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Review of the Televue Nagler 31mm, T5

Posted by Ed Moreno   01/12/2007 08:00AM

(AstroMart Editor's Note: At the end of this review, you will find a brief dialogue between the author an Al Nagler, designer of this eyepiece)

I was surprised to find that there weren’t that many reviews of the T 31mm Nagler, but hey, even if there were, I would STILL write my own, because well, I LIKE writing reviews and I REALLY want to thank the readers that send me notes telling me that they liked READING one of my reviews.

So, here goes.

I am a VERY lucky man. My girlfriend is ALMOST as good looking as I am. In her own way, she is just about as SMART as I am. I like that. TOO smart and I start to feel challenged. And it gets better. She is RICHER than I am. WOW. Am I a lucky boy, or WHAT! So, what has THAT got to do with anything you are asking? Well, EVERYTHING in terms of the fact that you wouldn’t be reading this if she hadn’t decided buy me a 31mm Nagler for my BIRTHDAY! WOW…

See, I previously took the LOW road. Now there isn’t much new in that piece of news. I have well over $12,000 of astronomy gear sitting around, and honestly, I have decided that astronomy is getting expensive, and I decided that I wanted to some of that money I was spending on astro grear and spend it frivolously instead. This means that the HUGE obstacles to owning a Nagler have proved insurmountable (all 640 of them).

My primary wide field eyepiece for 5 years has been the Televue 35mm Panoptic eyepiece. And WHAT an eyepiece. NOTHING I have previously put up against it has proven to be as good to use for getting a reasonably wide, HIGH QUALITY, low power field of view. I have tried to get a bigger low power fields. I tried mid priced 40mm wide field eyepieces, and mostly found them to be unacceptable. They were too soft off center. Half-way out from the center, the UO and the Meade both started to degrade. On axis, they were nice, but by 60%-70% percent of the way to the edge, both of these eyepieces fell apart in my f/5.7 140mm refractor. In my SCTs they were better, but only a bit. Now I VALUE a field that is sharp across MOST of the field and none of these other alternatives really did it for me.

I also tried some discount 32mm widefield eyepieces in the refractor, so I wouldn’t have to keep switching the 35mm Panoptic back and forth between my big SCTs. The 35mm Panoptic was my WORKHORSE low power eyepiece, and I rode that pony so hard that I had to put it away wet more than once. Well, the 32mm widefields were actually even WORSE to me than the 40mm eyepieces were. The extra magnification seemed to make the edge of field seem even MORE distorted simply because it magnified the smudges more.

Finally, I settled on a CHEAP eyepiece for my WIDEST FIELD eyepiece, and it was better than either the 32mm or 40mm mid-priced selections at providing a wide, sharp field. That eyepiece was the Meade 56mm Super Plossl. This eyepiece was sharper than any of these other eyepieces off axis, and when I wanted a bit of extra field for some of the larger open clusters when using my C11 and later, my C14, I would plug in the 56mm Meade. The 56mm offered the widest true field I could achieve in my C11 and C14, and in fact, it was QUITE sharp across 90%. But there was a big downside. In my C14, the 56mm presented a 70x view with a 5.0mm Exit Pupil. Now the problem with this is that if you are like me and live under light polluted sky conditions, a big exit pupil works AGAINST you, because it lets the foreground light pollution really wash out fainter objects due to lack of sufficient contrast. This is nothing to do with the eyepiece itself. It is simply a function of the physics of the human eye. Anyway, the 56mm DID let me stretch the field of view from about .61 degree with the 35mm to about .71 degree. This doesn’t seem like much of a difference, but when M37 is so big that it almost totally fills your field of view, a few extra arc minutes can make a big difference by allowing you to put a darker sky “Frame” (albeit a very THIN frame) around the object. The 56mm DID this, but at the same time, in the example of M37, it did it with the accompanying affect of washing out the cluster a bit, so that the view looked somewhat dull, and many of the very dimmest stars simply merged into the bright background. So the ability to FRAME objects improved, but the ability to resolve the faintest features decreased. In many instances, I have to admit that I simply preferred the view through the 35mm Panoptic because of the extra contrast afforded by the higher magnification and correspondingly smaller exit pupil.

