Spectrum Scientifics Blog

Continuing our series of Frequently asked questions we get about telescopes at the store we continue from part 1

“HOW MUCH DOES THIS TELESCOPE MAGNIFY?”

Now this question is more relevant but by no means the most important thing one should ask about one’s telescope.

First of all the answer is “magnification depends on the eyepieces you use”. A telescope’s magnification is determined by dividing it’s mirror or lens’ focal length by the focal length of the eyepiece. So if you have a 25mm eyepiece and a 800mm telescope you will get (800/25 = 32) 32x Power. Most telescopes come with a 25mm and 10mm (but not all) so you can do the math easily enough with each telescope.

That having been said, some folks want to know how much you can crank up the magnification with other eyepieces, barlows and such. A rule of thumb there is to not increase magnification beyond 2x per mm, or 50x per inch of the telescope’s aperture. So a 70mm telescope shouldn’t go above 140x. This is because the image will break down and you will not get any benefit to the increased magnification. Even the largest telescope should not go above 300x.

In truth, in most cases magnification should only be ‘cranked up’ on small, bright objects such as planets or the Moon. Otherwise you can usually stick to low and medium magnifications.

But most of all you should remember critical rule with telescopes: The aperture of the lens of the mirror is more important than the magnification! This is why amateur astronomers are always telling folks to stay away from telescopes that advertise excess magnifications (such as “600X!”). Magnification should be a distant second consideration. It is always more important to gather light before increasing the size of the image.

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Filed under: Telescopes, Uncategorized