A Celestron Omni 4mm Plossl in the Sky-Watcher Evostar 100ED gives 225x, a 0.22 degree true field and a 0.4 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Celestron Omni 4mm Plossl in the Sky-Watcher Evostar 100ED: magnification and field

What the Celestron Omni 4mm Plossl does in the Sky-Watcher Evostar 100ED, worked from the published focal lengths and apparent field.

Field of view

ring = 0.22 degrees true
dot = 0.4 mm exit pupil

Computed optics

Magnification
225x900 mm / 4 mm
True field
0.22 degrees50 deg / 225x
Exit pupil
0.4 mm100 mm / 225x
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 Celestron Omni 4mm Plossl gives 225x with a 0.22 degree true field and a 0.4 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 4 mm is 225x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 0.22 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 100 mm over 225x, which is 0.4 mm. That makes this a high-power lunar, planetary and double-star setting.

The eyepiece to buy

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Related

Sky-Watcher Evostar 100ED100 mm f/9
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Best eyepiece for the Sky-Watcher Evostar 100EDexit-pupil match

Questions and answers

Celestron Omni 4mm Plossl in the Sky-Watcher Evostar 100ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 100ED the Celestron Omni 4mm Plossl gives 225x with a 0.22 degree true field and a 0.4 mm exit pupil, inside the telescope's useful range.