Mirror seeing effect and thermal deformation are two major effects brought by sunlight radiation to large ground-based solar telescopes, which compromise the imaging quality. To mitigate the effects, a thermal control system (TCS) is required for the primary mirror of large ground-based solar telescopes. Several studies, including ours, have discussed the TCS, about how to control the temperature difference between the primary mirror surface and the ambient air. But few of them refer to the temperature homogeneity control for the mirror surface. The temperature inhomogeneity across the mirror surface introduces thermal deformation of high spatial frequency, which cannot be compensated for through defocus aberration. So it is important to achieve the temperature homogeneity across the mirror surface. We propose a passive method to control the temperature homogeneity for the mirror surface. First, a model is built to estimate the temperature differences across the mirror surface under different cooling conditions. Based on the model, we make an estimation of the parameters of the TCS under given temperature homogeneity requirement for the mirror surface. The estimation should make a good reference for the TCS design of large ground-based solar telescopes. Then, based on the 60-cm prototype of open solar telescope (POST), we make a numeric analysis and experimental validation of the model and obtain a proper engineering coefficient of about 2.42 in the experiment. Finally, with the proposed model, we estimate the parameters and performance of the TCS for the 1.8 m Chinese large ground-based solar telescope (CLST). The results show that the velocity uniformity of the 297 air flows in the TCS for the CLST should be better than 4.86% when the temperature homogeneity requirement across the mirror surface is within ±0.5 ° C.
A light-weighted primary mirror is the most important optical element for a large ground-based solar telescope. It receives solar radiation of more than 1000 W / m2, of which about 10% is converted into heat energy, bringing in mirror seeing effect and surface shape distortion. Thus, a thermal control system (TCS) is necessary and important. Many studies have discussed the factors that influence the temperature difference between mirror surface and ambient air, but few refer to TCS modeling and optimization for the sake of parameter estimation. A thermal control model for the parameter estimation is proposed. The analytical process and the numerical analysis results of the model on the Chinese large ground-based solar telescope are given. A 60-cm prototype of open solar telescope (POST) is developed, on which practical experiment is taken place. According to the parameters estimated results, we select devices for the TCS of the POST. The experiment results show that the maximum temperature difference was restricted within ±0.5 ° C, despite the temperature variation of the ambient air, whereas without TCS, the maximum temperature difference rises up to 6.5°C. That validates the feasibility and effectiveness of the proposed model, and it can be referred for other large ground-based solar telescopes.
The 1.8-m primary mirror of solar telescope is heated by the solar radiation and introduce harmful mirror seeing degrading the imaging quality. For the Chinese Large Solar Telescope (CLST), the thermal requirement based on the quantitative evaluation on mirror seeing effect shows that the temperature rise on mirror surface should be within 1 kelvin. To meet the requirement, an active thermal control system design for the CLST primary mirror is proposed, and realized on the subscale prototype of the CLST. The experimental results show that the temperature on the mirror surface is well controlled. The average and maximum thermal controlled error are less than 0.3 and 0.7 kelvins respectively, which completely meets the requirements.
In order to study some special solar activities, such as the emergence, evolution and disappearance progress of the sunspot and magnetic flux, and the key role of magnetic field, a new 1.8-meter size high-resolution solar telescope —the CLST will be built in the Institute of Optics and Electronics(IOE), Chinese Academy of Science(CAS), which locates in Chengdu, China. The CLST has a classic Gregorian configuration, alt-azimuth mount, retractable dome. Besides that, a large mechanical de-rotator will be used to cancel the image rotation, and finally it will cooperate with another kind of mechanical de-rotator to cancel both of the pupil rotation and image rotation. Φ3 arc-minute field of view will help the CLST to observe the whole solar activity region, and if necessary the FOV can be enlarged to Φ 6 arc-minute. A 1.8m primary mirror with honeycomb sandwiches structure made by using ULE material will reduce about 70% of weight. Thermal controlling system will also be equipped for the CLST, which including Heat-Stop, primary mirror, tube truss, mount and the other optics elements. An experimental system for validating thermal controlling of primary mirror and Heat-Stop has been built, and the temperature tracking results will be illustrated in this paper. Currently, we have finished the detailed design of the CLST, and some important components also have been manufactured and finished. In this paper, we describe some important progresses and the latest status of the CLST project during these two years.
