A major driver in the design of the telescope facility is the requirement in the DCT error budget which states that dome seeing shall not degrade the natural atmospheric seeing by no more than 10% of the natural site conditions.
Dome seeing is the "blurring" effect that thermal gradients inside and near the telescope enclosure have on an optical image due refraction of light when it travels through air layers of varying temperature.
This detrimental effect of temperature variations within a telescope chamber on dome seeing leads to the requirement that all air and surfaces inside a telescope chamber should track the outside temperature as closely as possible during times of observation. Historically this led to a philosophy that a telescope effectively standing in the open is the objective towards which designers must strive since this would eliminate any temperature difference attributed to a building in close proximity to the telescope. The disadvantages of this approach are increased dusting and exposure to fluctuating and strong wind conditions. The wind causes vibration of the mirrors and structure, which becomes more problematic with larger telescopes and larger mirror surface areas.
During the conceptual design of the telescope, several computational fluid dynamics (CFD) analyses were conducted. The analyses indicated that a passive ventilation approach is acceptable for the DCT site; the DCT site has the advantage of a consistent breeze of at least approximately 5 mph (~2 m/s). Therefore, the DCT approach is to insulate the air inside the telescope chamber during the day, and to open a system of ventilation doors at the beginning of nightly observing operations.
Design principles followed to minimize dome seeing are:
1. Most turbulence found close to ground, raise mirror;
2. Daytime air conditioning to keep air and surfaces within 2K of set point;
3. No dark paving near telescope site;
4. Use building materials with low thermal inertia in telescope chamber;
5. No exposed concrete in telescope chamber;
6. Insulate the dome and floor;
7. Cool local heat sources.
A recent study of local seeing at the DCT facility was conducted by telescope design expert Dan Blanco in 2008. Read the results of the study here.
FIRST LIGHT INSTRUMENTS