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Home / 13mm T6 Nagler or the 16mm T5 Nagler
by Pete Rasmussen 01/11/02

Is it going to be a "lucky" 13mm, or a "lucky" 16mm this time around? A tough call based on the pros 'n cons between each of two of the latest Nagler ultra-wides.
Eyepieces... eyepieces... what 'm I gonna do!

The Observing Setting:

  • Friday, 12/28/01, 11:00 PM to 1:30 AM CST, 35.5 degrees N Latitude, Arkansas, USA.
  • Nearly full moonlight ~20 deg. off away from Jupiter.
  • Local temperatures ~30 deg. F.
  • Light intermittent winds, some beautiful wave clouds and high thin patches.
  • Variable seeing (mostly average) but with added short periods as VG to EX.
  • Fully collimated/acclimated 17.5" f/6 Dob Newtonian.
  • "Up 'n down" a 6 ft. step ladder for 2 hrs. 30 mins.
  • Test EPs: 13mm Nagler T6 (205x) and 16mm Nagler T5 (176x).
  • Control EPs: 13.8mm SWA (193x), 12mm Konig II (222x), and 12mm SMC-ED Ortho (222x).

The Comparison in Summary:

I like the newest 13mm T6 Tele Vue compact "Nagler". It is a product made in Japan vs. Taiwan where the recently introduced 16mm T5 is from. They both bear the same price tag of around $300 USD. The 13mm is a 7 element design and the 16mm is a 6. Eyepieces in this FL range are often the most useful for general (usually medium) powered viewing.

This latest 13mm model may well could be the very best product namesake yet designed. It is obvious, from the initial glance over, that Mr. Nagler took it unto himself to brilliantly craft something yet nearer towards perfection, with visual astronomy involvement in mind, and used by non-eyeglass wearers.

The non-specific eye relief data Tele Vue provides states 12mm for this EP. I measure < 11mm off from the top of the retracted rubber eyeguard surface. That, BTW, is the most realistic way to quantify this specification and the only one if applied to the needs of eyeglass wearers.

There is an alternate choice in providing this important product specificaton and is what I believe Tele Vue may have chosen for these. That is to gauge a measurement as between the front of the eye and to the center of the outer glass surface itself. This method can often be very misleading and can easily shortchange the end user's expectations or needs. The reason why is that the top of the EP is higher up away from the glass and the resulting useable eye relief will be less than stated.

The newly compressed outward size and appearance of the Nagler 13mm T6 eyepiece itself (vs. the old and lumbersome 13mm T1 made a few years back) offers potentially huge advantages. This is especially so if the optic design proves suitable to pair them together for that special two-eyed opportunity allowed for through a good binoviewer!

The image below shows most of the test EP line up. The weight and size of the 13mm T6 model (left of center) is 180g and 42mm dia. x 78mm tall. The 16mm T5 (right of center) is 200g and 44mm dia. x 78mm tall. The original style Meade 13.8mm SWA in on far left and University 12mm Konig II on far right for reference.

The 13mm T6 possesses a nicely placed, well defined field stop with surprisingly low chromatic fringing around it. The 16mm T6 in comparison has a significant fringe. However, the severity of lateral color error seems equivalent overall, in use, between them.

The kidney bean effect in this new 13mm EP does not appear significantly intrusive or that bad while under most night viewing conditions. Still it remains in a minor sort of way to disturb the visual acuity involved with viewing a bright moon or by doing daylight terrestrial work. To view by the brighter light conditions, one would most usually have to pull back away from the EP to eliminate the KB effect at the expense of angular field size and reduction in ability to hold the view.

The 16mm Nagler doesn't effectively have this aberration and works much better by day. That indicates the 16mm is possibly the better model between the two when if paired up. However, this does not rule out the 13mm used that way, particularly so when at night when the eye pupil diameter is expanded.

With most viewing, the behavioral impact of this 13mm model, towards the human eye, is quite similar to what is found in a very good large-sized 2" ultra-wide super EP. There is a definite and enticing "wow" image factor eminating outwardly from within the small (1.25") package.

