Telescope magnification calculator
This telescope magnification calculator takes the focal length and aperture of your telescope plus eyepiece and Barlow lens and shows the magnification, exit pupil, true field of view and the useful range straight away.
Example: Telescope focal length 1,000 mm · Aperture (diameter) 200 mm · Eyepiece focal length 25 mm · Apparent field of view 52 ° · Barlow factor 1 × → Magnification: 40× magnification, exit pupil 5 mm. Source: Wikipedia – Magnification, section Telescopes (M = f_objective / f_eyepiece). Updated: .
How it is calculated
A telescope with 1000 mm focal length and a 25 mm eyepiece magnifies 40×; with 200 mm aperture the exit pupil is 5 mm and the useful upper limit is about 400×. The calculator gives these and more figures.
How it is calculated
- Magnification: telescope focal length divided by eyepiece focal length, times the Barlow factor if you use one.
- Exit pupil: aperture divided by magnification. It should be no larger than your dark-adapted eye pupil, about 5 to 7 mm.
- True field of view: the eyepiece's apparent field divided by the magnification.
- Resolution: the Dawes limit, 116 divided by the aperture in mm, in arcseconds.
- Useful range: from aperture ÷ 7 up to twice the aperture in mm.
Which magnification for which target?
Wide star fields and galaxies look best at low power with a large exit pupil, while the Moon and planets take 150× to 250× when the air is steady. More magnification does not help if the optics or the atmosphere cannot keep up.
Related tools: the sun position, the crop factor for telescope photography and the depth of field calculator.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope focal length by the eyepiece focal length. 1000 mm divided by 25 mm gives 40×; a 2× Barlow lens doubles that to 80×.
What is the maximum magnification of a telescope?
As a rule of thumb twice the aperture in millimetres, so 400× for a 200 mm telescope. In poor seeing the usable limit is often well below that.
What is the lowest useful magnification?
Aperture divided by 7 mm, so that the exit pupil is not larger than the eye's pupil. For 200 mm that is about 29×.
What does focal ratio f/5 mean?
It is focal length divided by aperture. Low numbers like f/5 are "fast" and show wider patches of sky; high numbers like f/10 magnify more with the same eyepiece.
What is the Dawes limit?
It is the smallest angle at which two equally bright stars still look separate: 116 arcseconds divided by the aperture in mm. For 200 mm that is 0.58 arcseconds.
Sources and legal basis
- Wikipedia – Magnification, section Telescopes (M = f_objective / f_eyepiece)
- Wikipedia – Dawes' limit (Auflösung = 116 / D in mm)
- Wikipedia – Exit pupil (Austrittspupille = D / M)
- Wikipedia – Limiting magnitude (limiting brightness from the aperture)
As of: · Editorial policy
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