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How to Calculate Telescope Magnification

Whether you're gazing at the moon, Saturn, the Orion Nebula or a distant object, the magnification of the telescope plays an important role in your experience. Calculating magnification is relatively easy, and it can help in decision-making when purchasing and using your telescope and eyepieces.

Things You'll Need

Focal length of the telescope
Focal length of the eyepiece
Calculator or pencil and paper

Instructions

- 1
  Find the focal length of the telescope. Focal length is the distance light travels from the telescope's lens or primary mirror to where it is focused as an image. You can find this number on the telescope or in the accompanying manual.
- 2
  Find the focal length of the eyepiece. That number, usually in millimeters (mm), is located on the eyepiece.
- 3
  Divide the focal length of the eyepiece into the focal length of the telescope. For instance, if the telescope's focal length is 1000mm and the eyepiece's focal length is 25mm, then the magnification is 1000/25, or 40x.

Astronomy