Large Diameter Telescope Research Articles

Focal surface attitude detection for LAMOST

With telescope apertures becoming larger and larger, the deployment of large-field telescopes is becoming increasingly popular. However, optical path calibration is necessary to ensure the image quality of large-field and large-diameter telescopes. In particular, focal plane attitude calibration is an essential optical path calibration technique that has a direct impact on image quality. In this paper, a focal plane attitude detection method using eight acquisition cameras is proposed based on the calibration requirements of the wide-field telescope, LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope). Comparison of simulation and experimental results shows that the detection accuracy of the proposed method can reach 30 arcsec. With additional testing and verification, this method could be used to facilitate regular focal plane attitude calibration for LAMOST as well as other large-field telescopes.

Velocity Field Diagnostics of the Quiet Sun Using the Lambda-Meter Method: Si I 1082.7 nm Line

The validity of the lambda-meter method for determining the quiet Sun velocity field using the Si I 1082.7 nm line is investigated. To this end, the intensity profiles of this line were calculated for the solar disk center by means of NLTE simulations in a three-dimensional model atmosphere describing the small-scale magnetic activity in the quiet solar photosphere. The velocity field recovered using the lambda-meter method from theoretical NLTE profiles of the Si I 1082.7 nm line was compared with the velocity field from the model atmosphere. The influence of atmospheric and instrumental effects on the results is considered. These effects are atmospheric turbulence and light diffraction by telescope aperture, such as VTT, GREGOR, and EST/DKIST. It is shown that, in the case of observations of the Si I 1082.7 nm line on large-diameter telescopes like GREGOR and EST/DKIST with a spatial resolution substantially better than 0.27″, the lambda-meter method provides reliable values of the velocity field for the lower and upper solar photosphere. For the middle photosphere, the correlation between the inferred and the real velocities is worse, particularly when using the smaller diameter telescopes like VTT. Under a poor spatial resolution exceeding 2″, information about the velocity field can be obtained only for the uppermost photospheric layers. For this case, the lambda-meter velocities turn out to be noticeably smaller than the real values.

A New Double-Sided Flux Reversal Arc Permanent Magnet Machine With Enhanced Torque Density Capability

This paper proposes a new double-sided (DS) flux reversal arc permanent magnet (PM) machine (DS-FRAPMM), which could be applied on large-diameter telescope drive. The proposed DS-FRAPMM incorporates the merits of the APMM, the stator-PM machine, and the DS machine. The machine topology and operation principle of the DS-FRAPMM are first proposed. Then, different slot/rotor pole combinations and main structural parameters of the proposed machine are investigated and optimized for maximizing the torque and minimizing the torque ripple based on the response surface method. Finally, finite-element analysis (FEA) is used to comprehensively analyze and compare the electromagnetic performance of the proposed DS-FRAPMM, single-sided FRAPMM (SS-FRAPMM), DS surface-mounted PMs APMM (DS-SPMAPMM), and DS flux-switching APMM (DS-FSAPMM). Compared with DS-SPMAPMM, DS-FRAPMM has lower PM usage and higher torque in per unit PM usage, while compared with SS-FRAPMM, DS-FRAPMM shows higher flux linkage and higher output torque. In addition, compared with DS-FSAPMM, DS-FRAPMM has a lower torque ripple with a slightly lower output torque, which is suitable for large telescope drive.

A Study on a Single Continuous Deformable Mirror for Correction of Atmospheric Disturbance

The deformable mirror is the main component of the adaptive optical imaging system, which removes the atmospheric disturbance and acquires the target image without any loss and the beam transmission system, which transmits the laser beam to a long distance. The mirror transforms the optical wavefront distortion caused by atmospheric disturbance into the opposite wavefront shape thereby correcting the distorted wavefront. In order to develop the deformable mirror, it is necessary to analyze the adaptive optics system and the usage environment where the application is required. In the present work, a prototype of a deformable mirror with a single continuous mirror was developed. In order to design the deformable mirror, the main factors of the adaptive optics system and the large diameter telescope were analyzed. Subsequently, the mirror material and the actuator were selected by confirming the amount of deformation through the finite element analysis. Based on the data collected, we designed and fabricated the prototype of the deformable mirror, and the Gaussian model was derived by analyzing the influence function of the deformable mirror through the design data. It was observed that the derived Gaussian model matched with the Zernike polynomial. Apparently, the fitting ability of the deformable mirror was confirmed.

