🔭 Telescope Calculator
Enter your telescope's focal length, aperture and eyepiece to get magnification, focal ratio, exit pupil and resolving power.
Magnification
120×
Focal ratio
f/8
Exit pupil
1.3 mm
Max useful mag
300×
Dawes' limit
0.77″
Rayleigh limit
0.92″
Magnification = scope FL ÷ eyepiece FL; focal ratio = FL ÷ aperture. Resolving power: Dawes' limit (116⁄aperture-mm, empirical double-star) and Rayleigh (138⁄aperture-mm, diffraction). "Max useful" ≈ 2× aperture in mm is a rule of thumb — seeing usually caps it lower. 🔒 In your browser.
How the telescope calculator works
Magnification is the telescope's focal length divided by the eyepiece's; the focal ratio (f-number) is focal length divided by aperture. The tool also gives the exit pupil (aperture ÷ magnification), the maximum useful magnification (about 2× the aperture in mm), and the resolving power from Dawes' limit (116 ÷ aperture in mm) and the Rayleigh criterion (138 ÷ aperture in mm).
Aperture matters most: it sets how much light you gather and how fine a detail you can resolve. Dawes' limit is an empirical double-star figure; Rayleigh is the theoretical diffraction limit. Real-world magnification is usually capped by atmospheric "seeing" well below the 2×-aperture rule of thumb.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length. A 1,200 mm telescope with a 10 mm eyepiece gives 120× magnification. Swap eyepieces to change the power.
What is the focal ratio (f-number)?
The focal length divided by the aperture — a 1,200 mm scope with a 150 mm aperture is f/8. Lower f-numbers (faster scopes) give brighter, wider fields; higher ones suit planets and the moon.
What is the maximum useful magnification?
Roughly 2× the aperture in millimetres (or 50× per inch) — about 300× for a 150 mm scope. Beyond that the image just gets dimmer and blurrier, and atmospheric turbulence usually limits you sooner.
What is Dawes' limit?
A measure of a telescope's resolving power: 116 ÷ aperture in mm, in arcseconds. It's the smallest separation of two equal stars a scope can split. A 150 mm telescope resolves about 0.77 arcseconds.
What is the exit pupil?
The width of the light beam leaving the eyepiece: aperture ÷ magnification. Keeping it under about 7 mm (the dark-adapted human pupil) ensures you're not wasting light; small exit pupils suit high-power planetary views.