Telescope Limiting Magnitude
Calculate the faintest star visible through a telescope based on aperture and sky brightness conditions.
Inputs
mm
mag/arcsec²
Results
Practical Limiting Mag
14.71 mag
Theoretical Limit
14.21 mag
Naked Eye Limit6.93 mag
Magnitude Gain7.28 mag
Light Gathering816.3× eye
Dawes Resolution0.58 arcsec
Rayleigh Resolution0.69 arcsec
Stars Visible (est.)33,349,551
How to Use This Calculator
- Enter your Telescope Aperture (mm) — a 200 mm (8-inch) Dobsonian is a common deep-sky instrument.
- Enter Sky Brightness (mag/arcsec²): city skies are ~18, suburban ~20, rural ~21.5, excellent dark sites ~22.
- Read the Practical Limiting Magnitude — objects fainter than this number will not be visible.
- Compare with Naked Eye Limit (~6.5 mag) to see the Magnitude Gain your telescope provides.
- Use Dawes Resolution (arcsec) to determine whether close double stars can be split at your aperture.
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Formula
m_lim ≈ 2.7 + 5 × log₁₀(aperture_mm)Related Calculators
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