Geography

While astronomers are mostly concerned with what is above their telescopes, the engineers who design and build observatories must consider the properties of the land on which they are constructed.

Earthquakes

Earthquakes can cause minor to significant damage to observatory domes and precision telescopes and instruments. Unfortunately, high mountains and earthquakes seem to go together, so some “earthquake-proofing” must be included in the design of any observatory. The more active an area is, the greater the structural accommodations that must be made, and thus the more expensive the construction. Consequently, a low-earthquake activity site is preferable.

Keck Telescopes on Mauna Kea, HI
View of the Keck Telescopes from the north. Note the extensive cloud layer far below the summit. At 14,000 ft, the top of Mauna Kea is usually above the clouds that frequently cover the lowlands below. W. M. Keck Observatories.

Topography

Topography is also an important consideration. A high elevation reduces the amount of air the telescope must look through. High, isolated mountains poke up through the surface winds and turbulence into smoother air. High mountains are also often above the cloud layers that form in the lower atmosphere. Experience has shown that the best telescope sites tend to fall into two topographic patterns: isolated high mountains on islands, and mountain ranges near an ocean with a cold offshore current.

Atmospheric Dust

Atmospheric dust can also be a problem, as seen in the satellite image below:

canary islands dust
Strong wind currents drive a curtain of dust from the Sahara Desert out over the Atlantic Ocean and Canary Islands in this true-color Aqua MODIS image from December 12, 2003. NASA Image.

The dry air that astronomers prefer at observing sites usually means that the surrounding lands are also dry, that is deserts. In some cases, the dry land is covered with dust and sand that can be picked up by high winds and carried for large distances. In the image above, the observatory site is the European Northern Observatory on the Isla La Palma in the Canary Islands, and the desert is the Sahara of Northern Africa. La Palma is the medium-size, triangular-shaped island next to the clouds at upper left. The edge of the Sahara is visible on the mainland of Africa at right, and extends many thousand miles to the east. Strong trade winds (see Weather) blowing westward off the desert carry dust far out to sea, blanketing the Canary Islands. In this case, however, the dust is confined to a layer in the atmosphere near the surface and so thin that the higher mountains of the Canaries, including La Palma, stick up through the dust into clear air. That is not always the case, and dust can be a severe problem.

Measuring Geography

Earthquake statistics can be obtained for any location in the world through the U.S. Geological Survey Earthquake Hazards Program home page.

Qualitatively, you can just look at recent earthquake activity at each site. If you want a more quantitative comparison, select a large, consistent sample of quakes for each site, compile numbers and magnitudes of earthquakes over an extended period of time—say 50 years—for each site. Then compare all the sites together.

Topographic maps of any location in the world can be found at the following web pages:

http://www.primidi.com/2005/01/07.html

http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html

Download the highest resolution map you can find for each site and see what the topographic setting is for each.

Deserts may or may not have lots of dust on the surface readily picked up by the wind. To get a good sampling of conditions for each site, go to the MODIS web site:

http://modis.gsfc.nasa.gov/gallery/index.php

and click on “Enter the Image gallery.” Then do a search on the name of the island or country each site is in. You should get a selection of images over time that you can carefully examine for dust clouds.