Adaptive optics is the key technology of large-aperture optical telescopes to eliminate the effects of atmospheric interference on optical imaging. In adaptive optical systems, the distortion of the optical system is detected by the wavefront sensor and corrected by the deformable mirror to improve the resolution and image quality of telescopes. The contactless thin adaptive mirror is considered as the most competitive solution for the large-aperture optical telescope, which uses voice coil motors to drive the deformable mirror and capacitive displacement sensors to measure the mirror shape for feedback control. However, capacitive displacement sensor is very expensive and sensitive to environmental contaminants, such as dust, moisture, etc. In this paper, eddy-current displacement sensor is proposed for the shape measurement of contactless thin adaptive mirror in harsh environment. The structural design and working principle of the single unit with a voice coil motor and an eddy-current displacement sensor are explained. A complete model is built for the simulation of magnetic field interference among the voice coil motor, eddy-current displacement sensor, and other components. The influence of voice coil and permanent magnet on the performances of detecting coil is analyzed. The influence of the detecting coil, conductive target, and other components on the actuation of voice coil is also estimated. The simulation results indicate that the influence of eddy-current displacement sensor on the voice coil motor is limited. But the voice coil motor has a significant influence on the eddy-current displacement sensor, for which special attention should be considered.
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).
Adaptive optics (AO) systems are trending towards miniaturization and cost reduction, with wavefront sensorless adaptive optics systems (WFSless AOSs) emerging as a field of interest due to their simple structure and application versatility. The advent of deep learning has propelled the use of convolutional neural networks (CNNs) to extract aberration information from CCD images. Nevertheless, CNNs often fail to focus on the regions of images where effective information is concentrated, which limits the accuracy in aberration extraction. This paper introduces a novel Swin-UNet-based model for WFSless AOSs based on point source that employs an attention mechanism to target relevant areas within CCD light intensity images, thus addressing CNN shortcomings. Furthermore, the proposed model fuses in-focus and out-of- focus image information to directly output the reconstructed wavefront image, enhancing the overall wavefront reconstruction process. Our simulations across various D/r0 ratios reveal significant improvements with the Swin-UNet-based model: a reduction in rms wavefront error from 0.0219 to 0.0061 wavelengths at D/r0=1, from 0.0806 to 0.03825 at D/r0=6, and from 0.1241 to 0.0991 at D/r0=11. Correspondingly, the Strehl ratio improved from 0.9950 to 0.9988, from 0.8380 to 0.9567, and from 0.6814 to 0.7522, indicating enhanced image quality post-correction. Compared with existing CNN-based technology, our Swin-UNet approach can more effectively concentrate on the relevant areas of the image and mitigate the influence of invalid regions on the analysis, thereby substantially improving the effectiveness and robustness of the correction performance in WFSless AOSs.
Active optical primary mirror segmented technology is one of the most critical core technologies for ground-based large optical infrared telescopes. The co-phase segment of mirror surfaces is the fundamental guarantee for giving full play to the optical performance of large primary mirrors. To achieve this goal, the performance of the position actuators presents a huge challenge. Considering the two key indicators of control accuracy and power consumption, we developed a new compound position actuator, which consists of a fine-tuning mechanism and an active offloading mechanism. For this new type of actuator, we have developed a high-performance control system based on the active disturbance rejection control algorithm. The experimental results show that the position actuator system we developed can achieve high-precision position tracking and position control, can meet the index requirements.
In this paper, we propose a novel control method for high precision position control of large deformable mirrors with voice coil actuators. The controller consist of a low frequency force feedforward and a high frequency local position feedback.using force feedforward control, the dynamics of mirror can be contolled in faster settling times, and the local position controller based on the active disturbance rejection control(ADRC) algorithm guarantee the final stability of the high precision shape. we present the progress of the unit construction with a 7-actuator sub-system used to test the control method. Finally, by using experimental test, results confirm the validity of the proposed control scheme.
Adaptive optics is an essential technology for large ground-based telescopes to correct atmospheric disturbances. Presently the large-aperture adaptive deformable mirror uses piezoelectric actuator or voice coil actuator, however both of them have their own problems. Then Giant Magnetostrictive Material (GMM) actuator has great advantage as the actuator for large aperture adaptive mirror, which has large stroke, high bandwidth, high precision and doesn’t require high drive voltage. The research ‘s aim is to use the GMM actuator to drive the deformable mirror, and to build a large-aperture adaptive mirror calibration experiment platform to study this new material actuator in the adaptive mirror, at the same time adaptive deformable mirror’s control method would be studied. This research could be considered as a pre-research for GLAO system of Chinese large optical infrared telescope.
Large adaptive deformable mirror is becoming a trend for ground base telescope, at present almost all the large adaptive deformable mirrors in use are driven by voice coil, simultaneously the capacitive sensor is used as a feedback component. While the Eddy Current Sensor could have a lot of advantages over the capacitive one in large deformable mirror. In this research a prototype of 150mm deformable mirror is built, which is driven by 7 voice coil actuators, the ECSs are used in this prototype instead of capacitive ones. In the prototype experiment some standard low-order aberrations applied on the mirror could be corrected as expected, so it could be proved that the ECS could be used in large adaptive deformable mirror to take place of the capacitive one.
