A Tele Vue 13mm Ethos in the Sky-Watcher Evostar 120 gives 77x, a 1.3 degree true field and a 1.6 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 13mm Ethos in the Sky-Watcher Evostar 120: magnification and field

What the Tele Vue 13mm Ethos does in the Sky-Watcher Evostar 120, worked from the published focal lengths and apparent field.

Field of view

ring = 1.3 degrees true
dot = 1.6 mm exit pupil

Computed optics

Magnification
77x1000 mm / 13 mm
True field
1.3 degrees100 deg / 77x
Exit pupil
1.6 mm120 mm / 77x
Max useful mag
283xpublished
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 120 the Tele Vue 13mm Ethos gives 77x with a 1.3 degree true field and a 1.6 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1000 mm divided by 13 mm is 77x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1.3 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 120 mm over 77x, which is 1.6 mm. That makes this a medium-power general setting.

The eyepiece to buy

Shop Tele Vue 13mm Ethos on AmazonaffiliateFind Tele Vue 13mm Ethos on eBayaffiliate

Related

Sky-Watcher Evostar 120120 mm f/8.3
Tele Vue 13mm Ethos13 mm, 100 deg
Best eyepiece for the Sky-Watcher Evostar 120exit-pupil match

Questions and answers

Tele Vue 13mm Ethos in the Sky-Watcher Evostar 120: magnification and field?

Good fit. In the Sky-Watcher Evostar 120 the Tele Vue 13mm Ethos gives 77x with a 1.3 degree true field and a 1.6 mm exit pupil, inside the telescope's useful range.