When the eye is not being flooded with bright lights (hence maintaining a larger eye pupil) it is close to twice as easy to hold the view in the 13mm as compared to the 16mm. Since both are seemingly optimized (but differing) designs, much of the advantage here is likely due a relationship between the larger 18mm exit aperture diameter of the 13mm model, and the ability of the eye to laterally reposition itself enough to take in the huge field of view.

The 16mm EP has a smaller 15mm exit aperture diameter, and the eye must be positioned in tighter and closer to the instrument center axis to work well. The non-specific eye relief data Tele Vue provides states 10mm for this EP. I measure < 7mm off from the top of the retracted rubber eyeguard surface. The eyelens outer surface is steeply concave. Much more so than the one on the 13mm T6.

There is a minor degree of blackout flutter that tends to crop up in the 16mm, and it leaves things roughly a third harder to maintain visually when getting that good full field view. The 16mm has significantly less "wow" factor involvement partly due its smaller, almost peephole-sized exit aperture to work with. It is my opinion the 16mm T5 should have been made as an 18mm, instead, for the needed "cush" with eye relief and exit aperture by design.

Now, the 12mm Konig II also has a small exit aperture (10mm) outbound of its 60 deg. AFOV design. It, and the 13.8mm SWA (at 16mm exit aperture and 66 deg. AFOV) show useability differences akin to the Nagler comparison.

The image of Jupiter through the 13mm T6 was a bit warmer toned, than in the 16mm T5, where with it, the image was cooler and hence closer to a neutral tone. The difference in color throughput is easily visible under the illuminated white paper test but my rather low resolution camera unfortunately would not show it.

There seems to be a very slightly higher degree of chromatic error produced within the 13mm against the 16mm. It was hard to see any differences well, and therefore probably is for the most part irrelevent.

There is a slight bit more pincushion distortion seen in the 13mm than in the 16mm. Outer field sharpness was found as slightly better in the 16mm. There is much more angular magnification distortion in the 13mm's outer field. An example highlighting that is where Jupiter appears stretched directly away from the EP center point when it is viewed nearer towards the field edge.

When a dealer or optic company representative says their eyepiece is "sharp to the edge", don't believe it! Having better field corrections may be one thing, but all of these EPs are with spherical shaped glass surfaces (vs. aspherical) and will show aberrated fields with most telescope set ups.

The 16mm seems to have slightly more effective internal baffling than the 13mm but both are still excellent. See image below that shows the bright internal ring surface reflections for each while under oblique angle lighting.

The image contrast and color saturation properties inherent to each of the two test eyepiece is very similarly high.Jupiter was, for the most part, presented in grand style in each. The levels of these attributes is quite close to that of the quality available from a Pentax SMC-ED modified ortho. I believe this special Pentax ortho line has some of the finest coatings and careful internal baffling ever combined and made available commercially. The 13.8mm SWA and 12mm Konig II, OTOH, presented a bit less contrast of the planet image. Very slightly more so with the Meade than the University EP. This I believe, for the most part, is due coating differences between makes and the number of elements. There are 4 in the Konig II and 6 in the SWA. Color saturation is also lower in these two over the others tested.

Granted, with the longer FL, the 16mm is brighter. But it also seems much sharper than the 13mm than normal. This is probably a reasonable approximation towards fact, and is based on my experience and in taking the 15% magnification difference between the two into consideration. For example, an even shorter FL model 12mm Konig II seemed crispier than the 13mm T6 in comparison on Jupiter. The 16mm T5, OTOH, is a known and amazingly crisp model.

The 13mm T6 is still very sharp but maybe not as ultimately so as I had initially hoped for. Nothing great always just comes easy and this one leaves me guessing. Alas, there can be much wild speculation among potential trade-offs involved with the pulling off of an exotic view.

Careful daylight resolution comparisons speak the same on the issue of relative sharpness. My second 13mm T6 is to arrive shortly and will allow me to put them head to head for potentially detectable product variances. I also intend to pair and carefully study them further in my binoviewer.

