The AFOV for almost all eyepieces is provided by the manufacturer and it is easy to derive the magnification of any telescope/ocular combination. The easy way and the method I use is to divide the apparent field of view (AFOV) of the ocular by the magnification of the system. There are two ways to calculate the true field of view (FOV) in degrees of a telescope and eyepiece combination. Always make sure you're using the same units! True Field of View TELESCOPE FOCAL LENGTH / CLEAR APERTURE = FOCAL RATIOįor example, a telescope with a focal length of 1000mm and a 100mm (4") clear aperture has a focal ratio of f/10 (1000/100=10).Īn example calculation of focal ratio and magnification for a scope with a 25 mm focal length, eyepiece, 102 mm aperture, and 900 mm focal length, telescope. In other words, the focal ratio is the ratio of the focal length and clear aperture. The focal ratio, or f/stop, of any lens system (including telescopes), is computed by dividing the focal length by the clear aperture (usually expressed in millimeters). TELESCOPE FOCAL LENGTH / OCULAR FOCAL LENGTH = MAGNIFICATIONįor example, a telescope with a 1000mm focal length using a 10mm ocular is operating at 100x magnification (1000/10=100). It is calculated by dividing the focal length of the telescope (usually marked on the optical tube) by the focal length of the eyepiece (both in millimeters). The magnification of an astronomical telescope changes with the eyepiece used.
To illustrate the various formulas below, I will use a telescope with a 100mm (4") clear aperture and a prime focal length of 1000mm. If they are not, the manufacturer's web site usually has the basic numbers, as should the instrument's Owners Manual. Most of the values you need to know to work these formulas, such as prime focal length, focal ratio and clear aperture, are usually printed right on the optical tubes of astronomical telescopes, binoculars and the barrel of eyepieces. (Binoculars are just two small telescopes linked together, so the same formulas work for them, too.) Armed with a simple calculator and the following formulas, even I can work out the vital statistics of my telescopes and eyepieces.
#Telescope resolution calculator how to#
However, sometimes it is good to know how to calculate some of the numbers commonly mentioned by telescope owners and in telescope articles. I am about as far from being a mathematician as it is possible to get.
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