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First Look: Tele Vue 8mm Ethos
Jul 02 2008 05:44 AM |
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First
Look: Tele Vue 8mm Ethos
Tom Trusock 7/08  13mm Ethos, 8mm Plossl
and 8mm Ethos
I've had the 13mm since they first hit the streets last year and
have become quite enamored of it. (BTW - those of you who may be
wondering where my next Nagler installment is - blame the Ethos).Why Ethos? At this point everybody knows WHAT the Ethos is, but there are some folks who simply don't realize the advantages of such a wide field. This is similar to the challenge that the Nagler line faced when they were introduced, and the answer remains much the same as then. The fact is, there are several ways to look at the benefits of uberwide fields. The first and most obvious is that they pile on the Wow factor – but after that initial emotional high wears off, what's the real reason for using one? Tele Vue eyepieces are known for their superior aberration correction – and the ethos is no exception. This frequently means that stars which would be distorted off axis (and often become invisible) are still focused to points and remain in view. Another key advantage for undriven telescopes lies in the very wide field – this coupled with the excellent off axis correction means that you have more time to inspect an object before you have to concentrate on moving the telescope. Now consider the following. In order to maintain the true field of view, you can select between the following eyepieces: 13 Ethos, 17 Nagler, Pentax 20 or a 26mm Plossl. In a typical 8” f6 dobsonian, these yield 94x and a 2.15mm EP, 72x and a 2.82mm EP, 61x and a 3.33mm EP and finally 47x and a 4.32mm. By raising the power you increase apparent contrast, optimize the exit pupil and with the uberwide field – maintain the TFOV. Because of the increase in apparent contrast per magnification I'm coming to think that wide field eyepieces should not only be compared at similar magnifications, but at the view presented at similar true fields of view.  Apparent field size comparision for similar focal lengths In my opinion, the correct way to select the proper eyepiece is to go for the highest power that seeing will support which properly frames the target while taking the sky background to black. Beyond that, contrast is simply lost between the target and the sky and increased magnification becomes counterproductive. This is the Nagler / Tele Vue basic premise for eyepiece selection and (one must think) the raison d'etre behind wide field design. Roger Clark explores this idea of an optimum magnification (but neglects the field size) in his book Visual Astronomy of the Deep Sky (highly recommended). Designed by Tele Vue's Paul Dellechiaie, the 8mm is the 13's smaller, lighter cousin - you know the one that I'm talking about: The one that chooses to fill up on fruit rather than visit the sundae bar. Although smaller, the 8 shares the large eye lens and 15mm eye relief of it's cousin. It also, of course, shares the 100 deg afov. For those wondering, I was informed at NEAF that the 8mm is not a scaled version of the 13mm. I'm pleased to say (especially after laying out the cash for it) that after preliminary night time deep sky runs with the eyepiece (I have not yet attempted to evaluate daytime, or lunar and planetary performance), the 8mm appears to give everything you'd expect - IE: just like the 13mm only with 1.625x more power. In short – it's a winner. In my 18", this eyepiece yields around 300x (with a 1.55mm exit pupil) and really excels on globs and planetary nebula. It barlows well both with the Antares 1.6x (for an effective 5mm) and the Tele Vue 2x (for an effective 4mm). Image quality held up very well when amplified, and both barlows present very nice images when coupled with the 8mm. Eye relief was just about perfect for me (a non eyeglass wearer). Blackouts and kidney bean were not issues, and I was easily able to take in the entire AFOV using my peripheral vision.  13mm and 8mm
For this preliminary report I spent several hours with the 8mm in my 18” and had many memorable views. The ring (M57) easily showed the central star as well as definition in the edges and across the center of the nebula. In a first for me tiny IC 1296 (the mag 14.3 galaxy off the edge of the ring) was easily visible and even had some hints of structure. M13 almost filled the massive field of view and was resolved across the core while clearly revealing showing the propeller. The Cheeseburger (NGC 7026) in Cygnus is a superb planetary nebula in a moderate sized telescope, and while I typically like to increase power on planetary nebula to reveal detail, my best views of this object were at ~300. The 8mm clearly revealed a bi-lobed structure and hinted at more detail inside. The HII regions in M51 stood out quite well with the 8mm, although I think the 13mm provides a more optimal magnification for this particular target. As per coma, Tele Vue recommends using the 8mm in 1.25” mode with the paracorr set at 4. But frankly, performance was just fine when using it in 2” mode with the paracorr all the way out. As I hinted at earlier, aberrations are controlled quite well. Although this wasn't an in depth examination, I noted no astigmatism or lateral color. Color tone appeared quite nice – and cooler than other TV lines. If there is distortion (and I'm told there is a small amount), I found it completely unnoticeable for this application. Frankly, the Ethos line appears to be one of Tele Vue's best corrected lines ever. And that's saying something.  13mm, 8mm Ethos and 8mm
Plossl
I've been asked a couple of times to compare the 8mm to the 13mm with the Antares 1.6x barlow. The 13mm barlow combination is quite a bit more cost effective, and if cost is a major concern - that would be the route I would recommend - at least initially. However, there does appear to be a bit more even edge illumination with the 8mm than in the 13mm/1.6x combination - somewhat odd as I'd never really noticed it before. It's not a major issue by any means. As per aberrations, neither setup is a clear winner - both perform admirably. Perhaps the biggest advantage the 8mm has over the 13mm/1.6x combination is it's size and weight - or more appropriately - lack thereof. I'm not really much of a barlow fan (sorry Larry), but last year the massive AFOV of the Ethos convinced me that it was worthwhile to invest in a couple. However, when coupled with the Paracorr, the 13 Ethos + Barlow combination gets to be rather lengthy and heavy. I've had no issues in my larger scopes with the size and weight, but it's something of a pain in the smaller ones. And then of course, there's the fuss factor of inserting another element in the train, and for glass minimalists the issue of additional lens elements (although frankly with modern coatings I'm not personally much of a glass minimalist - but like any tool, it does have it's applications). Of course, the 8mm (vs the 13mm barlow combination) is preferred if you are thinking about binoviewing. If you're looking at getting one Ethos, the 13mm would be my recommendation. Not because of any optical differences, but in general I think 13mm is a bit more flexible focal length. That said, folks who have the cash for both the 13 and the 8 will certainly want to pick up both as the 8 is an excellent addition to the stable - "especially" for glob lovers. In general I've found throughput high, eye relief just where I like it, color rendition excellent and basically no aberrations worth speaking of. We'll see how it does after an extended evaluation, but so far it appears the 8mm TV Ethos is another stunner - and for me well worth the price. |
- Live_Steam_Mad, Jeff Struve, JuhaH and 2 others like this
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