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Telescope Resolving Power Calculator

Work out telescope resolving power instantly with clear inputs, formula shown and shareable results.

About Telescope Resolving Power Calculator

Dawes' empirical limit for splitting double stars is 116/aperture in millimetres, giving 0.58 arcseconds for a 200 mm telescope. The theoretical Rayleigh criterion at 550 nm is slightly larger at 0.69 arcseconds. That resolution corresponds to about 1.3 km of lunar detail, and the same aperture reaches roughly magnitude 14 visually.

Formulas

Dawes limit

resolution(arcsec) = 116 / aperture in mm

Rayleigh criterion

resolution = 1.22 lambda / D, converted to arcseconds

Worked example

With the default values below, Telescope Resolving Power Calculator returns Dawes Resolution Limit of 0.580. Change any input above and every figure here is recalculated in your browser.

Inputs used

Apertureminimum 1 mm
200 mm
Observing wavelengthminimum 1 nm
550 nm

Results

Dawes Resolution Limit
0.580 arcsec
Rayleigh Criterion
0.692 arcsec
Approximate Limiting Magnitude
14.01
Smallest Lunar Detail Resolved
1.29 km

Step by step

  1. 1Values used
  2. 2Dawes limit
  3. 3Rayleigh criterion
  4. 4Dawes Resolution Limit= 0.580 arcsec
  5. 5Rayleigh Criterion= 0.692 arcsec
  6. 6Approximate Limiting Magnitude= 14.01
  7. 7Smallest Lunar Detail Resolved= 1.29 km

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why do Dawes and Rayleigh disagree?

Dawes is an empirical result for equal-brightness double stars, where the eye can detect an elongated blur. Rayleigh is the formal diffraction criterion.

Does atmospheric seeing matter more?

Usually yes. Typical seeing is 1-2 arcseconds, so apertures above about 150 mm are limited by the atmosphere rather than optics.

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