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Posted by Ed Moreno   05/19/2009 07:00AM

I have seen this question come up a few times: For those wanting to get a really high quality wide-field viewing experience which is better: the Televue 31mm Nagler, or the Televue 41mm Panoptic.

The answer as it turns out is not simple at all. See, the eyepiece is just one part of an optical system, and because of this, it is hard to say that this or that eyepiece is better without respect to the rest of the optical system, and for this reason, I usually test eyepieces in a variety of telescopes. As it turns out, I think that some eyepieces COMPLEMENT some scopes more than others.

See, telescopes have vastly different off-axis performance capabilities, and for a wide-field eyepiece, the TELESCOPE’S off axis performance has a DRAMATIC effect on what you will see when you put ANY eyepiece into the focuser. Please keep that in mind when you read this review.

Now first, I have to say that the 31mm Nagler is one of my FAVORITE eyepieces of all time. It is without a doubt one of the most thrilling wide field eyepieces ever produced. In fact, I often set up two telescopes and sometimes even three telescopes, and often found myself wishing I had TWO 31mm Naglers because the eyepiece was getting worn out from swapping it back and forth! I VERY seriously considered buying another one.

Ah, but I didn’t. I decided to take a chance on the 41mm Panoptic rather than adding another 31mm Nagler. Why? Read on.

I won’t go into the technical descriptions of these eyepieces. It is well known that they are big, heavy, and beautifully finished.. I will focus exclusively on the testing and why and when I prefer one over the other.

First, I plugged both eyepieces into Buffy, my old 6” F/8 Astro-Physics refractor. Now what you have to know about the old 6” AP is this: the field in this telescope is INCREDIBLY flat and as best as I can tell absolutly free of coma or astigmatism. Now I often read that this or that Televue eyepiece has field curvature and I even MISTAKENLY said that in a review once when I MEANT to say that it had pincushion distortion.

Field curvature is where the telescope focuses the rays of light at the edge of the field slightly ahead or slightly behind the focal plan as defined by the CENTER of the field. When this happens, if you focus on the center of the field, the stars at the outer edge of the field may not be in sharp focus. Depending on whether the field is inward or outward curving, it MIGHT be possible for some observers to use “Accomadation” to keep the entire field sharp.. For example, if the field curves outward, an observer with really good accommodation (maybe 3 diaopters for a younger observer, maybe 1 to 1.5 for an older observer) can focus on the OUTSIDE edge of the field to get it in sharp focus, and use accommodation to let their eye re-focus so that the center of the field is in good focus when they look directly at it. If the telescope has a flat field, and the eyepiece has a flat field, no accommodation is required by the observer IF THE EYEPIECE also has a flat field.

Again, many of the reviews or postings on the web that I read seem to indicate that this or that Nagler or Panoptic has a “Curved" Field (and again, I made this accidental mistake once myself). My personal observation though is that in general, Televue Naglers and Panoptics have fields that are essentially “Flat” What this means is if the telescope has a flat field, the center and outside edges of the field will both be in essentially perfect focus at the same time. I personally believe MOST eyepieces today have fairly flat fields an that most desingers trade off axis sharpness in the form of astigmatism to KEEP the fields flat so the eyepieces will work optimally what ALL scopes. But this is typiecally the difference between cheap wide-fields and expensive ones.. The desingner choose to keep the field FLAT AND to keep the field free of astigmatism.. You CAN do both if you want, but it involves far more complex glass shapes and more exotic glasses.

When I put the 31mm Nagler in the flat field refractor, this is exactly what I see. I see an essentially PERFECT 2 degree field of view. The telescope is essentially coma free, and as best as I can determine, completely free of ANY off axis aberration when used visually. With the 31mm Nagler in place, I can place any star or double-star at the center of the field, allow it to drift to the extreme edge (and even where one component of a double drifts under the field stop) and see the stars remain EXACTLY the same in terms of appearance. The field is literally PERFECT. And if you have never really seen this, I assure you that it is (As Mr. Nagler would say) MAJESTIC. The field STILL dazzles me even almost a year and a half of using this combination. Simply put, it is optical wide-field nirvana.

The 31mm Nagler give 39.2x with an exit pupil of 3.9mm. In this particular scope, that is a good exit pupil for observing under suburban light conditions because it tends to give the impression of a darker sky than a lower power, larger exit pupil, and the eyepiece is in fact my all time favorite sweeping eyepiece. There is some pincushion distortion, and while Mr. Nagler says that this eyepiece has LESS pincushion distortion than a 31mm Panoptic, I THINK that because the magnification is a bit higher, as you sweep, the magnified image gives the IMPRESSION that the field has a bit more pincushion distortion. When sweeping, it can take a bit of getting used to, especially if you sweep at high slew speeds as I often do. Still, this is a pretty common characteristic of most decent wide-field eyepieces and the Telvue mantra is that a Sharp edge of field is far more preferable than a less sharp eyepiece with less pincushion distortion. I agree 100%.

