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9mm Nagler T1, 10.5mm Pentax XL, and 10mm Radian. So which premium eyepiece is REALLY best?

A line of of excellent optics - from left to right: 13mm Nagler T6, 9mm Nagler T1, 10mm Radian, 10.5mm Pentax Xl, 20mm TV Plossl

I recently had a chance to get 10mm Radian on loan from Allister St. Claire as part of his outreach program. I have to admit, I was really curious about how the radian would perform. I have been using Panoptics and Naglers for quite some time now, and have grown extremely fond of them. When I contacted a friend of mine and let him know that I was going to have the 10mm radian to compare to the 9mm nagler, he immediately offered his 10.5 mm Pentax into the mix - now I had three premium eyepieces to stack up against one another - how would they fair?

Let me say straight off that my 9mm nagler is one of my favorite eyepieces. The other two selections were going into the fight with that bias already against them. But as you can see, it didn't make a difference, I was surprised by the performance of all three. To say that they were close is an understatement. Yet in the end, all three had their good points and areas in which they pulled ahead of the others. Would there be one overall winner?

These eyepieces were used in my TV-102 and my Pronto over the course of several nights on many different objects both planetary and deep sky.

Nagler 9mmT1 Pentax10.5mmXL Radian10mm
AFOV 82 65 60
FL 9mm 10.5 10
Elements 7 6 6
ER 12mm 20mm 20mm
Field stop 12.3mm ?? 10.5
Weight 410 grams 436 grams 227 grams
Warrenty 5 years N/A 5 Years
Cost $175-$240 $225 $240

left to right: 13mm Nagler T6, 9mm Nagler T1, 10mm Radian, 10.5mm Pentax Xl, 20mm TV Plossl

Mechanicals and Impressions

All three are imposing eyepieces. I have to admit that I do not like the instadjust on the Radians, it seems far to loose, and if this had been my personal 10mm Radian, I would certainly have tightened it up. I do like the screw eyecup on the Pentax. In addition, I also like the generous amount of plastic on the Pentax, as it didn't get quite so cold (and consequently painfull to handle) in our Michigan spring weather.

All three can be used in 1.25" focusers, and the Nagler can also be used directly in a 2" focuser. The TeleVues have the trademark TV coatings - blue green, and the Pentax has the SMC coatings that make their camera lenses so famous. Although the Nagler has been discountinued, it is still available new from some dealers, and there is a healthy supply of them available for purchase used.

Pentax XL 10.5mm

This is a wonderful eyepiece. The 65 deg afov is (much to my surprise) noticeably larger than the Radian's 60. There is a very slight amount of lateral color when used on the moon. Very difficult to tell which eyepiece has the least, but if pressed I would have to say that it's the Pentax.

Please note, it was not objectionable in ANY of the eyepieces. The Pentax was nearly as sharp as the Radian both on and off axis (The difference was very slight, but if pressed, I would have to give the edge to the Radian).. I do prefer the screw eyecup to the instadjust found on the Radian. Eye relief was generous, and made this eyepiece extremely comfortable to use. While I could get it to blackout, it was very very difficult, and probably wouldn't happen in normal observing.

Transmission was high, although perhaps not quite as high as the Nagler or Radian, it was an extremely small difference. It was sharp to the edges of the field in my TV102 and Pronto, and displayed no signs of astigmatism or other optical aberrations. This clearly, is a VERY nice eyepiece. About the only really significant difference I noted between the Pentax and the TeleVue eyepieces is that the Pentax (presumably because of it's different coatings) gave a warmer (slightly yellowish) tone to the images. This was most noticeable on the moon. While the impression I got was that it was not as accurate as the TeleVue eyepieces, it was not objectionable in the least (in fact, some may prefer it). It barlowed well with my Celestron Ultima Deluxe.

TeleVue Radian 10mm

Another very fine eyepiece. High transmission, a slight amount of lateral color on the moon, and an easier tendency to blackout and/or kidney bean. It was not as easy to view through as the Pentax, but by no means did I find it hard to use. Once you get adjusted to it, it is a very comfortable eyepiece (although not quite as comfortable as the Pentax.). It was (by a very slight margin) the sharpest eyepiece on axis, and presented what I felt to be a very true color image. It was sharp to the edges of the field in my TV-102 and Pronto, and displayed no signs of astigmatism or other optical aberrations (outside of a slight amout of lateral color). Light transmission was very high.

Star colors were perhaps a little more apparent in the Radian than the Pentax (This perception may be due in part to the warmer/yellower tone found in the Pentax, however I did have the impression that the Radian and the Nagler offered a truer picture of coloration). In all honesty, it reminded me of nothing so much as a 60 afov Nagler with 20mm eye relief. The instadjust was NOT my favorite feature. I found it to be too loose, and if it had been my personal eyepiece, would have followed the instructions given on the TeleVue home page to tighten it up. I also discovered that I hate the pupil guide. While I have heard some say that it helps provide a higher level of contrast, the only thing it did for me was to make me worry about scratching my eyeball. This of course is a personal issue and YMMV.

TeleVue Nagler 9mm T1

Yet another excellent eyepiece. High transmission, and there were slight amounts of lateral color on the moon, more lateral color than either the Radian or the Pentax displayed. Star and planet colors seem very true to life.

