Youming Guo

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 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.

A method for selecting M dwarfs with an increased likelihood of unresolved ultracool companionship

Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation models, the measurement of sub-stellar dynamical masses and radii, and for testing ultra-cool evolutionary models. We present an optimised method for identifying M dwarfs which may have unresolved ultra-cool companions. We construct a catalogue of 440,694 candidates, from WISE, 2MASS and SDSS, based on optical and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a sub-sample of 36,898 M dwarfs and search for possible mid-infrared M dwarf + ultra-cool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1,082 M dwarf + ultra-cool dwarf candidates for follow-up. Using simulated ultra-cool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultra-cool companions amongst our M dwarf + ultra-cool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.

Multichannel-Hadamard calibration of high-order adaptive optics systems

we present a novel technique of calibrating the interaction matrix for high-order adaptive optics systems, called the multichannel-Hadamard method. In this method, the deformable mirror actuators are firstly divided into a series of channels according to their coupling relationship, and then the voltage-oriented Hadamard method is applied to these channels. Taking the 595-element adaptive optics system as an example, the procedure is described in detail. The optimal channel dividing is discussed and tested by numerical simulation. The proposed method is also compared with the voltage-oriented Hadamard only method and the multichannel only method by experiments. Results show that the multichannel-Hadamard method can produce significant improvement on interaction matrix measurement.

Adaptive optical system based on deformable secondary mirror on 1.8-meter telescope

In 2009, A 127-element adaptive system had been manufactured and installed at the Coude room of the 1.8-meter telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. A set of new adaptive optical system based on a 73-element deformable secondary mirror is being developed and will be integrated into the 1.8-meter telescope. The 73-element deformable secondary mirror with convex reflecting surface is designed to be compatible with the Cassegrain focus of the 1.8-meter telescope. Comparing with the AO system of Coude focus, the AO system on the deformable secondary mirror adopts much less reflections and consequently restrains the thermal noise and increases the energy transmitting to the system. The design and simulation results of this system will be described in this paper. Furthermore, the preliminary test result of the deformable secondary mirror in the lab is also presented.

Solar adaptive optics for 1m new vacuum solar telescope

Adaptive Optics (AO) has become the requisite equipment of the ground-based large solar telescope to correct the wavefront aberration induced by the atmospheric turbulence. Two generation solar AO systems, one is the 37-element loworder AO system with 2100Hz frame rate and the other is 151-element high-order AO system with 3500Hz frame rate, were successfully developed in 2013 and 2015 respectively. In this presentation, the development of the two AO systems for 1-m New Vacuum Solar Telescope (NVST) at Fuxian Solar Observatory (FSO) will be introduced and the solar high resolution observational results are presented.

Error analysis of the de-crosstalk algorithm for the multianode-PMT-based quadrant tracking sensor.

For the multianode-PMT-based quadrant tracking sensor, one of the tracking error sources is the crosstalk. The crosstalk can be reduced by de-crosstalk algorithm, so the tracking error of the de-crosstalk algorithm for the multianode-PMT-based quadrant tracking sensor are analyzed in theory and verified by experiments. Both the theoretical analysis and the experimental results showed that the spot displacement sensitivity could be improved by the de-crosstalk algorithm, but the spot centroid detecting error increased at the same time. So the de-crosstalk algorithm could not improve the tracking accuracy effectively.

Progress on solar multi-conjugate adaptive optics at the New Vacuum Solar Telescope

Multi-conjugate adaptive optics (MCAO) has been proved to obtain the high resolution images with a large field of view in solar observation. A solar MCAO experiment system had been successfully developed and tested at the 1-meter New Vacuum Solar Telescope (NVST) of Fuxian Solar Observatory. It consists of two deformable mirrors (DMs), a multidirection Shack-Hartmann wavefront sensor (MD-WFS), and a real-time controller. In order to command the two DMs, five guide regions were selected from the MD-WFS to retrieve a three-dimensional measurement of the turbulent volume based on atmospheric tomography. This system saw the first light in October, 2017, and a series of MCAO-corrected high resolution sunspots images were acquired. In this presentation, the MCAO experiment system is introduced, and the observation results are presented. Furthermore, a new MCAO system based on our proposed MCAO configuration with a high order ground layer adaptive optics and low order high altitude correction will be developed for the NVST as a regularly operating instrument for scientific observations of the sun.

Comparison of correlation algorithms with correlating Shack-Hartmann wave-front images

Correlating Shack-Hartmann wavefront sensor is widely used in solar adaptive optics in which the relative shift between different subapertures by correlation algorithm is computed, and then the control voltage by wavefront reconstruction can be estimated to use for correcting the wavefront distortion induced by atmospheric turbulence. In this paper, several different correlation algorithms including Cross-Correlation Coefficient, Absolute Difference Function, Absolute Difference Function-Squared and Square Difference Function are used to estimate relative shift in correlating Shack-Hartmann wave-front sensor with the different observed solar structure such as sunspot, solar pore and solar granulation. The measurement noise RMS error is computed to compare the performance of the correlation algorithms. The results show the correlation algorithm precision is directly related to the solar structure. The measurement noise is relatively small with the relatively high contrast target, and vice versa. At the same time, the size of reference image also could influence the measurement noise, the larger size of the reference image, the smaller the measurement noise is.

Astronomical AO in Key Laboratory of Adaptive Optics, Chinese Academy of Sciences

The AO progresses for astronomy in the Key Laboratory of Adaptive Optics, Chinese Academy of Sciences are reported in this presentation. For night-time astronomical observations, the recent AO technological developments, such as Laser Guide Star, Pyramid Sensor and Deformable Secondary Mirror, are introduced. The solar AO researches are also presented for day-time astronomical observations. Furthermore, we will show the on-sky high resolution observational results in the 1.8m telescope at Gaomeigu site, Yunnan Observatory and the 1-m New Vacuum Solar Telescope (NVST) at Fuxian Lake Solar Observatory respectively.

First light of the deformable secondary mirror-based adaptive optics system on 1.8m telescope

An adaptive optics system (AOS), which consists of a 73-element piezoelectric deformable secondary mirror (DSM), a 9x9 Shack-Hartmann wavefront sensor and a real time controller has been integrated on the 1.8m telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. Compared to the traditional AOS on Coude focus, the DSM AOS adopts much less reflections and consequently restrains the thermal noise and increases the energy transmitting to the system. Before the first on-sky test, this system has been demonstrated in the laboratory by compensating the simulated atmospheric turbulence generated by a rotating phase screen. A new multichannel-modulation calibration method which is used to measure the DSM based AOS interaction matrix is proposed. After integration on the 1.8m telescope, the closed-loop compensation of the atmospheric turbulence with the DSM based AOS is achieved, and the first light results from the on-sky experiment are reported.