…continued10 Top Telescope Questions
The easiest way is look in the scope’s manual. The magnification of your telescope will almost certainly be printed somewhere in the instructions. But if your scope didn’t come with a manual, or if you’ve gotten a new eyepiece, you can run the numbers yourself.
Simply take the focal length of your telescope’s objective (its main mirror or lens) and divide it by the focal length of your eyepiece. For example, if your telescope has a 1,200-mm focal length and you’re using a 25-mm eyepiece, the magnification is 48× (1,200 ÷ 25). To change the magnification or “power,” change eyepieces: put in a 10-mm eyepiece, and the magnification becomes 120× (1,200 ÷ 10).
If you want to calculate them right now, here's a handy calculator:
Usually the focal length of your objective is printed on a sticker somewhere on the telescope (look for FL or F), while the focal length of the eyepiece is marked on its barrel, near the lens you look through. You’ll find both values given in millimeters (mm).