Some of the things are
very simple about selecting good sites for telescopes. Perhaps the
most fundamental is that you want the sky to be clear. With a radio
telescope we don't care if there are clouds. But if we're looking
with a normal optical (telescope), or we're looking at the infrared
part of a spectrum, then we have got to have a clear sky. So the first
thing we would do is look at whether the sites we're interested in
have a few clouds, but not too many clouds.
I think the next practical
thing is that we have got to be able to get to it. When given the
whole choice of the United States, we can probably find plenty of
places that don't have too many clouds, but it's better if there is
a road to where we want to go, or to at least close to where we want
to go. With a site where we are going to build permanent telescopes
with engineering staffs, we'd like to have a town that's not too far
away where these people can live.
On the other hand, we
can't be too near a big city because big cities generate a lot of
light. The natural brightness of the sky is very dark, but if we are
within 100 miles of a big city, then that can really light up the
sky and spoil our visibility of faint objects. We have to choose a
place that we can get to, but it has to be reasonably distant from
any big city. Those are the basics: clear sky, and we have got to
be able to get to it.
The next thing we are
looking for is a mountain. The ideal observing site would be in space
where the Hubble Telescope is, where we don't have to look through
the atmosphere. But if we're working from the ground, then the best
we can do is get high up on a mountain. This isn't because it brings
us closer to the stars; that's a completely negligible effect. But
we can get through some of the air that will block some of the light,
particularly in the infrared. The air blocks off some of the wavelengths
that we would like to see. The higher we get up, the more we can see
The second thing that
the air does is to cause the images to be blurred. Even when the air
is very still at its very best, it causes quite a lot of blurring.
The simple way to avoid that is to get out of as much of the air as
we can. Other things being equal, we would like to go to a high mountain.
Now, often, high mountains collect clouds, so we have to strike some
balance between getting up very high (and cloud cover). But if it's
in a permanent amount of cloud, that's not good.
It turns out to be that
the shape of the mountain is interesting. If we get into a range of
mountains, then the effect of a lot of mountain peaks will be to make
air turbulent. If we're in the middle of the Himalayas or somewhere,
the air is very turbulent. In fact, the stars twinkle a lot there.
You would prefer to be on a single mountain peak which just pokes
up into air and is pretty much undisturbed by other mountains. That's
when we get the sharpest images: when we have a single peak poking
up into the air.
Another thing that upsets
the light coming through the atmosphere is the presence of water vapor
which absorbs some infrared wavelengths. Given a chance, we'd like
to have air that's pretty dry. But it turns out, strangely enough,
the main thing that determines how dry the air is, is just height.
It doesn't matter whether your mountain is in the middle of the ocean
or in the middle of the desert. If you're up at some pretty high altitude,
then the amount of water in the air is about the same.
In summary, we need somewhere
where there are clear skies. We want to be able get to it. We'd like
to have a road or be able to get a road in without too much effort.
We don't want to be too close to a city because of light pollution.
We don't want to be too far away, or we're going to have trouble getting
people to come to the observatory. We'd like to be on a mountain,
and we'd like that mountain to be on its own and separated from others.
Adapated with permission from NASA's Astronomy Village CD-ROM