Telescope Limiting Magnitude Calculator

Limiting magnitude is the faintest star a telescope can detect under ideal conditions. The larger the aperture (lens or mirror diameter), the more light the telescope gathers and the fainter the objects it can reveal.

The formula: Limiting magnitude = 2 + 5 × log10(aperture in mm)

This formula assumes perfect dark-sky conditions and a well-adapted eye. In practice, light pollution, atmospheric conditions, and optical quality all reduce the actual limiting magnitude.

For comparison, the naked eye can see stars down to about magnitude 6 under dark skies. Each magnitude step represents a brightness factor of about 2.512. A magnitude 1 star is 100 times brighter than a magnitude 6 star.

Common telescope apertures and their limits:

• 60mm (2.4"): ~11.6 magnitude, good for Moon, planets, bright deep-sky objects
• 80mm (3.1"): ~12.2 magnitude, shows more detail in nebulae and clusters
• 130mm (5.1"): ~13.3 magnitude, resolves globular clusters, faint galaxies
• 200mm (8"): ~14.2 magnitude, serious deep-sky observing
• 300mm (12"): ~15.1 magnitude, faint galaxies, planetary nebulae
• 400mm (16"): ~15.7 magnitude, near the visual limit for amateur astronomy

Light-gathering power compared to the naked eye: Light gathering = (Aperture / 7)² where 7mm is the typical dark-adapted pupil diameter.

Magnification is a separate consideration. Maximum useful magnification is roughly 2× the aperture in mm (e.g., 200mm scope = 400× max). Higher magnification does not reveal fainter objects — only aperture does that.

Resolution (ability to split close double stars) is also determined by aperture: Dawes' limit (arcseconds) = 116 / aperture in mm