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How to Figure the Magnification Power of a Telescope

A basic telescope comprises two lenses, the main lens and the eyepiece lens. The main lens or primary mirror focuses a large area of light to a small point. The eyepiece captures that light and changes it to a size suitable for the human eye. The curvature, or focal length, of the two lenses determines a telescope's magnification power. It is important to find a telescope's maximum "useful" magnification before making a purchase. The width of the main lens determines useful magnification.

Instructions

- 1
  Find the focal length of the main lens or primary mirror. This length is a measure of the distance between the lens or mirror and the point where it brings light rays together. It usually is measured in millimeters (mm); you can find it on a label affixed to the telescope tube.
- 2
  Find the focal length of the eyepiece. You can usually find this number on the front of the eyepiece, in millimeters.
- 3
  Divide the focal length of the main lens by the focal length of the eyepiece to find the standard magnification of the telescope. For example, a telescope with a focal length of 500 mm for the main lens and 5 mm for the eyepiece has a standard magnification power of 100x.
- 4
  Check if the telescope has an amplifying lens such as a "Barlow Lens." These lenses increase the magnification by a set factor. An amplification lens marked "2x" will double the magnification power of the telescope.
- 5
  Determine the telescope's maximum useful magnification. No matter how powerful a telescope may be, you will not be able to see anything at long distances if there is not enough light coming from the main lens or primary mirror. A larger aperture allows more light to get through. As a general rule, you get about 50x of useful magnification for every inch in diameter of the main lens or mirror. A telescope with a 4-inch main lens has a maximum useful magnification of 200x, even though the box says 500x magnification.

Astronomy