Ground-based Solar Telescopes Research Articles

Reference optical turbulence characteristics at the Large Solar Vacuum Telescope site

Abstract Large ground-based solar telescopes are equipped with adaptive optics systems to correct wavefront distortions induced in the turbulent atmosphere. The design of the adaptive optics system strongly depends on the vertical profiles of the optical turbulence. In particular, the characteristics of the optical turbulence determine the design of tomographic adaptive optics systems, which provide image correction within a wide field of view. In the article, a new method to estimate reference optical turbulence characteristics from Era-5 reanalysis assimilated data is presented. This method is based on the dependence of the air refractive index structure constant $C_n^2$ on the vertical shears of wind speed as well as the outer scale of turbulence L0. The L0 parameter is estimated by minimization of the dispersion between the modeled and measured values of the refractive index structure constant $C_n^2$ within the surface layer. For the first time, parametrization coefficients and reference profiles of optical turbulence averaged for the period 1940–2022 are calculated for the Large Solar Vacuum Telescope (LSVT) site. The calculated optical turbulence profiles are representative; these profiles correspond to typical changes of the measured values of the Fried parameter, the isoplanatic angle, and the outer scale of turbulence at the LSVT site. The model turbulence profiles are verified taking into account the Shack–Hartmann wavefront sensor measurements at the LSVT. The higher accuracy of estimation of the optical turbulence characteristics makes it possible to refine parameters relevant to the LSVT adaptive optics system. The obtained results can be used in order to develop high-resolution solar adaptive optics technologies as applied to ground-based telescopes including those using the principles of atmospheric tomography.

PASAT: pathfinder in solar adaptive telescope

In the forefront of quantitative solar physics research using large-aperture ground-based solar optical telescopes, high-contrast observation along with high-accuracy polarimetric measurement in the solar active region are required. In this paper, we propose a novel high-contrast imaging telescope construction with a 60 cm medium aperture, namely, the PAthfinder in Solar Adaptive Telescope (PASAT), in which a deformable secondary mirror is used as the adaptive optical correction device and a symmetrical optical path design is employed, leading to the least Muller matrix polarization instruments. The telescope can provide a high-resolution magnetic field with high accuracy for the solar active regions, as well as high-contrast images with a superior signal-to-noise ratio and photometric accuracy of the solar photosphere and chromosphere. These data will be directly used for a better understanding of the evolution and release of magnetic energy, which will help in improving space weather forecasting. Meanwhile, PASAT will accumulate the relevant techniques for constructing similar, larger solar telescopes in the future.

Predicting the Arrival Time of an Interplanetary Shock Based on DSRT Spectrum Observations for the Corresponding Type II Radio Burst and a Blast Wave Theory

Since fast head-on coronal mass ejections and their associated shocks represent potential hazards to the space environment of the Earth and even other planets, forecasting the arrival time of the corresponding interplanetary shock is a priority in space weather research and prediction. Based on the radio spectrum observations of the 16-element array of the Daocheng Solar Radio Telescope (DSRT), the flagship instrument of the Meridian Project of China, during its construction, this study determines the initial shock speed of a type II solar radio burst on 2022 April 17 from its drifting speed in the spectrum. Assuming that the shock travels at a steady speed during the piston-driven phase (determined from the X-ray flux of the associated flare) and then propagates through interplanetary space as a blast wave, we estimate the propagation and arrival time of the corresponding shock at the orbit of the Solar Terrestrial Relations Observatory-A (STEREO-A). The prediction shows that the shock will reach STEREO-A at 14:31:57 UT on 2022 April 19. The STEREO-A satellite detected an interplanetary shock at 13:52:12 UT on the same day. The discrepancy between the predicted and observed arrival time of the shock is only 0.66 hr. The purpose of this paper is to establish a general method for predicting the shock’s propagation and arrival time from this example, which will be utilized to predict more events in the future based on the observations of ground-based solar radio spectrometers or telescopes like DSRT.

