Astranto

Practical stargazing guidance, from backyard to dark-sky site.

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Telescope magnification and exit pupil calculator

Enter the focal length of your telescope, the focal length of the eyepiece and the diameter of the objective to get the magnification, the exit pupil and the focal ratio.

The exit pupil is the diameter of the beam of light leaving the eyepiece. Focal length and aperture are given in the specifications of your telescope. A Barlow lens or a focal reducer changes the effective focal length: enter the resulting value.

Enter your values, then calculate.

Method and limitations

Magnification = telescope focal length ÷ eyepiece focal length, both in millimetres. Exit pupil (mm) = objective diameter ÷ magnification, which is also the eyepiece focal length ÷ the focal ratio. Focal ratio = telescope focal length ÷ objective diameter, written f/…. The magnification is shown to the nearest whole number, the exit pupil and the focal ratio to one decimal place.

Sources

No external reference is documented for this calculation; its method and limitations are described above.