A Celestron X-Cel LX 18mm in the Sky-Watcher Flextube 300P (12-inch Dobsonian) gives 83x, a 0.72 degree true field and a 3.7 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 Flextube 300P (12-inch Dobsonian): magnification and field

What the Celestron X-Cel LX 18mm does in the Sky-Watcher Flextube 300P (12-inch Dobsonian), worked from the published focal lengths and apparent field.

Field of view

ring = 0.72 degrees true
dot = 3.7 mm exit pupil

Computed optics

Magnification
83x1500 mm / 18 mm
True field
0.72 degrees60 deg / 83x
Exit pupil
3.7 mm305 mm / 83x
Max useful mag
about 610x2x aperture
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Flextube 300P (12-inch Dobsonian) the Celestron X-Cel LX 18mm gives 83x with a 0.72 degree true field and a 3.7 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1500 mm divided by 18 mm is 83x. The true field of view is the eyepiece's 60 degree apparent field divided by that magnification, 0.72 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 305 mm over 83x, which is 3.7 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 Flextube 300P (12-inch Dobsonian)305 mm f/4.9
Celestron X-Cel LX 18mm18 mm, 60 deg
Best eyepiece for the Sky-Watcher Flextube 300P (12-inch Dobsonian)exit-pupil match

Questions and answers

Celestron X-Cel LX 18mm in the Sky-Watcher Flextube 300P (12-inch Dobsonian): magnification and field?

Good fit. In the Sky-Watcher Flextube 300P (12-inch Dobsonian) the Celestron X-Cel LX 18mm gives 83x with a 0.72 degree true field and a 3.7 mm exit pupil, inside the telescope's useful range.