Steps toward 8m honeycomb mirrors VIII: Design and demonstration of a system of thermal control

If they are to achieve the best possible images, honeycomb borosilicate telescope mirrors must follow ambient temperature changes while maintaining low internal temperature gradients. This is best done by cooling or heating the internal, edge and back mirror surfaces at the same rate established by convection on the front surface. We propose to use directed jets of air at ambient temperature, arranged with more air flow on thicker sections so as to match cooling rates. A full scale glass thermal model of one honeycomb cell of an 8m mirror was built, along with a system to flow in air at controlled temperature. We find that the air jets realize high thermal coupling efficiency, and allow good control of internal gradients. With the air cooling steadily at 0.25°C/hour, typical of nighttime cooling at good sites, internal gradients were < 0.1°C and the overall lag between air and glass temperature was 0.25°C. This performance, achieved with a flow rate of 6-10 liters/sec per cell, will ensure negligible image degradation from convection at the mirror surface (mirror seeing) or from thermal distortion of the mirror substrate.