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According to the wikipedia page big bear, water can cool the observatory.

Big bear is in the southern California.

It is better to build it on a cold high mountain with stable and clear atmosphere?

What is the difference when building a solar and an optical telescope? Optical telescopes are usually not build on lakes.

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    $\begingroup$ I think if you re-read that article, you'll find that you've misunderstood it, and it actually answers your question. The lake cools the nearby atmosphere. $\endgroup$ – andy256 May 25 '15 at 3:52
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To amplify andy256's comment, the problem that solar telescopes face is that heating of the surrounding ground during the day gives rise to turbulence in the air near the ground, making the observing conditions worse (think of the heat shimmer just above the surface of hot pavement or a hot road -- that's turbulence bad enough for your naked eyes to notice). Since water has a high thermal inertia, it doesn't heat up as much as land does, so you get less turbulence. (Another approach is to put the telescope on top of a tower, above the worst of the turbulence.)

Note that Big Bear is at an altitude of 2,000 meters, so it is at a moderately high altitude.

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  • $\begingroup$ Because solar telescopes work at daytime, so they need more stable atmosphere. Although optical telescopes need stable atmosphere too, they work at night. Big bear is not far away from LA. Why do not we consider lakes in Canada? $\endgroup$ – questionhang Jun 5 '15 at 15:52
  • $\begingroup$ A slightly better way to put it might be that the atmosphere is more unstable during the day, but yes... I gather that Big Bear has two additional advantages: high altitude and good weather (clear skies); plus, it's far enough from L.A. that air pollution isn't really a problem. I'd guess that many lakes in Canada aren't at a high altitude and/or don't have good enough weather. $\endgroup$ – Peter Erwin Jun 5 '15 at 15:54
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Observatories are usually built on mountains where they are cold or not, and they are usually cold because it is higher in elevation for the following reasons:

  • Light from celestial objects are less distorted the higher altitude you are.
  • At higher altitudes the atmosphere is thinner allowing scientists to get more accurate readings while using various technologies that they use to view the universe. For example, infrared, x-ray, etc.
  • As you said, "stable and clear atmosphere", which is basically a 360 unobstructed view of the skies.
    • You wouldn't want to build an observatory in the city or close to it because there would be light pollution and possibly an obstructed view in certain directions.
    • It is more quieter up in the mountains, which allows scientists to think!
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    $\begingroup$ This doesn't address the question. $\endgroup$ – Mitch Goshorn May 25 '15 at 4:48
  • $\begingroup$ referring to "It is better to build it on a cold high mountain with stable and clear atmosphere?" $\endgroup$ – NuWin May 25 '15 at 7:30
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    $\begingroup$ Big Bear (the telescope the question mentions) is not built on a mountain, rather on a lake in southern California. The OP seems to be curious as to why it was built in a lake rather than a more typical elevated position, particularly if there are specific design considerations for solar telescopes that would make a mountain location unsuitable. $\endgroup$ – Mitch Goshorn May 25 '15 at 7:37
  • $\begingroup$ @Mitch not just big bear, there are other examples. I know optical and solar telescopes consider different factors, but do not know what factors exactly. $\endgroup$ – questionhang May 25 '15 at 15:24

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