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Thermal control of the MMT Observatory (MMTO) primary mirror (M1) has been optimized to reduce M1 thermal anomalies and mirror seeing while enhancing overall imaging quality. These refinements include 1) increased use of temperatures from outside, chamber, M1 glass, and U.S. National Weather Service sources for thermal control, 2) expanded monitoring and analysis of the M1 glass temperatures, 3) integration of multiple feedback-based PID (proportional-integral-derivative) controllers in M1 thermal control, and 4) extensive data analysis of thermal anomalies and trends within the M1 mirror and surrounding telescope enclosure. The newly deployed control strategy uses the minimum temperature from different sources (i.e., outside, chamber, and forecast) to regulate the temperature of the conditioned air used to cool the M1 mirror. The controlling temperature source commonly changes during the night. Before this current work, thermal control of the M1 ventilation system used a linear regression model to determine the setpoint of the main glycol chillers. This simple approach has been replaced by a combination of open-loop and closed-loop, feedback-based PID servo controllers that regulate the chillers and coolant valves along the M1 ventilation air path. Different feedback temperatures for the various PID servos are considered, allowing for more detailed and responsive conditioning of air within the M1 ventilation system. Comparison of M1 glass-air temperature contrasts to wavefront-sensor (WFS) seeing values define optimal performance conditions. This work has led to recommendations for operational changes that aim to improve thermal conditions for the M1 mirror and telescope chamber, including during the transition from daytime to nighttime activities.
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