A Tele Vue 21mm Ethos in the Celestron C9.25 optical tube gives 112x, a 0.89 degree true field and a 2.1 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 C9.25 optical tube: magnification and field

What the Tele Vue 21mm Ethos does in the Celestron C9.25 optical tube, worked from the published focal lengths and apparent field.

Field of view

ring = 0.89 degrees true
dot = 2.1 mm exit pupil

Computed optics

Magnification
112x2350 mm / 21 mm
True field
0.89 degrees100 deg / 112x
Exit pupil
2.1 mm235 mm / 112x
Max useful mag
555xpublished
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Celestron C9.25 optical tube the Tele Vue 21mm Ethos gives 112x with a 0.89 degree true field and a 2.1 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 2350 mm divided by 21 mm is 112x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 0.89 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 235 mm over 112x, which is 2.1 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Celestron C9.25 optical tube235 mm f/10
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Celestron C9.25 optical tubeexit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Celestron C9.25 optical tube: magnification and field?

Good fit. In the Celestron C9.25 optical tube the Tele Vue 21mm Ethos gives 112x with a 0.89 degree true field and a 2.1 mm exit pupil, inside the telescope's useful range.