Optical Telescopes

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Research telescopes that use ordinary (optical) light are basically light funnels. Their purpose is not so much to make things look bigger or closer, but to make them look brighter. Many astronomical objects are big enough to see with the naked eye; they just aren't bright enough. Therefore the most important feature of an optical telescope is the size of the area over which light is collected - the bigger the size, the more photons are collected, the brighter the image becomes.

The first astronomical telescope was designed and used by Galileo Galilei. He used two lenses, one an objective lens to focus the light, and one an eyepiece. Such a telescope is called a refracting telescope.

An alternative was designed by Isaac Newton, which uses a mirror to bring the light to a focus. Such telescopes are called reflecting telescopes.

There are problems with both kinds of telescope. Refractors have the difficulty that lenses refract different wavelengths of light by different amounts, so the focus is at a different point for blue light than for red light—this is called chromatic aberration. Also, the objective lens must be held by the edges. This means that bigger more powerful lenses become difficult to deal with, since a huge piece of glass has to be held by a very thin edge.

Almost all astronomical research telescopes are now reflecting telescopes. These can be supported all across the back of the primary mirror, and this has many advantages as the mirrors get large. The problem with a reflector is where to put your head (or photographic plate or detector). If you stare down at the mirror, you get in the way of the light coming from the star. The solution is generally to put another smaller "secondary" mirror in front of the primary mirror, to deflect the light to some other position where detectors can be placed. Various configurations of secondary and additional mirrors are in general use, such as prime, cassegrain and nasmyth focii. Very big telescopes sometimes use a prime focus cage where the secondary mirror ordinarily sits—the observer (or instrument) then rides around in this cage at the front end of the telescope while observations are being carried out.

For many years the biggest reflecting telescope was the Palomar 200" (5m) telescope, a marvel of mid-20th century engineering. Within the past decade, technical advances have made it possible to make bigger mirrors. While the 200" is still in active use, there are now many larger telescopes in use or under construction. Modern telescopes also have improvements in resolution and field of view that make them more powerful than the 200" and its smaller contemporaries. One of the first examples of a new technology telescope was the WIYN 3.5m telescope at Kitt Peak, Arizona, completed in 1994.

Interestingly, the famous Hubble Space Telescope (HST) is not a particularly large telescope—it has only a 2.4m mirror, about half the diameter of the Palomar 200", and a quarter that of the Keck telescopes. The advantage of the HST is that it is above the atmosphere, and therefore does not have degraded resolution due to the Earth's atmosphere. Shortly after launch, HST was found to have a problem with the primary mirror which reduced its resolution considerable. After this problem was solved, the resolution (measured by a quantity known as seeing) was about 10-15 times better than a typical ground-based telescope. It is the high resolution that allows HST to create such spectacular images. But there are many astronomical projects for which Hubble is simply too small—for these projects, astronomers continue to rely on ground-based optical telescopes.

<< back to Telescopes

• Explore the biographies of Newton and Galileo

• Read more on chromatic aberration, and how it can be minimized

• The biggest refractor ever successfully used for astronomical observations was the 72" refractor built by Lord Rosse at Birr Castle in the 19th century.

• See a comparison between HST and ground-based observations.

• New developments in adaptive optics have enable ground-based telescopes to produce images almost as good as HST under certain conditions.