The ghost image some folks have mentioned in the T6 models is not an issue IMHO. As example, there is a light dancer of Jupiter's likeness seen brought to illumination left of axial center if the planet is placed at one quarter of the way from the right field stop. The 16mm T6 has a slower moving "dancer" in the same manner but it was only about half a bright. The 12mm Konig II has one of these, too. All of this is normal fair, and basically non-distractive even with a powerful light grasp instrument. It is simply the nature of many a wide-angle EP.

Mr. Nagler must have been listening to the ranting on sci.astro.amateur newsgroup regarding the 1.25" barrel sleeve safety undercut. The depth of this cut on the new 13mm is noticeably shallower than usual (about half as deep) compared to most others having this controversial feature. The portion of barrel a full 1.25" diameter near the top of barrel itself (just under the upper housing) is a bit longer, too. This will make for a more assured alignment fit when placing the EP into a drawtube sleeve. The barrel is also longer overall to keep curling fingers from coming in contact with the R1 field element surface. It is just too easy to touch the field lens surface on the 16mm T5 ...eek!

In Conclusion:

The decision on which to keep may well be based upon the ultimate binoviewing capacity of one, against the ultimate sharpness of the other.

From this first introduction, my feeling is that as paired, the 13mm Nagler T6 will sweeten up nicely enough in sharpness to become a rather spectacular performer. That will be unquestionably a good thing. Everything else so far gleaned (albeit initially) seems to suggest we have ourselves a reasonably close approximation to that of the proverbial "perfect eyepiece".

Of eminent importance, we will likely experience first hand what visual "wowing" potential is available from this otherwise unassuming sized newcomer. Reeling in wide-eyed views from both left and right eyes to near 90 deg. apparent field would be spectacular, and delivered scorchingly hot from a good "flamethrower" of a telescope.

To Be Continued...

Pete Rasmussen
12-29-01

Part Two: A CO-OP of Eyes Champions the Night!

The second 13mm T6 Nagler finally arrived (01-07-02)... better late than never. See them paired (pictured below) pretty in a row, in the big Zeiss binoviewer, atop the big yellow 17.5" f/6 Dob. The scope, BTW, was built by long time friend and fellow enthusiast, Ross Workman of Kentucky.

Under brief visual inspection, both example eyepieces received appeared as virtually alike cosmetically, and by way of their coatings. Into the Zeiss and 102mm f/5 achromat refractor they go for a daylight two-eyed look-see...

Immediately, the invaluable nature of the traditional binocular-type rubber eyecups on these became apparent. I had supected earlier that there was something significant about them since they were so perfectly placed in relation to the exit pupil distance of the optic design. What happens is the eye socket can "lean" into the eyecup skirt. This assistance with eye-to-eyepiece placement (and maintenance) is important with this rather exotic model so as to best take in the large view without disturbance from inherent optical aberrations.

With my head propped into position by resting the bottom of the eyecups on the edge of my lower eye socket, and with eyes leaned back somewhat away from the eyelens, about 70 deg. AFOV was visable for use. A panoramic scene within that AFOV can be reasonably utilized without much kidney bean, or other distortions to struggle with. Most of the acheivement is by way of both of the eyecups together pressed against the face to keep the eyes and head well positioned.

If for daylight use (or a bright moon for that matter), and to include panning in the mix, I would call the new 13mm T6 useable but would not give it much better than a fair rating. The reason being is the squeamish nature of utilizing this especially wide optical design in conjunction with a smaller eye pupil size. There are much more suitable models to use for daylight applications that can maintain high contrast and wide fields of view. There is also significant glare off from the wide metal eyelens retainer ring. It was more distractive at night under my small town suburban conditions. The short rubber eyecups do not block incoming stray light to the eyepiece top.

Fortunately, at night, this part of story really got much sweeter. When the eye pupil dilated to perhaps > 4mm, the ability to pay full attention to the panoramic effects of a vast image field improved tremendously. This, of course, refers to the freedom from aberrations that would otherwise plague the pleasantries of the view.

