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CN Report: The Nagler Eyepieces, Part 1 - T1's, T2's, and the Nagler Zooms
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Tom T’s – The Nagler Eyepieces
Part 1: Type 1’s, Type 2’s and the Nagler Zooms
For the last few years, there’s been a growing need for a fairly comprehensive review of the Tele Vue Nagler eyepieces. The sheer number of eyepieces bearing the name Nagler leads to inevitable confusion in the marketplace – especially when confronted with all the different models available over the years.
Type 1’s, Type 2’s, Type 4’s, Type 5’s, Type 6’s and zooms – what are the differences, the strengths and weaknesses of each, and how does one choose? It’s a bewildering dilemma for a neophyte looking to sample the market leaders wares. With two notable exceptions, all Naglers have one thing in common – they provide a stunning 82 degree field of view. Where does one start?
And so, I thought I’d throw my hat into the ring and attempt to develop a semi-comprehensive review with an eye towards helping out folks who are looking to purchase. So what Naglers am I going to cover? All of them.
Yes. All of them.
I said comprehensive, didn’t I?
Ok, ok, so it’ll only be valid for a year or two, but hey – till then…
I’ve previously written an article entitled “Nagler Mythos” for which I summarized the differences between the lines, answered some frequently asked questions, and talked a bit about the history of the eyepieces. By and large the current document(s) isn't going to rehash that, so if you’re interested, you might want to check it out (see the end of this document for the link).
All told, there have been 24 eyepieces that have borne the name “Nagler” over the years, 16 of which are still in production as of this writing (late 2007). The sheer number of eyepieces available forces this into a 4 part series.
Before I get started, I’d like to bring forward a couple of somewhat neglected or often misunderstood points.
First off, a couple of words about testing – I’ve used these eyepieces with many different telescopes over many different years. I’ve had them in apo and achromatic doublets, triplets and petzvals from 66mm up to six inches, newtonian reflectors from f4 to f7.5 (6 to 20+ inches) and a wide variety of SCT’s and other less common designs. What you see at the eyepiece will reflect what system you’re using it in. Eyepieces tend to perform better in slower telescopes, but all the Naglers have been tested at f4, and are designed for use in fast telescopes – a rather large selling point for many people.
When considering reports of performance, it’s important to understand if the reviewer is talking about the performance of the system, or of the eyepiece itself. All too often today, people ascribe properties to eyepieces that are really inherent in the system. When we look through an eyepiece, we can forget that we’re really seeing almost the end result of the entire optical train: Target, sky (seeing, transparency), objective, secondary or diagonal, eyepiece, eye and brain. All of these things work together to give us our final image. It’s important to recognize what contributes different elements, and sort out the extraneous items as much as possible. (As a side note, it’s certainly valid to talk about system performance, because in the real world, we will never use an eyepiece in isolation.) However, while appropriate, it’s a lot more restrictive if we simply talk about performance in one type of telescope. Eyepieces are a worthwhile investment because they will accompany you from telescope to telescope, but this makes it even more desirable for us to have an understanding of their individual performance. Therefore, in this document I’ve attempted to isolate the eyepiece from the system. This is certainly possible if you know the parameters of your system, but the best telescope for this sort of work has to be the Tele Vue NP101. The NP101 presents a flat, color free (intra, extra and infocus), field which makes eyepiece evaluation much easier. Every scope is a trade off but, outside of some minor field illumination issues (which for all practical purposes, is completely insignificant for visual use), the NP101 design is near optical perfection.
Before we begin, there are a few misconceptions that should probably be addressed: field curvature, resolution and contrast.
First, field curvature -
Right upfront, I'd like to state that Tele Vue states that all Nagler eyepieces were designed nominally for flat fields. Therefore, reports of field curvature differences between Naglers are likely to be due to field curvature differences between the telescopes used for the observations. This can be verified by using a flat field telescope such as the NP101 - The grandfather of the NP101, the Tele Vue's MPT, was initially designed to test eyepeices and remains in use today at the Tele Vue facility for just that purpose.
