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Calculators for Telescopes

Use the Telescope Nights calculators for quickly working out metrics for telescopes:

Focal ratio (f/)

How to calculate telescope magnification

How to calculate the magnifying power of a telescope? Telescope magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece and multiplying the result by the magnitude of the Barlow lens (if using). Or, you can simply use the following tool to calculate telescope magnification. It’ll guide you on the usefulness of the magnification also.


Telescope focal Length

True field of view

The true field of view (measured in degrees) is how much of the sky you are likely to see through your eyepiece when using your telescope.

How to calculate the true field of view of your telescope? It is the apparent field of view divided by the magnification. You typically find the apparent field-of-view (AFOV) value listed in the product description for the eyepiece and magnification is simply the telescope focal length divided by the eyepiece focal length.

Field of view Guide

The following is a guide to the field of view for the different optics:

Naked eye — 100º
Pair of binoculars — 5º–8º
Spotting scope — 2°–1.4°
Low power telescope — 0.5º or 30 arcminutes (‘)
High power telescope — 0.166º or 10′

Calculators for Binoculars

For binocular users…

Binocular Field of View in Linear Measure

Linear measure of width (feet @ 1000 yd): Angle (º) X 52.5
(based on: at 1000 yards one degree is 52.5 feet).

See Field of View in Astronomy Explained for background.

Binocular field of view in Degrees

Angular measure (degrees): Feet @ 1000 yd ÷ 52.5

Charts / Quick References

Dobsonian collimation

Useful when collimating Dobsonian telescopes…For more details see our article on collimating a Dobsonian.


Star testing magnifications


Planet viewing

Rough idea of magnification to use when wanting to observe planets or the moon. See also our article on the best telescopes for planet viewing.



The following is for education purposes. We are happy for you to use these graphics on your page or social media account, but ask that you acknowledge us by adding a link to our site: – thank you.


Moon phases…

phases of the moon infographic
Phases of the moon infographic


Features, metrics, and moons of Jupiter…

Jupiter Galilean moons infographic
Jupiter infographic and measurements of moons


Features, metrics, and moons of Saturn…

Saturn infographic showing its moons
Saturn infographic showing its moons


Our Sun and stars like our Sun…See also our article on the type of stars in space.

Sun infographic showing sun spots and zones
Sun infographic showing zones and other features