A Celestron X-Cel LX 18mm in the Sky-Watcher Evostar 100ED gives 50x, a 1.2 degree true field and a 2 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Celestron X-Cel LX 18mm in the Sky-Watcher Evostar 100ED: magnification and field

What the Celestron X-Cel LX 18mm does in the Sky-Watcher Evostar 100ED, worked from the published focal lengths and apparent field.

Field of view

ring = 1.2 degrees true
dot = 2 mm exit pupil

Computed optics

Magnification
50x900 mm / 18 mm
True field
1.2 degrees60 deg / 50x
Exit pupil
2 mm100 mm / 50x
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 X-Cel LX 18mm gives 50x with a 1.2 degree true field and a 2 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 18 mm is 50x. The true field of view is the eyepiece's 60 degree apparent field divided by that magnification, 1.2 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 100 mm over 50x, which is 2 mm. That makes this a medium-power general setting.

The eyepiece to buy

Shop Celestron X-Cel LX 18mm on AmazonaffiliateFind Celestron X-Cel LX 18mm on eBayaffiliate

Related

Sky-Watcher Evostar 100ED100 mm f/9
Celestron X-Cel LX 18mm18 mm, 60 deg
Best eyepiece for the Sky-Watcher Evostar 100EDexit-pupil match

Questions and answers

Celestron X-Cel LX 18mm in the Sky-Watcher Evostar 100ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 100ED the Celestron X-Cel LX 18mm gives 50x with a 1.2 degree true field and a 2 mm exit pupil, inside the telescope's useful range.