And now a word (or three) about resolution -
I’ve heard people discuss if one particular Nagler is “sharper” than the other. But Tele Vue states that all their eyepieces are diffraction limited at f5.
Diffraction limited - An optical system with the ability to produce images with angular resolution as good as the instrument's theoretical limit (Wikipedia)
Ergo, you shouldn’t see resolution differences.
And yet people often talk about seeing resolution differences. I’ve even done so myself in the past. What’s going on here? Are there really resolution differences? Well, the short answer is no. The long answer – well, it’s a bit more involved, but not much.
I recently called up Tele Vue to discuss just this issue, and after some discussion concerning the design particulars of their eyepieces, it became evident to me that we should not be seeing resolution differences due to design. After some time internalizing, I’ve come to realize that this stems from a misunderstanding of the effects of contrast.
When we talk about resolution, we mention the ability to observe fine detail – either on the Moon, the planets or daytime views. But that’s not all there is too it. Contrast impacts more than we might first realize. If we work from the standpoint of a design view, all the Naglers are roughly equivalent in terms of resolution, how then do we explain these apparent differences?
There are a couple of factors at play here. When we observe objects that are at the limit of detection, absolute contrast can make all the difference in the world. As amateurs we know and accept this. However, consider the following - if we consider two eyepieces with differing contrast, you’ll be able to pick up fine detail easier with the eyepiece having the greater contrast. Because we find small details (like fine lunar features) easier to pick out in one eyepiece, we’re tempted to say that this is due to a resolution difference. Not so. It’s actually the effect of greater contrast.
Given two eyepieces of the same resolution, contrast rules.
So what factors govern contrast? Scatter. What governs scatter? Coatings, number of elements and mechanical construction (edge blackening, micro baffle / anti-reflection threads etc). In the past (with primitive coating technology) we increased contrast by decreasing the number of elements. Today, we can have those contrast gains on multi-element eyepieces through improved coating technologies.
This is where the low resolution and light loss ideas started, and why they seem so mythical to many amateurs today.
Now with that out of the way, lets get down to brass tacks.
I’ll start with the originals. Initially simply called Naglers, they are now referred to as the Type 1’s. I’m not going to discuss the type 1’s and 2’s in the same detail that I intend to go into with the current production items, but they do deserve mention – both for their historical value, and the fact that they are still readily available on the used market and frankly are still good choices for certain types of observing.
The Type 1's were the ones that really started it all: the wide field revolution and the reign of the high priced eyepiece. Wide field eyepieces existed before Al Nagler and Tele Vue, that much is certain – however, with a few exceptions, the pre-Nagler eyepieces didn’t have the same massive wide fields of view and they certainly didn’t have the same level of aberration correction.
One thing you have to keep in mind when talking about the older Naglers is that they were different designs for different times. Observers expect different things today then what they did back then. Back then big fast dobs were a new notion. One didn’t have the plethora of fast apos we do today. Amateurs in those days were happy with what we’d today consider minimal eye relief. In fact, the eye relief of the 4.8mm Nagler (considered rather tight by today’s standards) was a definite luxury – longer than anything else at that focal length then available. Finally, of course, we now have access to different coating technologies which are responsible for decreasing scatter, while increasing both throughput and contrast. The original Naglers were only fully multicoated on the eye lens. (This was done to minimize eye reflections.)
All T1's are scaled designs. Al Nagler found by putting a negative lens ahead of the focal plane, he could compensate aberrations from the upper section and deliver well corrected views in the fastest telescopes then available, and while extending eye relief.
Designed in 1979, revised in 1980, then commercially produced and sold in 1981/1982, the 13mm was the first out of the gate. An awesome eyepiece in its day and age, the 13mm was a valid focal length for nearly any telescope and a sweet spot for many. Today, the 13 still makes for a good deep sky eyepiece, however it’s not recommended for lunar or daytime use. Overall, aberrations are well controlled but the 13 is prone to kidney bean / blackout, making eye positioning critical. Modern alternative(s), the 13mm type 6, 12 type 4, 13mm Ethos all have improved contrast and require less critical eye positioning.
