Strategies for thermal control of the 6.5-meter diameter borosilicate honeycomb primary (M1) mirror at the MMT
Observatory have included: 1) direct control of ventilation system chiller setpoints by the telescope operator, 2) semiautomated
control of chiller setpoints, using a fixed offset from the ambient temperature, and 3) most recently, an
automated temperature controller for conditioned air. Details of this automated controller, including the integration of
multiple chillers, heat exchangers, and temperature/dew point sensors, are presented here. Constraints and sanity checks
for thermal control are also discussed, including: 1) mirror and hardware safety, 2) aluminum coating preservation, and
3) optimization of M1 thermal conditions for science acquisition by minimizing both air-to-glass temperature
differences, which cause mirror seeing, and internal glass temperature gradients, which cause wavefront errors.
Consideration is given to special operating conditions, such as high dew and frost points. Precise temperature control of
conditioned ventilation air as delivered to the M1 mirror cell is also discussed. The performance of the new automated
controller is assessed and compared to previous control strategies. Finally, suggestions are made for further refinement
of the M1 mirror thermal control system and related algorithms.