Maybe it's because I am used to this one, but I found the Nagler to have less tendency to kidney bean or blackout than the Radian. Of the three, this one is the least sharp on axis, that's not to say its NOT sharp, it is extremely sharp. It is only that the other ones are slightly better. There is a slight amount of pincushion distortion evident - you would only notice it if you were REALLY looking for it. The Nagler does have one HUGE thing going for it that the other two lack - its 82 degree AFOV. Amazingly, for as many elements as it has, it still maintains a very high transmission rate. Any star that was visible in either of the other eyepieces was visible in the Nagler. The eye relief is just about perfect as long as you do not wear glasses when observing.

It does lack the adjustable eyecup of the Pentax or the Instadjust of the Radian, but for me at least, the rubber eyecup that is there does the job perfectly. There is some lateral color in the eyepiece on extremely bright objects. Amazingly, this eyepiece barlows very well. When barlowed, eye relief improves, and I actually liked the barlowed view with the Nagler better than the barlowed view with the Radian. This is one of my favorite eyepieces, and has been for quite a while. In my opinion, this is one of Al's best efforts.

So - which one is the best?

On one hand we have the Radian, which clearly excels at planetary observing. It provides some of the sharpest high power views going, a wide 60 degree field and manages all this while providing 20mm of eye relief and an adjustable eyecup. Where the Radian line REALLY excels, in my opinion, is in providing short focal length eyepieces (3mm and 4mm) with its full 20mm of eye relief. I have a 4mm radian, and find it a joy to use.

On the other hand, we have the Nagler which is sharp on axis, excels at deep sky work, and is second to (almost) none in the amount of well corrected true field it supplies. This is extremely important if you observe on an undriven mount or are a spacewalk junkie. While it does not have the eye relief of either the Pentax or the Radian, it is comfortable to use IF you don't wear glasses while observing. If you do, you would be better served by one of the other selections.

The Pentax, on the other hand (three hands, I know ), is an excellent all around eyepiece that is very comfortable to use, very sharp, with minimal aberrations, good transmission, semi-wide AFOV and nice warm tone. The Pentax is probably the most comfortable to use of the three.

The biggest difference between the three (outside of AFOV) was the warmer tone that the Pentax provided, and even this was a very slight difference. If I weren't doing an A to B comparison on the moon, I am not sure that I would have noticed it.

One thing that I found very interesting was that I tended to prefer whichever eyepiece I started the evening off with. If I started with the Pentax, I tended to prefer the views through the Pentax, and vice versa.

It's probably best to make your selection by your observing preferences.

If you are a planet observer, and observing on a driven mount, then the Radian is probably the best eyepiece for you, with the Pentax coming in a VERY close second. The Nagler pulls a very respectable third. Of course if you are observing with an unmotorized alt-az, then this changes things a bit, and you might prefer the Pentax or the Nagler - it all depends on how much you want to touch your mount. The Radian does have an advantage in that it's available in shorter FL lengths.

Because of the warmth/tone of the Pentax, I would probably keep my higher power eyepieces in the same line, just to increase the probability that you are going to see things the same with each step in power.

If you are a deep sky observer, the choice is a little easier (as long as you don't need to wear glasses) and the Nagler comes in first with the Pentax second, and the Radian third.

All of these eyepieces provide very similar levels of performance. While the Radians and the Pentax XL's are very nice, I don't see any reason for my to get rid of my 9mm Nagler for any of the others, and I also don't feel a need to get a Radian or Pentax in addition.

As an aside, I should probably say a little something about how these stack up to the Speers-Waler I had. Although I don't have it any longer, I do feel that all three were better optically, but not by much. There was a larger difference between the Speers and any of these than between the Pentax, Radian and Nagler themselves. I do remember the Nagler as being slightly sharper than the Speers, and that the AFOV wasn't as large, so that should give you an idea how it fits into the scheme of things.

I have to admit that the Pentax line is really growing on me. (Maybe it's time to acquire a second line of premium eyepieces? ) I still prefer Naglers for the bulk or the observing that I do (Deep Sky), and for the occasional high power work, I am very happy with my 4mm Radian, but admittedly, all three are absolutly excellent eyepieces.

Update to: 9mm Nagler T1, 10.5mm Pentax XL, and 10mm Radian 5/29/02

There has been considerable discussion about the Pentax / Radian color issue. I have received several e-mails on this topic, and it is interesting to note that, some observers have agreed with me in stating the Pentax provides a warmer tone, and others feel it is actually the Radian which has a color issue. It has been suggested to me that perhaps the Pentax actually has a truer color than the Radian, and perhaps I thought it was the other way because of my extended experience with Tele vue eyepieces. (i.e. I am "used" to the color in the Radian.) I am willing to concede that this could be true.

I have also been informed that there were a batch of Pentaxes where the coatings were "less than optimum" One individual wrote me to say that he had a Pentax that looked very similar to what I described, whereupon he exchanged it and the problem went away. I have used two different Pentax eyepieces, and both have displayed this warmer tone. I would hope that both eyepieces were not defective, and to be honest, I really don't feel they (the two I have used) are. Color is a very subjective thing. I should note that it would be possible to say that the "Radian was flatter than the Pentax." Instead of saying the "Pentax was warmer than the Radian." To me, this means much the same thing. Nevertheless, there is a color difference between the two. Which line you prefer will probably depend on your personal preferences and the eyepieces that you already have.

Tom is an amateur astronomer from Michigan's thumb who is currently establishing a home for wayward and homeless eyepieces. In his spare time, he watches his 1 year old daughter dance, and wishes the clouds away.


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