Olaf Homeier, Oktober 2026

In order to give anyone interested the opportunity to calculate the resolving power of their own optics using the Rayleigh criterion or to calculate which optics would be necessary for a certain object at a certain distance, we provide a calculation tool here.

Therefore, I have developed the tool in such a way that it can be used for calculation with individually adjustable values. The optics required for observing an object at a given distance or the still solvable object size can be calculated with your own optics.

Have fun with it!

Rayleigh criterion calculator

Calculate required aperture D Calculate resolvable object size x

Underlying basis values J The Rayleigh criterion (α = 1.22 · λ / D)

Describes the theoretical limit at which an optical system can still perceive two closely spaced points as separate. It is strictly limited by the wavelength of light (λ) and the diameter of the aperture (D). Applies equally to refracting and reflecting telescopes.

Wavelength of light (λ):

0.000000400 m (UV / blue light) 0.000000550 m (visible light / green) 0.000000750 m (near infrared / red) 0.000002000 m (infrared / James Webb)

Rayleigh factor:

1.22

Small-angle approximation:

active (tan(α) ≈ α)

1. Input values

Target distance quick selection:

— Free manual selection —Space station ISS (approx. 400 km)The Moon (average: 384,400 km)Mars (minimum distance: approx. 56 million km)

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Distance to object (s):

384,400 km

Object size quick selection:

— Free manual selection —One-euro coin (approx. 2.3 cm)One-meter reference (1.0 m)Apollo Lunar Rover (3.10 m)Apollo descent stage (3.20 m)Football field width (105 m)

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Desired object size (l):

3.20 m

Telescope model quick selection:

— Free manual selection —Amateur Newtonian (200 mm / 0.2 m)Dobsonian telescope (300 mm / 0.3 m)Professional observatory (1.0 m)Hubble Space Telescope (2.4 m)Given reference mirror (10.0 m)Extremely Large Telescope (39.3 m)Theoretical super telescope (200.0 m)

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Given aperture (D):

2.40 m

2. Required Aperture (D)

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Calculating…

3. Calculation steps

Calculations based on the diffraction limit of light (Rayleigh criterion).