A Tele Vue 13mm Ethos in the Sky-Watcher Classic 250P (10-inch Dobsonian) gives 92x, a 1.08 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 13mm Ethos in the Sky-Watcher Classic 250P (10-inch Dobsonian): magnification and field

What the Tele Vue 13mm Ethos does in the Sky-Watcher Classic 250P (10-inch Dobsonian), worked from the published focal lengths and apparent field.

Field of view

ring = 1.08 degrees true
dot = 2.8 mm exit pupil

Computed optics

Magnification
92x1200 mm / 13 mm
True field
1.08 degrees100 deg / 92x
Exit pupil
2.8 mm254 mm / 92x
Max useful mag
about 508x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Classic 250P (10-inch Dobsonian) the Tele Vue 13mm Ethos gives 92x with a 1.08 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: 1200 mm divided by 13 mm is 92x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1.08 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 254 mm over 92x, which is 2.8 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Sky-Watcher Classic 250P (10-inch Dobsonian)254 mm f/4.7
Tele Vue 13mm Ethos13 mm, 100 deg
Best eyepiece for the Sky-Watcher Classic 250P (10-inch Dobsonian)exit-pupil match

Questions and answers

Tele Vue 13mm Ethos in the Sky-Watcher Classic 250P (10-inch Dobsonian): magnification and field?

Good fit. In the Sky-Watcher Classic 250P (10-inch Dobsonian) the Tele Vue 13mm Ethos gives 92x with a 1.08 degree true field and a 2.8 mm exit pupil, inside the telescope's useful range.