By "leaning" onto the eyecups, a much greater appreciation of the vastness of the huge field could be ascertained. The 13mm T6 seems to have proven it worth in gold! A magnificent acheivement tailored to bring forth exquisite mid-range views ... and deliver them through two eyes! With this EP duo, my eyes could exercise wide angular sweeps, at medium motion speeds, from one side of the field to the other. That is very good and acceptable form for capturing panorama from the night sky through a telescope. About 75-78 deg. AFOV is the useable maximum with this new model for night viewing fun and adventures.

Now, in using this moderately small package of optical ingenuity, there remained a residual factor of confinement from (or separation of) the eye's involvement with the view. This, versus the overt utility of a much larger ocular system, that would hence allow the peering through of a much bigger picture window, to start with.

Imagine things as something similar to this... You are to try on two types of round framed spectacles. One has a large enough aperture ring that you hardly notice their presence wearing them. The other, a pair that still allows you to view a full 90 degrees in front of your eyes, but, still you can notice them being there as a peripheral artifact. Both provide fully opened up vistas to the eyes, but, only the one projects the user effectively through and into the image itself.

What about the visual acuity improvements while using two eyes with Jupiter you ask? Try a three fold image improvement or greater! This is the very first time I've utilized large aperture in front of my binoviewer, and that may have something to do with the results. Comparisons done with much smaller scopes (6" and under) seem a bit less pronounced -- perhaps a two fold gain. Another finding, with the 13mm T6, was that eye repositioning comfort improved the viewing concentration a bit over an equivalent FL ortho. There is, of course, the major advantage with longer viewing stays if the scope is not driven in right ascension.

I had to shorten the scope truss poles to achieve focus with the binoviewer. The reduced telescope length also meant that with the heavy binoviewer and eyepieces in place, things would remain conveniently balanced as before!

There was no barlow (optical relay) involved in this set up. It was straight through the beam splitter prism assembly from scope to EP. Dr. Baader, owner of Baader Planetarium, Germany, once told me that my large Zeiss viewer would work best at f/6 and above in a Newtonian while used straight through without barlow interface. My new viewing experience seemed to play this out as true. Jovian detail was highly resolved and there was virtually no color fringing on the limb itself. Maybe this translates to very high accuracy with the Zeiss optics.

One of the by-products of revising the Dob was that the 2.60" ma diagonal effectively became undersized. It will need to be replaced with a 3.1" mirror as minimum. My calculations showed that when I was viewing at high power (250-333x) I was using approx. 13.5" of effective aperture, with a 20% central obstruction, all at f/7.8. At moderately low magnification (109x), I was only eliciting about 11.5" of aperture, with a 24% central obstruction, all at f/9.

The images in the 13mm T6 Nagler pair were sharp and quite flat-fielded across the majority of their fields. This was perhaps while working at an effective f/ratio of 8 or so. I retain my earlier impression and belief that the lens system of this eyepiece model is close to (but still not having) ultimate sharpness on-axis. The reinforcement this time based on final comparisons against a pair of 12mm Pentax SMC-ED orthos with Jupiter. However, what is especially good is the contrast and color saturation in this new Nagler. These two aspects, for the most part, are _very_ close to the same as the "ultimate" perhaps found in the Pentax orthos. Lastly, there is probably a slight bit of light throughput loss, over the Pentax, but it seems insignificant with the latest combination of glass and coatings.

On deep sky, the magic of moderate telescope magnification coupled with a nearly perfect ultra-wide field is a highly advantageous combination for any scope. Maybe it's better to simply call it perfectly compelling! But, the fact remains, in the real world, true fields are narrowed down in the process of scaling up the image.

To regain the often very desirable space of (or around) an object, one needs to employ the widest possible apparent field eyepiece that is comfortable for the eye(s) to work with. Now this "possibility" has become a worthwhile reality.

I believe that once enough visual astronomy folks have taken the required two-eye test run, they will be changed a little and forever. It would be fairly safe to say that Al Nagler scored a major victory with this new compact portal into our favorite spatial medium.

All in all it was an enlightening experience...

May your eyes enjoy great wide views :)

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