Ethos v 17T4 impressions

[Stephen65]

On Thursday night Ricky came over and we tried my Ethos against his 17T4. Since these have almost exactly the same FOV we were curious to see how they matched up. Test scope was a WO FLT132 refractor (f/7) which produces a nice flat field.

Comparing the Ethos and 17T4 the first thing you notice is that while the Ethos is slightly larger, the 17T4 is noticeably heavier, in fact the 17T4 is as heavy an eyepiece as I have ever held until you start getting up into the 28mm plus widefields.

Optically our conclusion was that the Ethos was superior to the 17t4 in every way. On-axis sharpness and contrast were better (the contrast would follow from the increased magnification if nothing else) and it was perfectly flat and aberration free to the edge of the field - and I mean perfectly flat. Which is quite amazing for a 100 FOV EP. It was also difficult to induce blackouts.

Switching to an 82 degree FOV eyepiece from an Ethos is the only time 82 degrees looks narrow to me. The 17T4 was very good on-axis but the Ethos with higher mag on the same field produced a nicer image and was at least as sharp. Off-axis there were some noticeable aberrations in the last 5-10% or so of the 17t4 field, whether it was astigmatism, field curvature or both was difficult to diagnose. But the stars definitely looked slightly blobbier and spikier than in the centre. The 17T4 also was more finicky with eye placement, it was easier to induce kidney beaning blackouts.

The 17T4 has a longer listed ER (17 v 15) but both had sufficient ER to be comfortable, though neither of us were using glasses.

All in all it was a comprehensive triumph for the Ethos.

[Rodstar (Rod)]

That is a very interesting comparison, Stephen.

Before the Ethos came on the market, I was planning to get both a 13T6 and 17T4. I formed the view when the Ethos came out that the Ethos would satisfy the benefits of BOTH of those eyepieces, in one.

It sounds as though the Ethos not only covers both such focal length choices, but is better than the sum total of such parts. It makes the purchase price a little more justifiable to those who do not already have those focal lengths covered in their eyepiece kit.

I do find that the Ethos is not perfectly flat across the entire FOV when put to work on a newtonian mirror, but is sharp for 85% of same, and certainly makes a Paracorr entirely redundant.

[ausastronomer (John Bambury)]

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

[Rodstar (Rod)]

Originally Posted by ausastronomer

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.

Cheers,
John B

Those are well made points John.

I have been fortunate so far with the seeing in that the Ethos has held up with tight stars each time I have used it (about 5 times now).

My previous scope, the 10" LX200, had the same focal length as the Mary Rose. The LX200 did not marry up well with my old 13T6 on nights of bad seeing. I think the larger light grasp of the Mary Rose makes seeing at that 190x mark more consistent, so that I should be able to use the Ethos almost every night of observing. Perhaps the owner of an SCT with similar focal length may not be able to use the Ethos quite as much.

I am fortunate enough not to suffer from astigmatism in my eyesight, so for now glasses free observing does make the Ethos much easier to use. I am sure that for eyeglass wearers, the 14mm Pentax XW remains a very attractive option at this focal length.