Large Optical Telescopes Research Articles

Extremely large ground-based optical telescopes

Extremely large ground-based optical telescopes

Application of variable intelligent trusses in large aperture optical telescopes

Application of variable intelligent trusses in large aperture optical telescopes

Improved etching uniformity using equivalent electrodes on an unconventional, irregular membrane optical element for large aperture diffractive optical telescopes.

The etching uniformity of diffractive membrane optical elements with an irregular shape was investigated. A deteriorative uniformity of electron number density and electron temperature was found according to finite element analysis of plasma discharge. A designable equivalent electrode was proposed to weaken the influence of introducing the unconventional, irregular sample. Improved uniformity of etching depths was demonstrated experimentally, assisting by the designable equivalent electrode. The demonstration of the designable equivalent electrode provides a beneficial solution for the fabrication of unconventional optical elements and an effective means for adjusting and controlling plasma characteristics.

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°C, 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.

High-power repetition rate- and pulse width-tunable 589 nm versatile laser for adaptive optical systems.

Compact high-power yellow laser is a critical part for sodium beacon adaptive optical systems. A narrow-linewidth quasi-continuous-wave (QCW) solid-state 589 nm laser with high-power and high beam quality simultaneously is investigated here, operating in hundreds-microsecond pulse duration with a tunable repetition rate of 400 to 1 kHz, which is flexible to allow the telescope to move in observing direction. The laser source is based on employing sum-frequency generation between 1319 and 1064 nm QCW Nd:YAG amplifiers. For a 100 µs pulse duration and 400 Hz repetition rate, the yellow laser provides a highest output power of 86.1 W with beam quality M2 = 1.37. The central wavelength can be precisely tuned to sodium-D2a line at 589.159 nm with a ∼440 MHz linewidth. This is the maximum power-reported for all-solid-state sodium guide star laser demonstrated to date. The result represents a key step toward solving the requirement of multi-conjugate adaptive optics for large adaptive optical telescopes.

Atmospheric parameters at the 6-m Big Telescope Alt-azimuthal site

ABSTRACT In this paper, we introduce the results of the statistical analysis of atmospheric characteristics at the site of the Big Telescope Alt-azimuthal (BTA) of the Special Astrophysical Observatory (SAO) of the Russian Academy of Science (RAS). The BTA is the largest optical telescope in Eurasia and is located near Mt Pastukhova in the northern part of the Caucasus Mountains, at an altitude of 2070 m above sea level. The atmosphere of the Earth is a major challenge for observing and it limits the quality of astronomical images obtained by ground-based telescopes. The study of the atmosphere above astronomical observatories is important for the planning of observing time, for the optimization of instrument performance and for the development of adaptive optics systems. We discuss the results of a study of the meteorological conditions at the BTA site: total cloud cover, wind speed at the pressure level of 200 hPa, vertical motions, vertical profiles of the wind speed employing data from the ERA-Interim and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis data bases.

Aberration correction based on wavefront sensorless adaptive optics in membrane diffractive optical telescope

The membrane diffractive optical telescopes are one of areas of active research of large aperture optical telescopes. However, due to the processing technology of thin membrane mirror, the changes of temperature and humidity, etc., the imaging performance of membrane diffractive optical telescope is degraded sharply and it is difficult to reach the design target. Adaptive optics (AO) technology is a potential solution to improve the imaging performance of this telescope. Our work is the first time to correct system aberration of hundred millimeter-level aperture membrane diffractive optical telescope based on AO technology. The point target and extended target imaging experiment of the telescope with WFSless AO system in the laboratory are accomplished. The experimental results demonstrate the effectiveness of AO correction in improving the imaging performance of membrane diffractive optical telescope. Especially when there are unexpected aberrations introduced in the telescope, such as disturbances caused by assembly and operation, AO correction will be an important guarantee for the telescope to maintain good imaging performance.

