Telescopes · Eyepieces
Telescope magnification calculator.
Enter your telescope and eyepiece to see the magnification, how much sky you will see and whether the power is practical.
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How telescope magnification is worked out
Safety: never point a telescope or binoculars at the Sun without a certified solar filter; it can cause permanent eye damage.
Magnification is the telescope's focal length divided by the eyepiece's focal length. The true field of view is approximately the eyepiece's apparent field of view divided by the magnification. This is accurate to about 4 percent only up to roughly 40° apparent field; at wider fields such as the default 52° the error is larger (Wikipedia, Eyepiece, cites about 10 percent at 60°). The exit pupil, the width of the beam leaving the eyepiece, is the aperture divided by the magnification.
Worked example. A 200 mm f/6 telescope (focal length 1200 mm) with a 25 mm eyepiece of 52° apparent field gives 1200 / 25 = 48x. True field = 52 / 48 = 1.08°, about 2.1 Full Moons wide using NASA's mean full-Moon apparent diameter of 1896 arcseconds. Exit pupil = 200 / 48 = 4.17 mm.
Astronomics gives the upper end of useful power as about 50x to 60x per inch of aperture, 25x to 30x per inch as a sensible level in average seeing, and 3x to 4x per inch as the lowest useful power. These are guidelines; steady air matters more than any formula.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope focal length by the eyepiece focal length. A 1200 mm telescope with a 10 mm eyepiece gives 120x.
What is the maximum useful magnification?
Astronomics suggests the point where faint objects get too dim is about 50x to 60x per inch of aperture. For a 200 mm scope that is about 394x at 50x per inch. Poor seeing usually limits you well below that.
What does exit pupil tell me?
It is the aperture divided by the magnification. In the worked example the exit pupil is 4.17 mm at 48x. If it is larger than your own dark-adapted pupil, some light is wasted.
How do I find the apparent field of view?
It is a number from the eyepiece maker, often printed on the box or listed in the product specs. If you do not have it, the field of view result is left out.
Sources
Checked
- Wikipedia: Eyepiece (magnification from focal lengths; true field of view from apparent field)
- Wikipedia: Exit pupil
- Astronomics: Determining Magnification
- NASA NSSDCA: Moon Fact Sheet
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Scope
Magnification limits are common rules of thumb from retailers, not published standards. Real limits depend on optics quality, seeing and the object.