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Telescope Calculations

From just four pieces of information you can enter below we can calculate, for a given telescope with a particular eyepiece:
  1. Telescope Ratio or f number: This describes the relation between the aperture and focal length of the telescope
  2. Magnification: How many times bigger the object will appear
  3. True Field of View: The area of sky seen through your telescope/eyepiece combination
  4. Exit Pupil: The diameter of the cone of light coming out of the eyepiece

You can find out more about these values and what they mean in the E-course
Basic Astronomy with a Telescope
Telescope Information Eyepiece Information
Focal Length of Telescope in mm:

Diameter of lens or mirror (aperture) in mm:

Focal Length of Eyepiece in mm:

Apparent Field of View in degrees:

Please enter numbers only (no units) into the boxes above.