Experimental study of the air-knife thermal control system for a large-aperture primary mirror

The thermal induced effect errors including the surface distortion of heated mirror and micro-thermal turbulence fluctuation at the optical surface dramatically degrade the image quality of the telescope. To address the problem, we have proposed an air-knife system consisting of an annular flushing subsystem and a central sucking subsystem and reported its simulation analysis. This paper presents the detailed experimental performance of the air-knife thermal control system. The scaling experiment is conducted in a thermo-cycling experiment room with different environmental conditions, where the temperature fluctuations and wavefront perturbation of the scaling mirror can be accurately measured. It is shown from the experimental results that the approximately laminar forced air flow at the optical surface does blow away the turbulence fluctuation and not induce novel low order wavefront aberrations. Meanwhile, the air knife system contributes to the stability of the thermal boundary layer and enhances the convective heat exchange between mirror and air around. As a result, the air-knife system significantly decreases the surface-to-air temperature difference and improves the image quality with a thermal response. Furthermore, it is found that thermal control efficiency is less significant with the increase of the air intake flow or the decrease of the surface-to-air temperature difference. The scaling experiment results demonstrate the practicability of the air-knife thermal control system for large-aperture primary mirror.