Hi,
and thank you all for your replies, very good points (once again...).
Yes it is very hard to get the same view to the pictures of what one can see with naked eye. Those pictures I posted were not exception but I took multiple frames and took the ones I thought would be the "average" ones...First picture was inside ("past focus") and second one outside ("before" sharp point) focus. Even with naked eye it is hard to see everything correct as your eye needs to be centered also.
I have looked different light sources quite a lot with my Kowa and also with Zeiss Diascope 85 FL and doing star tests. What I can see with my eyes and what differs from the images taken:
a) the inside figure does look like regular round circle without any dents (Joachim probably was right about what caused them to be seen in the picture) but if I unfocus to just 3 rings, I would say the circle is not perfectly round but just slightly elliptical, compressed from up and down. I cant really see it if I unfocus more rings visible.
b) the outside circle shows when similarly unfocused (just about 3 rings visible) 90 degree elliptical form compared to the inside focus circle, which also disappears when unfocused to more rings.
c) with naked eye and unfocused just to 3 rings, the inside pattern shows slightly coma, very hard to see but the centre seems to be just little downward. But I cannot see the same thing with outside pattern.
What the pictures dont lie about is that the outside pattern really has brighter outer ring and inner rings are much more harder to see clearly, it looks pretty much the same I can see with my naked eye.
Already when I first tested this sample (before I bought it), it was clear that it wasn't a cherry. But nevertheless I thought (and still do) it was good or maybe very good sample, most certainly not a lemon at least...I concluded that worst issue in star test with this sample was the undercorrection of spherical aberration. It may have also very minimum amount of astigmatism and even coma but those are very hard to see. As said, I'm also not able to tell if there are other aberrations involved and thats why I'm asking you guys here. If I compare the image quality overall to the two other samples of Kowa Prominar 883 I have seen, this one is way way better than either of those. I did do a star test to one of those two other samples and it showed significant astigmatism and coma. I would never use a scope with image quality like they both showed and star test wouldn't have to been done to conclude that. My sample size is small but I have to say that I'm not generally impressed by Kowa quality control...Luckily I was able to return the first bad sample of the Kowa I had and switch it to this one I now have. The seller agreed about the findings I made through star tests and even pre-selected the next sample he send to me, to ensure I would be happy with it.
I have compared this Kowa to Zeiss Diascope 85 FL and in a star test the Diascope shows just as little astigmatism as Kowa, coma is totally absent I think and spherical aberration may be better corrected since in the outside pattern the inner rings are certainly more clear than with Kowa. Also in side by side comparison (@60x) Zeiss shows just little better resolution when reading small size black text on a white paper. Also in the field Zeiss seems to be sharper.
But what really interests me is when I put the 1.6x extender and zoom about to 60x, the image and resolution seems to better than without the extender and zoom cranked to 60x...Could this be possible or am I just imagining? Somehow it feels logical if without the extender the image starts to worsen say, beyond 40-50x mags. and if all that extender does, is that it magnifies that image.
In practise, I have found the extender to work extremely well with my Kowa; I can't see any brightness loss and it certainly gives more resolving power all the way to 96x (allthough beyond say 80x there is very slight benefit to gain). Of course when exit pupil gets too small, the image gets dim and you need to have good light to get the benefit. Few days ago I was watching flying subadult golden eagle over quite a distance and as atmospherics and light were fine at the time, the image was very impressive @96x. It's too bad that often heat haze makes it hard to use even the 40x mags (or sometimes even 25x), so using the extender all the time on the scope is not very pleasant. Another broblem is vibrations when using mags like >60x, you loose gains if wind is too strong and it really doesn't need to be stormy winds to cause shaking when zooming to 96x and if you are touching the scope, that alone causes disturbing shaking...
Few days ago I also tried the extender and read some serial numbers from electric transformers in electric poles high above ground. There was one about 400m away and the numbers are about 2,5 cm high, that one was already easy to read @50x with my earlier scope (not so good sample of Leica Apo televid 82) and was of course not even a challenge to Kowa with 50x mag. What surprised me was that I could read another numbers from 1.2 km away when I zoomed 96x with my Kowa and air wasn't even completely still! That was something that with Leica and it's extender (1.8x resulting to 90x max mag) I never even dreamed of.
I will try to take more star test pictures with my Kowa and also with Diascope for comparison. Maybe this evening as weather might be good.
Until then I think this sample of Kowa gives me little mixed feelings, because it could be even better sample and Diascope 85 with even a smaller aperture has better resolution (allthough little), but sometimes it's image quality at high mags. just surprises me very positively...At least, after all it certainly is a lot better than my former Leica scope.
Best regards,
Juhani