A common misconception among those new to astronomy is that magnification is the most important factor when it comes to choosing an astronomical telescope. This is simply NOT true. Telescopes which are advertised on the basis of high magnification are usually of poorer quality and should therefore be viewed with suspicion. While the ability to magnify an image is of course necessary, the most important feature of a telescope is its light gathering ability. The objects we see in the night sky are very distant from us and therefore appear very faint. The primary purpose of the telescope is to gather as much of the incoming light as possible, in order to show these faint objects better.
The measure of a telescope's light gathering ability is given by its aperture size. The aperture size is the diameter of the telescope's primary mirror, or lens. The larger the aperture the brighter the images the telescope will give. (This is why many of the telescopes used in astronomy research are very large, this allows more and more faint objects to be brought into view which would otherwise not be visible in a smaller aperture telescope). It is often stated that the mimimum useful aperture for astonomical purposes is 3" for refracting telescopes, and 6" for reflecting telescopes.
So what role does magnification play? Magnification enlarges the image so that distant objects appear to be closer, thereby allowing us to study them in greater detail. However magnification comes at a price: each time an image is magnified, the already scarce light which was gathered by the telescope is `diluted', so that the image appears dimmer. In addition, the resolving power of a telescope is limited by the quality of the optical components and by a physical effect known as diffraction; so that even if we push up the magnification, no extra detail will be revealed. At very high magnifications images therefore appear blurred and dim.
So what is the maximum usable magnification of a telescope? This depends on many factors, including: the aperture size, the quality of the optical components, the degree of atmospheric disturbance at the time (which causes the image to appear `wavy' at higher magnification - analogous to rising hot air from a radiator), the location above sea-level (if you are at the top of a mountain, the starlight travels through less atmosphere to reach you so that images appear brighter and more steady). As a rule of thumb the maximum useful magnification (under ideal seeing conditions, and excellent optics) is approximately 50x per inch of aperture. This means that the maximum magnification for, say, a 6" reflecting telescope will be 300x.
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