TeleVue 21mm Ethos Eyepiece
|The 21mm Ethos features an incredible 100 degree apparent and is an ideal low power eyepiece for fast Newtonian optical systems. |
The newest and largest of the TeleVue Ethos family, the 21mm was announced in August of 2009 for delivery in December. This eyepiece features the same incredible 100 degree apparent field of all Ethos eyepieces.
We were fortunate to do some observing with the 21 Ethos at Stellafane 2009, courtesy of Al Nagler who brought a couple to show. I liked it even more than the 8mm model (previously used on three occasions) since I favor lower power rich-field views -- and the 21 certainly delivered them! The field was flatter than expected, with decent star images even near the edges of the field -- no paracorr was needed. One does have to look around a little through this "porthole" in a virtual spaceship, given the 100 degree apparent field -- but with my well developed peripheral vision, I could just about get the whole view in at once. Variations in eye position were tolerated better than with competing brands. With a stated eye-relief of 15mm, I had to remove my glasses to get the maximum effect, but Scott Ewart of Tele Vue noted that a DiopTRX can be added if desired -- but I didn't think I needed it since moderate eye astigmatism has little perceived effect at lower powers. The weight is barely over 2 pounds -- about the same as the 31 Nagler or the 28 Swann, and way less than the 3 pound Mead 30mm Ultra. The BIG advantage to the Ethos is to deliver more of my big mirror to my retina while retaining a large true field-of-view. My 56 year old eye only dilates to 4.75mm these days. The exit pupil with a paracorred Swann 28 (75.7x in my 16" f4.5, with a 1.08 deg. fov) is 5.4mm, which means my 4.75mm pupil effectively "masks" (stops-down) the mirror down to 14". The 21 Ethos alone delivers 87.8x in the same 16" scope with a 4.62mm exit pupil -- using ALL of the mirror to deliver higher power and contrast with a 1.14 degree true fov! Obviously each astronomer's numbers for their scope and eye will vary, but the 21 Ethos is definitely a winner in my book.
- John Symborski, Delaware Valley Amateur Astronomers