Sky Talk September 2009: Telescope “Power”

Whenever the word “telescope” is mentioned to most people, it invariably and immediately elicits the question “How powerful is it?” But, in fact, there are really three different kinds of “power” involved and the one typically referred to — magnification — is actually the least important of the three!

The primary reason astronomers continue to build ever-larger telescopes is to collect more light from distant parts of the universe. This function is known as light-gathering power and it’s just as important to amateur stargazers as to professional ones. The bigger the telescope, the more light it collects and the brighter the image seen. And it’s not a linear relationship — when you double the size of the aperture, you quadruple the amount of light it collects since the area of the objective lens or primary mirror goes up as the square of the aperture! Thus, the images in a 6-inch scope are four times as intense as in a 3-inch one.

Second in importance is the ability to see fine detail in an image, which is known as resolving power. Given equally good optics, the bigger the scope the sharper the image it shows (assuming steady atmospheric conditions). And in this case the relationship is linear — doubling the size doubles the ability to resolve fine detail on the Moon and planets, split double stars, and resolve deep-sky wonders like tightly-bound globular clusters.

Finally, we come to magnifying power. Making objects bigger is not even a consideration in most cases to professional astronomers. But it’s quite definitely uppermost in the mind of the typical beginning stargazer! And it is important in its own right, for an image needs to be magnified in order to make it big enough to see the detail it contains (full resolution requiring what’s technically known as the resolving magnification, which is around 25X per inch of aperture). But novices have a tendency to overpower their telescopes, thanks largely to misleading advertising by low-end telescope manufacturers. The rule of thumb is that the useful magnification range is 7X (and as low as 4X for short-focus scopes like the Edmund Astroscan Plus #30050-01) to a maximum of 50X per inch of aperture. That’s 28X to 200X on a 4-inch glass. The higher powers can certainly provide spectacular close-ups of bright objects like the Moon and planets, but require steady nights of good atmospheric “seeing” to do so. Note also that the lower the power, the wider, brighter and sharper will be the views. Observers typically equip their scopes with a range of low, medium and high power eyepieces. But for all practical purposes, a magnification of 30X to 50X will show you just about everything you could ever want to see in the heavens. (Be sure to browse Edmund’s Telescope category to see what is available.)

The author has logged over 20,000 hours of stargazing time during the past 50 years. Some of this has involved using huge observatory instruments capable of reaching magnifications in excess of 3,000X! And yet, virtually all of my observing has been — and continues to be — conducted with powers of 15X to 100X on scopes 2.4- (60mm) to 8-inches in aperture.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.


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