PARALLEL IMAGE RECONSTRUCTION FOR NEW VACUUM SOLAR TELESCOPE

New Vacuum Solar Telescope (NVST) possesses a world-class solar telescope which observes the sun by high resolution image instruments and multi-wave spectrometer in visible and near infrared bands. NVST has gathered millions of Flexible Image Transport System (FITS) files since it has been for the routine observation and thousands of files are generated in a day. To search efficiently for a subset of these extremely large and linearly increasing FITS files with low latency and response time goes beyond traditional database systems. In this paper, We present an NVST indexing and querying framework (NIQF) based on an implementation of word-aligned hybrid bitmaps to deal with the issue, and design several simple and useful APIs, which are convenient for integrating with NVST data archiving system and meet the requirements of fast data indexing of massive data processing in NVST. We study the performance of NIQF by comparing with MySQL. The results demonstrate that the performance of NIQF is more efficient than MySQL in a huge amount of solar data set. This study indicates potential performance and feasibility of NIQF for large astronomical data indexing and querying in NVST.

NVST chromosphere data interference fringes removal based on NSCT and PCA

This paper is devoted to the primary spectro-polarimetric observation performed at the New Vacuum Solar Telescope (NVST) of China since 2017, and our aim is to precisely evaluate the real polarimetric accuracy and sensitivity of this polarimetry by using full Stokes spectro-polarimetric observations of the photospheric line Fe I 532.4 nm. In the work, we briefly describe the salient characteristic of the NVST as a polarimeter in terms of technology and then characterize its instrumental polarization based on the operation in 2017 and 2019. It is verified that the calibration method utilizing the instrumental polarization calibration unit (ICU) is stable and credible. The calibration accuracy can reach up to 3 × 10−3. Based on the scientific observation of NOAA Active Region 12645 on 2017 April 5, we estimate that the residual cross-talk from Stokes I to Stokes Q, U and V, after the instrumental polarization calibration, is about 4 × 10−3 on average, which is consistent with the calibration accuracy and close to the photon noise. The polarimetric sensitivity (i.e., the detection limit) for polarized light is of the order of 10−3 with an integration time over 20 s. Slow modulation rate is indeed an issue for the present system. The present NVST polarimeter is expected to be integrated with a high-order adaptive optics system and a field scanner to realize 2D vector magnetic field measurements in the following instrumentation update.

In this paper we illustrate the technique used by the New Vacuum Solar Telescope (NVST) to increase the spatial resolution of two-dimensional (2D) solar spectroscopy observations involving two dimensions of space and one of wavelength. Without an image stabilizer at the NVST, large scale wobble motion is present during the spatial scanning, whose instantaneous amplitude can reach 1.3″ due to the Earth’s atmosphere and the precision of the telescope guiding system, and seriously decreases the spatial resolution of 2D spatial maps composed with scanned spectra. We make the following effort to resolve this problem: the imaging system (e.g., the TiO-band) is used to record and detect the displacement vectors of solar image motion during the raster scan, in both the slit and scanning directions. The spectral data (e.g., the Hα line) which are originally obtained in time sequence are corrected and re-arranged in space according to those displacement vectors. Raster scans are carried out in several active regions with different seeing conditions (two rasters are illustrated in this paper). Given a certain spatial sampling and temporal resolution, the spatial resolution of the composed 2D map could be close to that of the slit-jaw image. The resulting quality after correction is quantitatively evaluated with two methods. A physical quantity, such as the line-of-sight velocities in multiple layers of the solar atmosphere, is also inferred from the re-arranged spectrum, demonstrating the advantage of this technique.

<p>In this report, we present a prominence observed by New Vacuum Solar Telescope (NVST) in Hα wavelength.  We use morphological approach to identify the rising or descending knots in the prominence. The rising knots are often found on the top of the prominence, while more knots are seen to descend from anywhere of the prominence.</p><p>The optical flow, referring to the apparent proper motion of a feature across the image plane, may be used to determine the velocity field from two images. The technique of local correlation tracking (LCT) optical flow has been widely used in the solar research. The Demon algorithm, which has been  used to match medical image, performs image-to-image matching by determining the optical flow between two images. We have examined the performance of the two methods applying the Hα images, and we noted that the Demon algorithm outperforms traditional LCT  methods.</p><p>The result of Demon optical flow allows us to estimate the velocity and acceleration of the moving knots. The descending speed of the knots near the solar surface is higher than that far away from the solar surface. This indicate that most of knots are more possible to descend across the horizontal magnetic field.</p>

Ellerman bombs (EBs) are believed to be small-scale reconnection events occurring around the temperature minimum region in the solar atmosphere. They are often identified as significant enhancements in the extended Hα wings without obvious signatures in the Hα core. Here we explore the possibility of using the 1700 Å images taken by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) to study EBs. From the Hα wing images obtained with the New Vacuum Solar Telescope (NVST) on 2015 May 2, we have identified 145 EBs and 51% of them clearly correspond to the bright points (BPs) in the AIA 1700 Å images. If we resize the NVST images using a linear interpolation to make the pixel sizes of the AIA and NVST images the same, some previously identified EBs disappear and about 71% of the remaining EBs are associated with BPs. Meanwhile, 66% of the compact brightenings in the AIA 1700 Å images can be identified as EBs in the Hα wings. The intensity enhancements of the EBs in the Hα wing images reveal a linear correlation with those of the BPs in the AIA 1700 Å images. Our study suggests that a significant fraction of EBs can be observed with the AIA 1700 Å filter, which is promising for large-sample statistical study of EBs as the seeing-free and full-disk SDO/AIA data are routinely available.

