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What will I see? Telescope magnification calculator

Magnification, exit pupil, true field, resolution, faintest star and the size of Jupiter, for any telescope and eyepiece.

By the editorial team of Aperture Math

Updated November 7, 2026

Aperture Math is paid through affiliate links: a purchase through a link to Unistellar or High Point Scientific may earn us a commission, at no extra cost to you. As of November 7, 2026, no affiliate program has accepted the site yet. Commissions never decide a ranking. How we choose.

Choose a telescope from our catalog (its published aperture, focal length and largest included eyepiece fill in) or type your own. Every result is arithmetic on published figures; each formula and its source is listed under the tool.

Every formula, and where it comes from

magnification = telescope focal length / eyepiece focal length, x 2 or x 3 with a BarlowCelestron knowledge base: "a 20 mm eyepiece used on a 1000 mm focal-length telescope would yield a power of 50x"; "A 2x Barlow lens will double the magnification".
highest useful magnification = 50 to 60 x aperture (in)Celestron knowledge base. When the brand publishes its own figure for the model, the tool shows that figure instead.
exit pupil = eyepiece focal length / focal ratio = aperture / magnificationWikipedia, Exit pupil: "dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope". Night pupil by age from the same page: 7.3 mm at 20, 6.6 at 30, 6.2 at 40, 5.8 at 50, 5.6 at 60, 5.2 at 70, 4.9 at 80.
true field = apparent field / magnificationWikipedia, Eyepiece: "accurate to 4% or better up to 40° apparent field of view, and has a 10% error at 60°". Default 50°: the apparent field Celestron publishes for its Omni Plössl eyepieces (Omni 15 mm spec sheet).
Dawes limit = 116 / D (mm) arcsec; Rayleigh = 1.22 x 550 nm / DWikipedia, Dawes' limit and Wikipedia, Angular resolution: "θ ≈ 1.22 λ/D". We use 550 nm, green light; brands print slightly different Rayleigh figures, shown beside ours.
faintest star = 5 log10(D in cm) + 7.5 (range with N = 6.8 to 8.7)Wikipedia, Limiting magnitude. N = 7.5 reproduces Celestron's and Sky-Watcher's published figures (see the check on the home page).
apparent size = angular size x magnificationAngular sizes from the NASA NSSDCA planetary fact sheets: Jupiter 46.9" and Saturn's globe 18.8" at an average opposition, Mars 17.8" (25.6" at its closest), full Moon 1896".

Two worked examples

8-inch Dobsonian, 1200 mm, with its 25 mm eyepiece (the default): 1200 / 25 = 48x. Exit pupil 203 / 48 = 4.2 mm. True field 50 / 48 = 1.04°, about two full Moons. Jupiter: 46.9" x 48 = 37.5 arcmin, a bit more than the naked-eye Moon. Dawes: 116 / 203 = 0.57" (Celestron publishes 0.57" for its 8-inch SCT). [calc]

Same telescope, 10 mm eyepiece and 2x Barlow: 1200 x 2 / 10 = 240x, under the 480x Celestron publishes for its StarSense Explorer 8". Exit pupil 0.8 mm; true field 0.21°. Jupiter: 187.6 arcmin, about six naked-eye Moons wide. [calc]

Next: does it fit your car and your back? Will it fit? Or compare every model: telescope comparison.

Frequently asked questions

How much magnification do I need to see Saturn's rings?

The calculator shows how big Saturn's globe looks at any power: at 50x, its 18.8 arcsecond globe (NASA, average opposition) appears about 16 arcminutes wide, half the naked-eye Moon. Whether the rings show cleanly also depends on the air and the optics, which no formula captures.

Why is there a highest useful magnification?

Because the image dims and softens past it. Celestron's rule is 50 to 60 times the aperture in inches under ideal conditions; the brand's own figure for the model is shown when it publishes one.

What is exit pupil and why does my age matter?

The exit pupil is the beam of light leaving the eyepiece: eyepiece focal length divided by the focal ratio. If it is wider than your eye's pupil, light is lost. Wikipedia's table of night-time pupil sizes goes from 7.3 mm at 20 to 4.9 mm at 80; the tool uses it.

Does this work for smart telescopes?

Partly. Resolution and light grasp depend on aperture for any telescope, so they are computed. A smart telescope has no optical eyepiece: magnification depends on your screen, so the tool shows the field of view and the limiting magnitude the brand publishes instead.