So, MAGNIFICAITON is really your friend in the fight against light pollution. For really big clusters where the larger amount of stars was BRIGHT, the 56mm worked well enough to provide better framing, so the humble 56mm Meade remained in my inventory.

Now, I have done the math again and again… A 35mm Panoptic offers a .61 degree field in a C14. The Meade 56 offers a .71 degree field. A 31mm Nagler would come in at only .65 degrees. On one hand, there were those 640 obstacles to getting a 31mm Nagler, and on the other hand, the appeal of SLIGHTLY higher magnification and SLIGHTLY wider field of view offered by the 31mm Nagler as opposed to the 35mm Panoptic seemed like a tough sell. In the end, I was TOO STINKING CHEAP, so I just went with the 35mm Panoptic and stuck in the 56mm in those cases where I needed a bit more field. And you know what? MOSTLY, I was satisfied with this.

But we all have wish lists. And a 31mm Nagler was on mine. Still, I had a LOT of inertia against PURCHASING one. But remember the girlfriend??? Well, SHE came through! WOW. A 31mm Nagler of my VERY OWN, and it didn’t cost me a CENT (but maybe those diamond earrings and that diamond necklace that I gave as presents over the last couple of years helped….YA THINK???).

Anyway, like I say, I am a lucky man. And now, without further delay, I offer my review.

First, I will tell you that when I got it, it was DAYTIME! DARN IT! I am TYPE A when it comes to wanting to play with my toys. But I was determined to use the eyepiece, so at my first opportunity. So I just held it out in front of me and STUCK MY EYE DOWN INTO THAT BIG HOLE…. And I was not happy. With my glasses on, I could not get close enough to get the edge of field into view. The lens on my glasses would hit rubber flip-down eye-guard before I could get close enough to actually see the field stop out there 41 degrees either side of center! I was a bit worried. SOMETIMES, I have to wear my eyeglasses in my fastest scope (140mm - f/5.7) to handle the astigmatism of my eye. Did I say already that I was a bit worried? But I knew that in the 140mm, the 35mm Panoptic, which has a 6.3mm Exit pupil and worked at 22x COULD be used sometime without my glasses. I was hoping that the 31mm with its 25x and slightly smaller exit pupil (5.6mm) would negate the need for glasses when using the 31mm. Worse case, I would always have the 35mm, which has excellent eye relief, and use it with my glasses.

Well clouds and rain made it impossible to try the eyepiece under the stars, so it was several days later before I first used the 31mm behind glass, and I started on the first clear DAY by sliding that enormous puppy (you have seen pictures of it, and EVERYONE knows that the 31mm is ENORMOUS) into the 2” diagonal that sits behind the ED lens of my excellent little Orion 80ED and aiming it at a TELEPHONE POST! I KNOW that telephone post. I have used it MANY TIMES to check telescopes and such. So I KNEW how much detail I should expect to see.

And what did I see? Well, in the BRIGHT daylight, I was not all that thrilled. Sharp? Yes. Contrasty? Indeed. But the CURVATURE OF FIELD was MASSIVE. I thought that there was a mini-black Donut (an exotic form of black hole) just outside and surrounding the field, and it was SUCKING any part of the phone pole that came close to it OFF of the edge! “Help Me, I’m Melting” seems appropriate for the way the phone pole looked.

And there were COLORS. Man, there were all KINDS of odd colors around the edge of the field. There as a huge orange/red ring at the edge of field, and other odd colors.

Now I was a bit MORE worried. I took off my glasses so I could get my eye closer, and the orange ring at the edge of field went away as I moved my eye closer so that I could see the full field stop at the edge of field. But Blackouts became PRONOUNCED. I had to be VERY steady to keep them from occurring, and only when I was DEAD on axis would the kidney-bean blackouts stop.

I was worried. Yes, contrast was SUPERB, and the eyepiece was SHARP, but it was NOT working for me (for this one target at least).

Night falls.

Stars twinkle into view.

A stack of eyepiece covers grows in the deepening darkness under the Polaris mount.

The 31mm slides into the barrel of the Swan diagonal.

There, off in the darkness, the faint glow of M45. The red dot of the finder drifts across the sky until its journey intersects the pale glow.

