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Tests of Two TSN-883 Scopes (1 Viewer)

henry link

Well-known member
I’ve recently had the opportunity to test two examples of the Kowa TSN-883. A friend purchased the first scope from a mail-order dealer and allowed me to do an evaluation. I did the usual optical tests, measuring the resolving power with a USAF 1951 test pattern and star testing with an artificial star. I used both an indoor set-up at 10m and an outdoor set-up at about 40m. Fortunately it was quite easy to adapt a 4mm TMB Supermonocentric eyepiece to the scope for testing at 125X, and I also used the standard 20-60x zoom. For comparison purposes I used two high quality astronomical scopes with similar apertures, a Takahashi SKY90 and an Astro-physics 92mm Stowaway. The Stowaway is essentially aberration free. The Tak is not quite as good, but uses a Fluorite doublet of about the same focal ratio (f/5.6) as the Kowa. The Kowa uses a triplet with the Fluorite element in the middle.

This scope turned out to be a less than perfect specimen. It showed both miscollimation and astigmatism in a star test. The result was an image that looked decidedly inferior to the reference scopes at 60x or even at 30x. Resolution on the USAF chart measured about 1.47”, well below the reference scopes (1.3"-Tak, 1.25" AP), however by birding scope standards that’s actually quite a good measurement. The smallest bars resolved on the test chart couldn’t quite be seen at 60x, so it ‘s unlikely that any real detail was lost at 60x, but the image just didn’t look completely sharp or comfortably focused compared to the reference scopes. I would add that without a star test and the reference scopes it would not have been at all obvious that this scope had problems. Its image could easily be described as “stunning” or “awesome” from simply looking through it at 60x or less. It falls into an unfortunate gray area: not grossly defective, but performing below what the optical design allows when properly executed.

Since I had no prior experience with the Kowa I conferred with Kimmo Absetz who thought this was probably a below average specimen, so my friend returned the scope in exchange for another. The second one, I’m happy to say, turned out to be much better. Its star test still shows a touch of astigmatism and collimation is a tiny bit off, but the image quality is much better at 30-60x. It compares quite favorably to the SKY90 at 60x, a bit darker but with better correction of longitudinal chromatic aberration. In fact CA correction is far better than it really needs to be for a 60x scope. I increased the magnification to 250x and still saw remarkably little false color, a true APO. Resolution for this specimen measured 1.35”, by far the best I’ve measured for a birding scope and better than ANY 80-82mm scope can possibly achieve even with perfect optics. Unfortunately this excellent resolution can’t be seen at 60x, the highest magnification possible with a Kowa eyepiece. The best I could do at 60x was about 1.8". My measurements were done at 125x. The smallest resolvable details might be visible at 80x, but probably not much less. On another Kowa thread here: http://birdforum.net/showthread.php?t=105037 ,John Wright (Sirius Birder) has described a homemade adapter he fashioned which allows the use of standard 1.25” eyepieces on the Kowa for magnifications higher than 60x.

The zoom eyepiece is quite excellent. It has the best off-axis performance I’ve seen in a zoom, with remarkably little astigmatism or field curvature all the way to the edge of the field at every magnification. I could wish for wider apparent fields, which vary from about 42* at 20x to 62* at 60x. This is not as wide as the Zeiss zoom at any magnification (50* at 20X, 70* at 60x), but the difference in real fields between the two eyepieces is not as large as the apparent fields suggest. That’s because there is very slight barrel distortion at all magnifications in the Kowa and pincushion distortion in the Zeiss, so real field differences turn out to be about half as much as the apparent field differences.

Overall this is certainly the best birding scope I’ve seen. At magnifications of 60x or less it gives up very little to a scope with perfect optics. Perhaps the only really obvious compromise is in brightness compared to an astronomical APO with fewer elements. The Kowa, like other birding scopes, is complex and that appears to limit the light transmission to something in the mid 80% range.

