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Padawan
2005-Jan-29, 07:04 PM
Hehe, a second topic today:

Is there a reaon that observatory domes are often white?

Plat
2005-Jan-29, 07:10 PM
Who knows....by the way I like that Nova Celestia site

Evan
2005-Jan-29, 07:17 PM
To minimize heat buildup during the day.

Doodler
2005-Jan-29, 08:28 PM
To minimize heat buildup during the day.

Huh? Are you saying that observatory domes are uninsulated? If so, it doesn't matter what color you paint the dome, you're going to get WILD variations in temperature. Metal conducts outside air temperatures like a wire conducts electricity, unless its a VERY expensive paint with an inherent R-value, painting makes no difference in controlling heat gain. Even then, the paint is only one layer of protection and its far from the most effective.

ngc3314
2005-Jan-29, 08:41 PM
To minimize heat buildup during the day.

Huh? Are you saying that observatory domes are uninsulated? If so, it doesn't matter what color you paint the dome, you're going to get WILD variations in temperature. Metal conducts outside air temperatures like a wire conducts electricity, unless its a VERY expensive paint with an inherent R-value, painting makes no difference in controlling heat gain. Even then, the paint is only one layer of protection and its far from the most effective.

There have been whole mythologies about this. (And by the way, they're not all metallic; we have one that's wood with copper sheet outside). If such a thing is on a brick base, you might think white or silver would help keep solar absorption down (and there have been major fights about whether white or silver - bare metal - is superior). You do have to work hard to avoid big temperature swings in large domes, where it's hardest to get things calmed down. Some places have used very large refrigetation coils in the floor, but by now pure air flow seems to work best. You do want to start this before sundown for best results. Large domes are now built with large sections that can open as louvers for wind flow, assisted by large fans. The one at Kitt Peak are affectionately known as "B-29 engines" to those in other domes who an't help hearing when they turn on.

Kristophe
2005-Jan-29, 09:12 PM
There's an old dome on campus that's made of copper. It's a very pretty shade of green. The new one is aluminium.

Evan
2005-Jan-29, 11:09 PM
Metal conducts outside air temperatures like a wire conducts electricity, unless its a VERY expensive paint with an inherent R-value, painting makes no difference in controlling heat gain.

It isn't the outside air temperature that is the concern, it is the solar flux. A white metal panel doesn't become nearly as hot as a black one and therefore won't radiate as much infrared into the interior. Paint color make a BIG difference to solar heat gain. You wouldn't paint the pipes in a solar collector white, would you?

Kristophe
2005-Jan-29, 11:37 PM
Also, you want the air inside the dome to be the same temperature as the air outside the dome.

Weird Dave
2005-Jan-30, 12:16 PM
The Institute of Astronomy at Cambridge has white domes:
http://www.srcf.ucam.org/astronomy/northumberland_small.jpg
and a black one:
http://www.srcf.ucam.org/astronomy/thorrowgood_small.jpg

I don't have enough experience to know if the black shed has heat problems - it always seems to be freezing cold whenever I go observing :( .

[edit to remove a large picture].