Ethos
100° & 110° Apparent Field Eyepieces
TeleVue.com: Eyepieces > Ethos > 21mm
21-mm
Model #ETH-21.0
Focal Length (mm)21
Barrel Size (in.)2"
Apparent Field of View (deg.)100
Eye Relief (mm)15
Weight (lb / oz)2.25 / 36.0
Field Stop (mm)36.2
Compare to similar focal lengths
“You've got very innovative engineers always managing to break the laws of optics. This Ethos is out of this world!”
—J.G., France


The 21mm focal length completes the logical set of 21mm/13mm/8mm and nicely juxtaposes the 17mm/10mm/6mm combination. Each step within these two sets approaches a 3x field area gain. The 21mm's effective field stop diameter is 36.2mm, bringing its true field close to the 35mm Panoptic (38.7mm Field Stop diameter). Its true field is also larger than the 26mm Nagler with its 35mm effective field stop diameter. In an f/4 Dobsonian, the exit pupil is just 5.25mm using the 21mm Ethos instead of 8.75mm with a 35mm Panoptic, or 6.5mm with a 26mm Nagler.

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