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CN Report: The Nagler Eyepieces, Part II - The Type 4's
Dec 13 2007 02:47 PM |
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The Nagler Eyepieces - Part II of IV
The Type 4 Naglers Tom Trusock - 12/07  Welcome to the second article in my (most likely futile) attempt to cover the Tele Vue Naglers. In Part I we talked about testing methodology, scopes that I used and then had a short synopsis of the Type 1, 2 and Nagler zooms. I'd recommend that you start there and move on to this article. For those that have already read the previous piece, there are a few things that I think could be added or reiterated. First and most importantly the design of your telescope will affect your perception of the performance of the eyepiece. The primary off axis aberration in a Newt is coma, but they also suffer from (usually minor) amounts of field curvature. Even if the eyepiece is perfect (which no eyepiece is), when used in a newt, you will see the effects of the telescope. And not just the telescope. When you're looking through an eyepiece, you're looking at an entire chain of events: target, atmosphere, objective, secondary, corrector (if used), eyepiece, eyeball and brain. This makes it very difficult to talk about absolute performance of any eyepiece. There are however, a few things that can be done to minimize these effects. Foremost is the knowledge that you will be held back by the weakest link in the chain. There are some things you can control - there are others you can't, at least not completely. But if you want to examine an eyepiece critically, I believe you need to do the following:
"the
eyepiece is in such a low power-space that it is impossible to see any
image change due to thermal changes of the lenses. This myth seems to revolve around the 31mm Nagler which produces low power not suitable to observe changes in performance due to equilibration of the optics. " In my opinion, the absolute best platform to test an eyepiece is a flat field scope like the NP101/127. The second best is a moderately long focal ratio Dobsonian with a Paracorr - however, in my opinion it absolutely MUST have a Paracorr. In a well corrected eyepiece, you'll quickly find the coma from the primary in a fast dob will swamp many (if not all) of the aberrations remaining in the eyepiece.  Let's talk for a minute about distortion. The wider the AFOV the more significant it can be to deal with in terms of eyepiece design. There are two types of distortion which cannot be simultaneously corrected because they are defined differently and their effects manifest themselves differently at the eyepiece. (Some math is required here, please bear with me for a bit...) Rectilinear Distortion plots the location of a point moving across the field of view using the formula: h=f(tan(theta)) where h=the "height" of the image position measured laterally in the focal plane, f=eyepiece focal length, and tan(theta) is the tangent of the angle from the optical axis Angular Magnification Distortion plots the location of a point across the field, using: h=f(theta) where theta is in radian measure It is mathematically impossible to correct for both types of distortion since the tangent plot is a non-linear slope. Looking at specific cases, we see that in a 50 degree eyepiece, the difference between theta and tan(theta) at 25 degrees off axis is 6.8% while in an 82 degree eyepiece the difference between theta and tan(theta) at 41degrees off axis is 21.5%. Correction for Rectilinear Distortion keeps straight, vertical lines at the edge of the field straight but sacrifices the shapes of small objects and apparent angle between stars. Correction of Angular Magnification distortion keeps the apparent separation of objects the same any place in the field but creates a "pincushion" effect where a star traveling near the edge of the field appears to move in an arc. Tele Vue's design philosophy is to correct as best as they can for Angular Magnification Distortion since we don't look at picket fences, telephone poles or brick walls in the sky. However, we do look at planets and double stars which all should appear naturally shaped and spaced anywhere in the field. If choosing to correct for Rectilinear Distortion, double stars, for example, would appear to change their distance relative to each other when moved across the top, bottom or sides of the field. To keep the apparent distance (angle) between the stars constant requires correction for Angular Magnification Distortion.  The rate of the pincushion effect can be varied thereby making it possible to reduce its visibility. This explains why we see a difference in apparent pincushion between the Type 4 Naglers and the Type 5 and 6 even though they are all 82º apparent field. The type 4 Naglers are closer to the f theta ideal. The last point I'd like to cover has to do with throughput (again). Studies have shown that it typically takes around a 10% difference in throughput before someone notices a difference in brightness. Modern coatings greatly pare down light loss on multi-element eyepieces so much so that the old stories about "light robbing widefields" no longer hold much basis in fact. But when one looks through some of these eyepieces, one often can see a difference. What then, causes it? Not throughput but, contrast. See my first article in this series for more discussion of contrast. With that out of the way, let's get down to brass tacks. Taking a quick look at all the available Naglers, we immediately see the following: If you want a short focal length, wide angle Nagler - your choice is simple: the Type 6's. Ranging from 2.5 up to 13mm, these provide good eye relief, great contrast and the trademark 82 deg AFOV. They are also fairly small and light and make a good match (in the longer focal lengths) for a binoviewer. Where the t6's leave off at 13mm, the t5's pick up. The 16mm is the shortest focal length in the type 5's, and they continue up to the vaunted 31mm. The purpose of the t5 series is to provide the widest true field of view for both 1.25" and 2" barrel sizes in an 82º AFOV. Where, then, do the t4's come in? In the lower range, they overlap with the t6's, and the upper range overlaps with the t5's. Some amateurs have been fooled by the Type 4 designation into thinking these are obsolete, but the truth couldn't