Selecting an Observatory Construction Site

A Virtual Lecture by Dr. Roger Angel

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 these.

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