Advances in optical engineering for future telescopes

Significant optical engineering advances at the University of Arizona are being made for design, fabrication, and construction of next generation astronomical telescopes. This summary review paper focuses on the technological advances in three key areas. First is the optical fabrication technique used for constructing next-generation telescope mirrors. Advances in ground-based telescope control and instrumentation comprise the second area of development. This includes active alignment of the laser truss-based Large Binocular Telescope(LBT) prime focus camera, the new MOBIUS modular cross-dispersion spectroscopy unit used at the prime focal plane of the LBT, and topological pupil segment optimization. Lastly, future space telescope concepts and enabling technologies are discussed. Among these, the Nautilus space observatory requires challenging alignment of segmented multi-order diffractive elements. The OASIS terahertz space telescope presents unique challenges for characterizing the inflatable primary mirror, and the Hyperion space telescope pushes the limits of high spectral resolution, far-UV spectroscopy. The Coronagraphic Debris and Exoplanet Exploring Pioneer(CDEEP) is a Small Satellite(Small Sat) mission concept for high-contrast imaging of circumstellar disks and exoplanets using vector vortex coronagraph. These advances in optical engineering technologies will help mankind to probe, explore, and understand the scientific beauty of our universe.

Site testing campaign for the Large Optical Telescope at the Ali site

The Large Optical/infrared Telescope(LOT)is a ground-based 12 m diameter telescope which is proposed to be built in western China.The site selection for LOT in China began in 2016,and Ali was listed as one of the three candidate sites.Remote studies and local surveys have been carried out for more than 15 years in western China,and the results show that Ali is a promising site with comprehensive quality in terms of atmospheric and supporting conditions.An overview of the site testing campaign at the Ali site from 2016 to 2019 is presented.After the two years of data collection,the overall median seeing value is found to be 1.17 arcsec,the observable nights are 81.71%and the good observable nights are 71.76%.The weather conditions as follows,the median night temperature value is-5.18℃,the median night relative humidity value is 41.25%,the median night atmospheric pressure value is 540.92 hPa,the median night wind speed value is 7.41 m s-1 and the mainly wind direction is southwestern(SW).The median night sky background value is 22.07 magV.We also discuss the wind speed at different locations on-site,the possibility of light pollution and the effect of wind speed on differential image motion monitor(DIMM)seeing measurements.

A Tilt-correction Adaptive Optical System for the Solar Telescope of Nanjing University

A tilt-correction adaptive optical system installed on the 430 mm Solar Telescope of Nanjing University has been put in operation. It consists of a tip-tilt mirror, a correlation tracker and an imaging CCD camera. An absolute difference algorithm is used for detecting image motion in the correlation tracker. The sampling frequency of the system is 419 Hz. We give a description of the system's configuration, an analysis of its performance and a report of our observational results. A residual jitter of 0.14 arcsec has been achieved. The error rejection bandwidth of the system can be adjusted in the range 5-28 Hz according to the beacon size and the strength of atmospheric turbulence.

Optical System Design of Inter-Spacecraft Laser Interferometry Telescope

The fundamental measurement of space gravitational wave detection is to monitor the relative motion between pairs of freely falling test masses using heterodyne laser interferometry to a precision of 10 pm. The masses under test are millions of kilometers apart. The inter-spacecraft laser interferometry telescope deliver laser efficiently from one spacecraft to another. It is an important component of the gravitational wave detection observatory. It needs to meet the requirements of large compression ratio, high image quality and extraordinary stray light suppression ability. Based on the primary aberration theory, the method of the large compression ratio off-axis four-mirror optical system design is explored. After optimization, the system has an entrance pupil of 200 mm, compression ratio of 40 times, scientific field of view (FOV) of ±8 μrad. To facilitate suppressing the stray light and delivering the laser beam to the back-end scientific interferometers, the intermediate images and the real exit pupils are spatially available. Over the full FOV, the maximum root mean square (RMS) wavefront error is less than 0.007λ, PV value is less than 0.03λ (λ = 1064 nm). The image quality is approached to the diffraction-limit. The TTL noise caused by the wavefront error of the telescope is analyzed. The TTL noise in the image space of 300 μrad range is less than 1 × 10-10 m whose slope is lower than 0.6 μm/rad, which is under the noise budget of the laser interferometer space antenna (LISA), satisfying the requirements of space gravitational wave detection.