be further off. These Naglers replaced the t2's and are a unique, very valid, and potentially even a preferable choice. Why would one choose the t4's over the t5's or t6's? In a nutshell - longer eye relief, dioptrix compatibility (if you wear eyeglasses, these are your Naglers), lower pincushion distortion, and a greater sense of immersion. In my opinion, the t4's are quite possibly the most immersive Naglers. There's a long running debate about what actually causes the feeling of immersion one experiences with various eyepieces. In the case of the Naglers, a direct comparison is often made between the 13t6 and the 12t4. While the 13 is a spectacular eyepiece, most amateurs will tell you that it's just not as immersive as the 12t4. The visceral punch, that feeling of floating in the depths of space is just not quite the same with the 13t6 as it is with the 12t4. While I can't give an exact reason, I can tell you that I'd guess it has to do with a couple of factors - the first being the size of the eye lens. An observing buddy recently remarked that even though the 12 and the 13 have the exact same AFOV, sometimes they just don't feel that way. Big eye lenses are physically striking. That's a lot of glass to be presented to your eye, and I believe there's a subconscious effect at work. Personally, I think it's from the sense of a larger "window" into space. Secondly, I suspect it's also partly because of the Instadjust eye guard. If you've not used a type 4 Instadjust is simply a sliding eye guard. You pull or push it into the proper place to help keep your eye in position and block ambient light. The correct way to set the Instadjust is to first pull the housing all the way up and put your eye to the eyepiece. As you look through, you'll notice that you don't see the entire field. Now simply click that housing down a step or two till the edge of the field stop is sharp in your peripheral vision. A proper setting allows one to see the entire field, and also prevents you from getting in too close to the eyepiece and thereby vignetting the exit pupil. Instadjust does an excellent job of blocking extraneous light - in my opinion, noteably better than the fixed eye guards on the t5's and t6's. Instadjust does have one small drawback; older versions can make your heart skip a beat. You'll understand what I'm talking about if you've ever picked up one of those t4's, and felt that weight shift in your hand as the Instadjust slides. It feels like it's going to fall out of your hand, and I've caught myself preparing to make a dive for it on occasion. In newer models, this has been addressed by modifying the internal design to tighten the Instadjust eye guard. A retrofit is available to help out folks with older models. Before we take a look at the individual members of the Type 4 family (and some of their counterparts in the other lines), I'd like to say that while the t4's are not a scaled design, performance is so similar I felt I was looking at the exact same eyepiece, but at different magnifications. 
In a large Dob, this is an excellent moderate to high power eyepiece. The immersion factor and true field the 12 t4 yields is truly only surpassed only by the Ethos with its 100 deg AFOV. This is my second favorite Type 4. Under detailed examination, I found there to be some slight uncorrected rectilinear distortion and a small amount of lateral color towards edge of field. If you are careful to keep your eye on axis, the color error is minimized. This makes me think I'm accidentally vignetting the light bundle a bit - not something that is inherent in the eyepiece, but rather in how I'm interacting with it. To be clear, there is a little color at the edge of field stop (blue - this is due to illumination of the field stop by a bright object in the field), but this is a very very minimal amount. I found blackout to be intrusive during daytime use and wouldn't recommend it for that. On the flip side, used where intended, I found the eyepiece much easier to view through at night. The field is flat, and despite some claims I've read on various forums, I wasn't able to detect any angular magnification distortion. I wonder if people who claim to see this are actually confusing the effects with pincushion at the extreme edge. Eye position is somewhat critical on the 12 t4, but using the Instadjust to best advantage helps immensely. Another thing that will help some observers here is the bit of plastic that TV includes with the Type 4 eyepieces labeled "The Pupil Guide". This is a circle of plastic that helps the observer keep their eye on axis and thus minimize the users propensity to drift and thus vignette the light bundle, as well as limiting environmental stray light not blocked by your head. The pupil guide is another tool that when properly used in conjunction with the Instadjust forces your eye to be at the correct location for viewing. The pupil guide was originally developed for the short focal length Radians, where the resulting small exit pupil, large eye-lens and long eye-relief all contributed to frustration in locating the eye to the exit pupil for beginners. Tele Vue provides it as a further aid for those who have trouble finding the exit pupil as it tends to help center your eye to the large eyelens. This becomes particularly more helpful if the exit pupil is very small. Personally, I don't find it necessary - but some might. As compared to the 11 and 13 t6: I felt these had slightly greater pincushion than the 12 t4, although not by much. (Truly, it's very hard to tell the difference.) Eye positioning is still somewhat critical during daytime use, but it's not as easy to blackout the 13 as the 12. On the other hand, the 11 is much easier to black out than the 13. If you get your eye in too far, then a larger portion of the field blacks out. I felt (but couldn't put my finger on it exactly) that the t6's had a very slight edge in performance at night, most likely due to increased contrast. The field in the T6's is flat. Color error in both t6's is minor to nonexistent. For daytime use, or in a binoviewer, the t6's are to be preferred over the 12t4. For immersion - the 12 t4 wins hands down. As a friend of mine put it - it's really hard to believe that these have the same AFOV. The huge curved eye lens of the 12 t4 really hugs your eyeball and adds to the immersive feeling. 
