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in F r e i s t i l 10.10.2018 08:18
von xuezhiqian123 • 2.598 Beiträge

There are two primary types of telescopes: refracting telescopes (which use lenses) and reflecting telescopes (which use mirrors).

Refracting telescopes use a spherical or parabolic concave primary mirror to reflect incoming (parallel) rays of light from astronomical objects to a focus. Since this focus sits in the path of the incoming light Cheap Carlos Dunlap Jersey , reflecting telescopes have to use one of several different strategies to get the image formed at this focus to the observer.

Isaac Newton developed the first reflecting telescope in 1668, which used a flat secondary mirror, located in front of the primary mirror and set at an angle, to reflect light into an eye piece. Telescopes following this simple design Cheap Tyler Eifert Jersey , now known as Newtonian telescopes, are popular with amateur astronomers today.

Another design for a reflecting telescope was published in 1672 by Laurent Cassegrain. This telescope used a primary concave mirror and a smaller secondary convex mirror to reflect light onto the secondary, though a hole in the primary mirror and onto an eyepiece. This design allows for a shorter telescope tube relative to the diameter of its mirror. Telescopes following this design, known as Cassegrain telescopes Cheap Jeremy Hill Jersey , are used by amateur and professional astronomers today.

While the first telescopes, including the telescope used by Galileo to discover Jupiter’s moons, were refracting telescopes, most large telescopes used by amateur astronomers and virtually all research telescopes used by professional astronomers are reflecting telescopes. While both kinds of telescopes can collect and focus light from distant objects Cheap Geno Atkins Jersey , mirrors have several advantages over lenses when used in telescopes.

1) Chromatic aberration: As light passes through glass at an angle, different wavelengths (colours) of light will bend at different angles. This is useful when you are trying to separate the colours of light with a prism, but less useful when you are trying to focus a source of white light to a single point. All glass lenses will exhibit chromatic aberration — blurry fringes of different colours at the edges of an object. While careful design can reduce chromatic aberration, it cannot eliminate it.

The angle that light reflects off of a mirror does not depend on its wavelength (colour) Cheap Andy Dalton Jersey , so reflecting telescopes focus all colours of light to the same point. Thus, they do not experience chromatic aberration.

2) Very large mirrors are easier to manufacture than very large lenses. In addition, to be used in astronomy, lenses have to be perfect throughout. Mirrors only have to be perfect and polished on one side.

3) Since the primary mirror is mounted at the end of a telescope Cheap John Ross Jersey , it can be supported along its base. Lenses used in refracting telescopes can only be supported on their sides. Thus, extremely large lenses (larger than about 1 m) tend to sag under the effects of gravity, distorting the image. This is not a problem for large mirrors.

Why are large mirrors so important? While many people think that the primary purpose of a telescope is to magnify astronomical objects, this is not the case. While telescopes may magnify objects to some extent Cheap A.J. Green Jersey , their primary purpose is to act as “light buckets” — to collect as many photons from a distant object as possible. Telescopes also allow observers to resolve small details and make sharp images. Both the light collecting ability and the resolving power of the telescope depends on the size of the mirror — in general, the larger the mirror, the more powerful the telescope. How Many LED Downlights Are Required For A Greater Lightening Effect? Sherrie Niblock
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