For better understanding and forecasting of solar activity, high resolution observations for the Sun are needed. Therefore, the Chinese Large Solar Telescope (CLST) with a 1.8-m aperture is being built. The CLST is a classic Gregorian configuration telescope with an open structure, alt-azimuth mount, retractable dome, and a large mechanical de-rotator. The optical system with an all reflective design has a field of view of larger than 3 arc-min. The 1.8-m primary mirror is a honeycomb sandwich fused silica lightweight mirror with an ultra lower expansion material and active cooling. The adaptive optics system will be developed to provide the capability for diffraction-limited observations at visible wavelengths. The CLST design and development phase began in 2011 and 2012, respectively. We plan for the CLST’s start of commission to be in 2017. A multiwavelength tomographic imaging system, ranging from visible to near-infrared, is considered as the first light scientific instrument. The main system configuration and the corresponding postfocal instruments are described. Furthermore, the latest progress and current status of the CLST are also reported.
For better understanding and forecasting of the solar activity and the corresponding impacts human technologies and life on earth, the high resolution observations for Sun are needed. The Chinese Large Solar Telescope (CLST) with 1.8 m aperture is being built. The CLST is a classic Gregorian configuration telescope with open structure, alt-azimuth mount, retractable dome, and a large mechanical de-rotator. The optical system with all reflective design has the field of view of larger than 3 arc-minute. The 1.8 m primary mirror is a honeycomb sandwiches fused silica lightweight mirror with ULE material and active cooling. The adaptive optics system will be developed to provide the capability for diffraction limited observations at visible wavelengths. The CLST design and development phase began in 2011 and 2012 respectively. We plan for the CLST’s starting of commission in 2017. A multi-wavelength tomographic imaging system with seven wavelengths range from visible to near-infrared wavelength is considered as the first light scientific instruments. In this paper the main system configuration and the corresponding post focal instruments are described. Furthermore, the latest progress and current status of the CLST are also reported.
According to the thermal environment of a 1.8m open structure solar telescope, a passive thermal control scheme of the
mechanical structures is designed to reduce the effects of air turbulence and thermal deformation in this paper.
Geometric characteristic of the open structure and the coating properties of mechanical structure are discussed.
The stringent thermal requirements for the telescope mechanical structure should be satisfied to avoid the thermal
inhomogeneity of ambient air at an intolerable level. A detailed finite element model with environmental conditions is
presented to analyze the thermal response of the telescope mechanical structure. Thermal deformation and stress are also
studied with the same finite element model. The numerical results demonstrate that the approach of passive cooling is
effective and feasible.
We are developing a sodium guide star adaptive optics system for the 1.8 meter telescope, which consists of three
main parts: (i) 20W microsecond pulsed laser system, (ii) Φ200mm laser launch telescope and (iii) 37-elements adaptive
optics system. All of these three parts are mounted on the 1.8 meter telescope which is located in Gaomeigu site of
Yunnan Astronomical Observatory, Chinese Academy of Sciences. The pulsed laser system and the launch telescope are
rotated with the azimuthal base of the telescope. A miniaturized 37-elements low-order adaptive optics system including
a 37-elelment deformable mirror and a 6x6 array Hartmann-Shack wavefront sensor is mounted at the Cassegrain focus
taking account of the pulsed laser mode. A separate tip-tilt correction loop is also integrated into the system. This paper
describes the details of this system, the simulation result and the test result in the lab. After the indoor test, the whole
system will be shipped to 1.8 meter telescope. The latest commissioning status and results is presented also in this paper.
In 2009, A 127-element adaptive system had been manufactured and installed at the Coude room of the 1.8-meter
telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. A set of new
adaptive optical system based on a 73-element deformable secondary mirror is being developed and will be integrated
into the 1.8-meter telescope. The 73-element deformable secondary mirror with convex reflecting surface is designed to
be compatible with the Cassegrain focus of the 1.8-meter telescope. Comparing with the AO system of Coude focus, the
AO system on the deformable secondary mirror adopts much less reflections and consequently restrains the thermal
noise and increases the energy transmitting to the system. The design and simulation results of this system will be
described in this paper. Furthermore, the preliminary test result of the deformable secondary mirror in the lab is also presented.
A microsecond pulsed sodium has been developed in TIPC laser physics research center, the power of this laser is
around 20W and the length of the pulse is about 120 microseconds. In 2011, an experiment to project the TIPC prototype laser to the sky and measure the photon returns of the laser has been held on the 1.8 meter telescope in Yunnan observation site. During the sky test, an artificial sodium beacon has been successfully generated, and the brightness of the sodium beacon is around 8.7M in V Band. In the 2012 test campaign, the sodium column density facility has mounted on the telescope to test the local sodium density and structure and the sodium density test result is around 2.2x1013/m2.