Design of a thermally balanced entrance window for the NFIRAOS thermal enclosure

NFIRAOS is the first-light adaptive optics system for the Thirty Meter Telescope (TMT). It features a sub-cooled thermal enclosure (ENCL) operating at -30 degrees Celsius (degrees C), which contains all of the opto-mechanics for the system. A unique entrance window system (NWIND) features a pair of windows separated by vacuum to allow light from the telescope to enter NFIRAOS, while precisely controlling the surface temperatures of the window optics to match the internal and external thermal environments of the enclosure. This ensures that convective currents are not created at the glass surfaces, which are both near the focal plane of the system. NWIND uses a set of multiple temperature controlled warm and cold radiative baffles positioned in the vacuum space between the optics to counteract and balance the radiative thermal load between the two optics, which are typically operated at +5 degrees C and -30 degrees C. Presented here are the requirements, design, and engineering of the NFIRAOS entrance window.