Today we’d say the 4.8 has minimal eye relief but at the time, it was quite long eye relief for a short focal length eyepiece. When compared to modern Naglers, and even others in the t1 series, I’ve found it lacks the defining spacewalk feeling because of it’s short eye relief. If you’re willing to sacrifice field however, it still makes a fairly nice casual planetary eyepiece for an undriven scope. This was the first Nagler I ever bought, and it was also the first Nagler I ever sold. However, outside of the price, the modern 5t6 is clearly superior to the 4.8. Another interesting alternative would be the Nagler Zoom 3-6, and I’ll discuss that more in a bit.
The 7mm is the shortest focal length in the t1 series that gives a fairly modern Nagleresque/spacewalk feeling. While it does not hold up to the modern Naglers in terms of contrast and eye relief, overall it’s a very comfortable and nice eyepiece. If you’re looking for a deep sky eyepiece, the 7t1 is still highly recommended. If you’re looking for lunar and planetary, I’d go with the modern alternative, the 7mm type 6 for its improved contrast.
The 9mm t1 was an extremely popular eyepiece in its day, and is still an excellent performer. This is a dual skirted piece for use in either size drawtube/diagonal, but as are all dual skirted units I find it’s really more at home in a 2” barrel than in a 1.25”. If you’re looking for an inexpensive, used wide field in this focal length, you might keep an eye out for one of these on the used market. While the smooth sides have their fans, I’d recommend the later versions largely for the mechanical improvements: eye guard and grip ring. The modern alternative is 9mm type 6, and it surpasses it mainly in contrast.
Finally, the 11mm type 1 is something of a collectors piece. This one only exists in the smooth side version. It’s a fairly rare focal length, and I’ve not seen too many of them over the years. The ones I have seen have always commanded premium prices. As per it being “optically special” when compared to the others – well, it's not. It does give nice views, but it's simlpy scaled down from the 13mm t1. I haven’t seen anything that makes it stand out when compared to the other Naglers available. Still, if you have one of these – I’d hold on to it (from a collectors stand point). As is the trend, the modern alternative(s): 11mm type 6 and the 12mm type 4 surpass it mainly in contrast, but the t6’s are also a bit less critical of eye placement than the T1's in this focal range.
The Type 1's generally have at least two quite different iterations – the smooth sides, and the non-smooth sides. The smooth sides came first, and lacked any sort of eyecup, grip ring or eyelens cap. Later models added these features while maintaining the same optical design, and improving coatings when applicable. Downsides of the type 1’s as compared to later versions typically decreased contrast and lower light transmission. On the whole, the t1’s are still valid deep sky eyepieces, but the modern Naglers are a step up.
For those with series 1 eyepieces, in particular the 9 and 13 smooth side, retrofits were (and are) available with eye guards.
In a quest for longer focal lengths (and for the elimination of the kidney bean seen in the 11 and 13t1's), Tele Vue next delivered the T2's – in 12, 16 and 20mm. In general, the Type 2's are extremely sharp eyepieces (most observers feel they are a bit sharper than their type 1 counterparts), but tend to lack a little in eye relief in comparison with modern designs. As compared to the type 1's however, it’s interesting to note that the type 2's were specifically designed with the same amount of eye relief as the 9mm t1 (12mm), as informal polls showed it to be the most comfortable of the Type 1's. Limiting the eye relief enabled the T2's to be smaller and lighter than corresponding focal length T1's would have been. It’s worth mentioning that the most common definition of eye relief (and the one Tele Vue uses) is from the center of the eyelens to the exit pupil. Thus if the lens is recessed and concave, it’s easy to see why an eyepiece that’s spec'ed with a relatively large amount of eye relief might actually seem somewhat tight.