Automatic segmentation of the fine structures of sunspots in high-resolution solar images

Context. With the development of large-aperture ground-based solar telescopes and the adaptive optics system, the resolution of the obtained solar images has become increasingly higher. In the high-resolution photospheric images, the fine structures (umbra, penumbra, and light bridge) of sunspots can be observed clearly. The research of the fine structures of sunspots can help us to understand the evolution of solar magnetic fields and to predict eruption phenomena that have significant impacts on the Earth, such as solar flares. Therefore, algorithms for automatically segmenting the fine structures of sunspots in high-resolution solar image will greatly facilitate the study of solar physics. Aims. This study is aimed at proposing an automatic fine-structure segmentation method for sunspots that is accurate and requires little time. Methods. We used the superpixel segmentation to preprocess a solar image. Next, the intensity information, texture information, and spatial location information were used as features. Based on these features, the Gaussian mixture model was used to cluster different superpixels. According to different intensity levels of the umbra, penumbra, and quiet photosphere, the clusters were classified into umbra, penumbra, and quiet-photosphere areas. Finally, the morphological method was used to extract the light-bridge area. Results. The experimental results show that the method we propose can segment the fine structures of sunspots quickly and accurately. In addition, the method can process high-resolution solar images from different solar telescopes and generates a satisfactory segmentation performance.

Image Quality Specification for Solar Telescopes

Modern large ground-based solar telescopes are invariably equipped with adaptive optics systems to enhance the high angular resolution imaging and spectroscopic capabilities in the presence of the Earth's atmospheric turbulence. The quality of the images obtained from these telescopes can not be quantified with the Strehl ratio or other metrics that are used for nighttime astronomical telescopes directly. In this paper, we propose to use the root mean square (rms) granulation contrast as a metric to quantify the image quality of ground-based solar telescopes. We obtain semi-logarithmic plots indicating the correspondence between the Strehl ratio and the rms granulation contrast for most practical values of the telescope diameters (D) and the atmospheric coherence diameters ($ r_0$), for various levels of adaptive optics compensation. We estimate the efficiency of a few working solar adaptive optics systems by comparing the results of our simulations with the Strehl ratio and rms granulation contrast published by these systems. Our results can be used in conjunction with a plausible 50 system efficiency to predict the lower bound on the rms granulation contrast expected from ground-based solar telescopes.

A Spectroscopic Survey of Infrared 1–4 μm Spectra in Regions of Prominent Solar Coronal Emission Lines of Fe XIII, Si X, and Si IX

The infrared solar spectrum contains a wealth of physical data about the Sun and is being explored using modern detectors and technology with new ground-based solar telescopes. One such instrument will be the ground-based Cryogenic Near-IR Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope (DKIST), which will be capable of sensitive imaging of the faint infrared solar coronal spectra with full Stokes I, Q, U, and V polarization states. Highly ionized magnetic dipole emission lines have been observed in galaxies and the solar corona. Quantifying the accuracy of spectral inversion procedures requires a precise spectroscopic calibration of observations. A careful interpretation of the spectra around prominent magnetic dipole lines is essential for deriving physical parameters and particularly for quantifying the off-limb solar coronal observations from DKIST. In this work, we aim to provide an analysis of the spectral regions around the infrared coronal emission lines of Fe xiii 1074.68 nm, Fe xiii 1079.79 nm, Si x 1430.10 nm, and Si ix 3934.34 nm, aligning with the goal of identifying solar photospheric and telluric lines that will help facilitate production of reliable inversions and data products from four sets of solar coronal observations. The outputs can be integrated in processing pipelines to produce level 2 science-ready data.

Development of Adaptive-Optics Systems for Ground-Based Solar Telescopes

Development of Adaptive-Optics Systems for Ground-Based Solar Telescopes

The Solar Activity Monitor Network – SAMNet

The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a future UK-led international network of ground-based solar telescope stations. SAMNet, at its full capacity, will continuously monitor the Sun’s intensity, magnetic, and Doppler velocity fields at multiple heights in the solar atmosphere (from photosphere to upper chromosphere). Each SAMM sentinel will be equipped with a cluster of identical telescopes each with a different magneto-optical filter (MOFs) to take observations in K I, Na D, and Ca I spectral bands. A subset of SAMM stations will have white-light coronagraphs and emission line coronal spectropolarimeters. The objectives of SAMNet are to provide observational data for space weather research and forecast. The goal is to achieve an operationally sufficient lead time of e.g., flare warning of 2–8 h and provide many sought-after continuous synoptic maps (e.g., LoS magnetic and velocity fields, intensity) of the lower solar atmosphere with a spatial resolution limited only by seeing or diffraction limit, and with a cadence of 10 min. The individual SAMM sentinels will be connected to their master HQ hub where data received from all the slave stations will be automatically processed and flare warning issued up to 26 h in advance.