My eye moves closer to the eyepiece. Stars start to show in the small window before me. I move closer. The field grows wider, and the stars stream in from the outside edges of the huge field.

Closer. More stars. Big. Bright. And they keep coming in. Closer and closer. It seems like the field is NEVER going to end, like it is going to expand outwards forever.

FINALLY, the field stop comes into view. And I am FLOORED! WOW! Freaking WOW! I have NEVER seen M45 as beautifully in this scope. The shear PRESENCE of M45 is so PRONOUNCED. The view is VIBRANT!

For comparison, I slip the 35mm Panoptic in. The field of view is ALMOST the same (3.9 degree for the 35mm vs. 4.3 for the Nagler), but at the same time, M45 is not quite as lustrous in the 35mm. In fact, some of the faintest stars seem a bit harder to detect cleanly, ALMOST requiring some averted vision. But it looks quite beautiful in the 35mm. I mean it looks TERRIFIC, in the 35mm, but it looks even BETTER in with the 31mm Bagler.

Back to the 31mm. WOW. No doubt, the field is WIDER, the background is DARKER, and the stars appear BRIGHTER as a result. M45 LOOKs better. It is better framed, and I can SEE more in it. WOW!

REMEMBER, it is the MAGNIFICATION that is making MOST of this difference. I don’t think this is a function of the 35 having less light transmission, I think it is totally a function of having both smaller exit pupil, and having that subtle extra 14 degrees of apparent field of view to work with.

I plug in the 56mm, even thought the 7.2 degree exit pupil is wasting some light. Faint stars wash completely from view. The cluster looks pale.

I trade back and forth between the 35mm and 31mm, and after at least 20 swaps, I am convinced that the faintest stars are detectably brighter in the 31mm. Not much, but enough to say that there is a difference. Some of the faintest stars would fade in and out against the background in the 35mm, but would be a tiny bit more solid in the 31. Just a TINY bit, but to me, it was indeed detectable.

As for the field size, M45 is a BIG target, and in the 80mm ED scope, the overall impression was that it was even BIGGER (Due to magnification of course) and MUCH brighter. When viewing the WHOLE object, the 31mm just seemed to make M45 much more “Alive.” Lovely. Really. Absolutely Lovely.

I turned the scope to Orion. It seemed like I could fit in over half the constellation. The southern half of Orion was utterly fantastic. The sky looked blacker, and the stars were in tight swarms everywhere. Back to the 35mm.. Beautiful, but simply not QUITE as “Rich” looking. So yes, in the small refractor, the difference was actually much more beneficial than I would have expected. I was amazed at how brilliant that big piece of sky looked in the big Nagler. Now THIS is wide field viewing!!!!

The field in the f/6 refractor was quite sharp, only falling off in the VERY last couple of apparent degrees of the field. On axis, even without my glasses, stars looked quite sharp.

As for the Kidney-bean blackouts, they were far less intrusive when my eye had fully dilated in the darkness.

The next night, the 140mm Refractor came out. This is an f/5.7 800mm f/l scope, and the 35mm Panoptic used to be used in this scope almost exclusively in the summer. With the 35mm (22x), the field was QUITE sharp almost all of the way to the edge. The field was 2.9 degrees with the Panoptic. I started with the Panoptic, this time on the Double Cluster. MAN what a view. I LOVE the Double Cluster in this scope, and the view was incredible. I established a baseline, then went to the 31mm. Now here, the magnification only goes to about 26x. The field stretches out from 2.91 to 3.17 degrees. Now this was not nearly as noticeable as the difference when used in the 600mm f/l 80mm scope. So the increase in TRUE field size was not so apparent, but the 82 degree APPARENT field DID give a much more pleasing view. But this is a wide target, and I was aware of the pincushion distortion if I moved the telescope so that the nucleus of either cluster came near the outer 20 degrees of apparent field. I did not like the affect. My guess is that on some areas of the sky, such as the area around the spout of Sagittarius, this distortion could be VERY distracting. The amount of pincushion distortion in this eyepiece is EXTREME. It is FAR worse than an ANY eyepiece I have ever used. On a single target, placed at the very center of the field, you will not notice it, but on those giant Milky Way fields were lots of “Fake” clusters abide, the curvature of field may indeed be a serious problem when trying to squeeze two such regions into the same field. Believe me; the pincushion distortion is THAT bad. Remember that telephone post? I swear it had a curve so bad at the edge of field that it looked like it was slumping over.

