Refracting and reflecting are the two main types of telescopes. Refracting telescopes use lenses to collect and focus the light from distant objects. Reflecting telescopes use mirrors to collect and focus this same light. Because of several advantages, astronomers more commonly use reflectors for the large telescopes in observatories. Many higher-quality amateur telescopes also use mirrors instead of lenses.
The mirrors in reflecting telescopes are concave in shape. Such mirrors are curved inward, having a shape similar to a bowl. Different types of concave mirrors are used for telescopes. Spherical, parabolic and hyperbolic mirrors are all common and may be used singly or in combination to produce the desired optical effect.
When light hits a concave mirror, it reflects off the curved surface at an angle. If the concave mirror is built properly, all the light that hits it at any point on the mirrors surface reflects to a single focal point at the center of the image. Parabolic mirrors are often used in telescopes in place of spherical mirrors, as the parabolic mirrors help to avoid the problems associated with light reflected from the edges of the mirror.
The concept of using mirrors for a telescope has existed since just after the invention of the refracting telescope in the early 17th century. While early models did not work well enough to replace the telescopes with lenses, improvements in mirror design and construction led to the first working reflectors by the end of the century. Isaac Newton's 1668 telescope is often considered the first fully functioning reflector. Reflecting telescopes did not gain in popularity until the 19th century, when they began to be the telescopes of choice for professional astronomers.
The size of the mirror in a reflecting telescope determines the amount of light it can gather and therefore how effective the telescope is. Larger mirrors collect more light and so can observe more distant objects in space. The largest telescope mirrors now exceed 8 meters (26.2 feet) in diameter, with the largest single mirror (with a diameter of 8.2 meters or 26.9 feet) found in the Subaru telescope at the Mauna Kea Observatory in Hawaii.
Reflecting telescopes are more common today than refracting telescopes. Construction and maintenance of large mirrors are simpler than the same tasks for large lenses. Mirrors only need to be perfect on the surface, while lenses must have no imperfections throughout. Lenses, especially large lenses, also have a tendency to distort over time as the glass ages. Mirrors therefore have a longer lifespan in a telescope.
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