Results of the dome turbulence sensor at the Anglo-Australian Telescope

Abstract

Two major contributors to the overall seeing that degrades astronomical images are turbulence from the atmosphere and turbulence within the telescope dome structure. Dome seeing generally contributes less than 1 arcsec to the overall seeing. However, most existing telescope domes have not been characterized for dome seeing; there is an opportunity to significantly improve the overall seeing by optimizing the dome seeing. An instrument that measures a proxy to dome seeing was installed at the Anglo-Australian Telescope (AAT) at Siding Spring Observatory in Australia. The instrument is based on a similar ’dome seeing monitor’ built and tested by Bustos and Tokovinin for the 4 m Blanco telescope in 2018. The instrument consists of a collimated laser beam that propagates from the AAT’s primary mirror box, reflects off a flat mirror on the secondary strut, back down to the primary mirror box, and is imaged by a camera. The angle-of-arrival fluctuations are used to derive the seeing proxy in arcsec. Meteorology is recorded in parallel to the dome seeing proxy, including inside, outside, and mirror temperature, humidity, pressure, wind speed and direction, and telescope azimuth and elevation. These meteorology variables were tested for correlation to the dome seeing proxy. There are 77 nights worth of data, spanning from August 2021 to May 2022. The highly correlated variables were the outdoor/indoor and indoor/mirror temperature difference, the wind speed and humidity. Poorly correlated variables include the wind-to-dome slit angle, the sky/ambient temperature difference and elevation. Thermal convection conditions were found to significantly affect the dome-seeing-proxy compared to thermal inversion conditions.