When we think of telescopes we conjure up familiar images of old sea dogs scanning the horizon or astronomers in hilltop observatories gazing at the stars, but what actually ARE telescopes and how do they work?
A telescope is simply an optical instrument that makes distant objects seem nearer. It does this by using a lens or mirror to gather the light reflected by those distant objects and bring it to a focus. A second lens is then used to magnify the focused light and this magnified image is what our eye sees; We see the close-up image.
There are two main types of telescope: Refractors and reflectors.
The refracting telescope was the first to be invented and is still the more familiar type. A simple refractor consists of a tube with a large convex (thicker in the middle) lens - called the objective lens - at one end, and a smaller magnifying lens - called the eyepiece - at the other. The objective lens collects the light reflected by a distant object and focuses this light to form an 'image' inside the tube. This image is then magnified by the eyepiece. The distance between the eyepiece and the objective lens can be varied (or focused) to achieve a sharp image. If only two lenses are used the image will appear upside down to the viewer, so additional lenses called 'erecting' lenses are usually added to the tube to ensure the image is upright.
A disadvantage of refracting telescopes is that the lenses are susceptible to light distortions and aberrations. This distortion problem led to the development of the second main type of telescope: The reflector.
In a reflecting telescope the light enters the tube and hits a large concave mirror situated at the far end. This mirror collects the light and reflects it back onto a smaller flat mirror situated further up the tube nearer the entry which in turn reflects the light 90 degrees towards a magnifying eyepiece on the side of the tube.
Reflector telescopes are less problematical than refractors because they are less susceptible to light distortion. Nowadays most large telescopes are reflectors.
The first practical reflecting telescope was constructed in the late 17th century by Sir Isaac Newton and that is why they are often called Newtonian reflectors.