What is a telescope eyepiece’s field of view?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your practical astronomy ailments.

Close up of eyepiece of telescope with man's hand. Telescope tuning

What is a telescope eyepiece’s field of view?

Steve says: “The body of an eyepiece is normally marked with two important pieces of information; the focal length in millimetres and the apparent field of view (AFOV) in degrees (°).

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The AFOV figure is not, however, the field of view that you will observe through a telescope as this is dependent on the magnification of the telescope-eyepiece combination.

The true field of view identifies how many degrees of the sky are covered by the diameter of the circular view observed through the telescope, and can be determined by this calculation:

  • True field of view = apparent field of view / magnification
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Email your astronomy queries to Steve at scopedoctor@skyatnightmagazine.com and it could get answered in a future issue of BBC Sky at Night Magazine.