Thermal testing results of an electroformed nickel secondary (M2) mirror

To support higher-frequency operation, the Large Millimeter Telescope/Gran Telescopio Milimetrico (or LMT/GTM) is replacing its existing monolithic aluminum secondary mirror (M2). The new mirror is a segmented design based on the same electroformed nickel reflector panel technology that is already in use for the primary reflector segments. While the new M2 is lighter and has better surface accuracy than the original mirror, the electroformed panels are more sensitive to high temperatures. During the design phase, concerns were raised over the level of temperature increase that could occur at M2 during daytime observations. Although the panel surface is designed to scatter visible light, the LMT primary mirror is large enough to cause substantial solar heating, even at significant angular separation from the Sun. To address these concerns, the project conducted a series of field tests, within the constraint of having minimum impact on night time observations. The supplier sent two coupon samples of a reflector panel prepared identically to their proposed M2 surface. Temperature sensors were mounted on the samples and they were temporarily secured to the existing M2 mirror at different distances from the center. The goal was to obtain direct monitoring of the surface temperature under site thermal conditions and the concentration effects from the primary reflector. With the sensors installed, the telescope was then commanded to track the Sun with an elevation offset. Initially, elevation offsets from as far as 40 degrees to as close as 6 degrees were tested. The 6 degree separation test quickly passed the target maximum temperature and the telescope was returned to a safer separation. Based on these initial results, a second set of tests was performed using elevation separations from 30 degrees to 8 degrees. To account for the variability of site conditions, the temperature data were analyzed using multiple metrics. These metrics included maximum temperature, final time average temperature, and an curve fit for heating/cooling. The results indicate that a solar separation angle of 20 degrees should be suitable for full performance operation of the LMT/GTM. This separation not only is sufficient to avoid high temperatures at the mirror, but also provides time to respond to any emergency conditions that could occur (e.g., switching to a generator after a power failure) for observations that are ahead of the motion of the Sun. Additionally, even approaches of 10 to 15 degrees of angular separation on the sky may be achievable for longer wavelength observations, though these would likely be limited to positions that are behind the position of the Sun along its motion.