A Tele Vue 13mm Ethos in the Sky-Watcher Skymax 102 gives 100x, a 1 degree true field and a 1 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 Skymax 102: magnification and field

What the Tele Vue 13mm Ethos does in the Sky-Watcher Skymax 102, worked from the published focal lengths and apparent field.

Field of view

ring = 1 degrees true
dot = 1 mm exit pupil

Computed optics

Magnification
100x1300 mm / 13 mm
True field
1 degrees100 deg / 100x
Exit pupil
1 mm102 mm / 100x
Max useful mag
about 204x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Skymax 102 the Tele Vue 13mm Ethos gives 100x with a 1 degree true field and a 1 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1300 mm divided by 13 mm is 100x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 102 mm over 100x, which is 1 mm. That makes this a high-power lunar, planetary and double-star setting.

The eyepiece to buy

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Related

Sky-Watcher Skymax 102102 mm f/12.7
Tele Vue 13mm Ethos13 mm, 100 deg
Best eyepiece for the Sky-Watcher Skymax 102exit-pupil match

Questions and answers

Tele Vue 13mm Ethos in the Sky-Watcher Skymax 102: magnification and field?

Good fit. In the Sky-Watcher Skymax 102 the Tele Vue 13mm Ethos gives 100x with a 1 degree true field and a 1 mm exit pupil, inside the telescope's useful range.