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How to Calculate the Focal Length on a Telescope

The focal length of a telescope is the distance between the lens and the focus. The focus of a refracting telescope which uses lenses, it is behind the lens. In a reflecting telescope which uses a mirror, it is in front of the reflecting mirror. Focal length can be calculated using the f/ratio and the aperture (diameter of the lens). So, to calculate the focal length you would need to know both of these numbers. If you know the focal length of the lens and the magnification of the telescope, you could also find the focal length of the telescope using those numbers. These specifications can be found in the materials that come with your telescope and may be written on the side of the scope.

Things You'll Need

Calculator
Telescope manual

Instructions

Calculating Focal Length Using the F/Ratio
- 1
  Look in the manual for your telescope. If the manual is missing, consult the manufacturer's website.
- 2
  Find the number for the f/ratio (or f/number).
- 3
  Find the number for the aperture of the lens.
- 4
  Multiply the f/ratio by the aperture of the lens. This will give you focal length.
Calculating Focal Length Using Magnification
- 5
  Look in the manual for your telescope. If the manual is missing, consult the manufacturer's website.
- 6
  Find the magnification of the telescope.
- 7
  Find the number for the focal length of the eyepiece.
- 8
  Multiply the focal length of the eyepiece by the magnification. This will give you the focal length of the telescope.

Astronomy