The studies were carried out preliminary within the framework of the RSF project 15-19-20013 Creation of an adaptive system ensuring operation of a large-sized solar telescope under conditions of strong atmospheric turbulence. It is known that the use of adaptive optics (AO) systems is the most radical way for reducing the effect of atmospheric distortions. AO systems correct distortions introduced by the Earth's atmosphere into the image of a space object in real time. The creation of an AO system for large-aperture solar telescopes still is a scientific and engineering challenge. Currently, for the largest Russian solar telescopes, in particular, the Large Solar Vacuum Telescope (LSVT), the Automated Solar Telescope (AST) and the 3-meter Large Solar Telescope (LST) under development, promising new- generation systems are being developed and implemented based on the principles of multi-mirror adaptive correction.

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.

Timed solar eclipses in Chinese records and the secular changes in the Earth's rotation before the 8th century: Zhang Pei-yu & Han Yan-benPurple Mountain Observatory, Nanjing 210008

Studying solar filament dynamical evolutions is an important approach to reveal the driving mechanism of solar eruptions, which seriously impact on the Sun–Earth system and could cause disastrous space weather. To better understand the evolution process of solar filaments, here we investigate an active-region filament by employing observations from the New Vacuum Solar Telescope (NVST), Solar Dynamics Observatory, and Interface Region Imaging Spectrograph. The high-resolution NVST Hα images show that the northern footpoint of the filament gradually moved northward. Near the northern footpoint, there is an arch filament system (AFS). Between adjacent footpoints of the filament and the AFS, transient brightening, underlying magnetic cancellation, and bidirectional flows were detected, which jointly imply that it could be the magnetic reconnection between the filament and the AFS that changes the connection of filament threads and drives its footpoint to move northward. In addition, during the footpoint evolution, the filament with highly twisted structure underwent several untwisting motions. Meanwhile, transient brightenings were also observed and appeared as bright knots around several positions where filament threads might braid with each other. And some bright blobs were also detected to propagate outward from the brightening region. These observations suggest that magnetic reconnection might be responsible for the untwisting motion. This work exposes us to a dynamical scenario of the filament evolution driven by magnetic reconnection, which will promote our understanding of the formation and eruption of the filaments.

Solar Adaptive Optics (AO) shares many issues with night-time AO, but it also has its own particularities. The wavefront sensing is performed using correlations to efficiently work on the solar granulation as a reference. The field of view for that measurement usually is around 10". A sensor collecting such a wide field of view averages wavefront information from different sky directions, and the anisoplanatism thus has a peculiar impact on the performance of solar AO and MCAO systems. Since we are entering the era of large solar telescopes (European Solar Telescope, Advanced Technology Solar Telescope) understanding this issue is crucial to evaluate its impact on the performance of future AO systems. In this paper we model the correlating wide field sensor and the way it senses the high altitude turbulence. Thanks to this improved modelling, we present an analysis of the influence of this sensing on the performance of each AO configuration, conventional AO and MCAO. In addition to the analytical study, simulations similar to the case of the EST AO systems with FRiM-3D (the Fractal Iterative Method for Atmospheric tomography) are used in order to highlight the relative influence of design parameters. In particular, results show the performance evolution when increasing the telescope diameter. We analyse the effect of high altitude turbulence correlation showing that increasing the diameter of the telescope does not degrade the performance when correcting on the same spatial and temporal scales.

Themis is a 90 cm solar telescope which undergoes a rejuvenation of its scientific instruments. In particular, it is about to be equipped with an adaptive optics (AO) system with a bandwidth of at least 1 kHz and featuring a 97 actuator deformable mirror and 10×10 Shack-Hartmann wavefront sensor. Nowadays, the computational power required by such a system can be provided by current multi-core CPU. We have therefore implemented from scratch the real-time control system in pure software using Julia, 1 a new language for technical computations, and running on Linux OS. Our main motivation was to be able to exploit new advances in wavefront sensing and adaptive optics control. With a computational cost comparable to state-of-the-art but sub-optimal methods used in solar AO, our wavefront sensing algorithm estimates the local slopes and their covariances following a maximum likelihood registration method. Themis AO system has a modest size but can be used to assert the benefits of maximum a posteriori (MAP) wavefront sensing and control, 2, 3 of accounting of the covariances of the measure and of the temporal correlation of the turbulent wavefront.

The spatial analysis of the wavefront distortions registered by the Shack-Hartmann sensor of the adaptive optics (AO) system of Large solar vacuum telescope have been performed. In the paper the results of analysis of spatial structure of the wavefront within the field of view ~35 arcsec are discussed. It is shown that the wavefront distortions formed within the field of view ~35 arcsec are not random. There are the repetitive hills of spatial crosscorrelation functions in the field of view at the same intervals for different space subapertures. It is shown that spatial crosscorrelation functions do not contain pronounced kernel for finding the exact position of the atmospheric layer contributing to significant distortions in the wavefront incident on the telescope aperture.

Shift-and-add (SAA) is a simple image processing procedure. SAA was devised to reconstruct a diffraction-limited image from atmospherically degraded stellar images. Recently SAA has been applied to biological imaging. There are several variants of SAA. Here proposed is an SAA procedure incorporated with unsharp masking (USM). The SAA procedure proposed here encompasses an extended version of USM. The proposed SAA method retains the simplicity and easiness, and the basic features of SAA. The effectiveness of the proposed method is examined by restoring atmospherically degraded solar images. It is shown that the USM SAA reconstructed image exhibits high contrast and reveals fine structures blurred by atmospheric turbulence. It is also shown that the USM SAA performs better with a data frame selection scheme.

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.