A Tele Vue 17mm Ethos in the Orion SkyQuest XT8 Classic Dobsonian gives 71x, a 1.42 degree true field and a 2.9 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 17mm Ethos in the Orion SkyQuest XT8 Classic Dobsonian: magnification and field

What the Tele Vue 17mm Ethos does in the Orion SkyQuest XT8 Classic Dobsonian, worked from the published focal lengths and apparent field.

Field of view

ring = 1.42 degrees true
dot = 2.9 mm exit pupil

Computed optics

Magnification
71x1200 mm / 17 mm
True field
1.42 degrees100 deg / 71x
Exit pupil
2.9 mm203 mm / 71x
Max useful mag
about 406x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Orion SkyQuest XT8 Classic Dobsonian the Tele Vue 17mm Ethos gives 71x with a 1.42 degree true field and a 2.9 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 17 mm is 71x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1.42 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 203 mm over 71x, which is 2.9 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Orion SkyQuest XT8 Classic Dobsonian203 mm f/5.9
Tele Vue 17mm Ethos17 mm, 100 deg
Best eyepiece for the Orion SkyQuest XT8 Classic Dobsonianexit-pupil match

Questions and answers

Tele Vue 17mm Ethos in the Orion SkyQuest XT8 Classic Dobsonian: magnification and field?

Good fit. In the Orion SkyQuest XT8 Classic Dobsonian the Tele Vue 17mm Ethos gives 71x with a 1.42 degree true field and a 2.9 mm exit pupil, inside the telescope's useful range.