| from The Backyard Astronomer’s Guide, by Terence Dickinson and Alan Dyer
Some amateur astronomers and telescope dealers insist that the most important characteristic of an instrument is its aperture. They are right, up to a point. Larger telescopes do generally provide brighter, sharper images.
But what about magnification? We have ignored it, and so should you. The magnification of a telescope is a meaningless specification. With the right eyepiece, any telescope can magnify hundreds of times. The question is How does the image look at, say, 450x? Probably either very faint or blurry. Why? There are two reasons:
Not Enough Light: The telescope simply is not collecting enough light to allow the image to be magnified to that extent. When an image is enlarged, it is spread out over a greater area and becomes too faint to be useful. The telescope has been pushed beyond its limits. The only recourse is to move to a bigger telescope. The brighter images in larger telescopes will allow higher powers, theoretically. Because of the following, however, it does not always work that way in practice.
Blurry Atmosphere: The Earth’s atmosphere is always in motion, distorting the view through the telescope. Some nights are worse than others. At low power, the effect is usually not noticeable. But at high power, it can blur the image badly. Increasing the magnification only makes things worse; it becomes impossible to see any more detail. Instead, the image becomes fuzzier and fainter. Since big telescopes have to look through a larger column of air than do small telescopes, they are often more affected by atmospheric turbulence (astronomers call the condition “poor seeing”).
Most amateur astronomers find that about 300x is the practical upper limit for any size telescope, despite advertising claims to the contrary. In fact, a telescope touted solely on the basis of its magnification is surely a lemon advertised by a company that does not know what constitutes a good instrument.
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