TEC: The Eyepiece Calculator

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 aspects, 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 the calculator.

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.

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

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

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.