A Tele Vue 17mm Ethos in the Sky-Watcher Evostar 72ED gives 25x, a 4.05 degree true field and a 2.9 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 17mm Ethos in the Sky-Watcher Evostar 72ED: magnification and field

What the Tele Vue 17mm Ethos does in the Sky-Watcher Evostar 72ED, worked from the published focal lengths and apparent field.

Field of view

ring = 4.05 degrees true
dot = 2.9 mm exit pupil

Computed optics

Magnification
25x420 mm / 17 mm
True field
4.05 degrees100 deg / 25x
Exit pupil
2.9 mm72 mm / 25x
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 17mm Ethos gives 25x with a 4.05 degree true field and a 2.9 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 17 mm is 25x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 4.05 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 72 mm over 25x, which is 2.9 mm. That makes this a medium-power general setting.

The eyepiece to buy

Shop Tele Vue 17mm Ethos on AmazonaffiliateFind Tele Vue 17mm Ethos on eBayaffiliate

Related

Sky-Watcher Evostar 72ED72 mm f/5.8
Tele Vue 17mm Ethos17 mm, 100 deg
Best eyepiece for the Sky-Watcher Evostar 72EDexit-pupil match

Questions and answers

Tele Vue 17mm Ethos in the Sky-Watcher Evostar 72ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 72ED the Tele Vue 17mm Ethos gives 25x with a 4.05 degree true field and a 2.9 mm exit pupil, inside the telescope's useful range.