Light from street lights and industry can greatly increase the sky
background. The increased background light causes increased noise in astronomical
images, making it harder to detect faint sources. Therefore, the largest telescopes are placed
in regions of low light pollution. Figure 29 shows the artificial
sky brightness around the world. It can be seen that some of the regions which show low cloud
cover in figure 28, such as the sparsely populated western coasts of South
America and Africa, are also largely free of light pollution. Figure 29
also clearly shows why no major telescopes are now built in mainland Europe.
Selecting a site with good seeing
is one of the most important criteria. Lower seeing improves the spatial resolution of images.
Since the light from the stars is also spread out over fewer pixels, good seeing also improves
the signal-to-noise ratio. Seeing is caused by turbulence in the atmosphere and this has two
causes: heat from the ground rising through the atmosphere and strong winds higher in the
atmosphere (e.g the jet stream).
The atmosphere over the sea tends to be much less turbulent than the atmosphere over land, as the
sea exhibits an essentially smooth, constant-temperature surface compared to the land. Some of
the best astronomical sites are therefore located on small islands in the middle of oceans, such
as Hawaii and the Canaries, as these small land masses cause little additional turbulence. For
the same reason, coastal regions that receive winds predominantly from the direction of the
ocean, such as the western coasts of the Americas and Africa, also exhibit excellent seeing.
The higher the site of an astronomical observatory, the thinner the atmosphere above the
telescope. This reduces atmospheric extinction, and
can also help reduce the seeing if the observatory lies above a turbulent layer in the
atmosphere. High-altitude telescopes are also often above the local inversion layer in the
atmosphere, meaning that local cloud formation occurs below the telescope, significantly
increasing the number of usable nights at the observatory compared to a telescope sited below
the inversion level.
Additional factors, such as the difficulty of supplying utilities like water and power,
accessibility, humidity and wind speed all play a lesser role in locating professional
observatories. For a detailed discussion of many of these factors, see Vik Dhillon's old
Political stability is also a factor in the location of large telescopes. This has been a
particular issue in recent years, with protests from Native Hawaiians causing severe issues for the Thirty
Meter Telescope project's plans to build their telescope on Mauna Kea.
The world's largest telescopes
Figure 30 shows the sites of the worlds largest telescopes. For the reasons
discussed above these are clustered around a few major sites, namely Chile, Hawaii and the
Canary Islands. The gallery below shows photos of these observatories. These sites have low
cloud cover, low light pollution, good seeing and low humidity. They are all at high altitude
and are politically stable.