A Tele Vue 21mm Ethos in the Sky-Watcher Skymax 180 gives 129x, a 0.78 degree true field and a 1.4 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 Sky-Watcher Skymax 180: magnification and field

What the Tele Vue 21mm Ethos does in the Sky-Watcher Skymax 180, worked from the published focal lengths and apparent field.

Field of view

ring = 0.78 degrees true
dot = 1.4 mm exit pupil

Computed optics

Magnification
129x2700 mm / 21 mm
True field
0.78 degrees100 deg / 129x
Exit pupil
1.4 mm180 mm / 129x
Max useful mag
about 360x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Skymax 180 the Tele Vue 21mm Ethos gives 129x with a 0.78 degree true field and a 1.4 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 2700 mm divided by 21 mm is 129x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 0.78 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 180 mm over 129x, which is 1.4 mm. That makes this a high-power lunar, planetary and double-star setting.

The eyepiece to buy

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Related

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Best eyepiece for the Sky-Watcher Skymax 180exit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Sky-Watcher Skymax 180: magnification and field?

Good fit. In the Sky-Watcher Skymax 180 the Tele Vue 21mm Ethos gives 129x with a 0.78 degree true field and a 1.4 mm exit pupil, inside the telescope's useful range.