Telescope is a Greek word meaning “far-seeing”. There are two basic telescope types, namely refractors and reflectors. Refractors use lenses to collect and focus light. Meanwhile, reflectors, known as a “Newtonian” after its inventor, collect light using a curved mirror.
Reflectors And Refractors
For amateur astronomy purposes, the minimum useful aperture for a refracting telescopes is 3″ (80mm). And for a reflecting telescope 4″ (100mm) is a good entry point. Before buying a telescope, though, you might want to first read this article to help decide which telescope best suits your stargazing purposes.
Purpose Of Telescopes
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. Therefore, a larger aperture will allow more light to be collected for a greater detailed image.
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 magnification eyepieces offering low (under 50x), medium (50x to 150x) and high powers (over 150x). The human eye is similar to a small telescope with a magnification of 1x and an aperture of 7mm (pupil).
Night Sky Contains Many Large Objects
When starting out in stargazing, many people fail to appreciate the importance of aperture compared to magnification. What they fail to realize is that many objects in the night sky are actually large and don’t require much magnification. The Orion Nebula, for instance, appears larger than the Moon. Likewise, 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. By contrast, high power would be more useful for viewing much smaller objects, such as planets. A 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 (2.5mm) of telescope aperture. Or 300x for a 6″ reflecting telescope, and 200x for a 80mm refracting telescope.