A Tele Vue 40mm Plossl in the Sky-Watcher Evostar 120 gives 25x, a 2 degree true field and a 4.8 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 40mm Plossl in the Sky-Watcher Evostar 120: magnification and field

What the Tele Vue 40mm Plossl does in the Sky-Watcher Evostar 120, worked from the published focal lengths and apparent field.

Field of view

ring = 2 degrees true
dot = 4.8 mm exit pupil

Computed optics

Magnification
25x1000 mm / 40 mm
True field
2 degrees50 deg / 25x
Exit pupil
4.8 mm120 mm / 25x
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 40mm Plossl gives 25x with a 2 degree true field and a 4.8 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 40 mm is 25x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 2 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 120 mm over 25x, which is 4.8 mm. That makes this a low-power, wide-field and deep-sky setting.

The eyepiece to buy

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Related

Sky-Watcher Evostar 120120 mm f/8.3
Tele Vue 40mm Plossl40 mm, 50 deg
Best eyepiece for the Sky-Watcher Evostar 120exit-pupil match

Questions and answers

Tele Vue 40mm Plossl in the Sky-Watcher Evostar 120: magnification and field?

Good fit. In the Sky-Watcher Evostar 120 the Tele Vue 40mm Plossl gives 25x with a 2 degree true field and a 4.8 mm exit pupil, inside the telescope's useful range.