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Hi Rod,
As you know I have them both.
Some things to ponder upon.
When used in a paracorr the 17mm T4 gets much closer to the Ethos. ie. The 17mm T4 is improved noticeably more when combined with the Paracorr than the Ethos does, when it is combined with the paracorr.
Another consideration is scope focal length. If you only have a long focal length scope the 13mm of the Ethos is sometimes too much power, where the 17mm Nagler is perfect. eg. In Murrell's 25" Obsession (3150mm focal length) the Ethos gives 240X and often stars are mush because of thermal issues and seeing conditions. Under the same conditions the 17mm Nagler is giving 185X with nice tight stars.
Something else to consider is the eye relief. The Ethos is not useable with glasses on, whereas the 17mm T4 is.
All that having been said, the Ethos is an outstanding eyepiece and if I didn't wear glasses and had scopes with a focal length under 2.5 metres, I would take the Ethos over the 17mm Nagler every time. I am just raising these issues as there are often factors outside raw eyepiece performance you need to consider.
Cheers,
John B
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