A Tele Vue 21mm Ethos in the Sky-Watcher Evostar 100ED gives 43x, a 2.33 degree true field and a 2.3 mm exit pupil. The worked calculation and whether it sits in the useful range.

Updated 05 September 2026.

Eyepiece and telescope pairing

Tele Vue 21mm Ethos in the Sky-Watcher Evostar 100ED: magnification and field

What the Tele Vue 21mm Ethos does in the Sky-Watcher Evostar 100ED, worked from the published focal lengths and apparent field.

Field of view

ring = 2.33 degrees true
dot = 2.3 mm exit pupil

Computed optics

Magnification
43x900 mm / 21 mm
True field
2.33 degrees100 deg / 43x
Exit pupil
2.3 mm100 mm / 43x
Max useful mag
236xpublished
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Evostar 100ED the Tele Vue 21mm Ethos gives 43x with a 2.33 degree true field and a 2.3 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 21 mm is 43x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 2.33 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 100 mm over 43x, which is 2.3 mm. That makes this a medium-power general setting.

The eyepiece to buy

Shop Tele Vue 21mm Ethos on AmazonaffiliateFind Tele Vue 21mm Ethos on eBayaffiliate

Related

Sky-Watcher Evostar 100ED100 mm f/9
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Sky-Watcher Evostar 100EDexit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Sky-Watcher Evostar 100ED: magnification and field?

Good fit. In the Sky-Watcher Evostar 100ED the Tele Vue 21mm Ethos gives 43x with a 2.33 degree true field and a 2.3 mm exit pupil, inside the telescope's useful range.