Dome venting: the path to thermal balance and superior image quality

Abstract

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.

The CFHT Large Area U-band Deep Survey (CLAUDS)

“…To take advantage of the excellent natural seeing at the CFHT site, MegaPrime includes a wide-field corrector that delivers uniform image quality over the entire field of view and an image stabilizing unit that effectively removes telescope wind shake. Furthermore, just before the start of the CLAUDS observations, the CFHT enclosure was retrofitted with venting louvres that increase airflow through the dome (Bauman et al 2014) and are thought to improve seeing by ∼0.1 compared to previous CFHT MegaCam imaging programs such as the Canada-France Hawaii Telescope Legacy Survey (CFHTLS; Hudelot et al 2012) and the Next Generation Virgo Survey (NGVS; Ferrarese et al 2012).…”

OSSOS. VII. 800+ Trans-Neptunian Objects—The Complete Data Release

“…The difference in accuracy between the two r filters can be explained by two factors. The installation of vents in the dome of CFHT in late 2013 (Bauman et al 2014) improved the median seeing in r by ∼0 1 (Devost et al 2016) relative to earlier CFHT imaging (Salmon et al 2009). Additionally, the new R.MP9602 filter has better throughput (Section 2.8.3), resulting in deeper images.…”

Overview and status of the Giant Magellan Telescope project

“…16 Ventilation and control of airflow through the upper enclosure is critical to mitigating "dome seeing." 36 Thermal CFD modeling of the observatory, including telescope, enclosure, and enclosure features such as wind vents, wind screen, shutter doors, and moon screen will continue during the final design phase in 2019.…”