But on the Double Cluster, the components were still close enough that the distortion was not a problem. And once again, everything looked brighter in the 31mm. I was impressed. Really. The difference is a bit more than “Subtle.” When swapping eyepieces, even though I KNEW I was looking at the same target, it looked “Different.” Better. Richer. Brighter. Again, I think that this is due entirely to the fact that the smaller exit pupil helps boost apparent contrast. I do not think it has anything to do with better light transmission, but MAYBE it does… I simply don’t know. I can only say the double cluster looked better in the 31mm Nagler. Not as much difference as with the 80mm on M45, but enough that I could see it.

The other thing is that the slightly smaller exit pupil of the 31mm in this moderately fast telescope made the astigmatism of my eye suppressed JUST enough that I didn’t NEED my glasses, which was a VERY good thing. Even people that don’t require correction for astigmatism might start to experience some affects as the exit pupil grows larger because more of the eye lens is being used, and defects that don’t show in the doctor’s office WILL start to show when viewing through a telescope using a magnification that results in a very large exit pupil. But again, for me, the 140mm and 31mm Nagler combination WORKED without glasses. Now the f5.7 scope is the fastest I own, and the reason I don’t own faster scopes is partly BECAUSE of the problem of getting really sharp low power views ACROSS THE ENTIRE FIELD, so I was happy that I could dispense with the glasses. Sometimes I would actually NEED them with the 35mm Panoptic in this scope, but the 31mm seemed to be JUST enough past that level, so this was a VERY welcome development.

Well, fact is, the 3.9 degrees of field with the 80mm and the 2.9 with the 140 using the Panoptic were “Enough” for me for a long time, and while there WAS a an improvement with the 31mm, I have to tell you, it wasn’t THAT much that I couldn’t live without the 31mm. But there is no question in my mind that the difference was noticeable.

Now to the Big Gun.. The 3910mm focal length King Kong of portable SCTs, my awesome C14. HERE is the telescope that I REALLY wanted the Nagler for. As I mentioned, the 56mm DID frame objects quite well, offering a .71 degree field. But the object itself usually offered more contrast in the 35mm.

A note: You will HAVE to use a specialized visual back for the 31mm Nagler with a C11 or C14 to fully illuminate the field of view when using the 31mm Nagler. Without it, the field will dim in the outer 10 to 15 degrees of apparent field. The field stop on the T5 Nagler is around 41mm, and the standard back on a C14 is only about 36mm (I am not being exact in these figures, they are approximate, but I think I am pretty close). With the standard back, you can move a star towards the edge of field and see it dim a magnitude or two as it approaches the edge.

The Panoptic by comparison, and even the 56mm Plossl did not do this. Their field stops are close in size to the stock visual back’s opening. Only the LARGE SCTs are going to be able to fully illuminate the field (as opposed to SMALLER SCTs), so if you have a C9.25 or C8, pass on the 31mm, or do it just because you want too. But much of the value proposition of the 31mm Type 5 will be lost on these compound scopes. As an added caution: In refractors with long focusers, you might find that if you go inside focus very much, the front of the focuser draw-tube may actually cut into the light cone from the objective resulting in the same problem. I did not notice it on the Celestron 80ED, but it is a relatively slow scope (as 80mm scopes go) but on faster scopes, it MIGHT be a problem.

Ok, how about some eyepiece impressions from the C14. This time, the subject was M38. I started with the Panoptic (117x). M38 almost fills the field. WOW does it look great. I studied the field and made the swap to the 31mm (126x). Based on my experience with the smaller scopes, I guess I was expecting the same level of improvement, but honestly? HONESTLY???? I was disappointed. M38 DID look a TINY bit brighter, but the difference was HARD to detect. I looked at several field stars, and to be honest, I was not really able to see the same difference when sniffing out really faint members in the C14 as when used in the much smaller telescopes.

I moved to M37 and repeated this test, and the result was similar. Now when looking at the ENTIRE field, SOME of the effect DID seem to be there. M37 did indeed seem to be a bit more sparkly, but only a bit. Would a novice see the difference is a casual comparison? In fact, I think that they MIGHT, but to them it would be VERY subtle. To me, it was a TINY bit more than barely detectable, but not TOO much more.