SpUpNIC (Spectrograph Upgrade: Newly Improved Cassegrain): a versatile and efficient low- to medium-resolution, long-slit spectrograph on the South African Astronomical Observatory’s 1.9-m telescope

We report on the extensively upgraded Cassegrain spectrograph on the South African Astronomical Observatory (SAAO) 1.9-m telescope. The introduction of new collimator and camera optics, a new detector and controller, a rear-of-slit viewing camera to facilitate acquisition, and a new instrument control and quick-look data-reduction software (to take advantage of the entire system now being governed by a programmable logic controller) has revolutionized this workhorse instrument on Africa’s second largest optical telescope. The improvement in throughput over the previous incarnation of the spectrograph is ∼50 % in the red, increasing to a factor of four at the blue end. A selection of 10 surface-relief diffraction gratings is available to users, offering a variety of wavelength ranges and resolutions, with resolving powers between ∼500 and 6500. SpUpNIC (Spectrograph Upgrade: Newly Improved Cassegrain) has been scheduled for ∼80 % of the time available on the 1.9-m since being installed on the telescope in late October 2015, providing the single-object spectroscopic capability to support the broad research interests of the SAAO’s local and international user community. We present an assortment of data obtained for various observing programs to demonstrate different aspects of the instrument’s enhanced performance following this comprehensive upgrade.

Surface Correction Control Based on Plasticized Multilayer P(VDF‐TrFE‐CFE) Actuator—Live Mirror

AbstractThe interdisciplinary approach presented here creates next‐generation large mirrors using electroactive polymer (EAP) actuators without classical glass abrasive polishing (“live mirrors”). The outstanding electromechanical coupling properties of terpolymer are taken advantage of, particularly when doped with plasticizer, e.g., diisononyl phthalate (DINP). This doped terpolymer creates a large strain response as well as excellent mechanical energy density under relatively low electric fields. Classical EAPs (e.g., polyurethane, silicone) require extremely high input voltages to reach sufficient mechanical strain. Using the high‐permittivity doped terpolymer and the concept of stacking multilayers, high displacements and large forces are generated. The actuation performance of multilayered terpolymer filled with DINP has been proven to shape mirror glass with a preliminary prototype of an 8‐layer actuator stack. The experimental results demonstrate surface deformations under load conditions of several microns. This is large enough to usefully control large optical telescope mirrors. This technology may enable much larger high‐quality optical mirror systems for ground‐ and space‐based astronomy and communications telescopes.

Calculation of the Optical Telescope Mirror

This work considers the problem of bending a thick annular plate of variable thickness on point supports under the action of its own weight. The problem describes the stress-strain state of the primary mirrors of large optical telescopes when the mirror axis is directed to the zenith. The main feature of this problem is the transverse displacements of the plate reference surface, which coincides with the rear flat base of the plate where the supports are located, and the front surface from which the incident light is reflected. Errors of the wavefront of the reflected light, including the standard deviation and the magnitude of the wavefront, are associated with the transverse displacement. The problem is solved with the use of two nonclassical theories of plates, the Timoshenko-Reissner theory of plates and the Palii-Spiro theory of mean thickness shells. The case of the optimal location of the supports corresponds to the smallest values of the deviation of the wavefront magnitude and the standard deviation. Calculations according to the Timoshenko-Reissner and the Palii-Spiro theories gave the same optimal location of the supports. The Palii-Spiro theory, which takes into account the variation of the transverse displacement along the thickness of the plate, is preferable for calculating the distortion of the reflecting surface of the primary mirrors of optical telescopes.

A Novel Region-Division-Based Tolerance Design Method for a Large Number of Discrete Elements Distributed on a Large Surface

The array structure is widely used in precise electronic products such as large phased array antennas and large optical telescopes, the main components of which are a large surface base and a large number of high-precision discrete elements mounted on the surface base. The geometric error of discrete elements is inevitable in the manufacturing process and will seriously degrade the product performance. To deal with the tolerance design of discrete elements, a region-division-based tolerance design method is proposed in this paper. The whole array was divided into several regions by our method and the tolerance of discrete elements was correlated with the region importance on the performance. The method specifically includes the following steps: first, the sensitivity of the product performance to geometric errors was analyzed and the statistical relationship between the performance and geometric errors was established. Then, based on the sensitivity matrix, the regional division scheme was developed, and the corresponding tolerance was optimized according to the established relationship function. Finally, the optimal tolerance was selected among the multiple solutions to achieve the best performance. Taking a large phased array as an example, a simulation experiment was performed to verify the effectiveness of the proposed method.

