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Refractors magnified view of the heavenly bodies by refracting or bending the path of light as it passed through the lenses. The early refractors suffered from several defects that blur the image.
Lens-type or refracting telescopes are widely used by amateur astronomers
today because they are small and able to give excellent results.
A refractor uses two lenses (an objective and eyepiece) to form an image. The objective is a converging lens that has a long focal length that gathers lights from distant objects and forms an image. The eyepiece that is also converging lens, views and magnifies the image. The object is mounted at the front of the main telescope tube while the eyepiece is mounted in a tube that can slide in and out of the main one to focus the image.
Refractors are difficult to make in large sizes because of the way they
are made. They can only be supported on the edge. Large lenses are heavy
and difficult to mount in this way without being distorted. This is why
the largest refractor, at Yerkes Observatory in Wisconsin USA has a lens
only 100cm across.
How refractor works
Light usually travels in a straight line, but it can be bent or "refracted"
by passing it through substances of differing densities. This laser beam
above seems to bend as it is directed at a rectangular-shaped container
of water because the light is passing through three different media --
water, glass, and air.
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Disadvantage
It has two main optical defects or aberrations that caused blurred images. One is spherical aberration that results from the lens having slightly the wrong curvature. Grinding the lens into shape with the utmost accuracy can reduce this.
The other defect is chromatic (colour) aberration. This is a colour blurring of the image caused by lens refracting light of different wavelengths (colour) by slightly different amounts. It can be corrected by using achromatic lenses.
How a lens magnifies
When a convex lens is held between the eye and an object, the object appears larger because the lens bends the rays of light inwards. The eye naturally traces the rays of light back towards the object in straight lines. It sees a "virtual image, which is larger than the original image. The degree of magnification depends on the angles formed by the curvature of the lens.