Design and optimization of kinematic lateral support on 2 m SiC primary mirror

The support structure based on kinematic principle had been wildly used in the axial support on large-diameter telescope. But it is not maturely applied in the lateral support system. A lateral support structure based on kinematic principle is explained. Based on a primary mirror with a diameter of 2.04 m, a support structure with more details was designed and analyzed by the ANSYS Parametric Design Language. Selected the Root Mean Square (RMS) of the primary mirror when the optical axis was vertical as the objective function, the support reaction was optimized by the simulated annealing algorithm. Final surface error RMS was 16.67 nm. Surface error with different obliquities were calculated, which meets the requirement that the RMS should be less than λ/3.

Analysis of measurement accuracy of ephemeris parameters for pulsar navigation based on the X-ray space observation

The pulsar ephemeris that maintains the time-space benchmark for pulsar navigation is an important part of Xray pulsar navigation system. The parameters of pulsar timing model which are contained in the pulsar ephemeris can influence directly the accuracy of pulsar navigation. Some studies have shown that 100 m target of X-ray pulsar navigation should need 1 mas angle position and 100 ns pulse time of arrival, the high-level precision of parameters of some pulsars can be reached by ground radio observations with large-diameter telescope. Owing to the development of high-performance X-ray detector and stable space observation platform, the technology that the parameters of pulsar ephemeris are measured by space X-ray observations may be achieved, so the feasibility of this technology is studied in this paper by reconstructing the analysis process. The process includes mainly three parts. Firstly, the methods of simulating X-ray pulsar signals, replicating pulse profile and getting the time of arrival between the observed pulse profile and the standard one from analyzing observation data of the RXTE and Chandra satellite are studied, then the accuracies of X-ray space observations for four pulsars are estimated by using the large sample duplication events. Secondly, the process of fitting model for ephemeris parameters is established and realized by computer program in C++ language. Finally, the relationships between the accuracies of ephemeris parameters and those of the following factors are analyzed: the observation accuracy, the observation duration, the observed frequency. Those results of four pulsars (Crab, B1937+21, B1821-24 and B1509-58) are concluded below. 1) The X-ray space timing precisions of Crab pulsars in the observation durations of 1000 s and an hour are 1.41 s and 0.83 s respectively, the ones of other 3 pulsars in three different observation durations of 1000 s, an hour, and a day are also gained. 2) The ephemeris parameters of four pulsars are achieved by the X-ray space simulation observations, which are similar to the result of ground pulsar radio timing, the precision of right ascension is better than that of declination. 3) The precisions of ephemeris parameters can be improved by increasing the times of observation. 4) If each pulsar can be observed for 1000 s by space satellite every half an month with a 1 m2 effective area detector, the precisions of the estimated parameters (RA, DEC and Period) for Crab pulsar are 23.4 mas 806.0 mas, 8.121310-8 s, those of other three pulsars are gained and analyzed. However, owing to the low-flux radiation characteristics of millisecond X-ray pulsar and the demand for light and efficient large detector, the high-precision ephemeris parameters can be achieved difficultly by using the space X-ray observations, but can be established and maintained well by the ground radio observation technology. The suggestion for promoting the construction of some large-diameter telescopes is made, and the method that the behavior of X-ray emissions from pulsar is predicted by the ground radio observations still needs be studied.

