Telescope Resolution Calculator

Estimate a telescope’s theoretical angular resolution using the Rayleigh criterion. Then see what that means for details on the Moon and planets (approximate).

Example: 70, 130, 200 mm.
Wavelength affects resolution slightly.
Typical backyard seeing: ~1.5–3.0″.
How we calculate

Rayleigh criterion: θ ≈ 1.22 λ / D (radians), converted to arcseconds. “Realistic” resolution is approximated as the worse of diffraction limit and seeing.