The 12mm type 2 is the shortest focal length in the series, and has a hybrid 1.25/2” skirt for use in either size focuser. In comparison to the modern Type 4 Naglers, some observers find eye relief a bit tight (I find it fairly comfortable). While the coatings don’t measure up to modern technology, this is still a heck of an eyepiece. Counterparts currently for sale new include the 11 and 13 t6, the 12 t4 and the 13mm Ethos (ok, that’s stepping out of the Nagler line up a bit, but it still deserves mention). Modern technology buys you improvements in coatings (contrast), binoviewer use, eye relief and apparent (as well as true) field of view depending on which one you’re looking at.
The 16 type 2 is the longest focal length of the type 2 dual skirted Naglers. A little smaller than the 12 t2, by modern standards, some observers may find the useable eye relief a little tight – I’d put it at around 10mm or a bit less. It’s comfortable for me. The field is flat and sharp. Modern options in this focal length include the 16mm t5 and the 17mm type 4, while observers looking for a similar true field can also consider the 13mm Ethos.
The 20mm type 2 has a special place in many amateur hearts and kits. This is one heck of an eyepiece. There are those who consider this to be the finest pieces of glass to ever come out of Tele Vue, and I’d agree that it was truly a superb eyepiece. As compared to modern eyepieces, it was lacking in a couple of minor areas- weight (this thing is a beast - it's still the heaviest eyepiece Tele Vue has ever produced!) and optical coatings. Modern counterparts to the 20 type 2 include the 22 type 4, and more recently the 20 type 5, and offer decreased weight and improved contrast. Although some feel that the 20 t2 has a certain quality of image that the modern eyepieces lack my personal preference is for the modern 20t5.
Type 1's and Type 2's recommended for:
Type 1’s and Type 2’s (generally) not recommended for:
And with that, we conclude our discussion of the historical Naglers, and we’re on to present day Let’s begin with the smallest subset of the modern Naglers – both in selection and focal length.
The Nagler Zooms
Some of you may not actually think of these as Nagler eyepieces – the defining factor for most people is the trademark 82 degree field of view. However, Al made a conscious decision to brand these with his name because they are so unlike any zoom that has come before. They are truly revolutionary eyepieces, and was in fact developed as Tele Vue's idea of the ultimate planetary eyepiece..
There are two Nagler Zooms, a 2-4mm and a 3-6mm. (The 8-24mm is more of a standard zoom, was not designed by Al Nagler and not covered in this series.)
The 3-6 was introduced in 2001, followed afterwards by the 2-4. Both spec out with five elements in three groups (for six air to glass surfaces), 10mm of eye relief, and both zooms provide click stops: the 3-6 at 1mm intervals, and the 2-4 at .5mm intervals.. The field stop of the 2-4 varies from 1.7-3.3mm, while that of the 3-6 ranges from 2.6-5.1mm. Tele Vue places them in parfocal group B with the Type 6 Naglers, the 16mm type 5, the Radians, most of the Tele Vue Plossls, and a couple of the Panoptics.
Right off the bat, we can see these are specialized eyepieces and not for every scope. However, you’d be surprised at how useful these can be even in longer focal lengths – a bit more on that later. The Nagler Zooms are stated to be par focal, with a fixed apparent field of view of 50 degrees. This is fairly unusual for a zoom eyepiece, as both field size, eye relief and focus points tend to vary a bit. Further, until the Nagler zooms, zoom eyepieces had a rather poor reputation. In general, fixed focal lengths were usually a bit better optically and tended to have less internal reflections.
In their early days, the Nagler Zooms had to work fairly hard to overcome those zoom preconceptions. Several observers simply discounted them, and the eyepieces just weren’t given their due. Time does tell however, and today opinons are very favorable.
They are targeted as lunar and planetary eyepieces for shorter focal length telescopes, and the 2-4mm in particular is an excellent match for optics with a focal length 600mm or less (Tele Vue’s own NP101 comes to mind) as it provides magnifications from 150x to 300x at that particular focal length. Most observers will probably find the 3-6 zoom a little more useful. In the short end of the scale, there’s a massive difference between 4mm and 6mm. I won’t single out these eyepieces however, as extended testing showed they perform identically in the regions where they overlap, and the only significant differences are in their range.
