. Site testing is proceeding in western China (e.g. northern Yunnan Province and Tibet). The CGST is a Chinese solar community project originated by the Yunnan Observatory, the National Astronomical Observatories, the Purple Mountain Observatory, the Nanjing University, the Nanjing Institute of Astronomical Optics and Technology and the Beijing Normal University.