The research of wavefront sensor based on focal plane and pupil plane

Abstract Large aperture telescope with its high concentrated capability and high resolution plays an important role in many fields. The demand for large aperture imaging telescope is increasingly urgent, there are a lot of challenges with the increase of telescope aperture during the optical processing and testing. In order to solve the above problems, this paper mainly studies wavefront sensing technology which is the key technology in the adaptive optics system, launches a comprehensive in-depth study on the pupil plane wavefront sensor and focal plane wavefront sensor, introduced the pupil wavefront detector and focal plane wavefront detector, and then compares both of them, this work has important implications for theoretical research and engineering applications, providing a favorable guidance for the practical application of adaptive optics system is also needed for the large-diameter telescope distortion and provides a reference wavefront aberration detector.

DIRECT IMAGING IN THE HABITABLE ZONE AND THE PROBLEM OF ORBITAL MOTION

High contrast imaging searches for exoplanets have been conducted on 2.4-10 m telescopes, typically at H band (1.6 microns) and used exposure times of ~1 hr to search for planets with semi-major axes of > ~10 AU. We are beginning to plan for surveys using extreme-AO systems on the next generation of 30-meter class telescopes, where we hope to begin probing the habitable zones (HZs) of nearby stars. Here we highlight a heretofore ignorable problem in direct imaging: planets orbit their stars. Under the parameters of current surveys, orbital motion is negligible over the duration of a typical observation. However, this motion is not negligible when using large diameter telescopes to observe at relatively close stellar distances (1-10pc), over the long exposure times (10-20 hrs) necessary for direct detection of older planets in the HZ. We show that this motion will limit our achievable signal-to-noise ratio and degrade observational completeness. Even on current 8m class telescopes, orbital motion will need to be accounted for in an attempt to detect HZ planets around the nearest sun-like stars alpha Cen A & B, a binary system now known to harbor at least one planet. Here we derive some basic tools for analyzing this problem, and ultimately show that the prospects are good for de-orbiting a series of shorter exposures to correct for orbital motion.

SPICA sub-Kelvin cryogenic chains

SPICA, a Japanese led mission, is part of the JAXA future science program and is planned for launch in 2018. SPICA will perform imaging and spectroscopic observations in the mid- and far-IR waveband, and is developing instrumentation spanning the 5–400μm range. The SPICA payload features several candidate instruments, some of them requiring temperature down to 50mK. This is currently the case for SAFARI, a core instrument developed by a European-based consortium, and BLISS proposed by CALTECH/JPL in the US.SPICA’s distinctive feature is to actively cool its telescope to below 6K. In addition, SPICA is a liquid cryogen free satellite and all the cooling will be provided by radiative cooling (L2 orbit) down to 30K and by mechanical coolers for lower temperatures. The satellite will launch warm and slowly equilibrate to its operating temperatures once in orbit. This warm launch approach makes it possible to eliminate a large liquid cryogen tank and to use the mass saved to launch a large diameter telescope (3.2m). This 4K cooled telescope significantly reduces its own thermal radiation, offering superior sensitivity in the infrared region.The cryogenic system that enables this warm launch/cooled telescope concept is a key issue of the mission. This cryogenic chain features a number of cooling stages comprising passive radiators, Stirling coolers and several Joule Thomson loops, offering cooling powers at typically 20, 4.5, 2.5 and 1.7K. The SAFARI and BLISS detectors require cooling to temperatures as low as 50mK. The instrument coolers will be operated from these heat sinks. They are composed of a small demagnetization refrigerator (ADR) pre cooled by either a single or a double sorption cooler, respectively for SAFARI and BLISS. The BLISS cooler maintains continuous cooling at 300mK and thus suppresses the thermal equilibrium time constant of the large focal plane.These hybrid architectures allow designing low weight coolers able to reach 50mK. Because the sorption cooler has extremely low mass for a sub-Kelvin cooler, it allows the stringent mass budget to be met. These concepts are discussed in this paper.