When examined critically, there is some some minor pincushion. Again, like the 12, eye placement is critical, and Instadjust helps here. Lengthy forum discussions aside - by design, the field is flat, and it's sharp to the edge in a flat field telescope. Views are immersive and pull you into the scene. Any angular magnification distortion (if any exists) was not apparent. Again, I found some minor false color at very edge of the field, and keeping eye on axis helps. This is an extraordinarily immersive eyepiece. When it's in the focuser the telescope just disappears. As compared to the 16 t5: I found the eye relief on the 16 to be noticeably tighter, and wouldn't recommend the 16 for eyeglass wearers. The field stop has the blue ring that is so often ascribed to the t5's. I felt the 16 had a little more pincushion effect than the 17 t4, and to my eye the field is just a hair less flat. Most users probably won't notice the difference. I was somewhat surprised by the almost complete lack of blackouts with the 16 whatsoever. I also feel that contrast is slightly better in the 16. There may be a touch of angular magnification distortion present, but with the pincushion I find this very hard to quantify. Likewise, there may be just the smallest touch of astigmatism in the 17. I seem to notice it some nights, and not others. It could be my eyes or something else in the optical train. Because of it's size and weight, the 16 makes a better choice for small scopes, alt/az mounts and (the only choice) for binoviewers. If you're using the eyepiece in the daytime, I'd recommend the 16. However, if you 'll be using the eyepiece under the stars and can handle the size and weight the 17 t4 gets my unreserved recommendation. 
What happened? I fell victim to a common occurrence, one I warned you about at the beginning of the article. What I was looking at was the residual field curvature from the apochromatic doublet, not an aberration in the eyepiece at all. In these scopes the 22 is fantastic. Looking at it critically in the NP101 for this article I found it to have a flat field and some minor pincushion distortion. As compared to the 17, I found eye placement not quite as critical, and the extreme edge of field color to be less intrusive. This is an absolutely beautiful eyepiece. As compared to the 20t5: Its main competition in the Tele Vue line up, the 20t5, is another fantastic eyepiece. The 20t5's field is flat and there's some visible color at the field stop, typical of the type 5's. The 20 T5 also, as is typical of the Naglers, has a minor amount of pincushion. When used at night, I find the only time this is really noticable is when viewing Luna. The 20 t5 is fairly small and light, and might be a little better match for you if you're using a smaller telescope or are concerned with balance. On the flip side, the eye relief is a bit tighter.  For all of these eyepieces I found the most significant / noticable aberration to be pincushion. Some will decry the distortion at the edge, but eyepiece design is a trade off, and that's a natural consequence. Tele Vue has made the choice to let pincushion be the "allowable" aberration (to a degree). This is a philosophy I whole heartedly agree with. It's only when slewing that pincushion becomes noticeable. Personally, I don't see pincushion distortion as an issue on a stellar field or on targets with small angular size. All of the type 4's have essentially flat fields with only minor amounts of false color creeping in at the edge. All provide generous eye relief and excellent contrast. And when used in a flat field scope, all provide tack sharp images right to the very edge of the field. Further I found the T4's to be better corrected for angular magnification distortion and pincushion than their "competition". Type 4's are generally viewed as deep sky eyepieces but when barlowed they can yield some excellent lunar and planetary views, showing only a slight loss of contrast when compared to the simpler designs. With the exception of the 22, I probably wouldn't choose these for daytime use. They were designed to be used under the stars. My admittedly arbitrary preferences for the type 4's follow: the 17 is my hands down favorite, but the 12 is close runner, and the 22 comes in third place. The t4's yield an exceptional spacewalk experience which typifies what most folks come to think of as the "Nagler" view, and are wonderful eyepieces. They are excellent choices for the amateur looking to experience the "spacewalk" effect while not sacrificing comfort, long eye relief or contrast. Type 4's recommended for:
Bilblography / References / Credits / Sources / Recommended Reading Telescope Optics - Rutten and Van ven Rooj Tele Vue: A Historical Perspective, Company 7, http://www.company7.com/televue/telal.html Nagler Mythos, Tom Trusock, http://www.cloudynights.com/documents/naglers.pdf Various Articles, Tele Vue Optics Web Site, http://www.televue.com The Nagler Eyepieces, Part 1 - T1's, T2's and the Nagler Zooms, Tom Trusock, http://www.cloudynights.com/item.php?item_id=1723 Al and David Nagler |
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