Now for the 41mm. And what can I say. Once again, when used with this telescope, I found the field to be essentially perfectly flat. As stars approached the field stop, the remained (at least visually) PERFECTLY FOCUSED TINY LITTLE POINTS of light. There was NO sign of field curvature being introduced by the eyepiece. It was optical perfection.

Ah, but the view lacked the “Majesty” factor that Televue uses in its marketing for the Ethos eyepieces. Still, the 41mm produces an even larger 2.2 degree field at 29.7x and a 5.1mm Exit Pupil. I was pleased to see that the field was not all much more washed out due to the lower power. In fact, I would say that while the field seemed a bit brighter, there really wasn’t any star visible in the 31mm Nagler that wasn’t visible in the 41mm Panoptic.

Yes, but the Majesty factor really does count, and overall, I preferred the view through the 31mm Nagler. While I could see a bit more field in the Panoptic, and I could see EVERYTHING in the field in either eyepiece, I simply preferred the aesthetics of the 32mm Nagler. When used with a high quality, coma-free telescope, it produces the most spectacular wide-field viewing experience you are likely to have.

The other telescope I tested in was the C14. And one of the MAIN reasons I went with a 41mm Panoptic rather than with another 31mm Nagler was BECAUSE of the telescope that it would be used in.

See, the standard commercial SCT has two particularly notable off-axis performance issues. Even though the C14 is f/11, it has about as much coma as maybe an f /5 Newtonian. But that is not the half of it. The standard commercial SCT formula also means that the field of the telescope at the focal plane is quite strongly curved. The result is not only a comatic blur of stars at the edge of the field, but rather a DEFOCUSED comatic blur at the edge of the field.

My reasoning for purchasing the 41mm Panoptic over adding another 31mm Nagler is that I simply thought that it would be a slightly better match for this particular telescope. So, the question is, was I correct? Well, here is what I saw.

I put in the 31mm Nagler, and since I have used this eypeice many times in this telescope, I knew what I would see. Again, remember that in the 6” coma-free, flat-field refractor, the view with when using this eyepieces is essentially PERFECT. As best as I could see, there was NO effect from the eyepiece. So, what I saw was COMING FROM THE TELESCOPE ITSELF. And what I saw was this. If I focused on the center of the field, the field was fairly sharp out to maybe 15 apparent degrees from the field stop. Past this, I could easily start to see the increasingly out of focus comatic blur coming from the telescope. By the time I reached the edge of field, the stars were clearly defocused, and again, since this eyepiece was essentially PINPOINT in the flat field scope, then this defocus must be the result of the field curvature of the TELESCOPE.

Next, I attempted to use accommodation to focus the field. I focused the telescope on stars near the edge of the field and this greatly reduced the comatic blur simply because as the scope was brought closer to focus, the blur diameter got very small. It did not get exactly "Pinpoint" but it shank enough to get decently sharp.

Unfortunately, if I focused the outer edge of the scope, my eye did not have enough accommodation to bring the center of the field completely back into focus. Darn, I hate that.. I am getting older though, and that is what happens. Your eye experiences a degraded ability to accommodate.

I then focused on stars that were about 2/3rds of the way to the edge of the field. Doing this, I was able to accommodate the center of the field with my eye to bring it into focus, but I was still able to resolve the comatic blur at the edge of the field. In other words, I could never get the entire field reasonably sharp.

And THIS was my great hope for the 41mm Panoptic.. I was hoping that the lower power (and the consequently lower magnification of the comatic blur) would allow me to focus the outside edge of the field in the 41mm Panoptic and still have enough accommodation in my eye to keep the center of the field sharp when I shifted to it.

Did it work? Yes. When I put in the 41mm Nagler, even if I focus the center of the field as perfectly as possible, the lower magnification kept the defocused (remember this is coming from the telescope) comatic blur MOSTLY in check, but I could still resolve it at the edge of the field.. However, when I focused the edge of the field, all but the brighter stars started to shrink in size to the point that I could get them to be more or less “Pinpoints,” and when I looked back to the center, I was able to use accommodation to keep the center of the field sharp. It can be fatiguing to use accommodation, but for shorter observations, I have AT LAST found the key to getting a fairly sharper field in my SCT.