I am attaching some pretty crude photos of the out of focus star tests of the two scopes. At the eyepiece the diffraction rings looked much better defined, but the photos are good enough to show the astigmatism and miscollimation of the first scope (left two photos) and the slight astigmatism of the second scope (right two photos). Notice the oval shape caused by astigmatism, with the axis changing from vertical to horizontal when focus is changed from one side of focus to the other. The miscollimation of the first scope is most visible in the far left photo where you can see that the central spot is displaced toward the bottom of the diffraction pattern, which causes the rings toward the bottom to compress and those toward the top to spread.
 

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Henry,

Thanks for the excellent report. I'm glad the second scope turned out to be satisfactory. I'm sure your friend is very happy he let you evaluate the scope and that he took your advice and had it exchanged. Although it is no doubt true that the difference in the minimum resolvable detail between the two is not dramatic, I would bet that he readily sees that the image of the new scope is more satisfactory and offers significantly more relaxed views.

It's also really good to have the photos, since more and more people have been asking about star-tests recently. The miscollimation of the first scope is, I think, also pretty clearly visible in the second photo from the left, which shows the rings much brighter in the bottom half. Concerning the second scope's photos, I imagine seeing a trace of the prism roof line in the left-hand photo (3rd from the left). Just looking at the photos, I would dare guess the first scope was worse than any of the Kowa 883s I have seen, and the second one is what a good specimen could be expected to be. Even better specimen are probably out there, but getting one would require more than 50/50 luck.

Overall this is certainly the best birding scope I’ve seen. At magnifications of 60x or less it gives up very little to a scope with perfect optics. Perhaps the only really obvious compromise is in brightness compared to an astronomical APO with fewer elements.
. This was really nice to hear. Not many of us at all have the privilege to test birding scopes against a reference nearly as good as your A-P Stowaway.

So, are you going to get a Kowa for yourself now?

Kimmo
 
This is a test that was set up at this years british birdfair on the kowa stand. It was taken with the 883 + zoom eyepiece and my P5100.

I am not really sure if it is a good test but you can see some of the smallest lines and I should mention it was about 30m away.

Paul
 

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Well that just shows the value of star testing, we can see some astigmatism but we don't know what is causing it. That's how I understand it at any rate.

I'm a bit surprised Kowa would demo such a specimen at a bird fair. I would have thought they would put out the best specimen.

Nev
 
Paul,

As Nev mentioned the only thing I can tell for certain from the photo is that there is some astigmatism, in the scope or the camera. Notice that the horizontal lines in the groups labeled "10" and "12.5" are much better resolved than the vertical lines and one set of diagonals is also better resolved than the other. That indicates astigmatism with the axis tilted slightly.

Kimmo,

Wish I'd bought one two weeks ago when I still had some net worth!

Henry
 
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Henry,

Yes, a good sample of the Kowa retains its full value in any stock market crash - fortunately, so does your A-P.

Kimmo
 
Henry your right there must a problem with the vertical prisms. Although would you see the difference when viewing birds. so the moral of the story is try a few before you buy.
 
Overall this is certainly the best birding scope I’ve seen. At magnifications of 60x or less it gives up very little to a scope with perfect optics. Perhaps the only really obvious compromise is in brightness compared to an astronomical APO with fewer elements. The Kowa, like other birding scopes, is complex and that appears to limit the light transmission to something in the mid 80% range.

Thanks for the excellent report. I've looked though the Kowa a few times in the field and thought it was excellent optically at all magnifications. Your report confirms how good it really is, provided you've got a good specimen of course. It also shows quite clearly how important it is to check the quality of a scope before you buy it because there definitely are a few lemons about.

I personally feel the excellent CA correction of the Kowa also shows at fairly low magnifications, even though it's of course especially important at high magnification. It's shame Kowa doesn't offer a zoom with higher magnifications, with such an eyepiece the Kowa would really be on top of virtually every other scope on the market. Even a dedicated fixed 90x for all those really long-range difficult ID's would be better than nothing. It wouldn't have to be an extreme wideanlge, something like 55-60 degrees FOV would be sufficient for such a specialist eyepiece.