Modeling and experiment validation of thermal control for the heat-stop of the Chinese Large Solar Telescope

Thermal control system is one of the most important components of large ground-based solar telescope. Heat-stop is one of the key components for controlling thermal effects for a large solar telescope, which reflects over 95% of solar radiation protecting the relay optics and post-focus instruments. Meanwhile, a part of solar radiation has also converted into thermal energy, which will lead to internal seeing effect. Thus, the thermal control for heat-stop is a necessary work. We deduce a mathematical model to express the relationship between thermal response of heat-stop and its structure design, ambient environment, and coolant input. We also experimentally validate the feasibility of the proposed method based on the Chinese Large Solar Telescope, which is the largest solar telescope in China. The experimental results show that the temperature difference between the experimental results and calculated results based the proposed model is <2 ° C. Moreover, the temperature difference between the surface temperature of the heat-stop and the ambient temperature can satisfy thermal control requirement of ±5 ° C. Therefore, it can be verified that our model is correct and consistent with the actual situation.

The development of the adaptive optics systems for the ground-based solar telescopes

The development of the adaptive optics systems for the ground-based solar telescopes

Tracking Downflows from the Chromosphere to the Photosphere in a Solar Arch Filament System

Abstract We study the dynamics of plasma along the legs of an arch filament system (AFS) from the chromosphere to the photosphere, observed with high-cadence spectroscopic data from two ground-based solar telescopes: the GREGOR telescope (Tenerife) using the GREGOR Infrared Spectrograph in the He i 10830 Å range and the Swedish Solar Telescope (La Palma) using the CRisp Imaging Spectro-Polarimeter to observe the Ca ii 8542 Å and Fe i 6173 Å spectral lines. The temporal evolution of the draining of the plasma was followed along the legs of a single arch filament from the chromosphere to the photosphere. The average Doppler velocities inferred at the upper chromosphere from the He i 10830 Å triplet reach velocities up to 20–24 km s−1, and in the lower chromosphere and upper photosphere the Doppler velocities reach up to 11 km s−1 and 1.5 km s−1 in the case of the Ca ii 8542 Å and Si i 10827 Å spectral lines, respectively. The evolution of the Doppler velocities at different layers of the solar atmosphere (chromosphere and upper photosphere) shows that they follow the same line-of-sight (LOS) velocity pattern, which confirms the observational evidence that the plasma drains toward the photosphere as proposed in models of AFSs. The Doppler velocity maps inferred from the lower photospheric Ca i 10839 Å or Fe i 6173 Å spectral lines do not show the same LOS velocity pattern. Thus, there is no evidence that the plasma reaches the lower photosphere. The observations and the nonlinear force-free field (NLFFF) extrapolations demonstrate that the magnetic field loops of the AFS rise with time. We found flow asymmetries at different footpoints of the AFS. The NLFFF values of the magnetic field strength help us to explain these flow asymmetries.

