Ken's Telescope Calculator

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. The eyepiece data has not been updated since circa 2000. You can find my contemporary website at:

This telescope calculator takes the objective diameter, f ratio, focal length, eyepiece apparent field, and eyepiece focal length as required to calculate the estimated objective performance, telescope magnification, magnification per inch, true field, and exit pupil size. It is a great way to compare the characteristics of different eyepieces. The calculator requires a JavaScript compatible browser.

I have already entered the apparent field for many popular eyepieces from Brandon, Celestron, Meade, Pentax, Tele Vue, and Vixen. You can enter values for any other eyepiece.

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

Note: The calculations to estimate the performance of your telescope objective work in units of either inches or millimeters. Unfortunately, the eyepiece calculations require units of millimeters. If you want to do the eyepiece calculations below, then you must use millimeters.

Objective Parameters

Units of Measurement
Diameter of Objective
f Ratio of Objective
Focal Length of Objective
Barlow Lens X Factor (blank if none)
Focal Length of Objective Adjusted for Barlow

Based on the objective diameter, one can infer the following performance for the telescope's limiting magnitude and resolution (all very approximate). Atmospheric seeing limits resolution to approximately 1/2 second, regardless of the objective size. Note that no compensation is made for light loss in the optics or for the quality of the optics. Aperture gain and faintest star calculations are based on a 7mm pupil size and 6th magnitude visibility limit for the naked eye.

Estimated Objective Performance

Aperture Gain
Faintest Star, Visually (magnitude)
Max Field of View for Questar 3.5 at Visual (degrees)
Max Field of View for 1.25" Focuser (degrees)
Max Field of View for 2" Focuser (degrees)
Max Field of View for 4" Focuser (degrees)
Resolution: Dawes (seconds)
Resolution: Rayleigh (seconds)
Resolution: Working Value (seconds)

Unless already entered, type a value for the apparent field and focal length, then press Compute. If you change the objective parameters above, you must press Compute once again to update the eyepiece information.

Note: The eyepiece calculator works only in millimeters. Information you entered about your telescope objective (above) must be in millimeters in order to do eyepiece calculations.

I have used data published by the eyepiece manufacturer. I have not verified any of these values. You should expect some variation from the published values.

Estimated Eyepiece Characteristics

Eyepiece Name (barrel size)
per inch
True Field
of Tele-

Note: JavaScript does not currently allow the printing of field values. If you want a printed record of your calculations, you must use the Print Screen key which will copy to the clipboard the visible portion of the current window.

(c) Copyright Kenneth L. Hess 1996-2023, All Rights Reserved