A space surveillance satellite for cataloging high-altitude small debris

This paper proposes a space surveillance satellite whose mission is to detect, catalog and maintain space debris as small as 10 cm in the Geosynchronous Earth Orbit (GEO) region. To fulfill the objective, the satellite is designed to be placed on a zero-inclination orbit of 4163 km in altitude. The payload is a large optical telescope with a 1.5 m Entrance Pupil Diameter (EPD) and a 20° × 20° Field of View (FoV). Based on these configurations, a 60-day simulation experiment is organized to evaluate the debris detecting and cataloging performances of the satellite. The simulation results show that the satellite is capable of detecting small debris higher than 4163 km. About 94.7% of 10 cm debris in the GEO region are detected more than 60 times over the 60-day simulation span by the satellite. In addition, the rules-based cataloging and maintaining capability of the satellite is close to 50.0% with regard to an entire 10 cm reference population of 3267 debris. For larger size debris, the results are more optimistic. This paper also assesses the performances of our Initial Orbit Determination (IOD) and Un-Correlated Track (UCT) association methods for 74 debris in the GEO region. The IOD success rate is 97.4% by applying a range-search based method. The True Positive (TP) association rate is 89.7% and the False Positive (FP) rate is 1.8%. Accordingly, 66 of the 74 space objects are correctly identified that each has at least two detection arcs, making accurate orbit determination possible.

A New Mechanical Resonance Suppression Method for Large Optical Telescope by Using Nonlinear Active Disturbance Rejection Control

Aiming at solving the problem of the multi-low-frequency mechanical resonances appearing in the large optical telescope control system, this paper proposes a novel control method based on nonlinear active disturbance rejection control (NADRC) and proportional-integral (PI) control. In the proposed control framework, a nonlinear tracking differentiator (NTD)-based feedforward control is designed to improve the tracking performance of the system. Then, the principle of suppression of mechanical resonance of this method is analyzed. Compared with the most commonly used acceleration feedback control (AFC) method, the theoretical analysis shows that the proposed method is more effective for suppressing the low-frequency mechanical resonance. Finally, the proposed method is applied to a large optical telescope, and the experimental results show that the proposed method is better than AFC.

Analytical decomposition of Zernike and hexagonal modes over a hexagonal segmented optical aperture

Zernike polynomials are widely used to describe common optical aberrations of a wavefront as they are well suited to the circular geometry of various optical apertures. Non-conventional optical systems, such as future large optical telescopes with highly segmented primary mirrors or advanced wavefront control devices using segmented mirror membrane facesheets, exhibit a hexagonal geometry, making the hexagonal orthogonal polynomials a valued basis. A cost-benefit trade-off study for deriving practical upper limits in, e.g., polishing, phasing, alignment, and stability of hexagons imposes analytical calculation to avoid time-consuming end-to-end simulations, for the sake of exactness. It is important to include global modes over the pupil for Zernike decomposition over a hexagonal segmented optical aperture into the error budget. However, numerically calculated Zernike decomposition is not optimal due to the discontinuities at the segment boundaries that result in imperfect hexagon sampling. In this paper, we present a novel approach for a rigorous Zernike and hexagonal mode decomposition adapted to hexagonal segmented pupils by means of analytical calculations. By contrast to numerical approaches that are dependent on the sampling of the segment, the decomposition expressed analytically only relies on the number and positions of segments comprising the pupil. Our analytical method allows extremely quick results minimizing computational and memory costs. Further, the proposed formulae can be applied independently from the geometrical architecture of segmented optical apertures. Consequently, the method is universal and versatile per se. For instance, this work finds applications in optical metrology and active correction of phase aberrations. In modern astronomy with extremely large telescopes, it can contribute to sophisticated analytical specification of the contrast in the focal plane in presence of aberrations.

Rapid fabrication strategy for Ø1.5 m off-axis parabolic parts using computer-controlled optical surfacing.

Off-axis parabolic parts (OAPs) or quasi-OAPs are mostly frequently used in large optical telescopes. Compared to the stressed mirror polishing, computer-controlled optical surfacing (CCOS) or other computer-controlled subaperture tools provide more flexibility. However, the fabrication efficiency needs to be promoted in tactical ways. In this paper, we present a large aperture CCOS lap equipped with a compound motion unit and go through the grinding and pre-polishing with its figure errors. A CCOS-based heterocercal tool is first used in large optics to restrain the edge effects. In the fine polishing stage, corrective polishing, smoothing, and ion beam figuring are applied in combination to finish. We experimentally test this strategy on an Ø1.5 m OAP, as a part of giant steerable science mirror (GSSM) in the Thirty Meter Telescope. Finally, the surface error of Ø1.5 m OAP is better than 1/50λ RMS (full aperture), and the mid-spatial frequency part is better than 0.64μrad in slope RMS (effective aperture). The effective fabrication duration is reduced to 2 months.

