High resolution observations of the Sun are of key importance in understanding fundamental astrophysical processes. Adaptive optics (AO) is an important tool that allows solar astronomers to achieve diffraction limited observations from existing ground based telescopes. AO is also a key technology required for a future 4m-class Advanced Solar Telescope (AST) that the international community of solar astronomers is planning to build. The history of the development of solar AO is reviewed and results from recent successful demonstrations of solar AO systems are presented. The main difference between solar AO and night time AO is the different, and more elaborate wavefront sensing technique that has to be applied in order to measure wavefront aberrations using solar granulation as a target. Different approaches to this problem are discussed. Multi-conjugate AO has been proposed as a technique to achieve diffraction limited resolution over a field-of-view (FOV) significantly larger than the isoplanatic patch. The Sun is an ideal object for the development and application of MCAO.

Solar adaptive optics

Adaptive optics (AO) has become an indispensable tool at ground-based solar telescopes. AO enables the ground-based observer to overcome the adverse effects of atmospheric seeing and obtain diffraction limited observations. Over the last decade adaptive optics systems have been deployed at major ground-based solar telescopes and revitalized ground-based solar astronomy. The relatively small aperture of solar telescopes and the bright source make solar AO possible for visible wavelengths where the majority of solar observations are still performed. Solar AO systems enable diffraction limited observations of the Sun for a significant fraction of the available observing time at ground-based solar telescopes, which often have a larger aperture than equivalent space based observatories, such as HINODE. New ground breaking scientific results have been achieved with solar adaptive optics and this trend continues. New large aperture telescopes are currently being deployed or are under construction. With the aid of solar AO these telescopes will obtain observations of the highly structured and dynamic solar atmosphere with unprecedented resolution. This paper reviews solar adaptive optics techniques and summarizes the recent progress in the field of solar adaptive optics. An outlook to future solar AO developments, including a discussion of Multi-Conjugate AO (MCAO) and Ground-Layer AO (GLAO) will be given.

/pdf/solar-adaptive-optics-2ifcrjexdi.pdf

Solar Adaptive Optics

Calculations of backscatter emission of mesospheric sodium atoms in a laser guide star that is excited by pulses ranging from 30-ns to 0.9-μs duration are described. The efficient use of such pulses at saturating irradiance values is shown to require ~3 GHz of spectral broadening to provide access to the full absorption spectrum of the D2 line. The broadening is provided by frequency modulation. A set of density matrices was used to account for all 24 hyperfine states and inhomogeneous Doppler broadening. At the broadband (3-GHz) saturation irradiance of 4 W/cm2, both linearly and circularly polarized laser beams are shown to produce emission rates exceeding 60% of the maximum possible rate-equation rate for the 0.9-μs pulses. As expected, circular polarization produced more backscatter than did linear polarization, but the enhancement never exceeded 1/3 in the calculations that are reported. A brief estimate of state precession in the Earth’s magnetic field suggests that achieving even this enhancement will require that the time scale for optical pumping be held to less than 1 μs, which will require the use of irradiances greater than 0.7 W/cm2 and spectral coverage of the full 3-GHz hyperfine-plus-Doppler absorption profile, at least until most of the population is pumped out of the F = 1 ground states.

Efficient excitation of a mesospheric sodium laser guide star by intermediate-duration pulses

A high-order solar adaptive optics (AO) system including a fine tracking loop and a high-order wavefront correction loop has been installed at the 1 m New Vacuum Solar Telescope of the Fuxian Solar Observatory, in routine operation since 2016. The high-order wavefront correction loop consists of a deformable mirror with 151 actuators, a correlating Shack-Hartmann wavefront sensor with 102 subapertures of which the Absolute Difference Square Algorithm is used to extract the gradients, and a custom-built real-time controller based on a Field-Programmable Gate Array (FPGA) and multi-core Digital Signal Processor (DSP). The frame rate of the wavefront sensor is up to 3500 Hz and this is, to our knowledge, the fastest solar AO system. This AO system can work with a Fried parameter r(0), at the 500 nm wavelength, of larger than 3 cm. The first 65 modes of the Zernike aberrations can be efficiently corrected and the Strehl ratio of the corrected TiO image for the solar pore is superior to 0.75 with the Fried parameter r(0) larger than 10 cm. In this paper, the design of the system is described, and high-resolution solar observational images are presented. Furthermore, the performances of the AO system are evaluated according to the data recorded by the real-time controller.

https://iopscience.iop.org/article/10.3847/1538-4357/833/2/210/pdf

INSTRUMENT DESCRIPTION AND PERFORMANCE EVALUATION OF A HIGH-ORDER ADAPTIVE OPTICS SYSTEM FOR THE 1 m NEW VACUUM SOLAR TELESCOPE AT FUXIAN SOLAR OBSERVATORY

A 37-element solar adaptive optics system, which consists of a fine tracking loop with a tip/tilt mirror and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann wavefront sensor, and a real-time controller, was built and installed at the 26 cm solar fine structure telescope of the Yunnan Astronomical Observatory in 2009. In this system, the absolute difference algorithm is used. A new architecture with field-programmable gate array (FPGA) and digital signal processor (DSP) for the real-time controller based on systolic array and pipeline was developed. The computational latencies of the fine tracking loop and high-order correction loop are about 35 and 100 mu s, respectively. The tracking residual root-mean-square error is less than 0.1 arcsec, and the wavefront residual root-mean-square error is about 0.05 wavelengths (lambda = 550 nm) after correction. The observational results show that the contrast and resolution of the solar images are improved after the correction by this adaptive optics system. (C) 2010 Optical Society of America

Performance of the 37-element solar adaptive optics for the 26 cm solar fine structure telescope at Yunnan Astronomical Observatory

The current status of the adaptive optics system for the National Solar Observatory (NSO) 75-cm aperture, evacuated solar telescope at Sunspot, New Mexico, is described. This system is interfaced to a birefringent filter and two solar spectrographs. The optical system provides for the High Altitude Observatory Advanced Stokes Polarimeter (ASP) that measures solar vector magnetic fields. Recently the optical design has been modified to accommodate a 19-segment adaptive mirror and its 19-segment quad-cell tracker.

Richard B. Dunn

Papers

NSO/SP adaptive optics program

NSO/SP Adaptive Optics Program