As for the difference in the ability to “Frame” either of these objects, the difference between a .61 degree field and a .65 degree field turns out to be VERY small. In fact, I had to REALLY work to actually SEE the difference. I would go back and forth on M37 and some of the stars near the very edge would be JUST outside of the field in the 35mm and JUST inside the field in the 31mm… NOT SO MUCH DIFFERENCE!

For grins, I plugged in the 56mm. M37 washed out. But the step from the .61 field to the .71 field was a bit more noticeable in terms of framing the object. But the view wasn’t close to that offered by the 31mm Nagler. I put the 56mm away for the night (Forever??? Maybe not… More later).

So, I am a bit disappointed. The primary reason that I wanted the 31mm Nagler was to improve my ability to increase my field of view in the C14, and not suffer the contrast loss encountered in the 56mm. And yes, there WAS an improvement in field with the 31mm, and there WAS a tiny improvement over the 35 in contrast and field of view, but it was small. VERY small in fact. And THIS was the telescope that I MOST wanted the big eyepiece for!!. But an improvement in FOV is an improvement in FOV, and in the end, that is why I wanted a 31mm Nagler.

Ok.. A LONG review and a mixed summary.

DON’T buy a 31mm for daytime use. It is unacceptable in my opinion. Pincushion distortion is EXTREME. If this eyepiece will have a dual role (Daytime/Nighttime), the 35mm Panoptic would be a better choice. It induces far less pincushion distortion which will be more desirable for daytime observing, and will only give up a tiny amount of contrast at night.

For a large SCT, the 82 degree apparent field seems to be the MAIN benefit of the 31mm, but you know what? The 68 degree field of the 35mm Panoptic is ALMOST as good. I find that the aesthetic qualities of the “Spacewalk” effect to be not nearly as pronounced between the Nagler 82 degree apparent field and the Panoptic 68 degree apparent field. But there is NO denying that there IS a difference that DOES appeal to me. After all, I own a 12mm T4 Nagler, a 17mm T4 Nagler, and a 22mm T4 Nagler, and except for planetary viewing, these are the eyepieces I use almost exclusively in all of my telescopes… And I adore all of them. So the difference isn’t GREAT, but it is ENOUGH to make me a total Nagler fan.

If you live under DARK skies, I think that the humble 56mm Meade Plossl MIGHT be a FAR more cost effective way to reach MAXIMIUM field. The 56mm induces only slight pincushion distortion, and on widely separated dense clusters, it might provide a more pleasing view. I don’t know this for sure, but I suspect it may be true. But you would probably have to be under VERY dark skies to match the contrast of the 35mm and 31mm eyepieces. Under dark sky conditions, LOWER magnification becomes FAR more beneficial because the larger exit pupil works FOR you in those cases. That being said, in MY back yard conditions, I doubt that the 56mm will ever be used again in my C14… But I MIGHT use it in the C8, where the Nagler will vignette. And next time I go under really dark skies, I will do a 31mm to 56mm comparison. In previous trips to semi-dark skies, I used the 56mm much more than I do in my back yard. The Swan Nebula was stunning in the 56mm at the local “Dark Sky” sight (Mansfield Dam) so I think that under dark skies, the tiny extra field and the FLAT field of the Meade might save it from showing up on Astromart.

It was in the smaller scopes that I was MOST impressed, but once again, the price difference is VERY hard to justify. But as compared to a 41mm or 40mm widefield, the extra contrast that you will get from the 31mm Nagler under suburban skies WILL make a difference, and for a 4” to 6” refractor, I think the 31mm would be very, VERY hard to beat in that combination. In fact, I may wind up using the 31mm more in my Vixen 140 mm refractor more than in my C14 when I have them both outside at the same time, just so I don’t have to keep swapping the 31mm Nagler back and forth. I DO often set both of these scopes up together. WHAT a KILLER combination.

Bottom line? If you have a rich girlfriend, and she gives you a 31mm Nagler as a present, I promise it will be one of the BEST presents she might ever give you.

