Telescope Reviews: A Pair of 8mm Ethos in Denk II's

I decided to start a new thread as otherwise, people might think my post was yet another postponement due to bad weather

Finally, Wednesday night was a good clear night with reasonably good seeing - perfect for testing out the 8mm Ethos in the Denk II's. This is a horribly expensive setup - is it worth it even if you can afford it?

The scope was a TMB 105/650 with exceptionally fine optics (Strehl 0.988). This scope is wonderful for binoviewing as it has a sliding extension tube which allows almost all binoviewers to come to focus without an OCA.... except the Denk II's. However, the good news is that the lower power OCA can screw into the Denk's, thereby reducing the effective magnification and making it easier to swap between mono and binoviewing (with the Denk's the OCA normally goes ahead of the diagonal.

So.....cut to the chase - what can one see?

Well, with the Ethos8, when looking at the center of the FOV, I can see the field stop all around - all 100° is sensed.

Now I think it might be worth clarifying something here. EdZ, an exceptionally experienced observer, reports that he cannot take in the entire field of the Ethos even in mono mode.

There are two explanations here:

1. There's a physiological difference that allows some people take in a wide FOV while others can not. This is certainly not unreasonable - after all we see significant differences in acuity between observers.

2. We are not defining "taking in the entire view" in the same way. I am specifically saying that when staring at the center of the FOV, I can sense the field stop in all directions. This does not mean that without shifting my head, I can move my gaze to the edge of the FOV and still see it - that does not work - it just blacks out - to see the extreme edge, I have to shift my head in the opposite direction.

Obviously it would be nice if one could take in the entire FOV and shift one's eyes around to see the entire FOV but that's certainly not something I can even begin to do.

Second - with the Ethos8 you really have to jam your eyes into the eyepieces to take in the whole FOV. This results in jiggling the scope around, and also causes the eyepieces to dew over fast if it's a bit cold and very humid outside.

That being said, it is a breathtaking feeling viewing such a wide field. Switching back to the 65° Denk 21's, it seemed like I was looking through the proverbial soda straw(!)

So for me, at least, the answer is yes, I can take in the entire FOV, but it is not exactly comfortable to do that. On balance though the effect is very impressive, and besides which, backing off a little and losing part of the FOV is not a major problem - I like that one can quickly move all the way in.

I was comparing the pair of Ethos8 with pair of Pentax XF8.5. The XF's win hand down on ergonomics - they are much lighter and smaller - far better suited for binoviewing, with substantially more eye relief.

So what was the surprise? the Ethos was much brighter - so much so I had to keep on swapping the EP's in and out as I could not believe that there would be such an observable difference, but there it was, and it was dramatic. Maybe it was some kind of odd optical illusion, but there was no question I was seeing it, regardless.

Views through the Denk II's with the powerswitch were fantastic - I wish I knew what magnfication I was getting. Without an OCA, the 8mm would provide 81x, so I figure this was probably at least at 100-120x without the reducer/barlow. With the reducer set, I could easily split the double-double - no problem - at the highest power, the separation was - amazing and the stars were still quite sharp.

M13 filled 1/4 of the FOV and was just about resolved to the core with averted vision - certainly the best view I've ever seen of m13 through this scope. M4 looked great too with the central bar incredibly obvious. I looked at the two other globs in Scorpius which, while not resolved, provided a good image scale. In some ways M22 is even better than m13 because it's looser and resolves better in smaller scopes and provided yet another fine view.

Izar was an easy split- with the pale blue-green companion well separated from the primary (in the past, it's usually been sitting in one of the diffraction rings), and then, for the first time for me, Antares rendered a relatively easy split (I was surprised just how tight that double was).

I did just about all the Messiers up at this time - of course the images were a bit dim - this combo works best on small open clusters. By 2am, Jupiter had moved into the "viewing window" between my two tall trees - again a wonderful sight in the Denks. As with its 13mm brother, the 8mm just has superb sharpness (it was tack sharp to the edge of the FOV - the tighter pair of the double-double was still split at 98% out (i.e. just as it left the FOV). Detail in the bands was clear in spite of turbulence from the relatively low altitude.

I'm looking forward to lunar views - with a tack-sharp view to the edge of the FOV and a brighter object to start with, this should work well.

But all is not perfect - this rig weighs a lot. I really had to tighten up the FT lock, tighten up the clamp on the Teegul mount and more than once, the scope slid and the extension tube worked loose. The Ethos8's 2" sleeve slides over the tightening screw, making it quite awkward to lock the EP's into place. And 4lbs is a lot to carry on an OTA that only weighs 10lbs. Compared with a BV3 binoviewer with Pentax XF's the difference is just enormous.

I would like to have reported that the Pentax XF's only gave up FOV to the Ethos8 while providing better ER, but that was not the case.

With Saturn in the view, I could see the CD on a pretty regular basis with the Ethos, and the images snapped into view- relatively little focus-fiddling. I never did catch the CD in the Pentax XF8.5's (and I went back and forth several times, playing with the focus, etc.). And while the view was pretty sharp, it just didn't have that "snap-to" that the Ethos exhibited. When all was said and done, the only benefits of the XF over the Ethos were (1) size, (2) Weight and (3) Eye relief. The Ethos8 won in all other areas hands-down.

I'm not 100% sure I'll hold onto both the 8's - certainly the 8 in mono mode provided great views in the 4" - 81x is a nice adjunct to the 50x of the Ethos13, but for now, it's equally clear I'm not ready yet to let one of them go! I'm going to remember that view of m27 suspended in a 100° FOV for quite some time.

I'll post another report on this combination for lunar observing - I'm guessing this is really going to be a winner on that subject.

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Edited by CESDewar (06/27/08 12:06 AM)

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