System for precise temperature sensing and thermal control of borosilicate honeycomb mirrors during polishing and testing

A 1.8 m f/1 borosilicate honeycomb primary for the Vatican Advanced Technology Telescope (VATT) is being polished at the Steward Observatory Mirror Laboratory. To reduce thermal distortion of large borosilicate honeycomb telescope mirrors to a negligible level during polishing, the glass must be maintained isothermal to within 0.1 °C. It is also essential to minimize temperature variations in the air while a test is in progress. Testing of the surface is done in air using a laser and interferometer mounted above the mirror. To control refractive index variation in the light path of the laser, assuming some turbulence, the air is required to be isothermal to 0.2 °C. A system designed to achieve these goals has been installed in the Mirror Laboratory. Thermocouples are employed as temperature sensors in the polishing room, air ducts, and on the mirror itself. The output of each thermocouple is referred to one of two isothermal blocks of aluminum. By repeated reading, measurements are made to an accuracy of 0.005 °C at a rate of one thermocouple per second. Thermal control of the mirror is achieved by blowing air of controlled temperature into each cell of the honeycomb structure. Preliminary results presented here show that the new system is easily capable of keeping the glass within the 0.1 °C peak-to-valley limit, while maintaining adequate control of the thermal environment in the polishing room. A similar system will be used in polishing the next generation of large telescope primaries to be cast by Steward Observatory in the 1990s, and the system's temperature tracking capability makes it ideal for use in a telescope on-site to improve seeing.