TEC: The Eyepiece Calculator

Updated December 2, 2011

Fixed a rounding error that occured when entering aperture in mm instead of inches.

Updated April 7, 2004: ¡TEC: El Calculador Ocular, ahora está en Español! Pulsa aqui. Gracias a Francisco Javier Montero y Alberto Romero Vera.

Added a Barlow magnification column to the calculator. Thanks to Rich Covington for this suggestion.

Need a tool to log your search for Messier objects? TUMOL 3.0 is released! Visit the TUMOL page.

Need a place where you can keep track of all your star-gazing information? Check out more FREE software, The Simple Observing Log (TSOL). Visit the TSOL page.

Done with the Messier objects and looking for a little more? Check out even more FREE software, the Overlooked Object Log (OOLog). Visit the OOLog page.

What is TEC?

There are several aspects of your scope that change based upon the eyepiece you are using. The most obvious of these is the magnification, or power. Most of us know that if we switch eyepieces, we will change the power of our telescope. Less obvious perhaps is that the "field of view" for viewing that object will change. The least obvious is the "exit pupil" — the cone of light coming from the eyepiece — changes.

Speaking of "field of view," there are two types: "true" and "apparent." Apparent field of view is how much sky it appears you are seeing edge-to-edge through your eyepiece. The true field of view is how much sky you are actually seeing. So an eyepiece may seem as though it is filling your field of view when you look through it, giving you a wonderful "porthole" into space... but if you consider how much of the sky you are seeing at any one time, it is only a very small fraction.

There are fairly simple formulae for figuring out all of these aspect, and all of these formulae are inlcuded in the calculator below. Enter the values as needed, then click the "Compute" button to calculate all the values indicated in the shaded area. You may then print this chart and keep it in your eyepiece case or wherever.

The calculator contains some sample data. Simply replace the sample data with info about your own eyepieces.

Click here for a printer-friendly version.

Focal Length of Your Scope in millimeters (FLs)
Aperture of Scope in Inches (APi)
If you know the aperture of your scope in inches, enter it to the left. If you know it in mm, enter it to the right. Make sure the field you don't know is empty or 0.
fRatio of Scope
Aperture of Scope in mm (APm)

Barlow Power

(Enter Barlow Magnification)

Eyepiece Type/Description
(replace with your own)
Eyepiece Focal Length (FLe)
Eyepiece Apparent Field of View (FOV) °
Exit Pupil
Eyepiece True Field of View ° (T=FOV/M)
Barlow Factor

Click to see apparent fields of view for common eyepieces:

This calculator is based on JavaScript and should run on any browser. If you would like to see the formulas used and how they were coded, simply use your browser's "View Source" option.

If you like this calculator, and have a comment or would like to see features added — or you have an idea for another calculator — let me know.