Telescope Angular Resolution Calculator
Calculate a telescope's angular resolution limit using the Rayleigh criterion and Dawes limit.
Find what you can resolve on the Moon, planets, and beyond.
Angular resolution is the smallest angular separation a telescope can distinguish as two separate objects.
Rayleigh criterion:
θ = 1.22 × λ / D (radians)
In arcseconds:
θ = 252,000 × λ(μm) / D(mm)
For visible light (λ = 0.55 μm):
θ ≈ 138.6 / D(mm) arcseconds
Dawes limit (empirical, from visual observation):
θ = 115.8 / D(mm) arcseconds
Where D is the objective (primary mirror or lens) aperture in millimeters.
Physical resolution on the Moon (384,400 km away):
size_min = θ × d (in same angular/linear relation)
1 arcsecond at Moon distance = ~1.86 km on the surface.
Comparison of instruments:
| Instrument | Aperture | Resolution |
|---|---|---|
| Human eye | ~7 mm | ~1 arcminute (60") |
| 60 mm refractor | 60 mm | 2.3" |
| 200 mm reflector | 200 mm | 0.69" |
| Hubble Space Telescope | 2,400 mm | 0.058" |
| Event Horizon Telescope | ~10,000 km baseline | 20 microarcseconds |
Why space matters: Earth’s atmosphere causes “seeing” — turbulence that blurs stellar images to typically 1–2 arcseconds. All ground-based telescopes are limited by seeing, not diffraction, unless adaptive optics is used. Only space telescopes and interferometers achieve their theoretical diffraction limit.