Telescopes - research into types and properties of telescopes

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Telescopes are instruments that magnify distant objects. Astronomers use telescopes to study the planets, stars, and other floating bodies. In most telescopes a lens or mirror is used to form an image of an object. The image may be viewed through an eyepiece or recorded on photographic film or by electronic devices. Telescopes produce images of objects too far away to be seen by the unaided human eye. The Dutch optician Hans Lippershey designed the first telescope in 1608, when he mounted two glass lenses in a narrow tube. Within a year the Italian astronomer Galileo built a similar device and became the first person to use a telescope to study the sky.

Optical Telescopes

Optical Telescopes use a lens or mirror to collect and focus light waves. There are three main types of optical telescopes.

Refracting telescope

Refracting telescopes also known as refractors have a large lens called an objective lens at one end of a long, narrow tube. The lens is convex on both sides so that the middle of the lens is thicker than the edges. The glass slows the light rays as they pass through the lens. (A wave is slowed most in the middle of the lens where the glass is thickest). The lens therefore causes the entire crest of the wave to arrive at the focus at the same time. Refractors with a magnifying eyepiece invert the image so that it appears upside down as astronomical observations do not require an upright image.    

A ray diagram of a Refracting Telescope

 

Refracting telescopes do not have any small mirrors in their light path so they provide very sharp images. They are excellent for lunar, planetary and binary star observing especially in larger apertures.   They produce high contrast images with no obstructions.

However all refractor telescopes suffer from chromatic aberration. A production of rainbow colours around an Image. (Because of the wave nature of light, the longer wavelength light (redder colours) are bent less than the shorter wavelength light (blue colours) as they pass through the lens).

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There are a couple of ways to reduce chromatic aberration. One is to use multiple lenses to counteract this effect. The second uses a long focal length to minimise it. This is why early refracting telescopes were made very long.

Chromatic aberration caused by Refracting telescopes.

Reflecting Telescopes

Reflecting telescopes, also knows as reflectors, use bowl shaped mirrors instead of lenses. The mirror called the primary mirror has a surface shaped so that any line across the centre of the mirror is a parabola. A mirror with that shape, called a parabolic mirror (Plano concave) reflects light rays ...

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