But if you have to buy it YOURSELF??? It is a LOT of money for an eyepiece that offers LIMITED “Real” improvements over the 35mm Panoptic in larger scopes. Overall, I think that the 35mm Panoptic remains as one of the greatest VALUES in widefield eyepieces in the marketplace today.

But if I were looking for a wide field eyepiece (And didn’t already HAVE a 35mm Panoptic) and I had the money to spend over the cost of a 35mm Panoptic, or over the cost of a 41mm Panoptic, I would go hands-down for the 31mm Nagler. The benefit of the higher magnification while still being able to maintain a VERY large actual field of view makes the Nagler worth the extra money over either of these two other eyepieces… That is of course… IF you have the money.

In closing, I will say this…There really isn’t anything else in the market quite like the Televue 31mm Type 5 eyepiece, and I am a VERY lucky man to have a rich girlfriend…

Click here for more about this subject. -Ed.

Addendum #1 (by Al Nagler):

The design goals of the 31mm Nagler were to obtain more field with a smaller exit pupil than the 35mm Panoptic, retaining 6-elements so the contrast enhancement was maintained. Unlike the 35mm Panoptic, it was not designed for daytime use, and we always recommend the Panoptic for the daytime. The 31mm Nagler's prime purpose is for deep sky and retains its flat-field and full field sharpness even with the fastest scopes like our flat-field f/5.2 NP127, or with fast Paracorr corrected Dobsonians.

People often confuse the optical aberrations of "field curvature" and "distortion." Field curvature is a lack of flatness, so the edge and center of the fields focus in different places. Tele Vue eyepieces are designed to have flat-fields, so when using our Petzval type flat-field refractors, or Dobsonians with Paracorr, full field sharpness is achieved. (Note that doublet or triplet refractors do not have flat-fields and the shorter the focal length the greater the field curvature.)

Distortion is an aberration related to the location of points in the field, usually seen as a pincushion effect. As pointed out in the book "Telescope Optics," the correction for distortion in eyepieces should be linear, that is, an equal angle increment in the field of view corresponds to an equal linear dimension in the focal plane. This preserves the roundness of a planet as you move from center to edge. Examples of wide angle eyepieces well corrected for linear distortion include the Type 4 Naglers and Radians. Without getting into mathematical details, linear distortion correction in wide angle eyepieces naturally yield "pincushion" for straight lines at the edge of the field, producing the telephone pole bending observed by Ed. The type 5 Naglers and Panoptics are not fully corrected for linear distortion making the telephone poles appear to bend more. Actually, the 31mm Nagler is better corrected than the Panoptic for distortion at any given point in the field, so Ed's impression of "worse" distortion is based on his perception of the view in the larger 82 degree field. If we reduced the 82 degrees to the 68 of the Panoptic, it would be obvious.

However, the key point here is our philosophy. We think it more important to see sharp stars in the "wrong" places, than blurry stars in the "right" places. A final point is that Ed should consider our DIOPTRX to eliminate his eyesight astigmatism instead of trying to use eyeglasses. We suggest he and readers look at the DIOPTRX reviews on the Tele Vue website.

-Al Nagler

Response to Al's comments by the author:

My apologies to Al and to the review readers. As it turns out, I do indeed know the difference between a "Flat" field and a "Distorted" field, and while I said that the eyepiece introduced "Pincushion Distortion" throughout the review, in the CLOSE, I referred to the benefits of a "FLAT" feild... MY mistake, TOTALLY... I tried to be VERY correct in the usage of "Pincushion distortion" as the main abberation that I experienced with the 31mm and did so throughout the review, only to mess up in the closing.

Al also points out that the T4s Naglers are indeed better corrected for pincushion distortion. As I mentioned, I own a BUNCH of T4s, so maybe that is why the sensation of having the stars be sharp, but not in the right spot was so noticable to me. With the T4 Naglers, I am spoiled because I have BOTH! I simply wasn't used to seeing that effect, at least to the degree as I expeienced with the 31mm. If your point of reference is another T5 Nagler, then perhaps you would not be as sensitive to it as I was.

So again, apologies to Al and to you for my having screwed up my closing with the "FLAT" field reference. A total brain-check on my part. I worked hard to be consistent in the review, and blew it at the end.

-Ed Moreno