Image Data Processing of Spaceborne Fabry-Perot Interferometer Prototype

针对自主研制的星载法布里-珀罗干涉仪(Fabry-Perot Interferometer,FPI)样机, 介绍了Fabry-Perot (F-P)的测风原理和风速反演算法, 阐述了F-P测风干涉数据的自动化处理方法, 包括干涉条纹图像滤波、分割、细化和特征数据的提取.滤波包括中值滤波和背景补偿, 分割采用区域平均灰度值阈值法, 借助数学形态学计算填充和消除二值化条纹的断裂点和毛刺. 最后, 按照骨架提取思想细化成干涉条纹线条, 提取其特征数据. 应用VC++编程语言开发软件平台, 对上述F-P测风仪数据自动化处理方法进行了验证, 结果表明其可行性高、适应性强.

Optimization of a telescope movable support structure by means of Volumetric Displacements

The Purpose of this paper is to show the applicability of a methodology, developed by the authors, with which to perform the mechanical optimization of space truss structures strongly restricted. This methodology use a parameter call "Volumetric Displacement", as the Objective Function of the optimization process. This parameter considers altogether the structure weight and deformation whose effects are opposed. The Finite Element Method is employed to calculate the stress/strain state and the natural frequency of the structure through a structural linear static and natural frequency analysis. In order to show the potentially of this simple methodology, its application on a large diameter telescope structure (10 m) considering the strongly restriction that became of its use, is presented. This methodology, applied in previous works on continuous structures, such as shell roof and fluid storage vessels, is applied in this case to a space truss structure, with the purpose of generalize its applicability to different structural topology. This technique could be useful in the morphology design of deployable and retractable roof structures, whose use has extensively spread in the last years.

Differential absorption lidar probing of atmospheric ozone over a tropical urban station in India

An ultra-violet (UV) rare-gas halide XeCl excimer-Raman laser-based ozone lidar system has been developed and installed at the Indian Institute of Tropical Meteorology (IITM), Pune (18°43′N, 73°51′E, 559 m above mean sea level), India. This system essentially operates in the differential-absorption-lidar (DIAL) mode with laser emission at 308 nm (‘on’) wavelength as well as reference (‘off’) wavelength of 353 nm generated by stimulated Raman shifting (SRS) the 308 nm radiation in hydrogen. The receiving system consists of a large diameter telescope tailored with a signal detection and data acquisition/processing system with 5 ns–10.5 ms multi-channel scaler/averager. This paper deals with a brief description of the lidar system developed and discusses the methodology followed for the retrieval of ozone vertical distributions from the lidar back-scattered signals obtained at ‘on’ and ‘off’ wavelengths. These initial results are compared with those obtained from a collocated ozonesonde and multi-filter solar radiometer and also with coincident observations from TOMS satellite. They are found to be in fair agreement within the experimental limitations.

High-precision timing of PSR J1600−3053

We present the results of a high-precision timing campaign directed at the binary millisecond pulsar J1600-3053. Submicrosecond pulsar timing has long been the domain of bright, low dispersion measure millisecond pulsars or large diameter telescopes. This experiment, conducted using the Parkes radio telescope in New South Wales, Australia, and utilizing the latest baseband recording hardware, has allowed this pulsar, although distant and faint, to present residuals to a model of its spin behaviour of 650 ns over a period of more than 2 yr. We have also constrained the orbital inclination via Shapiro delay to be between 59° and 70° to 95 per cent confidence and obtained a scintillation velocity measurement indicating a transverse velocity less than 84 km s-1. This pulsar is demonstrating remarkable stability comparable to, and in most cases improving upon, the very best long-term pulsar timing experiments. If this stability is maintained, the current limits on the energy density of the stochastic gravitational wave background will be reached in four more years. © 2006 RAS.

