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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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