A Tele Vue 25mm Plossl in the Sky-Watcher Evostar 100ED gives 36x, a 1.39 degree true field and a 2.8 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 25mm Plossl in the Sky-Watcher Evostar 100ED: magnification and field

What the Tele Vue 25mm Plossl does in the Sky-Watcher Evostar 100ED, worked from the published focal lengths and apparent field.

Field of view

ring = 1.39 degrees true
dot = 2.8 mm exit pupil

Computed optics

Magnification
36x900 mm / 25 mm
True field
1.39 degrees50 deg / 36x
Exit pupil
2.8 mm100 mm / 36x
Max useful mag
236xpublished
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 100ED the Tele Vue 25mm Plossl gives 36x with a 1.39 degree true field and a 2.8 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 900 mm divided by 25 mm is 36x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 1.39 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 100 mm over 36x, which is 2.8 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Sky-Watcher Evostar 100ED100 mm f/9
Tele Vue 25mm Plossl25 mm, 50 deg
Best eyepiece for the Sky-Watcher Evostar 100EDexit-pupil match

Questions and answers

Tele Vue 25mm Plossl in the Sky-Watcher Evostar 100ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 100ED the Tele Vue 25mm Plossl gives 36x with a 1.39 degree true field and a 2.8 mm exit pupil, inside the telescope's useful range.