By the way, I'd like to see similar tests of *smaller* birding scopes some time, like the smaller Kowas or the Nikon EDIII. They show the quality of a scope in a far more meaningful way than most other tests I've seen so far, and I personally prefer smaller scopes since they're far easier to use on long hikes or on birding trips abroad. The bigger and heavier the scope the heavier the tripod and the tripod head need to be if you want to utilize the full range of magnifications of a scope.

Once again, thanks for your excellent report ... :)

Hermann
 
Hi Hermann, With Henry saying this "Resolution on the USAF chart measured about 1.47, well below the reference scopes (1.3"-Tak, 1.25" AP), however by birding scope standards that’s actually quite a good measurement."
For the first specimen Kowa 883 scope, I don't think I would call this scope a lemon. We don't want to forget what Henry is using for reference scopes.:)

Henry, Thanks so much for taking the time to test these scope. Excellent.:)
Regards,Steve
 
Henry,
Thank you for the test results and photo thumbnail attachments. The comments on the observation of Paul’s photo with horizontal, vertical and diagonal lines was helpful. It is great reading everyones opinions. After losing a Swarovski 80HD scope I purchased a Kowa 883 Prominar Flourite scope after many side by side comparisons using an artificial star with a 4mm or 5mm eyepiece as comparison tools. It was the only easy measurable tool I could find to keep the subjective opinion under control. One consistent observation was that many of the good scopes had a great inside focus ring differentiation but it was viewing/comparing the outside focus ring was where the separation between scopes was determined. One scope while viewing side by side with others the outside focus was so clear that I thought it was the inside focus, it was the one that stood out. The inside ring and outside ring were sharp and similar. Again thank you for your observations, with all the techological advances in optics, coatings I'm sure there will be many more. (I struggle using the USAF 1951 and the optics star target). I found Williams Optics 4mm eyepiece with a bayonet end fit into the Kowa, it did not engage but worked. When terestrial viewing with a spotting scope what 4mm/5mm eyepiece have you found to work best for allowing light and having a wide field of view? Has anyone heard if Kowa will come out with a 100x water proof eyepiece? It is nice for water but also for keeping the dust out of your scope compared to the astro eyepieces that just drop in.
Greg
 
Greg,

Strong rings inside of focus and weak ones outside indicates under correction of spherical aberration, but it can also mean that slightly unfocused colors in a nearly color free scope are interfering with ring formation on that side of focus, so it may not be as serious as it looks. I use a green filter to get a better idea of spherical aberration at one wavelength.

I've come around to the view that it's probably best for most people to limit star tests to detecting defects like astigmatism, miscollimation, bad roof prisms or pinching. Those are pretty easy to see, but analyzing other aberrations is much harder. The scope you found sounds like an excellent specimen. I've been using TMB/Burgess Planetaries for high magnification terrestrial beyond zoom eyepiece range.

What William Optics eyepiece are you referring to?

Henry
 
Henry,

The UWAN 4mm 1.25* Eyepiece that Williams modifies by mounting a bayonet end on it, this is part of the three (3) eyepiece accessories package with the SWAN 83 APO Spotting Scope. Thanks for the green filter tip.

Greg
 
I like Kowa scopes,always had them ,but this is a bit worrying,what does all this say about quaility control at Kowa,the average Joe birder probably wouldnt have a clue about all these tests..ive actually just sold my 821 on E-bay and bought a cheap astronomical type scope to use for birding (it suits my needs at the moment,and is excellent optically,and a decent lens to boot) until I can afford to buy the 883 or until it drops in price,how would I really klnow if it was perfect though (or is perfection an immpossibility)
 
I don't think Kowa is any worse than the other high end scope makers when it comes to quality control. Judging from Jan Meijerink's ongoing tests of sample variation it may actually be better than most others.

I'm afraid, if you want to know whether a scope is "perfect", you'll have to learn to star test. Resolution testing or comparisons to other scopes at high magnification should steer you away from the real lemons.
 
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