Characterization of flow in the BET enclosure following ventilation and application of low-emissive coatings

In 2002 the method of thermal control within the BET enclosure was dramatically changed by the installation of 300 square meters of ventilated area to the stationary walls below the rotating dome. During the month of July 2003 a nightly study of the airflow was undertaken at the BET. While dome seeing has shown substantial improvement, this study revealed a number of counter intuitive flow modes which indicate that additional seeing improvements can be achieved by selective ventilation of the BET dome above the ring-wall. Additional study of dome seeing revealed substantial sub-cooling of the dome skin. Measurements of the skin temperature of the un-insulated dome revealed night-time cooling of local ambient air by as much as 3.6degrees C. An investigation of coating alternatives resulted in the application of aluminum foil tape, which reduced the temperature drop to 0.3degrees C. Presently, the performance of the HET (as low as EE50=0.8 arc-seconds) is not thought to be limited by dome seeing.