Choosing eyepieces


This page provides some assistance in selecting your eyepieces.


  • For a "complete" set of formulas and definitions, check the ATM telescope formulas page on this site.
  • Definitions
    • Aperture: opening of the telescope (diameter of the objective). SYMBOL: D
    • Focal length of the primary: SYMBOL F
    • f-number: SYMBOL: f/ = F / D

Common Telescope Formulas


This page is an attempt to gather most of the formulas used with telescopes. I do not grant that all formulas here are correct, but I've been using most of them when building my own scope. Please always use the same units. I prefer working with millimeters. Some formulas are only applicable when one parameter is given in a certain unit that is then written next to the dimension, between square brackets, as in : D[mm].

General formulas

  • Definitions
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