focalyard.com
Calculate magnification first. Then buy eyepieces.
Focalyard works out what a telescope and an eyepiece will show before you buy either. Every number has a formula, and every formula is on the page.
A 25 mm eyepiece in a 1200 mm telescope gives 48x. Everything else follows from that division.
1200 mm ÷ 25 mm48x
Max useful 406x · exit pupil 4.2 mm
Chapter I · Calculators
Telescope Magnification Calculator
Telescope, eyepiece, Barlow. Get the power, the useful range and the exit pupil.
Run your numbers
Chapter II · Calculators
Eyepiece Calculator
Field of view and exit pupil for any eyepiece, with the Moon drawn to scale in the field.
Check an eyepiece32 · 15 · 6 mm3
Low, medium and high power
Chapter III · Eyepieces
How to Choose Telescope Eyepieces
Three focal lengths cover most nights. Here is how to find yours.
Read the guide4 · 6 · 8 · 10 in116″
Dawes limit ÷ aperture in mm
Chapter IV · What you can see
What Size Telescope Do You Need?
What each aperture shows on the Moon, the planets and the deep sky.
Compare apertures
Chapter V · Method
How We Research
Every number on this site comes from one constants file and a written formula. Here is the path from spec sheet to page.
See the methodEvery number has a formula.
- Magnification
- focal length ÷ eyepiece
- Useful ceiling
- aperture × 2
- Exit pupil
- aperture ÷ magnification
- True field
- apparent field ÷ magnification