Telescope magnification

I get asked a lot of questions from a lot of people. All of these questions are good, but some are so good they get asked a lot. This is one of those questions.

The magnification of a telescope is how much bigger your target looks compared to looking at it without the telescope. If you like, it's how much closer the target is going to look. Magnification is expressed in multiples, so if your telescope has a magnification of "50 times", it means that whatever you're looking at will look 50 times closer, or will appear 50 times bigger than without the scope.

Remember that your telescope can have several eyepieces. Changing eyepieces changes the magnification.

So, for watching the Moon, your 25mm eyepiece might give you just the view you want. However, if you want to look at Jupiter, all you get with a 25 is a dot, so you need more magnification. The 10mm eyepiece will give you a much closer image.

But to calculate it, there's some mathematics involved.

Your telescope, regardless of eyepiece, has a defined "focal length". You're likely to see this reported as, say, "F=650" on a sticker on the side of the scope. Typically, refractors or reflectors have focal lengths between 500mm and 1000mm, with Dobsonians being a bit longer, say, 1200mm. Cassegrains and Maksutovs (the ones with mirrors at both ends) can be much longer, up to 2800mm or even more for the giants.

Your magnification depends on both the focal length of your telescope and the focal length of the eyepiece you're using. In fact, the magnification is the ratio of the two.

So if you've got a scope with a 900mm focal length, and you're using a 25mm eyepiece, your magnification is 900/25=36 times. Change to the 10mm eyepiece and the magnification becomes 900/10=90 times.

If nothing else, this explains why the higher number eyepieces have lower magnification!

I know you're itching to ask about how to calculate magnification for photography. Err, it's complicated. I'll get to that another time.