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Reflectors And Refractors
"Telescope" is a Greek word meaning "far-seeing."
The two basic telescope types are refractors, which use lenses to collect and focus light, and reflectors, known as a "Newtonian" after its inventor, which collect light using a curved mirror.  Purpose Of Telescope
The primary purpose of a telescope is as an instrument which uses a large lens or mirror (aperture) to gather light and make very dim objects visible.
A larger aperture will allow more light to be collected for a greater detailed image.
Aperture is measured in millimeters or inches.
The secondary purpose of a telescope is as a magnifying unit which uses the eyepiece as a microscope to focus and magnify the image formed by the aperture. Most telescopes offer three changeable eyepieces offering low (under 50x), medium (50x to 150x) and high powers (over 150x).  |
Night Sky Contains Many Large Objects
When starting out in star gazing, many people fail to appreciate the importance of aperture compared to magnification, as they don't realise that many objects in the night sky are actually quite large and don't require much magnification.
The Orion Nebula, for instance, appears larger than the Moon, globular clusters are often around half the size of the Moon, and the Andromeda Galaxy (M31) would appear 6 times the width of the Moon, if it was visible to us with the naked eye.
Therefore, a low-power eyepiece may be all you need for observing such large objects, whereas a high power would be more useful for viewing much smaller objects, such as planets. Specifications
The human eye has a pupil opening of around 7mm in diameter, and is similar to a small telescope with magnification of 1x and an aperture of 7mm (0.276").
The mimimum useful aperture for astronomy purposes is 3" (80mm) for refracting telescopes, and 6" (150mm) for reflecting telescopes.
The telescope's magnification is restricted by its aperture beyond which sharp images cannot form. The maximum useful magnification for a telescope is approximately:
50x per inch of telescope aperture, (300x for a 6" reflecting telescope), or 2.5x per mm of aperture ( e.g. 200x for a 80mm refracting telescope)
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Telescopes |
