<title>Gemini primary mirror thermal management system</title>

This paper describes work done on the design of the thermal management system for the primary mirrors of the Gemini telescopes. The concept developed has a set of radiating plates behind the mirror, which can be used to heat or cool the mirror. In addition, there is a provision for heating the front surface of the mirror by passing a current through the reflective coating. It is shown that the heating and cooling together can be used to raise or lower the temperature of the surface of the mirror by about 1 degree per hour. Experiments and calculations are reported which show that the system can meet the target temperature range up to 90% of the time. The temperature gradients induced in the mirror have little effect on the optical performance. Experiments have shown that no degradation to the surface is caused by the current passing process. This approach potentially will allow thick mirrors of low thermal expansivity to follow rapid ambient air temperature changes, thereby avoiding mirror seeing.