A Tele Vue 21mm Ethos in the Sky-Watcher Flextube 300P (12-inch Dobsonian) gives 71x, a 1.4 degree true field and a 4.3 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 Flextube 300P (12-inch Dobsonian): magnification and field

What the Tele Vue 21mm Ethos does in the Sky-Watcher Flextube 300P (12-inch Dobsonian), worked from the published focal lengths and apparent field.

Field of view

ring = 1.4 degrees true
dot = 4.3 mm exit pupil

Computed optics

Magnification
71x1500 mm / 21 mm
True field
1.4 degrees100 deg / 71x
Exit pupil
4.3 mm305 mm / 71x
Max useful mag
about 610x2x aperture
Barrel match
2 in eyepiece in a 2 in focuser
Fits

Good fit. In the Sky-Watcher Flextube 300P (12-inch Dobsonian) the Tele Vue 21mm Ethos gives 71x with a 1.4 degree true field and a 4.3 mm exit pupil, inside the telescope's useful range.

Magnification is the telescope focal length divided by the eyepiece focal length: 1500 mm divided by 21 mm is 71x. The true field of view is the eyepiece's 100 degree apparent field divided by that magnification, 1.4 degrees.

The exit pupil, the width of the light beam leaving the eyepiece, is the aperture divided by the magnification, 305 mm over 71x, which is 4.3 mm. That makes this a low-power, wide-field and deep-sky setting.

The eyepiece to buy

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

Related

Sky-Watcher Flextube 300P (12-inch Dobsonian)305 mm f/4.9
Tele Vue 21mm Ethos21 mm, 100 deg
Best eyepiece for the Sky-Watcher Flextube 300P (12-inch Dobsonian)exit-pupil match

Questions and answers

Tele Vue 21mm Ethos in the Sky-Watcher Flextube 300P (12-inch Dobsonian): magnification and field?

Good fit. In the Sky-Watcher Flextube 300P (12-inch Dobsonian) the Tele Vue 21mm Ethos gives 71x with a 1.4 degree true field and a 4.3 mm exit pupil, inside the telescope's useful range.