Calculated thermal behavior of ventilated high precision radio telescopes

Radio telescopes that operate at millimeter and sub-millimeter wavelengths need a reflector-surface precision of a few tens of microns and a pointing accuracy of a few arcseconds. When built in a conventional way from steel and aluminum, as in the case of larger-diameter telescopes, thermal control must be applied to reduce temperature-induced deformations, in particular of the reflector backup structure. We illustrate that it is possible to make model calculations - for instance, during the design phase - that simulate the thermal behavior and the operation of a telescope when servo-loop-controlled ventilation or climatization (air-conditioned ventilation) of the backup structure is applied. We explain the technique of model calculations, and present as an example the calculated thermal behavior of a ventilated 64-m-diameter telescope and of the climatized 30-m IRAM telescope. It is explained that the thermal control of a telescope mount is less demanding if frequent pointing corrections can be made

The weakINTEGRALbursts GRB 040223 and GRB 040624: an emerging population of dark afterglows

We report here γ -ray, X-ray and near-infrared observations of GRB 040223 along with γ -ray and optical observations of GRB 040624. GRB 040223 was detected by INTEGRAL close to the Galactic plane and GRB 040624 at high Galactic latitude. Analyses of the prompt emission detected by the IBIS instrument on INTEGRAL are presented for both bursts. The two GRBs have long durations, slow pulses and are weak. The γ -ray spectra of both bursts are best fit with steep power-laws, implying they are X-ray rich. GRB 040223 is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10 h after the prompt event by XMM-Newton . The measured spectral properties are consistent with a column density much higher than that expected from the Galaxy, indicating strong intrinsic absorption. We carried out near-infrared observations 17 h after the burst with the NTT of ESO, which yielded upper limits. Given the intrinsic absorption, we find that these limits are compatible with a simple extrapolation of the X-ray afterglow properties. For GRB 040624, we carried out optical observations 13 h after the burst with FORS 1 and 2 at the VLT, and DOLoRes at the TNG, again obtaining upper limits. We compare these limits with the magnitudes of a compilation of promptly observed counterparts of previous GRBs and show that they lie at the very faint end of the distribution. These two bursts are good examples of a population of bursts with dark or faint afterglows that are being unveiled through the increasing usage of large diameter telescopes engaged in comprehensive observational programmes.

The Accuracy of the Center‐of‐Gravity Method for Measuring Velocity and Magnetic Field Strength in the Solar Photosphere

I investigate the accuracy with which the line-of-sight velocity and magnetic field strength in the solar photosphere can be recovered from spatially resolved spectral line profiles with the center-of-gravity (COG) method. For this purpose, theoretical non-LTE polarized line profiles of a series of Fe I lines were calculated through a two-dimensional slice from a snapshot of a three-dimensional solar magnetoconvection simulation. The calculated profiles were analyzed with the COG method for all positions along the slice, and retrieved values of velocity and field strength were compared with actual values at the heights of formation of the lines. The average formation heights of the employed lines range from 60 to almost 400 km above the average photospheric level. The COG method appears reliable for measuring velocities in the lower half of these formation heights and for measuring field strength over the whole range of heights, for fields up to intermediate strength. Moreover, it is shown that the COG determination is independent of spectral resolution, making it particularly suitable for applications that require high throughput and a correspondingly large spectral bandpass, such as high spatial resolution observations with a large-diameter telescope. Finally, the effect of broad-angle scattering, which includes a schematic representation of image deterioration through seeing, on the retrieved velocity and field strength was investigated.

Free-electron laser power beaming to satellites at China Lake, California

Laser power beaming of energy through the atmosphere to a satellite can extend its lifetime by keeping the satellite batteries in operating condition longer. An alternate propulsion system utilizing power beaming will also significantly reduce the initial insertion cost of these satellites, which now range up to $160 000/kg, for geosynchronous orbit. Elements of the power beaming system are a high power free-electron laser, a large diameter telescope to reduce diffractive losses, an adaptive optic beam conditioning system and possibly a balloon or aerostat carrying a large mirror to redirect the laser beam to the satellite after traversing the earth's atmosphere. China Lake, California has excellent seeing, averages 260 cloud-free days/year, has a large geothermal plant nearby for inexpensive power, the lake for water, and is thus an ideal site for such a laser power beaming system. Technological challenges in building such a system and installing it at China Lake will be discussed.