Solar Scintillometer (SoSci) and Lunar Scintillometer (LuSci) can efficiently assess the near-ground optical turbulence, which are crucial characteristic parameters for astronomical observatory sites. This paper presents the development and comparative analysis of two side-by-side SoSci / LuSci systems, aimed at determining the accuracy of turbulence data and its dependence on instrument calibration. Experiments conducted at both the park area of Nanjing Institute of Astronomical Optics & Technology and the Xuyi Observation Station of Purple Mountain Observatory in China demonstrate that atmospheric turbulence primarily affects solar or lunar flux fluctuations in the low-frequency range. The differences in the refractive index structure constant values obtained from the scintillometers with the identical configurations are minimal at the pivot points. Similarly, there are negligible differences in turbulence integrals, seeing values, and atmospheric coherence length values between them. Theoretical analysis and numerical computation of the weight function indicate that employing a normalized disk instead of the actual sun or moon image will lead the scintillometer to overestimate the turbulence near the ground. If the out scale of turbulence considered for data processing is larger or smaller than the actual value, the scintillometer will theoretically underestimate or overestimate high-altitude turbulence, respectively. However, the out scale of turbulence has minimal impact on the scintillometer's ability to assess low-altitude turbulence, making it reliable for profiling ground layer turbulence. Furthermore, the temporal trends observed in the seeing measurements via the Lunar Scintillometer are in good agreement with those from the Differential Image Motion Monitor, indicating that ground layer turbulence dominates the total turbulence.
Our article “Optical system for extremely large spectroscopic survey telescope” has been accepted on December 26, 2023, published online on March 7, 2024, and will be published in July 2024 Vol. 67 No. 7: 279511 in Sci. China-Phys. Mech. Astron. In this article, a pure reflecting optical system with a novel strip lensm (lens-prism) atmospheric dispersion corrector (S-ADC) is used, breaking through the glass material size limit of the lens atmospheric dispersion corrector, making possibility to have an aperture of 16 meters and a field of view 2.5 degrees for the spectral survey telescope. This paper gives a brief introduction to the previous article first, and then presents the progress of four important issues that were not discussed in depth in the previous article: (1) The study of Integrated Field Unit (IFU) observation of the galaxy extended sources in coudé focus; (2) The support structure and fabrication test of S-ADC; (3) A new special method of optical fiber positioning unit to resolve the problem of chief ray not perpendicular to the focal plane. (4) The preliminary telescope structure. Finally, this paper proposes to develop a spectroscopic survey telescope with a diameter of 14.5 meters. Its Nasmyth focus with a focal ratio of 4 or 3.5, a field of view diameter of 2.5 degrees (or 2 degrees), equipped with about 50,000 optical fibers, is mainly used for spectral survey of point sources. The coudé focus is used for integrated field spectral observation of extended sources (galaxies, etc.) with 4 arc-minutes and 2 arc-minutes field of view in diameter, equipped with about 50,000 or even more optical fibers. We refer to this Telescope simply as ESST (Extremely large Spectroscopic Survey Telescope).
The study introduces a technique and apparatus for assessing the atmospheric optical turbulence profile, utilizing the ring image obtained at the focal plane of a telescope fitted with a cone lens. Positioning of the cone lens is possible either ahead of the telescope or its focal plane, with the first option providing an expanded field of view and enhanced imaging efficiency. The algorithmic software analyzes the achromatic ring image from this optical arrangement to determine angular frequency signals indicative of wavefront scintillation. Subsequently, these signals facilitate the derivation of statistical data essential for analyzing turbulence, simultaneously forming a theoretical formula for the weight function that elucidates their correlation with turbulence. Inversion is used to restore the turbulence profile, by analyzing the link between the wavefront scintillation signal's angular power spectrum and the turbulence's intensity and weight function at a certain altitude. Remarkably, the suggested optical apparatus shows an absence of chromatic aberration throughout its full spectral span (400-1000nm). Given that the achromatic annular image's imaging surface aligns with the telescope's focal plane and the lens barrel is made of carbon fiber, the ring image's image quality remains unaffected by significant temperature variations, eliminating the necessity for additional focusing equipment. This method provides an economical solution with an uncomplicated hardware framework that is straightforward to set up and modify.
Jiao Tong University Spectroscopic Telescope (JUST) is a 4.8-m composite spectroscopic telescope which integrates the high-resolution observation in a small field of view (FoV) with the spectral survey in a wide FoV. The primary mirror of JUST adopts the latest segmented mirror active optics technology, which is composed of 18 hexagonal segments with a diameter of 1.1 meters each. There are two Nasmyth focal stations which can be switched by rotating the tertiary mirror. One station with a 10 arcmin FoV diameter and its image quality (full width at half maximum, FWHM) can be below 0.35 arcsec with the error budgets, which is suitable for installing high-resolution spectroscopic and imaging instruments. The other station is equipped with the wide-field correctors, which can achieve a FoV diameter of scientific 1.2 degrees and correct atmospheric dispersion. The wide-field Nasmyth focal station operates at a wavelength band ranging from 0.35 to 1.3μm and for the zenith distance ranging from 0 to 60 degrees, the image quality (FWHM) falls within a 0.75 arcsec diameter with error budgets. It is used for broad observations and mainly used for wide-field spectroscopic survey of stars, galaxies, etc. This article will describe the optical system design, the baffle design and stray light analysis, error budget, and thermal effect of the telescope in detail.
