Eyepiece Calculator

Calculate telescope magnification, field of view, exit pupil, and other optical parameters based on your equipment specifications.

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About This Tool

This comprehensive eyepiece calculator helps astronomers determine key optical parameters for their telescope setup. Enter your telescope's aperture, focal length, and eyepiece specifications to calculate magnification, true field of view, exit pupil, and other important metrics. Perfect for planning observing sessions and understanding how different eyepieces will perform with your telescope.

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How to Use

Enter your telescope's aperture (in mm or inches) and focal length (in mm).
Enter your eyepiece's focal length (in mm) and field of view (in degrees).
Optionally adjust the Barlow lens multiplier if using one.
Click 'Calculate' to see detailed results including magnification, true field of view, exit pupil, and more.
Use the 'Reset' button to clear all inputs and start over.

Frequently Asked Questions

Find Quick Answers

What is magnification and why is it important?

Magnification is how much larger an object appears through the telescope compared to the naked eye. It's calculated by dividing the telescope's focal length by the eyepiece's focal length. Higher magnification shows more detail but reduces brightness and field of view.

What is true field of view?

True field of view (TFOV) is the actual angular width of sky you can see through the eyepiece. It's calculated by dividing the eyepiece's apparent field of view by the magnification. Wider TFOV makes finding objects easier and provides more immersive views.

What is exit pupil and why does it matter?

Exit pupil is the diameter of the light beam exiting the eyepiece that enters your eye. It's calculated by dividing the telescope's aperture by the magnification. An exit pupil between 2-7mm is generally optimal for most observers, with smaller values for planetary viewing and larger for deep-sky objects.

Can I use this calculator with a Barlow lens?

Yes, simply enter the Barlow lens multiplier (e.g., 2x, 3x) in the calculator. The Barlow effectively increases the telescope's focal length, which increases magnification and affects all calculated parameters.

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