Often times you will see telescopes for sale that claim big magnifications such as up to 1000X power or 500X power telescope. And while this isn't a false claim because the telescope in reference will actually go to that magnification it is a worthless number because telescopes have definite limits to their magnification.
Need the formula to determine magnification for a telescope? It's near the bottom of this page.
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 an 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 qualilty is still acceptable because you are only magnifying by 3 or 5 or maybe 10. So it isn't distorted too much.
But what about if you were magnifying by 500? Not just the image but also the imperfections and distortions would be magnified by 500 and the image that you see would be useless to you.
Not only that but if you are holding a telescope with a magnification of 500 the movement of the telescope 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.
Thirdly you also have the problem of the atmosphere around the earth! Yup, the swirling atmosphere is a definite issue for telescopes to deal with and 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 optics with superb optics. The better the optics the more magnification power you can get out of the telescope before unacceptable distortion occurs.
And secondly we combat this problem of magnification by building sturdier mounts. If a telescope is built on a very sturdy mount that is solid and doesn't shake it will allow you to achieve higher magnification with your scope.
But these things cost money! Better optics and sturdier mounts cost more money to make! And this is part of the reason why telescopes can get expensive.
Okay, So how much magnification can telescopes typically go to?
There is a general rule of thumb when it comes to telescopes and I call it the 60 rule. And it applies to the diameter of the telescope.
You take the diameter of the telescope in inches and multiply it by 60 and this will give you the upper magnification limit of the telescope.
So, if you have a 6" telescope you multiply 6 by 60 and you get 360 so the upper limit of the telescope 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.
A couple of final thoughts about magnification and telescopes
You shouldn't be disappointed by the fact that your telescope won't magnify things by thousands of times. The magnification isn't really as important as people think. The big thing about the telescope is the light gathering power of it. Think of it in relation to your eyeball.
Your eyeball is a certain size and your telescope is how much bigger? See what I mean. The telescope is so much more bigger than your eye and it gathers so much more light. It is really quite an amazing thing. So light gathering power is much more important than magnification.
The Formula for Determining Magnification of a Telescope
This is an easy formula The magnification of a telescope is determined by dividing the focal length of the telescope by the focal length of the eyepiece. or Magnification = (objective focal length) / (eyepiece focal length)
Understanding the magnification limit of a telescope is also important when it comes to buying eyepieces for your telescope. Each sized eyepiece will give you a very specific magnification power on your scope so you want to calculate what powers are acceptable and then buy the eyepieces accordingly. If you have a 6" scope where the upper limit is around 360 you don't want to buy an eyepiece that gives you a magnification of 700. It would be useless.
This issue of magnification and eyepieces is a topic all to itself and I will be posting an article about calculating the magnification of eyepieces soon.