Often times you will see telescopes for sale that claim big magnifications such as up to 1000X power or 500X power. And while this isn’t a false claim because the instrument in reference will actually go to that magnification it is a worthless number because telescopes have definite limits to their useful magnification. This article explains the rule of thumb for how high a power you can get from typical telescopes.
What do I mean by limits in the magnification? Telescopes are sophisticated optical systems. They have to very carefully manipulate light without distortion. Did you ever look through a really inexpensive pair of plastic binoculars or a really inexpensive magnifying glass? Notice how the image is of a very low quality and seems to be distorted? Nevertheless, the low quality is still acceptable because you are only magnifying by 3 or 5 or maybe 10. So it isn’t distorted too much and the image is still recognizable. But what about if you were magnifying by 500? Not just the image, but also the imperfections and distortions in the plastic or glass would be magnified by 500, and the image that you see would be useless to you.
Not only that but if you are using a telescope with a magnification of 500 its movement is magnified 500 times! Which means that even the slightest, and I mean slightest movement, would turn into an uncontrollable shaking of the image which would also render it unusable. You have probably seen this phenomenon with a video camera and how shaky the image gets when you zoom into things. Yet your video camera probably only zooms by 5 or 20. Imagine the shakiness when you zoom in by 500? Thirdly you also have the problem of the atmosphere around the earth! The swirling atmosphere is a definite issue for consideration because any swirling action in the air is also magnified!
So what do we do about this magnification problem?Well, One of the things we do is to build telescopes with superb optics. The better the optics the more magnification power you can get out of the scope before unacceptable distortion occurs. Better optics mean higher magnification.
And secondly we combat this problem of magnification by building sturdier mounts and tripods. If the mount or tripod is very sturdy and very solid there will be significantly less shaking of the image at higher powers. But sturdier mounts are more expensive and this is a consideration when making a purchase.
Okay, So how much magnification can telescopes typically go to? There is a general rule of thumb called the sixty rule. And it gives you the upper limit of the scope as determined by its diameter times 60. You take the diameter of the telescope in inches and multiply it by 60 and this will give you the upper magnification limit. So, if you have a 6″ telescope you multiply 6 by 60 and you get 360. The upper limit would be 360 power. And this upper limit pretty much is assuming you have an extraordinarily calm and clear night sky. So it is the rough upper limit.
When purchasing a telescope or eyepieces for a telescope you just need to remember that higher powers don’t necessarily mean better because there is a definite limit to the useful magnification of the instrument. Just remember that the typical rule of thumb is sixty times the diameter of the scope and you will have a number that works well for most instruments.