Telescope Resolving Power (Dawes Limit) Calculator
Resolving power sets the finest detail your telescope can show and the closest double stars it can split. Enter the aperture to get the Dawes and Rayleigh limits.
Telescope Resolving Power (Dawes Limit) Calculator
Why use the Telescope Resolving Power (Dawes Limit) Calculator
Wondering if your scope can split a tight double star or show fine planetary detail? This is its resolution limit.
How to get your numbers
- Aperture in mm.
- Remember: steady “seeing” is needed to actually reach the limit.
Recommended for this job
How to use it
- Enter your telescope’s aperture in millimeters.
- Smaller arc-second numbers mean finer detail.
- Remember steady “seeing” is needed to reach the limit.
Frequently asked questions
What is the Dawes limit?
The resolving power of a telescope in arc-seconds, equal to 116 divided by the aperture in millimeters.
What is the difference between the Dawes and Rayleigh limits?
Both estimate resolution; the Rayleigh limit (138 ÷ aperture) is slightly more conservative than the Dawes limit (116 ÷ aperture).
Why can’t I see the detail the Dawes limit predicts?
Atmospheric turbulence usually caps real resolution at 1–2 arc-seconds, so a large scope rarely reaches its theoretical limit.
How do I split double stars?
You need both enough aperture (the Dawes limit sets the theoretical minimum separation) and enough magnification, on a night of steady seeing.
Does higher magnification improve resolution?
No — resolution is set by aperture. Magnification only enlarges the image; beyond a point it shows a bigger blur, not finer detail.
What aperture do I need to resolve a given detail?
Rearrange the Dawes limit: aperture (mm) ≈ 116 ÷ the separation in arc-seconds you want to resolve. Enter an aperture above to see its limit.
