A Tele Vue 20mm Plossl in the Sky-Watcher Skymax 150 gives 90x, a 0.56 degree true field and a 1.7 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 150: magnification and field

What the Tele Vue 20mm Plossl does in the Sky-Watcher Skymax 150, worked from the published focal lengths and apparent field.

Field of view

ring = 0.56 degrees true
dot = 1.7 mm exit pupil

Computed optics

Magnification
90x1800 mm / 20 mm
True field
0.56 degrees50 deg / 90x
Exit pupil
1.7 mm150 mm / 90x
Max useful mag
about 300x2x aperture
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Skymax 150 the Tele Vue 20mm Plossl gives 90x with a 0.56 degree true field and a 1.7 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1800 mm divided by 20 mm is 90x. The true field of view is the eyepiece's 50 degree apparent field divided by that magnification, 0.56 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 150 mm over 90x, which is 1.7 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Questions and answers

Tele Vue 20mm Plossl in the Sky-Watcher Skymax 150: magnification and field?

Good fit. In the Sky-Watcher Skymax 150 the Tele Vue 20mm Plossl gives 90x with a 0.56 degree true field and a 1.7 mm exit pupil, inside the telescope's useful range.