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Calculators for Telescopes
Use the Telescope Nights calculators for quickly working out metrics for telescopes:
- Tool for calculating telescope focal ratio
- Calculate telescope magnification
- Calculate true field of magnification for telescope
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
Useful when collimating Dobsonian telescopes…For more details see our article on collimating a Dobsonian.
Star testing magnifications
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: telescopenights.com – thank you.
Features, metrics, and moons of Jupiter…
Features, metrics, and moons of Saturn…
Our Sun and stars like our Sun…See also our article on the type of stars in space.