Advanced Technology Solar Telescope M1 Thermal Control System Design, Modeling and Prototype Testing

ABSTRACT The Advanced Technology Solar Telescope (ATST) project pl ans to implement thermal control of the primary mirror using jet impingement of temperature controlled air on the back side of the meniscus mirror. This technique will be used to minimize temperature rise of the optical surface due to coating absorption, minimizing mirror seeing effects. The performance of this system has been evaluated using numerical modeling techniques and weather data recorded at the proposed observatory site. To aid in the design of the M1 thermal control system for the ATST, a prototype test bed was designed, fabricated and tested. This paper reviews the progress and results of this development program Keywords: solar telescope, thermal control, mirror seeing 1. INTRODUCTION Telescope image quality is degraded by “seeing” or wavefront error due to non-uniform refractive index of the air within the optical path. In this case, the index variation is due to temperature variations within the convective heat transfer boundary layer adjacent to the optical surface when it is at a different temp erature from the surro unding ambient air. For a solar telescope, this effect, if uncontrolled, can be many orders of magnitude more pronounced than for night time telescopes due to the high level of incident solar radiation. To mitigate seeing, temperature control of all mirrors within the ATST will be required. Thermal contro l of the primary mirror is particularly problematic because the use of a low conductivity glass or glass ceramic meniscus design limits the options for effective control of front surface temperature. Engineering work began by assessing the potential impact of primary mirror seeing. General correlations were developed for seeing based on previous work as well as original analysis in the forced convection regime. Seeing dependence on temperature differences and wind speed over the mirror, or flushing , were then used to estimate seeing based on ‘average’ temperature and wind site characteristics. Th is work helped to both evaluate potential sites as well as provide initial input to image quality based error budgeting. After reviewing a number of potential concepts, the temperature control method selected for the primary mirror was jet impingement cooling on the back surface. This technique provides the high heat transfer coefficient required with moderate air jet velocities. Modeling the performance of such a system was done using a one dimensional heat transfer model subjected to historic temperature and wind conditions r ecorded on the proposed telescope site combined with the seeing correlations developed for the four meter primary mirror. The results are a statistical measure of optical image degradation relative to the error budget allocation. To further mitigate the risk associated with this novel thermal control concept a prototype testing program was developed and implemented. A subscale mirror was used to demonstrate the concept, pin the numerical modeling assumptions, check the optical surface temperature uniformity a nd evaluate edge effects. This information was then used to refine the final system design.