Pose Estimation of Space Targets Based on Model Matching for Large-Aperture Ground-Based Telescopes

With the development of adaptive optics and post restore processing techniques,large aperture ground-based telescopes can obtain high-resolution images(HRIs)of targets.The pose of the space target can be estimated from HRIs by several methods.As the target features obtained from the image are unstable,it is difficult to use existing methods for pose estimation.In this paper a method based on real-time target model matching to estimate the pose of space targets is proposed.First,the physicallyconstrained iterative deconvolution algorithm is used to obtain HRIs of the space target.Second,according to the 3D model,the ephemeris data,the observation time of the target,and the optical parameters of the telescope,the simulated observation image of the target in orbit is rendered by a scene simulation program.Finally,the target model searches through yaw,pitch,and roll until the correlation between the simulated observation image and the actual observation image shows an optimal match.The simulation results show that the proposed pose estimation method can converge to the local optimal value with an estimation error of about 1.6349°.

China's Next Optical/Infrared Telescope: The Debate Is On

China’s ambition to build a 12 m aperture optical/infrared telescope in Tibet has been recently challenged by intense disputes over its design options.The Large Optical/Infrared Telescope,or LOT,which will become the world’s biggest upon completion before the 30 m-level ones are in place,is expected to give Chinese astronomers a head start in exploring the early universe and exoplanets among other scientific goals.However,optical engineers and scientists are holding on to very different opinions as for what is the best technical route to get there.

The Jiao Tong University Spectroscopic Telescope Project

The Jiao Tong University Spectroscopic Telescope(JUST)is a 4.4-meter f/6.0 segmented-mirror telescope dedicated to spectroscopic observations.The JUST primary mirror is composed of 18 hexagonal segments,each with a diameter of 1.1 m.JUST provides two Nasmyth platforms for placing science instruments.One Nasmyth focus fits a field of view of 10′and the other has an extended field of view of 1.2°with correction optics.A tertiary mirror is used to switch between the two Nasmyth foci.JUST will be installed at a site at Lenghu in Qinghai Province,China,and will conduct spectroscopic observations with three types of instruments to explore the dark universe,trace the dynamic universe,and search for exoplanets:(1)a multi-fiber(2000 fibers)medium-resolution spectrometer(R=4000-5000)to spectroscopically map galaxies and large-scale structure;(2)an integral field unit(IFU)array of 500 optical fibers and/or a long-slit spectrograph dedicated to fast follow-ups of transient sources for multi-messenger astronomy;(3)a high-resolution spectrometer(R~100000)designed to identify Jupiter analogs and Earth-like planets,with the capability to characterize the atmospheres of hot exoplanets.

Design and application of an autonomous Master Control System for a multi-layer magnetic and helioseismic telescope

With the growing significance of space weather forecasting,multi-layer magnetic and helioseismic telescopes are emerging as a key area of research.However,owing to the diverse operational processes and sophisticated hardware configurations of these devices,there is an urgent need for efficient autonomous observation capabilities.An autonomous Master Control System(MCS)can ensure efficient performance,data consistency,and stability,and the prototype presented here adopts a microservices architecture,breaking down the hardware into multiple subsystems and converting their functions into individual services.A central decision-making system leads the operations,supported by three auxiliary systems and three device control systems.Through inter-subsystem service calls,it achieves rapid imaging and spectroscopic monitoring.To verify system stability and observation efficiency,the system was tested on the Solar Full-disk Multi-layer Magnetograph.Experimental results verify this system can operate automatically for 4 consecutive months,acquire photospheric vector magnetic and Doppler velocity fields within a 15-minute interval,and measure chromospheric longitudinal magnetic and Doppler velocity fields in under 180 s.This ensures consistent and stable solar monitoring and serves as a practical methodological benchmark for the development of similar devices.

Adaptive Optics System and Its Application Predictions at 1.2m Telescope of Yunnan Observatory

A 61 element adaptive optical system has been preliminary tested in the Coudé path of the 1 2m telescope at the Yunnan observatory this year. The whole system will be fully operated next year. This paper describes the AO system performances and its first experiment results, and the possible astronomical research topics.

