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The telescope is an important astronomical tool that gathers and focuses electromagnetic radiation. Telescopes serve to both increase the angular size and the brightness of objects. When we speak about telescopes we are usually referring to optical telescopes, but many other types of telescopes also exist for other spectrum of electromagnetic radiation. It is unclear as to who it was that actually invented the telescope but Galileo Galilei is credited as being the first to use a telescope for astronomical purposes in 1609.

Most people believe that the only thing a telescope does is magnify images. The problem is that the more you magnify an image the darker and blurrier it will become. For this reason, a telescope must also brighten and clear up an image in order to be useful. Telescopes have three basic functions. The first, and most important function of a telescope, is to gather light. By gathering light a telescope is able to make feint objects brighter and to make objects visible that are so feint that they are not visible to the human eye without a telescope. The telescope collects the light and concentrates it at a focus, the difference between the brightness when viewed through a telescope and the brightness as seen by the human eye is the telescopes light-gathering power. In general, the larger a telescope is the more light it will be able to gather, and the brighter objects will appear (How Does a Telescope Work?).

The second function of a telescope is to resolve fine detail, also known as angular resolution. Angular resolution refers to the telescope's ability to make objects in the heavens appear sharper and to separate objects that are close together in the sky. Angular resolution is usually expressed in terms of the minimum angle between two points that can be clearly separated. If the resolution of a telescope is greater than the angular separation of two stars it will be possible to view two separate and distinct objects. If the resolution of the telescope is less than or equal to the angular separation of two stars, the object will appear as a single image. The following sketch can help to better understand the angular resolution of a telescope. The angular resolution of the telescope is shaded. If the separation of the stars is equal or greater than this angular resolution, the stars will appear clearly as two stars. If the separation of the stars is anything less than this angular resolution the two stars will appear as one object (How Does a Telescope Work?).

The third, and final, function of a telescope is to magnify. This function is generally performed by the eyepiece at the end of the telescope rather than by the internal mirrors or lenses. Before an image reaches the eyepiece it is made into a small, clear, and bright image. The eyepiece then takes this image and magnifies it slightly in order for it to fit our pupil. When the image is magnified we lose some of the resolution and brightness, the amount the image is enlarged by compared to how much it is reduced in order to make it clear is very small though. If a telescope has a small lens and advertises a large magnification, it is likely that the image produced will be of poor quality. If a telescope has a larger lens it is able to magnify images to a much greater size while maintaining most of its clarity (How Does a Telescope Work).

There are three types of telescopes: refracting telescopes, reflecting telescopes, and catadioptric telescopes. The oldest, and simplest, form of the telescope is the refracting telescope. The refractor consists of a curved lens at one end known as the objective lens. This lens is used to bend the light towards a focal point towards the other end of the telescope. The eyepiece is just beyond the focus and allows for visual examination of the image. Advantages of refractor telescopes are that it is small and compact, requires little maintenance, has very high resolution, and it can be used during the day for terrestrial viewing without the object being flipped upside down. One disadvantage of this type of telescope is that as light is bent toward the focal point, each color is bent at a slightly different angle. If the viewer focuses on one color, it will bring the other colors out of focus causing a halo of color to form around the object being viewed. Other disadvantages of refractor telescopes include high prices for larger models, low light gather power, and they are awkward to use when pointing straight up, which is the best way to use a telescope (Telescopes).

Reflecting Telescopes gather light using a curved mirror. A parabolic mirror is first used to reflect the light back toward the object. A smaller mirror then deflects the light toward a focal point outside of the light path where it can be viewed. The most popular form of the reflector telescope is the Newtonian model, named after its inventor, Isaac Newton. The Newtonian telescope deflects the image through the side of the telescope off of a secondary mirror placed at a 45 degree angle toward the eyepiece (History of Telescopes).

Reflector telescopes have both advantages



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