Adaptive optical systems are originally developed for the field of astronomy to eliminate image blurring aberrations induced by atmospheric disturbance. In some complex applications, such as the contactless thin adaptive mirror for large-aperture ground-based optical telescopes, displacement sensors are needed to measure the deformation of the deformable mirror and construct a local position control loop. In the past adaptive secondary mirrors, capacitive sensors are designed to measure the mirror deformation. However, they suffer problems of manufacturing, maintenance, and environment. In this paper, a high-performance eddy current displacement sensor is proposed for the deformation measurement of adaptive secondary mirrors. Simulation and optimization of the detecting coil and conductive target are carried out. A deliberate signal processing circuit is designed for weak signal detection. Experimental results of the prototype sensor indicate a resolution up to 5 nm and a linearity better than 0.1% within the measuring range of 50 μm and bandwidth of 3 kHz, which meet the basic technical requirements of the adaptive optical systems.
The large adaptive deformable mirrors were adapted in several large aperture telescopes and has been proved to have great effect in the AO system, however the captive sensors used in the adaptive secondary are very sensitive to the circumstance such as temperature, moisture, dust, So a new adaptive mirror scheme using the Eddy Current Sensors has been proposed in this paper. The technical plan would be discussed in this paper, a number of simulations would be carried out and some experiments result would be posed in this paper.
The thickness of the thin shell used in the large aperture adaptive mirrors is usually less than 2mm, in contrast the shell’s diameter could be larger than several hundred millimeters, so that the shell’s stiffness could be low enough to adapt the high frequency deformation. However the shell could very easily be out of shape during the process when the pads were glued onto the mirror, some analysis based on the experiment would be presented in this paper, and force introduced by the glue process could be extracted by the method, so the result could be used during the polishing process to eliminate the influence of the bonding process on the shell
The spherical primary mirror (Mb) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is
segmented and composed of 37 hexagonal sub-mirrors, and segmented active optics method is successfully developed in
it. LAMOST project has passed through the project acceptance in 2009. The success of LAMOST makes deployable
primary mirror possible. The deployable large aperture space astronomical telescope is one of the most development
potential space observation spacecrafts in the future. This paper is targeted at the reflecting Schmidt telescope LAMOST,
which has a 6.67X6.05m primary mirror. The feasibility of the deployable structure of the large reflecting space
telescope's primary mirror has been mainly researched. The analysis of the design scheme for the deployable primary
mirror has been carried out, and according to the feature and the design of LAMOST, a subdivision type deployment
scheme has been given; The locating principle of the both side wings and the locking device after deployment has been
analyzed; In addition the problems in the process of deployment is also preliminary discussed. This paper is targeted at
the reflecting Schmidt telescope LAMOST, which has a 6.67X6.05 primary mirror. The feasibility of the deployable
structure of the large reflecting telescope's primary mirror has been mainly researched. The analysis of the design
scheme for the deployable primary mirror has been carried out, and according to the feature and the design of LAMOST,
a subdivision type deployment scheme has been given; The locating principle of the both side wings and the locking
device after deployment has been analyzed; In addition the problems in the process of deployment have been preliminary
discussed.
The Large Area Multi-Object Spectroscopic Telescope (LAMOST) is a meridian reflecting Schmidt telescope with a
40m optical axis between the reflecting Schmidt plate and the spherical primary mirror. In the middle is located the
spherical focal plane, through which there are corresponding 4000+ unit mounting holes for the fibers, and on its back,
there attached a support truss adapted from Serrurier concept. The mechanical stabilization of the focal plane system
naturally has magnificent impact on the observation efficiency of the LAMOST. A comprehensive Finite Element Model
of the focal plane system has been built to evaluate thermally induced degradation of its mechanical accuracy using the
nodal modification technique within ANSYS, and diverse temperature load cases have been considered on the Finite
Element model and related thermal analyses have been carried out to investigate thermal deformation of the focal plane.
Subsequently the calculated deflection of the working surface has been extracted and reconstructed with least square
fitting in MATLAB. The results show that temperature change around the telescope has little effect on the performance
of the focal plane within temperature variation requirements of the LAMOST. The methods of modeling and analyzing
used in this research are informative for future large telescope projects.
LAMOST is a quasi-meridian reflecting Schmidt telescope, which consists of a reflecting Schmidt corrector MA, a
spherical primary mirror MB and a focal plane. The telescope with its optical axis tilted by an angle of 25° to the horizon
tracks the celestial objects by the movements of MA to make the light throughout aim at MB, and MB mirror consists of
37 spherical segments hanged upside down on the truss which tilted by about 25° to the horizon. This paper presents a
design of MB sub-mirror segment handling manipulator for LAMOST, carries out a simulation analysis, presents an
engineering program and describes the program's design principles and ideas. The manipulator including grab, declutch,
elevation, stretch, pitching, rotation, automatic control and several other parts, and the control system can coordinate all
these movements so that each segment can be located installed and handled precisely. In the structural design process the
stiffness and positioning accuracy problems have been taken full account of.
The South Pole, especially the highest plateau Dome A, is the best place to locate telescopes in the world due to its
compelling environment, such as dry air, low humidity, low wind speeds, low values of sky noise, etc. For this site,
especially significant is its very clean air with the lowest concentration of atmospheric aerosols and its negligible
artificial light pollution. But the air temperature in Dome-A is extremely low. The lowest air temperature is about 89
degrees below zero. On the other hand, Dome A is covered with deep snow. In order to build and operate a telescope at
such low temperature successfully, some of the problems caused by low temperature should be considered and tested
through experiments. According to the Chinese Antarctic 2m LAMOST-style telescope project, a conceptual design of
tracking system for a 2 meter optical and infrared telescope fitting to operate at Dome-A, including axes structure,
materials, drive systems, lubricant and feasible ways to install telescope will be discussed in this paper.
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