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.
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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