From the March 2003 issue

Domes and enclosures

Land-based observatories come in a wide variety of shapes and sizes. What is the science behind their look?
By | Published: March 27, 2003
Palomar Observatory 200-inch Dome
Rudolf Danner / David Hogg / Palomar Video Project / California Institute of Technology
Classical observatories like Mount Palomar sport white hemispherical domes. But look on mountaintops and you’ll be hard-pressed to find these classic covers on today’s giants. Why are modern observatories abandoning the traditional look for sleek, silver alternatives?

It’s a turbulent question.

As it turns out, white paint tends to make a dome’s outer surface too cool at night, relative to the ambient air temperature. The result is micro-thermal turbulence around the dome — in other words, bad news for sharp, crisp images.

Sunset Over Gemini North
Gemini North’s enclosure with the setting sun (at left) lighting up the bottom half of the telescope through thermal vents (fully open). The observing slit is partially open revealing the truss and top end of the telescope.
Copyright 1999, Neelon Crawford (Polar Fine Arts)
Today’s solution involves a high-tech aluminized paint derived from stealth technology that doesn’t get too hot or too cool. (But coating your dome with this stuff can cost $30,000 or more.) Meanwhile, with the help of hydrodynamic and wind-tunnel tests, both Subaru and the Very Large Telescope (VLT) have concluded that a cylindrical enclosure is much more effective for channeling warmer ground air around and above the telescope.

Modern observatories go to extreme lengths to control the thermal environment immediately around the telescope. Most don’t even allow people to be inside the enclosure when the telescope is taking data, as the turbulence of even one warm body is too much. Likewise, many observatories have constructed completely separate buildings for all of the control and computing equipment to minimize vibration within the enclosures.

The Gemini Observatory effectively places their telescopes in the open air by using huge vents around the dome sides. Within ten seconds of opening the vents, the entire enclosure volume has the same temperature as the air outside. In addition, nearly all the heat generated during operations is pumped into the prevailing winds above Mauna Kea with huge fans located in a separate building. They even draw air through the floor to ensure that heat doesn’t rise under the telescope.