A Celestron Omni 4mm Plossl in the Sky-Watcher Evostar 120 gives 250x, a 0.2 degree true field and a 0.5 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 120: magnification and field

What the Celestron Omni 4mm Plossl does in the Sky-Watcher Evostar 120, worked from the published focal lengths and apparent field.

Field of view

ring = 0.2 degrees true
dot = 0.5 mm exit pupil

Computed optics

Magnification
250x1000 mm / 4 mm
True field
0.2 degrees50 deg / 250x
Exit pupil
0.5 mm120 mm / 250x
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 Celestron Omni 4mm Plossl gives 250x with a 0.2 degree true field and a 0.5 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 4 mm is 250x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 0.2 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 120 mm over 250x, which is 0.5 mm. That makes this a high-power lunar, planetary and double-star setting.

The eyepiece to buy

Shop Celestron Omni 4mm Plossl on AmazonaffiliateFind Celestron Omni 4mm Plossl on eBayaffiliate

Related

Sky-Watcher Evostar 120120 mm f/8.3
Celestron Omni 4mm Plossl4 mm, 50 deg
Best eyepiece for the Sky-Watcher Evostar 120exit-pupil match

Questions and answers

Celestron Omni 4mm Plossl in the Sky-Watcher Evostar 120: magnification and field?

Good fit. In the Sky-Watcher Evostar 120 the Celestron Omni 4mm Plossl gives 250x with a 0.2 degree true field and a 0.5 mm exit pupil, inside the telescope's useful range.