A Tele Vue 20mm Plossl in the Sky-Watcher Evostar 120 gives 50x, a 1 degree true field and a 2.4 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 20mm Plossl in the Sky-Watcher Evostar 120: magnification and field

What the Tele Vue 20mm Plossl does in the Sky-Watcher Evostar 120, worked from the published focal lengths and apparent field.

Field of view

ring = 1 degrees true
dot = 2.4 mm exit pupil

Computed optics

Magnification
50x1000 mm / 20 mm
True field
1 degrees50 deg / 50x
Exit pupil
2.4 mm120 mm / 50x
Max useful mag
283xpublished
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 120 the Tele Vue 20mm Plossl gives 50x with a 1 degree true field and a 2.4 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 20 mm is 50x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 1 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 120 mm over 50x, which is 2.4 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Sky-Watcher Evostar 120120 mm f/8.3
Tele Vue 20mm Plossl20 mm, 50 deg
Best eyepiece for the Sky-Watcher Evostar 120exit-pupil match

Questions and answers

Tele Vue 20mm Plossl in the Sky-Watcher Evostar 120: magnification and field?

Good fit. In the Sky-Watcher Evostar 120 the Tele Vue 20mm Plossl gives 50x with a 1 degree true field and a 2.4 mm exit pupil, inside the telescope's useful range.