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) is a special reflecting Schmidt telescope. LAMOST’s special design allows both a large aperture (effective aperture of 3.6 m–4.9 m) and a wide field of view (FOV) (5°). It has an innovative active reflecting Schmidt configuration which continuously changes the mirror’s surface that adjusts during the observation process and combines thin deformable mirror active optics with segmented active optics. Its primary mirror (6.67m×6.05 m) and active Schmidt mirror (5.74m×4.40 m) are both segmented, and composed of 37 and 24 hexagonal sub-mirrors respectively. By using a parallel controllable fiber positioning technique, the focal surface of 1.75 m in diameter can accommodate 4000 optical fibers. Also, LAMOST has 16 spectrographs with 32 CCD cameras. LAMOST will be the telescope with the highest rate of spectral acquisition. As a national large scientific project, the LAMOST project was formally proposed in 1996, and approved by the Chinese government in 1997. The construction started in 2001, was completed in 2008 and passed the official acceptance in June 2009. The LAMOST pilot survey was started in October 2011 and the spectroscopic survey will launch in September 2012. Up to now, LAMOST has released more than 480 000 spectra of objects. LAMOST will make an important contribution to the study of the large-scale structure of the Universe, structure and evolution of the Galaxy, and cross-identification of multiwaveband properties in celestial objects.

New vacuum solar telescope and observations with high resolution

The New Vacuum Solar Telescope （NVST） is a one meter vacuum solar telescope that aims to observe fine structures on the Sun. The main goals of NVST are high resolution imaging and spectral observations, including measurements of the solar magnetic field. NVST is the primary ground-based facility used by the Chinese solar research community in this solar cycle. It is located by Fuxian Lake in southwest China, where the seeing is good enough to perform high resolution observations. We first introduce the general conditions at the Fuxian Solar Observatory and the primary science cases of NVST. Then, the basic structures of this telescope and instruments are described in detail. Finally, some typical high resolution data of the solar photosphere and chromosphere are also shown.

Comparing different light-emitting diodes as light sources for long path differential optical absorption spectroscopy NO_2 and SO_2 measurements

In this paper, we present a comparison of different light-emitting diodes （LEDs） as the light source for long path differential optical absorption spectroscopy （LP-DOAS） atmospheric trace gas measurements. In our study, we use a fiberoptic design, where high power LEDs used as the light source are coupled into the telescope using a Y shape fiber bundle. Two blue and one ultraviolet （UV） LEDs with different emission wavelength ranges are tested for NO2 and SO2 measurements. The detailed description of the instrumental setup, the NO2 and SO2 retrieval procedure, the error analysis, and the preliminary results from the measurements carried out in Science Island, Hefei, Anhui, China are presented. Our first measurement results show that atmospheric NO2 and SO2 have strong temporal variations in that area and that the measurement accuracy is strongly dependent on the visibility conditions. The measured NO2 and SO2 data are compared to the Ozone Monitoring Instrument （OMI） satellite observations. The results show that the OMI NO2 product underestimates the ground level NO2 by 45%, while the OMI SO2 data are highly influenced by clouds and aerosols, which can lead to large biases in the ground level concentrations. During the experiment, the mixing ratios of the atmospheric NO2 and SO2 vary from 8 ppbv to 36 ppbv and from 3 ppbv to 18 ppbv, respectively.

Modeling the rail surface unevenness of a high-precision radio telescope

This study proposed a coarse-fine mixed model for describing the rail surface unevenness of an ultra-large fully steerable radio telescope （Qi Tai Telescope） with a diameter of 110 meters. The rail surface unevenness includes information on error arising from two different scales, i.e., the long-period- short-change and the short-period-long-change. Consequently, in this study an idea of a mixed model was proposed, in which trigonometric and fractal functions were, respectively, used to describe infor- mation on error from two scales. Key parameters were determined by using the least squares method and the wavelet transform method, and finally, a specific mathematical expression of the model was obtained by optimization. To validate the effectiveness of the new modeling method, the mixed model was then used to describe the rails of the Green Bank Telescope, the Large Millimeter Telescope, and a radio telescope in Miyun, Beijing. A comparative study revealed that the maximum error was less than 15 %, thus the result was superior to those of existing modeling methods.

Preliminary investigations of a potential optics system for wideband X-ray telescopes

We present preliminary investigations of a potential optics system for wideband X-ray telescopes.The optical design adopts the conical approximation of the Wolter-I configuration and a combination of multilayer coatings and silicon pore optics.The total number of mirror modules is 79,distributed in 8 rows with the radii at the intersection plane between 250 mm and 500 mm.The optimization of the total effective area using the figure of merits method suggests that the focal length is 30 m and the mirror coating is a combination of the W/Si and Pt/C multilayers.This fulfills the on-axis effective area requirements of 2000 cm^(2) at 10 keV and 300 cm^(2) at 60 keV and provides a broad energy response between 3 keV and 78.4 keV.With the current geometry and coating compositions,we implement a mass modeling of the telescope in Geant4 to predict mirror performances via the ray-tracing algorithm,including the angular resolution and effective area.With the presumed metrological data as input,this can provide precision and finishing requirements for the manufacture of optics.This work demonstrates the feasibility of combining multilayer coatings and silicon pore optics for potential use in wideband X-ray telescopes and advances the development and progress of such missions.

