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Lenses form images by refraction and are typically made of either glass or plastic. They are ground so that their surfaces are either segments of spheres or planes. If a lens is convex or converging, it takes parallel light rays from a distant object and bends them so that they converge to a single point called the focal point. The distance from the lens to the focal point is called the focal length of the lens.
If a lens is concave or diverging, it takes parallel rays and bends them so that they spread out. The rays will then appear to originate from a point in front of the lens. This point is also called the focal point, and its distance is measured in negative units.
The earliest telescopes, as well as many amateur telescopes today, use lenses to gather more light than the human eye could collect on its own. They focus the light and make distant objects appear brighter, clearer and magnified. This type of telescope is called a refracting telescope.
Most refracting telescopes use two main lenses. The largest lens is called the objective lens, and the smaller lens used for viewing is called the eyepiece lens.
The size of an image produced by a lens is proportional to the focal length of the lens. The longer the focal length, the larger the image. The brightness of an image from a telescope depends partly on how much light is collected by the telescope. The light-gathering power of a telescope is directly proportional to the area of the objective lens. The larger the lens, the more light the telescope can gather. Doubling the diameter of the lens increases the light gathering power by a factor of 4. Brightness of images also depends on how big an area the image light is spread over. The smaller the area, the brighter the image.
The magnifying power of a telescope is the ratio of an object's angular diameter to its naked eye diameter. This depends on the focal length of both lenses.
Magnification = focal length of objective lens/focal length of eyepiece lens
A small refracting telescope has an objective of focal length 100 cm. If the eyepiece has a focal length of 4.0 cm, what is the magnification of the telescope?
Magnification = 100/4.0 = 25 (usually written as 25×)
Magnification might seem like the most important aspect of a telescope, but there are limits to how sharp an image a telescope can produce because of the blurring effects of the Earth's atmosphere. Magnifying a blurred image makes it bigger, but not clearer, so the priority when telescopes are built is to have the greatest light-gathering power possible. Gathering more light makes brighter images, and brighter images make it easier to see faint details.
Galileo is credited with being the first person to use a telescope to make observations of the night sky. After hearing of the invention of the telescope in 1608, he built one of his own, called a Gallilean Telescope, in 1609 using a convex objective lens and a concave eyepiece lens. His telescope could magnify objects 3 times. Telescopes he made later magnified objects up to 30 times.
Lenses create a type of image distortion known as chromatic aberration. This occurs because as light passes through a lens, different colors are bent through different angles (like in a prism) and brought to a focus at different points. Because of this, stars viewed through a simple lens are surrounded by rainbow colored halos. This can be corrected for by adding a thin lens of a different kind of glass behind the objective lens.
Lenses present other optical problems including how difficult and expensive it is to make large lenses completely free of defects. Glass also absorbs most ultraviolet light, and visible light is substantially dimmed as it passes through a lens. In addition, lenses in telescopes can only be supported around the outside, so large lenses can sag and distort under their own weight. All of these problems affect image quality and clarity.