A Tele Vue 21mm Ethos in the Sky-Watcher Skymax 127 gives 73x, a 1.36 degree true field and a 1.7 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 127: magnification and field

What the Tele Vue 21mm Ethos does in the Sky-Watcher Skymax 127, worked from the published focal lengths and apparent field.

Field of view

ring = 1.36 degrees true
dot = 1.7 mm exit pupil

Computed optics

Magnification
73x1540 mm / 21 mm
True field
1.36 degrees100 deg / 73x
Exit pupil
1.7 mm127 mm / 73x
Max useful mag
about 254x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Skymax 127 the Tele Vue 21mm Ethos gives 73x with a 1.36 degree true field and a 1.7 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1540 mm divided by 21 mm is 73x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1.36 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 127 mm over 73x, which is 1.7 mm. That makes this a medium-power general setting.

The eyepiece to buy

Shop Tele Vue 21mm Ethos on AmazonaffiliateFind Tele Vue 21mm Ethos on eBayaffiliate

Related

Sky-Watcher Skymax 127127 mm f/12.1
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Sky-Watcher Skymax 127exit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Sky-Watcher Skymax 127: magnification and field?

Good fit. In the Sky-Watcher Skymax 127 the Tele Vue 21mm Ethos gives 73x with a 1.36 degree true field and a 1.7 mm exit pupil, inside the telescope's useful range.