The field of the zooms is quite well corrected, and shows no signs of astigmatism. (One of my pet peeves, astigmatism is the aberration I detest most and I’d trade off any other aberration to be rid of it.) Distortion and field curvature aren't issues.
About the only optical oddness is some subtle and exceedingly minor hints of color (blue) at the field stop – and only at high powers. It’s not lateral color. (After additional conversations with Tele Vue, I think we’ve narrowed it down to the fact I’m seeing the uncorrected visual edge of the field stop – the eyepiece is fully corrected for light entering via the negative lens group, however the field stop is between the negative group and the upper lens group. Thus the effect I’m actually seeing is due to the uncorrected upper section viewing the field stop. This is invisible at the lower settings, or when viewing objects with dark backgrounds (ie planets and deep sky objects). It’s a non-issue for lunar or daytime as well – even at higher powers I don’t find this intrusive. I only mention it here for completeness, and because I’ve noted it in the past as well.
As to the overall performance of the zooms, let me segue in -
I’ve owned, used, or looked through most of the eyepieces on the market, and personally consider the TMB supermonos one of the finest planetary performers available today. On axis, the eyepiece contrast is wonderful. But when I take into account the supermonos off axis performance and eye relief and the story changes a bit. A while back I compared the supermonos to the 3-6 zoom., and while the supermonos had a small edge in contrast, I noted two things – 1) there was nothing visible in the supermonos that wasn’t visible in the zoom (resolution wise, they were equal), and 2) the zoom performed much better off axis than the supermonos. In the years since, after owning all the short focal length monos and a host of other extreme planetary eyepieces, I’ve seen nothing to change my mind.
The Nagler Zooms performance is excellent and they remind me of nothing so much as an infinite plossl, but with better edge correction.
The other option for short focal lengths in the Nagler line lies with the Type 6s. The type 6’s do provide that massive 82 degree field of view, and thus have a distinct advantage on a mount without tracking. But you do loose that “zoom” ability. As per optical quality – well, it’s something of a toss up. At times I’ve felt there is a touch more contrast in the Type 6’s, but I’d hasten to note that others feel differently, and I’ve never been able to definitively confirm it. From various other reviews, forum posts and comments, most people don’t seem to see any difference, so one may not even exist.
To look solely at the optical qualities of the zooms is to neglect their most interesting aspect. The ability to – well – “zoom”. This is something that simply cannot be understated. With the zoom you can assume any focal length between the two extremes, thus providing a range of magnification. This aspect saves you both money and frees up space in your eyepiece case. But perhaps more importantly, it lets you precisely match magnification to the evenings seeing conditions. Another effect is particularly attractive to users who favor alt/az mounts. The zoom feature allows one to find the target and then zoom in on it without having to go to the fuss of changing eyepieces.
Don’t automatically assume the short focal length severely limits the telescopes that you can use the zoom in. Over the years I’ve used the 3-6 in everything from a 360mm TV60 (and they barlow quite well BTW - IF you actually need them too) to my 18” f4.5 Obsession where the magnification provided ranges from 394 to 788x! (I mainly use it for digging the central stars out of planetary nebula, as well as spotting faint galaxy groupings.)
There is one minor mechanical issue I’ve had with the zooms over the years. Some of them seem prone to “freezing up” in winter. When stored outside for an extended period of time in sub zero (-10F) weather I’ve noted that the twist mechanism can become sticky and even lock up. This could be a possible drawback to observers who store their gear in an unheated location, like a garage or an observatory. I’d stress this has not happened during an observing session when I bring the eyepiece outside from a warm environment. I give out long before they do. If this happens to you, don’t force the zoom mechanism, simply bring the eyepiece inside and let it warm up gradually.
Zooms recommended for:
Not recommended for:
In the next installment, I’ll be discussing the eyepieces that currently define the term space walk – the Type 4 Naglers. Stay tuned, we’ve got a lot to cover.
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
Al and David Nagler