A Explore Scientific 18mm 82 Degree in the Sky-Watcher Evostar 100ED gives 50x, a 1.64 degree true field and a 2 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Explore Scientific 18mm 82 Degree in the Sky-Watcher Evostar 100ED: magnification and field

What the Explore Scientific 18mm 82 Degree does in the Sky-Watcher Evostar 100ED, worked from the published focal lengths and apparent field.

Field of view

ring = 1.64 degrees true
dot = 2 mm exit pupil

Computed optics

Magnification
50x900 mm / 18 mm
True field
1.64 degrees82 deg / 50x
Exit pupil
2 mm100 mm / 50x
Max useful mag
236xpublished
Barrel match
1.25 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 100ED the Explore Scientific 18mm 82 Degree gives 50x with a 1.64 degree true field and a 2 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 900 mm divided by 18 mm is 50x. The true field of view is the eyepiece's 82 degree apparent field divided by that magnification, 1.64 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 100 mm over 50x, which is 2 mm. That makes this a medium-power general setting.

The eyepiece to buy

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Related

Sky-Watcher Evostar 100ED100 mm f/9
Explore Scientific 18mm 82 Degree18 mm, 82 deg
Best eyepiece for the Sky-Watcher Evostar 100EDexit-pupil match

Questions and answers

Explore Scientific 18mm 82 Degree in the Sky-Watcher Evostar 100ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 100ED the Explore Scientific 18mm 82 Degree gives 50x with a 1.64 degree true field and a 2 mm exit pupil, inside the telescope's useful range.