A Tele Vue 21mm Ethos in the Celestron C14 optical tube gives 186x, a 0.54 degree true field and a 1.9 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 21mm Ethos in the Celestron C14 optical tube: magnification and field

What the Tele Vue 21mm Ethos does in the Celestron C14 optical tube, worked from the published focal lengths and apparent field.

Field of view

ring = 0.54 degrees true
dot = 1.9 mm exit pupil

Computed optics

Magnification
186x3910 mm / 21 mm
True field
0.54 degrees100 deg / 186x
Exit pupil
1.9 mm356 mm / 186x
Max useful mag
840xpublished
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Celestron C14 optical tube the Tele Vue 21mm Ethos gives 186x with a 0.54 degree true field and a 1.9 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 3910 mm divided by 21 mm is 186x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 0.54 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 356 mm over 186x, which is 1.9 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Celestron C14 optical tube356 mm f/11
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Celestron C14 optical tubeexit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Celestron C14 optical tube: magnification and field?

Good fit. In the Celestron C14 optical tube the Tele Vue 21mm Ethos gives 186x with a 0.54 degree true field and a 1.9 mm exit pupil, inside the telescope's useful range.