Abstract
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.
Highlights
Promoted by the requirements of solar physics studies and space weather forecasting, large ground-based solar telescopes have been developing quickly.[1]
The solar flux ρ and the temperature of the ambient air Ta are related to the site and the time and can be estimated using the equations in Ref. 19 before the construction of a solar telescope. (Certainly, it is preferred to use the solar flux and the temperature of the ambient air that are recorded at the site.) the homogeneity requirement of the air flow velocities can be estimated according to the requirement of the temperature homogeneity across the mirror surface
We propose a passive control method of the temperature homogeneity across the primary mirror surface of large ground-based solar telescopes
Summary
Promoted by the requirements of solar physics studies and space weather forecasting, large ground-based solar telescopes have been developing quickly.[1]. The deviation of the flow rates only reached within Æ10%, which implied the method needed further improvement These studies were important and pathbreaking, especially for the development of large ground-based solar telescopes.
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