&lt;title&gt;Thermal control of classical astronomical primary mirrors&lt;/title&gt;

Bohannan, B.; Pearson, Earl T.; Hagelbarger, D. W.

doi:10.1117/12.391531

Cited by 6 publications

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“…In another approach, a 'cool reservoir' is created by lowering the the mirror temperature well below the temperature expected for the night observations. A near thermal equilibrium with ambient is accomplished in a resistive heating by passing electrical current through reflective coating on the front surface of the mirror (Bohannan et al, 2000;Greenhalgh et al, 1994). This approach does not quite suit the solar observations as the primary mirror is directly heated by sun's radiation.…”

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“…In addition, the mirror heating is modeled assuming fixed heat flux and fixed ambient air temperature which serves as the boundary conditions. The time scale for a mirror to reach a thermal equilibrium is of the order of few days while the ambient air temperature changes significantly in course of the day (Bohannan et al, 2000). This means, the usual mirror substrate with large heat capacity can never reach the thermal equilibrium with ambient unless some external cooling is used.…”

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Thermal characteristics of a classical solar telescope primary mirror

Banyal¹,

Ravindra²

2011

New Astronomy

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We present a detailed thermal and structural analysis of a 2m class solar telescope mirror which is subjected to a varying heat load at an observatory site. A 3-dimensional heat transfer model of the mirror takes into account the heating caused by a smooth and gradual increase of the solar flux during the day-time observations and cooling resulting from the exponentially decaying ambient temperature at night. The thermal and structural response of two competing materials for optical telescopes, namely Silicon Carbide -best known for excellent heat conductivity and Zerodur -preferred for its extremely low coefficient of thermal expansion, is investigated in detail. The insight gained from these simulations will provide a valuable input for devising an efficient and stable thermal control system for the primary mirror.Comment: 14 pages, 8 figures, Accepted for publication in New Astronom

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Thermal characteristics of a classical solar telescope primary mirror

Banyal¹,

Ravindra²

2011

New Astronomy

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“…Many studies have intensively discussed the factors that influence the temperature difference between the mirror surface and the ambient air, [11][12][13] and several practical TCSs are discussed. [14][15][16][17][18][19] But, few refer to a TCS modeling and optimization for the sake of parameter estimation and device selection, especially for ground-based solar telescopes. This paper proposes a thermal control model for the parameter estimation of the TCS.…”

Section: Introductionmentioning

confidence: 99%

Thermal control for light-weighted primary mirrors of large ground-based solar telescopes

Cheng

et al. 2019

J. Astron. Telesc. Instrum. Syst.

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Thermal control for light-weighted primary mirrors of large ground-based solar telescopes,"Abstract. 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∕m 2 , 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 AE0.5°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 Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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“…This means the temperature difference between the mirror surface and the ambient air should not exceed beyond ±2 • C [9]. More crucially, the temperature uniformity across the mirror surface has to be maintained within ±0.5 • C. Several approaches which include, air conditioning [10], fanning the optical surface [11], ventilating mirror interior, resistive heating [12,13] have been proposed to regulate the mirror temperature for ensuring high quality astronomical observations. Laminar air flow across the mirror homogenizes the temperature by disintegrating the 'thermal plumes' closer to the surface.…”

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confidence: 99%

Opto-thermal analysis of a lightweighted mirror for solar telescope

Banyal,

Ravindra,

Chatterjee

2013

Preprint

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In this paper, an opto-thermal analysis of a moderately heated lightweighted solar telescope mirror is carried out using 3D finite element analysis (FEA). A physically realistic heat transfer model is developed to account for the radiative heating and energy exchange of the mirror with surroundings. The numerical simulations show the non-uniform temperature distribution and associated thermo-elastic distortions of the mirror blank clearly mimicking the underlying discrete geometry of the lightweighted substrate. The computed mechanical deformation data is analyzed with surface polynomials and the optical quality of the mirror is evaluated with the help of a ray-tracing software. The thermal print-through distortions are further shown to contribute to optical figure changes and mid-spatial frequency errors of the mirror surface. A comparative study presented for three commonly used substrate materials, namely, Zerodur, Pyrex and Silicon Carbide (SiC) is relevant to vast area of large optics requirements in ground and space applications.

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<title>Thermal control of classical astronomical primary mirrors</title>

Cited by 6 publications

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Thermal characteristics of a classical solar telescope primary mirror

Thermal characteristics of a classical solar telescope primary mirror

Thermal control for light-weighted primary mirrors of large ground-based solar telescopes

Opto-thermal analysis of a lightweighted mirror for solar telescope

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