As an active amateur astronomer of 25 years I have to say that Televue has done it again... Created another amazing eyepiece. This occular provides the highest contrast and widest field of view of any 21mm eyepiece out there. I have owned and/ or viewed through just about every premium ultrawide field eyepiece in this focal length - and this eyepiece is in a league above them in every regard.
The field is very flat (well corrected) even in f/5 focal ration Newtonians. No distorted stars are visible at the edge of the field... even at the field stop, and this is without the use of a coma corrector.
The coatings / contrast are also exceptional. Faint galaxies and nebulae barely discernable in other eyepieces tend to "pop" into view through this eyepiece.
The giant 100 degree field of view speaks for itself... it is truly like a "porthole" into the heavens. It is less like looking into an eyepiece than looking through a window. After using the 21mm Ethos view through other standard ultrawides is like looking through a straw.
I highly recommend this occular if your budget can handle the expense (its only downfall).
Pros: Extremely well corrected, ultrawide field of view.
Cons: Very expensive
This review was written in the old system and had content requirements that are different than reviews written today.