Data reduction pipeline for MOF-based synoptic telescopes

There are strong scientific cases and practical reasons for building ground-based solar synoptic telescopes. Some issues, like the study of solar dynamics and the forecasting of solar flares, benefit from the 3D reconstruction of the Sun’s atmosphere and magnetic field. Others, like the monitoring and prediction of space weather, require full disk observations, at the proper sampling rate, combining H-alpha images and Doppler velocity and magnetic field. The synoptic telescopes based on Magneto Optical Filters (MOF) using different lines are capable of measuring the line-of-sight Doppler velocity and magnetic field over the full solar disk at different ranges of height in the Sun’s photosphere and low chromosphere. Instruments like the MOTH (Magneto-Optical filters at Two Heights), using a dual-channel based on MOFs operating at 589.0 nm (Na D2line) and 769.9 nm (K D1line), the VAMOS instrument (Velocity And Magnetic Observations of the Sun), operating at 769.9 nm (K D1 line), and the future TSST (Tor Vergata Synoptic Solar Telescope), using a dual-channel telescope operating at 656.28 nm (H-alpha line) and at 769.9 nm (K D1 line), allow to face both aspects, the scientific and the operative related to Space Weather applications. The MOTH, VAMOS and TSST data enable a wide variety of studies of the Sun, from seismic probing of the solar interior (sound speed, rotation, details of the tachocline, sub-surface structure of active regions), to the dynamics and magnetic evolution of the lower part of the solar atmosphere (heating of the solar atmosphere, identification of the signatures of solar eruptive events, atmospheric gravity waves, etc.), to the 3D reconstruction of the solar atmosphere and flare locations. However, the use of MOF filters requires special care in calibrating the data for scientific or operational use. This work presents a systematic pipeline that derives from the decennial use of MOF’s technology. More in detail, the pipeline is based on data reduction procedures tested and validated on MOTH data acquired at Mees Solar Observatory of the University of Hawaii Haleakala Observatories and at South Pole Solar Observatory (SPSO), at the Amundsen-Scott South Pole Station in Antarctica, during Antarctica Summer Campaign 2016/17.

Passive control of the temperature homogeneity for the primary mirror surface of large ground-based solar telescopes

Mirror seeing effect and thermal deformation are two major effects brought by sunlight radiation to large ground-based solar telescopes, which compromise the imaging quality. To mitigate the effects, a thermal control system (TCS) is required for the primary mirror of large ground-based solar telescopes. Several studies, including ours, have discussed the TCS, about how to control the temperature difference between the primary mirror surface and the ambient air. But few of them refer to the temperature homogeneity control for the mirror surface. The temperature inhomogeneity across the mirror surface introduces thermal deformation of high spatial frequency, which cannot be compensated for through defocus aberration. So it is important to achieve the temperature homogeneity across the mirror surface. We propose a passive method to control the temperature homogeneity for the mirror surface. First, a model is built to estimate the temperature differences across the mirror surface under different cooling conditions. Based on the model, we make an estimation of the parameters of the TCS under given temperature homogeneity requirement for the mirror surface. The estimation should make a good reference for the TCS design of large ground-based solar telescopes. Then, based on the 60-cm prototype of open solar telescope (POST), we make a numeric analysis and experimental validation of the model and obtain a proper engineering coefficient of about 2.42 in the experiment. Finally, with the proposed model, we estimate the parameters and performance of the TCS for the 1.8 m Chinese large ground-based solar telescope (CLST). The results show that the velocity uniformity of the 297 air flows in the TCS for the CLST should be better than 4.86% when the temperature homogeneity requirement across the mirror surface is within ±0.5 ° C.

Is the sky the limit?

We discuss the use of measurements of the solar granulation contrast as a measure of optical quality. We demonstrate that for data recorded with a telescope that uses adaptive optics and/or post-processing to compensate for many low- and high-order aberrations, the RMS granulation contrast is directly proportional to the Strehl ratio calculated from the residual (small-scale) wavefront error (static and/or from seeing). We demonstrate that the wings of the high-order compensated point spread function for the Swedish 1-m Solar Telescope (SST) are likely to extend to a radius of not more than about 2″, which is consistent with earlier conclusions drawn from stray-light compensation of sunspot images. We report on simultaneous measurements of seeing and solar granulation contrast averaged over 2 s time intervals at several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS beam). These data were recorded with the SST, which has been revamped with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of which were polished to exceptionally high optical quality. Compared to similar data obtained with the previous 37-electrode adaptive mirror in 2009 and 2011, there is a significant improvement in image contrast. The highest 2 s average image contrasts measured in April 2015 through 0.3−0.9 nm interference filters at 525 nm, 557 nm, 630 nm, and 853.5 nm with compensation only for the diffraction limited point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%, respectively. Similarly, the highest 2 s contrasts measured at 395 nm, 400 nm, and 484 nm in May 2016 through 0.37−1.3 nm filters are 16%, 16%, and 12.5%, respectively. The granulation contrast observed with SST compares favorably to measured values with SOT on Hinode and with Sunrise as well as major ground-based solar telescopes. Simultaneously with the above wideband red-beam data, we also recorded narrowband continuum images with the CRISP imaging spectropolarimeter. We find that contrasts measured with CRISP are entirely consistent with the corresponding wideband contrasts, demonstrating that any additional image degradation by the CRISP etalons and telecentric optical system is marginal or even insignificant. Finally, we discuss the origin of the 48 nm RMS wavefront error needed to bring consistency between the measured granulation contrast and that obtained from 3D simulations of convection.

