Types of Ground-Based Telescopes

History

• Historically, there have been two main types of ground-based telescopes: reflectors and refractors. The earliest telescopes were the refractors, which use lenses to magnify and focus the light from distant objects. Excellent for smaller telescopes, these refractors were less useful in large telescopes, in which the glass lenses tended to warp. Accordingly, most larger telescopes are reflectors that use curved mirrors to magnify and focus light.

Types

• There are four main types of ground-based telescopes. The most common are the small and portable telescopes. Astronomers can set up these telescopes in any location deemed suitable for observing, and the relatively-low costs of such telescopes make them accessible even to amateur astronomers. Larger optical telescopes are permanently mounted in observatories. These telescopes can be extremely large, with mirrors as wide as eight meters (26.2 feet) in diameter. Generally, these telescopes are housed in domed structures and are connected to electronic instruments like CCD cameras. Telescope arrays consist of multiple telescopes arranged far apart. The images from all telescopes are collected and combined to make a more precise single image. Radio telescopes are dish-shaped and detect the radio frequencies not visible to optical telescopes.

Size

• Portable telescopes necessarily remain small, with lenses and mirrors measured in inches. Observatory telescopes tend to contain mirrors from half a meter in diameter to as large as eight meters. The largest single optical telescope is the Gran Telescopio Canarias in the Canary Islands, with a segmented mirror of 10.4 meters (34 feet). The telescopes in arrays can be as close together as meters apart or as separate as hundreds of kilometers. Radio telescopes range in size from those that resemble large satellite dishes to dishes that can fill entire valleys. The Arecibo telescope in Puerto Rico is, at 305 meters (1001 feet) in diameter, the largest single radio telescope.

Geography

• Although there are some ground-based telescopes in cities, most telescopes are located in remote locations with little light pollution. High altitude locations like mountain-tops are favored for their thinner atmosphere, darkness and more constant atmosphere. Deserts are favored for similar reasons and have the added advantage of low levels of cloud cover. Major telescope arrays include the Very Large Array in New Mexico and the Very Long Baseline Array in 10 locations across North America.

Benefits

• Ground-based telescopes are extremely useful to astronomers. The first and most important benefit is that of proximity. Unlike the space telescopes, which must be controlled remotely and are difficult to adjust as a result, ground-based telescopes are immediately accessible. Any errors in positioning, breakages or mechanical defects can be swiftly and easily repaired.