Dome Venting: The Path To Thermal Balance And Superior Image Quality

The Canada France Hawaii Telescope operates a 3.6m Optical/Infrared telescope on the summit of Mauna Kea. As an effort to improve delivered image quality in a cost-effective manner, a dome venting project was initiated to eliminate local contributions to 'seeing' that exist along the optical path and arise to a large extent due to temperature gradients throughout the dome volume.The quality of images delivered by the telescope is adversely affected by variations in air temperature within the telescope dome. Air temperature differences are caused by the air's contact with large structures. They are different from ambient as a result of their large thermal inertias and the consequent inability of these structures to follow rapid air temperature changes.The dome venting project is an effort to add a series of large openings, "vents", in the skin of the dome with the purpose of allowing free stream summit winds to flush out "stagnant air". The term, "stagnant air", applies to thermally mixed air from the inside of the dome environment that, for one reason or another, has been heated or cooled by surfaces in the dome environment.The addition of vents to the CFHT dome is intended to facilitate the passive flushing of interior air by the local wind, thereby greatly reducing air temperature variations, a process that has been successfully demonstrated to improve image quality at other telescope facilities and supported by recent water tunnel tests conducted by CFHT staff.