Thermal control for light-weighted primary mirrors of large ground-based solar telescopes

A light-weighted primary mirror is the most important optical element for a large ground-based solar telescope. It receives solar radiation of more than 1000 W / m2, of which about 10% is converted into heat energy, bringing in mirror seeing effect and surface shape distortion. Thus, a thermal control system (TCS) is necessary and important. Many studies have discussed the factors that influence the temperature difference between mirror surface and ambient air, but few refer to TCS modeling and optimization for the sake of parameter estimation. A thermal control model for the parameter estimation is proposed. The analytical process and the numerical analysis results of the model on the Chinese large ground-based solar telescope are given. A 60-cm prototype of open solar telescope (POST) is developed, on which practical experiment is taken place. According to the parameters estimated results, we select devices for the TCS of the POST. The experiment results show that the maximum temperature difference was restricted within ±0.5 ° C, despite the temperature variation of the ambient air, whereas without TCS, the maximum temperature difference rises up to 6.5°C. That validates the feasibility and effectiveness of the proposed model, and it can be referred for other large ground-based solar telescopes.

Modern Ground-Based Solar Telescopes and Requirements for Their Automation Systems

This paper systematizes and generalizes data on the specific features, operation modes, and technical characteristics of world-class ground-based solar telescopes in order to obtain information required to develop and design an automated control systems for the large solar telescope included in the National Heliogeophysical Complex of the Russian Academy of Sciences.

Temperature gradient in the solar photosphere. Test of a new spectroscopic method and study of its feasibility for ground-based telescopes

Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient, might affect the irradiance. The synoptic measurement of this quantity along the activity cycle would improve our understanding of long-term irradiance variations. Aims. We intend to test a method previously introduced for measuring the photospheric temperature gradient from high-resolution spectroscopic observation and to study its feasibility with ground-based instruments with and without adaptative optics. Methods. We used synthetic profiles of the FeI 630.15 nm obtained from realistic three-dimensional hydrodynamical simulations of the photospheric granulation and line radiative transfer computations under local thermodynamical equilibrium conditions. Synthetic granulation images at different levels in the line are obtained by convolution with the instrumental point spread function (PSF) under various conditions of atmospheric turbulence, with and without correction by an adaptative optics (AO) system. The PSF are obtained with the PAOLA software, and the AO performances are inspired by the system that will be operating on the Daniel K. Inouye Solar Telescope. Results. We consider two different conditions of atmospheric turbulence, with Fried parameters of 7 cm and 5 cm, respectively. We show that the degraded images lead to both a bias and a loss of precision in the temperature-gradient measurement, and that the correction with the AO system allows us to drastically improve the measurement quality. Conclusions. Long-term synoptic observations of the temperature gradient in the solar photosphere can be undertaken by implementing this method on ground-based solar telescopes that are equipped with an AO correction system.

