Ken's Eyepiece Corner - Formulas

In 1996-1997 as an experiment in using Javascript, I wrote this calculator with a suite of additional pages focusing on eyepieces. At that time, these pages won an award from Griffith Observatory. You can find my contemporary website at: https://klhess.com

Enter two of the three values for the objective diameter, f/number, and focal length, then press Compute to calculate the remaining value.

f/Number = Objective focal length / Objective diameter

Objective focal length
Objective diameter
f/Number of objective

Enter two of the three values for the effective focal length, objective focal length, and Barlow X factor, then press Compute to calculate the remaining value.

Effective focal length = Objective focal length * Barlow X factor

Objective focal length
Barlow X factor
Effective focal length

Enter two of the three values for the eyepiece field stop, objective focal length, and field angle, then press Compute to calculate the remaining value.

Field angle (degrees) = (Eyepiece field stop / Objective focal length) * 57.3 degrees/radian

Eyepiece field stop
Objective focal length
Field angle (degrees)

(c) Copyright Kenneth L. Hess, 1996-2018, All Rights Reserved