The SONG telescope is part of the global SONG program, which includes 1-meter telescopes. It is located at the Lenghu Observatory in Qinghai, China. It is designed to serve two main scientific goals in stellar physics research: the detection of exoplanets by microgravitational lensing methods based on the Lucky Imaging Technique (LIT), and the study of the internal structure of stars using astroseismology methods based on apparent velocity. Telescope pointing accuracy is critical to scientific research, and high-quality data can provide more accurate and reliable results, thus advancing astronomical science. The telescope pointing error is the deviation between the actual pointing of the telescope and the expected pointing during observation. Considering the mechanical structure, driving system, atmosphere effects, sensors, and feedback errors of the telescope, the telescopes are often required to use pointing models to correct these errors. This article proposes a concept verification based on machine learning to reduce the direction error of the SONG Telescope. Using recent historical pointing data, the machine learning algorithm XGBoost is applied to train the model, which can effectively help to improve the precision of telescope pointing, thus enhencing the quality of observational data. At the same time, its results will provide effective information for the operation of the telescope in the future.
In this paper, a novel fiber bundle probe which can transfer energy and provide a real-time feedback signal of fiber positions simultaneously is proposed and demonstrated experimentally to improve the fiber positioning accuracy of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The fiber bundle probe is composed of four asteroid cores around a transmitting energy core at the center of the fiber bundle probe. The excitation characteristics were investigated numerically when a light spot illuminates on the end of the five-core fiber bundle probe. The calculated result shows that the fiber bundle probe can provide a real-time feedback signal when the offset between the center of end of probe and the light spot is in the range of -349.5 ~ -99.5 μm and 99.5 ~ 349.5 μm. Then the fiber bundle probe is fabricated by stacking-capillary method.
Except for the spectroscopic survey telescope LAMOST, there are only two 2m class general purpose telescopes for precision observation in China (2.16m in Xinglong and 2.4m in Lijiang). Chinese astronomical community unanimously agrees that a 10m class large diameter general purpose optical/infrared telescope is urgently needed in China for a wide range of scientific research. The configuration for LOT with primary aperture 12m has been selected by Chinese government for the Thirteen-five-years plan in July, 2016. The concept design introduced here has been approved by Chinese astronomical community and Chinese Academy of Sciences in Dec. 2017, and submitted into the formal funding procedure of Chinese government. For quite a long time, China will very likely have only one 10m class telescope, therefore LOT should be a general-purpose telescope including multi-foci. The Nasmyth focus, prime focus, Cassegrain focus and coudé focus have been considered or reserved. Also, LOT will closely combine with the development of new technologies, such as AO, GLAO, fiber and instrument related new technologies, to make it has powerful capability for the frontier sciences. The four-mirror Nasmyth system, optimized according to the GLAO requirements, has a f-ratio about 14 and field of view 14 arecmin with excellent image quality. Some off-axis four-mirror Nasmyth optical systems are also presented in this paper. The primary focus system has a f-ratio 2 and 1.5degree field of view with 80% light energy encircled in 0.5 arecsec, which will let LOT complementary with the coming 30m-class telescopes. A double–layer Nasmyth platforms are proposed to accommodate more instruments, such as the wide field imaging spectrograph, broad band medium resolution spectrograph, high resolution spectrograph and multi-object fiber spectrographs and so on. Not all optical systems will be constructed in the same time, which will be in stages depending on the science and funding situation.
The Maunakea Spectroscopic Explorer is designed to be the largest non-ELT optical/NIR astronomical telescope, and will be a fully dedicated facility for multi-object spectroscopy over a broad range of spectral resolutions. The MSE design has progressed from feasibility concept into its current baseline design where the system configuration of main systems such as telescope, enclosure, summit facilities and instrument are fully defined. This paper will describe the engineering development of the main systems, and discuss the trade studies to determine the optimal telescope and multiplexing designs and how their findings are incorporated in the current baseline design.
The closed-loop correction must be carry out before observation of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) to eliminate the low-frequency errors. A natural guide star S-H sensor in the focal plane of LAMOST is used to conduct wave-front sensing. The designed limiting magnitude of the S-H sensor is 10th magnitude, and the beacon must be located in the center of field of view, or slightly deviated from the center. The survey time of LAMOST is 2 hours before and after transit, wherefore the active optical correction should be completed within half of an hour, so it is necessary to make the wave-front sensing time as short as possible. Since the magnitude of guide star and atmospheric seeing have important effect on the efficiency of wave-front sensing, 9th magnitude or brighter stars are adopted in operation. For 9th magnitude stars, sky coverage will be about 100%, but at most of time, the beacons are not located in the center of field of view, so we propose to design a laser guide system based on Rayleigh scattering to provide a beacon whose brightness is equivalent to a 7th or 8th magnitude star and to launch the beacon in the center of field of view at any observational sky. In this paper, we describe the optical design of the implementation involved a laser system with 532nm in wavelength, beam diagnostics, a launch telescope with 350mm in diameter, and receiving system.
Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) is a large aperture and wide field telescope
whose image quality requirement at Xinglong station is 80% light energy within 2 arcsecond. In fact, the designed image
quality of the central field of view is diffraction limited under optical wavelength. Due to the 60m long light path and
poor natural seeing, dome seeing and other errors, the image quality is averaged about 0.5arcsecond to 1 arcsecond. We
consider deploying a low-order adaptive optics system on LAMOST to improve seeing conditions and the corresponding
image quality. Based on the sounding balloon results on Xinglong Station, we make the numerical simulation of the AO
performance and get Fried parameter, the final point spread function (PSF) characteristics of LAMOST including Strehl
ratio, full width at half- maximum (FWHM), and the residual variance.
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