Experimental validation of a CO2 refrigerant-based thermal control system for the Giant Magellan Telescope (GMT) primary mirror (M1) segments

Abstract

The Giant Magellan Telescope (GMT) employs seven 8.4m diameter borosilicate primary mirror segments whose optical performance depends on maintaining them at thermal equilibrium with ambient air during operations to minimize local seeing effects and to prevent figure distortions caused by temperature gradients. To meet these requirements, a prototype thermal control system that uses subcritical CO₂ refrigeration (R744) was developed and integrated into the Primary Mirror Subsystem (M1S) test mirror cell. Testing was conducted in a steady state thermal environment at the Richard F. Caris Mirror Lab using optical feedback to assess thermal performance as part of the Optical Performance Risk Reduction (OPRR) program. This paper presents the experimental validation of that system, including details on integration, calibration, control system evaluation, thermal model validation, thermal print-through identification, and mitigation. This paper will also discuss lessons learned and future development work for the final design of M1S Thermal Control.