There are other benefits as well. While I am trading some of the “Majesty” factor with the 68 degree appearent field, the magnification goes from about 126x with the 31mm Nagler to about 95x with the Panoptic. This also helps by reducing the size of the blur circle resulting from the effects of seeing.. On many nights with the C14, stars look bloated. This is not so much from the fact that it is an SCT (as common wisdom seems to believe) but rather due to the fact that it is a BIG TELESCOPE and with the 31mm Nagler, it was WORKING AT HIGHER POWER than many smaller scopes do when using much shorter focal length eyepieces! The 41mm Panoptic dropped the magnification to about 96x. This made stars all across the field seem quite a bit sharper when seeing is less than very good. The entire field was more pleasing even though I loose the 82 degre "Spacewalk" effect. For me, field sharpness has become far more important. The big coma-free refactors have really been an ephihany for me, and now I find myself really critical of the lack of off axis sharpness.

The field was also a bit bigger. Not much (about .67 degrees vs about .62 for the 31mm Nagler), and in a C14, this is pretty meaningful. Targets like the Wild Duck cluster literally fill the field of the 31mm Nagler, but the 41mm Panoptic adds JUST enough extra area that you can “Frame” a few more of these wonderful, larger clusters.

And there is ONE MORE advantage. In the 6” refractor, there was only a SLIGHT gain in exit pupil size, but in the big SCT, the difference in exit pupil was quite big going from about 2.8mm in the 31mm to about 3.7mm with the 41mm Panoptic.. Now as it turns out, THIS is a VERY good thing. While it is generally true that higher powers and smaller exit pupils can give a slight contrast boost when viewing under light polluted skies, on those rare nights of good transparency, a BIGGER EXIT PUPIL IS STILL A GOOD THING. And under dark skies for large faint nebula, it is VERY important.

I did several comparisons on several targets and here is an example. For M3, neither eyepiece allowed the resolution of more than maybe 10 or 15 stars against the nucleus. The difference seemed very minor. Also, even though the angular magnification was a bit smaller in the 41mm Panoptic, because of the 68 degree apparent field, I think the mind was tricked into making it seem to be almost the same size as it appeared in the 31mm Nagler. I was surprised by this, but that is the way it looked.. I guess like an optical illusion. The BEST view of the night was at 444x with the 8.8mm Meade 4000 UWA, because if you REALLY want to see how glorious some of these globulars can be, you need to really magnify the foo out of them. Still, for “Framing” the cluster in the region of space that it rests in, I actually felt like the 41mm Nagler did indeed do a better job in THIS TELESCOPE. For many targets in rich fields like those in the summer Milky Way, adding more context by including even a LITTLE more true field around the target makes you forget about the 82 degree AFOV.

Bottom line? Either of these eyepieces offer optical perfection. I find them to be essentially flawless and if you see ANY off axis aberration when using one, I believe it is most likely being introduced by your telescope. When used with a coma-free, flat field telescope, both render pinpoint performance right to the field stop.

Ah, but not all telescopes are created equal, and in some cases, you may find that one of these eyepieces may suit this or that scope better than the other. I indeed found this to be the case. I vastly preferred the view provided by the 31mm Nagler in my big refractor, but in the big SCT, I have to say that I give the edge to the 41mm Panoptic. It simply provided a sharper view to begin with (again, this was not because of the performance of the eyepiece, but rather because of the lower power), and by using accommodation, I was able to get my entire field quite sharp, while I was never able to completely accommodate the 31mm Nagler in the C14. I will say it one more time just to be perfectly clear. These differences in performance were differences in TELESCOPE performance, and not eyepiece performance. I found both eyepieces to be essentially perfect in their off axis performance and any abberations I observed in the C14 were COMING FROM the C14. In the 6" refractor, both were PERFECT.

So, 31mm Nagler or 41mm Panoptic? For use with a larger SCT (which is often what these 2 eyepieces are used for), I have to give the nod to the 41mm Panoptic. You trade a little of the 82 degree apparent field “Majesty,” but in return, the lower magnification gives you sharper stars in less than perfect seeing, and the ability for most users to comfortably accommodate the entire field. And don’t forget the much bigger exit pupil and slightly bigger true field.

Ah, but the 31mm Nagler is a magnificent eyepiece. IF you also own a telescope with a flat, coma free field, or if you own two telescopes and you will use the eyepiece in both telescopes frequently, it MIGHT be that you will enjoy it more. I am a lucky man.. I can afford both. But I fell like the decision to add the 41mm Panoptic rather than a second 31mm Nagler (I like them that much) was the BEST choice.

Consider this when you make your purchase decision. These eyepieces both represent practical optical perfection, but it MAY be that one is better suited to your needs than the other. My advice is to consider the off-axis performance characteristics of your own telescope and your accommodation power carefully before you make your selection. This is especially true if you are using the eyepiece with a scope with a sharply curved or coma-heavy field. As much as I admire the 31mm Nagler, I plan on using the 41mm Panoptic exclusivly in the C14. I think it is just a better match for this scope. And may be for yours as well.

I hope you enjoyed reading...

To all, my regards… And a toast to 10 more years of Hubble!