Adaptive optics-corrected solar image quality assessment based on image power spectrum and human visual system

In ground-based large aperture solar telescopes, the speckle image reconstruction technique combined with adaptive optics (AO) correction is generally used to get near diffraction-limited solar images. In order to select the high quality images from the AO-corrected high resolution solar image sequence, an automated no-reference image quality assessment (IQA) is needed. According to the noise characteristics of solar AO images, an IQA metric based on an image power spectrum and human visual system is developed. By the incorporation of noise masking and shifting the spatial frequency range of summation, our IQA metric could select sharper images with less noise than previous works based on the image power spectrum, even if there is image scaling. Compared with existing general-purpose IQA metrics and previous metrics specifically designed for solar IQA, experimental results verify that the proposed metric gains better performance whether on robustness to blur and noise, or on selecting high quality frames containing solar granulations, sunspots, or both of them. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

A global-shared and low-exchange parallel method of high resolution solar image reconstruction

The high resolution image reconstruction takes an important place in the solar physics research, but the solar high resolution observation has been hindered severely for a long time due to huge observation data volume, slow reconstruction speed and other factors. In order to treat the huge volume of quasi real-time solar observation data and cope with the computing burden of the same magnitude for high resolution reconstruction of solar image, a number of advanced ground-based solar telescopes at home and abroad have adopted the speckle masking, a method of reconstruction algorithm that can be parallely realized, to reconstruct the high resolution image. Good treatment results are obtained from this method. However, it is still hard to meet the demand for solar observation data treatment at the current efficiency, since the volume of solar telescope observation data is increasing. This paper is directed at the demands of China′s new solar telescopes such as the NVST (The 1 m New Vacuum Solar Telescope) and ONSET (Optical and Near-Infrared Solar Eruption Tracer). By analyzing the computing time of each module with the Triple-Spectral method through actual measurement, the paper reaches a conclusion that the data exchange performance is a key bottleneck affecting the reconstruction effect. On this basis, the paper puts forward a universal method of global-shared low-exchange parallel high resolution image reconstruction. This method takes the Triple-Spectral as the core algorithm for image reconstruction and uses the message passing interface (MPI) and shared memory mechanism, allowing the reconstruction computing process to read and write the data in the shared memory at high speed after algorithm optimization. The shared memory, which is created for storage of image data and image reconstruction results respectively according to the size of the image, will be subsequently mapped into each process, giving access to read the processing data and store the reconstruction results independently. While computing the image reconstruction, each child process will not apply the MPI communication to obtain the sub-block image data. Instead, it will read the associated data from the shared memory according to the sub-block image numbering. After the sub-block image reconstruction is done, each child process will store the reconstruction results directly into the shared memory according to the sub-block image numbering, instead of sending the results to the host process via MPI communication. In this way the communication between processes is reduced, the comunication time saved and the data exchange efficiency improved. The experiment results show that on one 16-core PC server, it takes only about 12.4 s for reconstructing the 100-frame ONSET 1660×1660 pixel image and 5.6 s for the 100-frame NVST 1024×1024 pixel image under this method. Good efficiency is achieved in the reconstruction of solar telescope data for ONSET and NVST with different apertures, proving that this method has certain efficiency and universality. The parallel combination of the Triple-Spectral and K-T has greatly reduced the communication process and data exchange in the course of image reconstruction, saved the communication time and improved the reconstruction efficiency. To be further mentioned, the achievement of good efficiency on one server would bring the method into flexible application and save the equipment cost substantially. With the aid of the research outcome of the paper, it is expected to tackle the puzzles remained in the high resolution reconstruction of NVST and ONSET, and bring down the data storage burden. Its fulfillment of real-time high resolution reconstruction has laid a solid foundation for the follow-up researches.

Noncontact temperature estimation method on the actively cooled primary mirror surface of large ground-based solar telescope

To control the mirror seeing effect and the thermal deformation, the actively cooled primary mirror is utilized in a large ground-based solar telescope. Due to direct solar illumination and high reflectivity of the mirror surface coating, the traditional contact or noncontact temperature measuring methods of the mirror surface are not available. A noncontact temperature estimation method based on the analytical heat transfer model of actively cooled primary mirror of a solar telescope is proposed. The experimental validation has been carried out on the actively cooled honeycomb mirror with 600-mm diameter. When the temperature on the mirror surface fluctuates between 23.7 deg and 26 deg, the corresponding estimation error is not more than 0.4 deg. The experimental results validate the correctness and accuracy of the proposed method.