Telescope Limiting Magnitude Calculator
Calculate the faintest star your telescope can see based on aperture size.
Compare visual limits across telescope types.
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