3D Measurement Simulation and Relative Pointing Error Verification of the Telescope Mount Assembly Subsystem for the Large Synoptic Survey Telescope.

An engineering validation of a large optical telescope consists of executing major performing tests at the subsystem level to verify the overall engineering performance of the observatory. Thus, the relative pointing error verification of the telescope mount assembly subsystem is of special interest to guarantee the absolute pointing performance of the large synoptic survey telescope. This paper presents a new verification method for the relative pointing error assessment of the telescope mount assembly, based on laser tracker technology and several fiducial points fixed to the floor. Monte-Carlo-based simulation results show that the presented methodology is fit for purpose, even if floor movement occurs due to temperature variation during the measurement acquisition process. A further research about laser tracker technology integration into the telescope structure may suggest that such laser tracker technology could be permanently installed in the telescope in order to provide an active alignment system that aims to detect and correct possible misalignment between mirrors or to provide the required mirror positioning verification accuracy after maintenance activities. The obtained results show that two on-board laser tracker systems combined with eight measurement targets could result in measurement uncertainties that are better than 1 arcsec, which would provide a reliable built-in metrology tool for large telescopes.

Preliminary research on focal plane calibration method in LAMOST based on flexible planar target

In the large sky area multiobject optical fiber spectroscopy telescope project, to capture the spectrum of a particular object, the optical fiber positioner must position the optical fiber end face to a specified location on the focal plane. The accuracy of the optical fiber positioner is guaranteed by feedback from photogrammetry. Photogrammetry accuracy is based on accurate calibration. However, given the complexities in the optical fiber focal plane and the optical fiber positioner, the accurate standard point is considerably difficult to obtain, which results in insufficient calibration accuracy. To solve this problem, a convenient calibration method based on the combination of small, planar targets is proposed. In this method, each optical fiber positioner positions the optical fiber to several designed locations, which are relatively accurate. These points form a high-precision, two-dimensional point array that can be used as the planar target. In this manner, each optical fiber positioner can be regarded as a small, high-precision planar target. All small, high-precision planar targets are assembled to form the flexible planar target, which is used for calibration. The experimental result indicates that this method is highly accurate and can be applied in focal plane calibration.

Estimating the mirror seeing for a large optical telescope with a numerical method

It is widely accepted that mirror seeing is caused by turbulent fluctuations in the index of air refraction in the vicinity of a telescope mirror. Computational Fluid Dynamics (CFD) is a useful tool to evaluate the effects of mirror seeing. In this paper, we present a numerical method to estimate the mirror seeing for a large optical telescope (∼ 4 m) in cases of natural convection with the ANSYS ICEPAK software. We get the FWHM of the image for different inclination angles (i) of the mirror and different temperature differences (ΔT) between the mirror and ambient air. Our results show that the mirror seeing depends very weakly on i, which agrees with observational data from the Canada-France-Hawaii Telescope. The numerical model can be used to estimate mirror seeing in the case of natural convection although with some limitations. We can determine ΔT for thermal control of the primary mirror according to the simulation, empirical data and site seeing.

Development of a Lunar Scintillometer as part of the national large optical telescope site survey

Ground layer turbulence is a very important site characterization parameter used to assess the quality of an astronomical site. The Lunar Scintillometer is a simple and effective site-testing device for measuring the ground layer turbulence. It consists of a linear array of photodiodes which are sensitive to the slight variations in the moon's brightness due to scintillation by the lower layers of the Earth's atmosphere. The covariance of intensity values between the non-redundant photodiode baselines can be used to measure the turbulence profile from the ground up to a height determined by the furthest pair of detectors. The six channel lunar scintillometer that has been developed at the Indian Institute of Astrophysics is based closely on an instrument built by the team led by Andrei Tokovinin of Cerro Tololo Inter-American Observatory (CTIO), Chile. We have fabricated the instrument based on the existing electronic design, and have worked on the noise analysis, an EMI (Electromagnetic Induction) resistant PCB design and the software pipeline for analyzing the data from the same. The results from the instrument's multi-year campaign at Mount Saraswati, Hanle is also presented.

大口径空间光学望远镜重力卸载点布局优化方法

大口径空间光学望远镜重力卸载点布局优化方法