A Tele Vue 21mm Ethos in the Sky-Watcher Evostar 72ED gives 20x, a 5 degree true field and a 3.6 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 Evostar 72ED: magnification and field

What the Tele Vue 21mm Ethos does in the Sky-Watcher Evostar 72ED, worked from the published focal lengths and apparent field.

Field of view

ring = 5 degrees true
dot = 3.6 mm exit pupil

Computed optics

Magnification
20x420 mm / 21 mm
True field
5 degrees100 deg / 20x
Exit pupil
3.6 mm72 mm / 20x
Max useful mag
170xpublished
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 72ED the Tele Vue 21mm Ethos gives 20x with a 5 degree true field and a 3.6 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 420 mm divided by 21 mm is 20x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 5 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 72 mm over 20x, which is 3.6 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 Evostar 72ED72 mm f/5.8
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Sky-Watcher Evostar 72EDexit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Sky-Watcher Evostar 72ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 72ED the Tele Vue 21mm Ethos gives 20x with a 5 degree true field and a 3.6 mm exit pupil, inside the telescope's useful range.