A Tele Vue 20mm Plossl in the Sky-Watcher Skymax 127 gives 77x, a 0.65 degree true field and a 1.6 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 20mm Plossl in the Sky-Watcher Skymax 127: magnification and field

What the Tele Vue 20mm Plossl does in the Sky-Watcher Skymax 127, worked from the published focal lengths and apparent field.

Field of view

ring = 0.65 degrees true
dot = 1.6 mm exit pupil

Computed optics

Magnification
77x1540 mm / 20 mm
True field
0.65 degrees50 deg / 77x
Exit pupil
1.6 mm127 mm / 77x
Max useful mag
about 254x2x aperture
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Skymax 127 the Tele Vue 20mm Plossl gives 77x with a 0.65 degree true field and a 1.6 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1540 mm divided by 20 mm is 77x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 0.65 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 127 mm over 77x, which is 1.6 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Sky-Watcher Skymax 127127 mm f/12.1
Tele Vue 20mm Plossl20 mm, 50 deg
Best eyepiece for the Sky-Watcher Skymax 127exit-pupil match

Questions and answers

Tele Vue 20mm Plossl in the Sky-Watcher Skymax 127: magnification and field?

Good fit. In the Sky-Watcher Skymax 127 the Tele Vue 20mm Plossl gives 77x with a 0.65 degree true field and a 1.6 mm exit pupil, inside the telescope's useful range.