First light on the 127-element adaptive optical system for 1.8-m telescope

A 127-element adaptive optical system has been developed and integrated into a 1.8-m astronomical telescope in September 2009.In addition,the first light on a high-resolution imaging for stars has been achieved（September 23,2009）.In this letter,a 127-element adaptive optical system for 1.8-m telescope is described briefly.Moreover,star observation results in the first run are reported.Results show that the angular resolution of the system after adaptive optics correction can attain 0.1 arcsec,which approaches the diffraction limit of 1.8-m telescope at 700-900 nm band.

Progress in monitoring high-temperature damage to rice through satellite and ground-based optical remote sensing

The occurrence of rice high-temperature damage (HTD) has increased with global warming. Cultivation of rice is seriously affected by the HTD in the middle and lower reaches of the Yangtze River, which directly affects food security in this region and in the whole of China. It is important to monitor and assess crop HTD using satellite remote sensing information. This paper reviews the recent development of monitoring rice HTD using optical remote sensing information. It includes the use of optical remote sensing information to obtain the regional spatial distribution of high temperatures, mixed-surface temperature retrieval for rice fields based on mixed decomposition information, the development of field and thermal infrared testing and modeling, and the satellite/ground-based remote sensing coupled method for monitoring rice HTD. Finally, the prospects for monitoring crop HTD based on remote sensing information are summarized.

Active Optics in LAMOST

Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is one of the major national projects under construction in China. Active optics is one of the most important technologies for new large telescopes. It is used for correcting telescope errors generated by gravitational and thermal changes. Here, however, we use this technology to realize the configuration of LAMOST,—a task that cannot be done in the traditional way. A comprehensive and intensive research on the active optics used in LAMOST is also reported, including an open-loop control method and an auxiliary closed-loop control method. Another important development is in our pre-calibration method of open-loop control, which is with some new features: simultaneous calculation of the forces and displacements of force actuators and displacement actuators; the profile of mirror can be arbitrary; the mirror surface shape is not expressed by a fitting polynomial, but is derived from the mirror surface shape formula which is highly accurate; a proof is given that the solution of the pre-calibration method is the same as the least squares solution.

MAIN OPTICAL SYSTEM OF CHINA'S 2.16-m TELESCOPE

This paper introduces configuration of the main optical system of China’s 2.16-m telescope and the results of its optical design. There are three foci in this telescope: the Cassegrain, the coude and the prime foci. Ritchey-Chretien (R-C) system is used as the Cassegrain system. The 2-lens and 3-lens correctors are prepared for the Cassegrain and the prime foci respectively. The most significant characteristic of this optical system is that the coude and Cassegrain systems share one secondary mirror. A relay mirror is added to the coude system. When the two systems exchange, the secondary mirror moves slightly, and the coude system obtained is free from both spherical aberration and coma simultaneously. Some other coude configurations and a special configuration for setting the focal reducer are also introduced in this paper.

Optical design for Antarctic Bright Star Survey Telescope

The exoplanet search is one of the most exciting research fields in astrophysics. The Antarctic Bright Star Survey Telescope(BSST), capable of continuous exoplanet observation on polar nights, is a Ritchey–Chretien telescope with a three-lens field corrector, and has a 300 mm aperture, 2.76 focal ratio, and a wavelength coverage ranging from 0.36 to 1.014 μm. Equipped with a 4 k × 4 k and 12 μm∕pixel CCD camera, the BSST can gain a field of view of 4.8°. This Letter presents the optical design, tolerance analysis, and the alignment plan for the BSST, and the test observation results.

Modified Multi-Resolution Telescopic Search Algorithm for Block-Matching Motion Estimation

This paper presents a modified multi-resolution telescopic search algorithm (MRTlcSA) for the block-matching motion estimation. A novel inverse telescopic search is substituted for the conventional telescopic search, that reduces the on-chip memory size and memory bandwidth for VLSI implementation. In addition, strategies of motion track and adaptive search window are applied to reduce the computational complexity of motion estimation. Simulation results show that, compared with the MRTleSA, the proposed algorithm reduces the computational load to only 30% while preserving almost the same image quality. Comparisons on hardware cost and power consumption of the VLSI implementations using the two algorithms are also presented in the paper.