A worked example
A 1,000 mm, 200 mm aperture telescope with a 25 mm eyepiece gives 40×, 5 mm exit pupil and f/5.
Magnification
Calculate magnification, exit pupil and effective focal ratio from your optical setup.
YOUR LITTLE WORKBENCH
Magnification
Start with an example and change it to your setup. Results update as you type.
Calculation method+
Magnification = effective focal length ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification.
YOUR LIVE RESULT ✦
Magnification
40 ×- Exit pupil
- 5 mm
- Effective focal ratio
- 5 f/
- Effective focal length
- 1,000 mm
Optical factor multiplies focal length, not aperture. Results describe geometry and do not predict seeing, image quality or usable magnification.
THE METHOD / 01
Know what goesinto your result.
Magnification = effective focal length ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification.
Keep the assumptions clear
Optical factor multiplies focal length, not aperture. Results describe geometry and do not predict seeing, image quality or usable magnification.
A FEW USEFUL ANSWERS
Good to know.
Are my inputs uploaded?
No. Calculations run locally in your browser. The host still receives normal requests for website pages and assets.
Can I keep my result?
Use the download controls for the current tool or print from your browser. Typing scores and camping plans can also be saved locally; see the privacy page for details.
Do I need an account?
No sign-up, paid API or subscription is required. Both language versions use the same calculation logic.
SKYKIND / TOOLKIT
A little help for every step.
02
Eyepiece fields
Compare two eyepieces on the same telescope with an approximate true-field diagram.
Open tool03
Camera field
Calculate rectangular camera field of view and compare it with an entered angular target size.
Open tool04
Pixel scale
Convert pixel size and focal length to arcseconds per pixel, with binning and a user-entered seeing value.
Open tool