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Kowa Prominar 883 star test pictures (1 Viewer)

BoldenEagle

Well-known member
Hi,

here are inside and outside focus star pictures shot through my Kowa 883 + 25-60x zoom + 1.6x extender (zoomed 60x * 1.6x =96x).

The artificial star I used was made by instructions given by Joachim in various posts here on Birdforum. Thin aluminium foil perforated with needle and placed on a bright led flashlight. The star was about 30 meters from the scope. Air was otherwise quite calm but it was raining a bit.

I'm an amateur in evaluating star test images but I can't see any major astigmatism or coma but how do you pros (Henry Link, Joachim, Binastro etc.) would evaluate these images, especially regarding spherical aberration? Are there anything else to be detected through these images?

Regards,

Juhani
 

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jring

Well-known member
Hi,

First of all, the first image is ok-ish (except for the pattern having a few dents but not being elliptic - the dents are probably air currents - either inside or outside of the instrument). The second image is not so great a no rings beside the outer one are really visible.

Also , which image is inside and which one outside of focus?

I agree that no astigmatism can be seen. As for coma, the 2nd image looks a bit funny with the bright bit on the inside neither in the middle of the outer ring nor looking round, but it could also be due to the fact that the image is not great in general. Coma, if it was there, should be equally visible inside and outside of focus.

There is some (mainly secondary) SA visible, I'd say, well defined rings on one side and only a bright outer ring and little in between on the other.

https://www.telescope-optics.net/star_testing_telescope.htm

But another set of images in more quiet air (and with a well cooled down instrument) would be good. Also make one without the extender - to rule out that one...

Joachim
 

henry link

Well-known member
I agree with Joachim about the turbulence. It might look like a slow boil that doesn't damage the image much through the eyepiece, but when frozen in a photo it distorts shapes and obscures details. Even in steady air photos tend to make weak diffraction rings look weaker than they appear through the eyepiece. Everything else looks like it's probably OK.

Having said that I still suspect there is too much SA in this unit to call it a cherry. It's probably closer to an "average" specimen of the 883 for SA. That means I would pass on it, but if I'm going to fork over a few thousand dollars for a small telescope all I want is a cherry.
 

mayoayo

Well-known member
This method is difficult.. I have done these tests many many times in the past, and the image can vary based on many things..and then you have to take a picture..I don't think much can be said about the optics with those two images only.. Sometimes the hole is too big or not even round, sometimes is not well centered on the light,most of the times the camera is not well alligned and so on.. For these type of home evaluation a number of tests are needed , involving sun reflections, artificial light "stars" and actual stars of different magnitudes and in different conditions. This would help you determine if the scope is or not a cherry, an average unit or a poor sample. Keep trying and testing and remember, It Is Really challenging to take a good picture of the results....
 

BoldenEagle

Well-known member
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
 

BoldenEagle

Well-known member
Hi again,

I just did some star testing in bright sunlight using steel ball bearing and also flashlight-foil combination.

I bit of accidentally found that I can see prism roof line in my Kowa star test pattern. If I defocus enough far or bring the light source more near to my scope, it is very clearly seen as an yellow vertical line both sides of sharp focus.

I think trace of it can be seen on the second picture I posted and what I believe Joachim already noticed and wondered in his post.

Zeiss Diascope doesn't show that line no matter how I tried.

There was too much turbulence to take pictures and also found that my camera adapter won't fit to Diascope eyepiece so I can't get pictures through it.

This prism defect might explain the difference in resolving power between my Kowa and Diascope? Their star test patterns are otherwise quite equal but Diascope has also little better correction for SA I think.

I didn't detect astigmatism or coma this time with neither of the scopes. It may have been that the hole in the foil wasn't perfectly round and the hole was not centered on the flashlight either last time and when I took the pictures. Also the light coming through the hole was not equal when I aimed the light on a wall; one side was clearly brighter than other. It also seems that if your eye is not properly centered, the pattern may seem like oval shaped...

Regards,

Juhani
 

BoldenEagle

Well-known member
Here are two more pictures taken last evening without the extender and zoomed to 60x. Light source was again led flashlight through hole on a aluminium foil, I think I managed to get it quite round and even this time.

Prism roof line is apparent in these pictures. Collimation may be just little bit off and I also would see that inside pattern still is compressed from up and down and outside pattern from it's sides so just a touch of astigmatism present? There are also some other artefacts in these pictures but which are likely not scope origin.

If I have interpreted these right, what would be the most significant factor to worsen the image quality from an ideal? Is the prism defect more damaging than presumed slight astigmatism and miscollimation added together or SA? If I compare the outside pattern inner rings to what I can see through Diascope sample mentioned before, I don't think that would be alone so much better with Diascope than Kowa and would explain the difference in resolution. All inner rings can't be counted or separated with Diascope either but its just little more clear than with Kowa.

The Diascope sample I used for comparison by the way has interesting history: It was bought new and it was fell down with tripod soon after, cracking its front lens to pieces. It was sent to Zeiss to repair and I think Zeiss did quite excellent work (allthough they could have corrected the SA at the sime time when the new lens element was mounted...). I cant't see any miscollimation with Diascope but it might have just the same little amount of astigmatism as my Kowa.

Regards,

Juhani
 

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jring

Well-known member
Hi,

now these images are really good - the depiction of the star test result that is - the result itself is not terrible but not not a cherry either.

The dominating aberration is certainly SA - I'd say about quarter wave, which alone would make it just diffraction limited. The astigmatism and coma are really minor.

I'd expect a nice and sharp image at 60x though - maybe some degradation at 96x...

Joachim
 

BoldenEagle

Well-known member
Hi,

Thank you Joachim for your reply. You didn't mention prism roof line at all, is it that it won't have any major effect on image quality? The image quality with this Kowa sample is very good @60x and focus snaps easily (allthough with high mags. it needs little fiddling because the slow focus knob is way too fast, just about 3x slower than coarse focus, which is very fast and I rarely use that coarse focus at all). When using extender @96x, then it's sometimes really difficult to focus the sharpest point because dof is so thin and slow focus knob is really not so slow...

Bill, I used Opticron universal digiscoping adapter and my Samsung Galaxy phone camera to get those pictures. I will attach below couple photos of the gear I was using.

It's very important to get the camera lens centered and well aligned with the eyepiece, with that universal adapter it is possible but may take time to get it right. The light source must be enough small, round and placed far enough from the scope (I used about 30 meters distance). I used led flashlight and thin aluminium foil as Joachim has recommended on many of his posts.

Air turbulance must be absent or at least minimum, you can see the difference between my first photos which were taken in worse air conditions and last two which were taken late evening with much less air turbulence (but also with less magnification 60x vs. 96x). With the first two photos the hole in the foil wasn't perfectly round and light coming through it wasn't even (you can check that by pointing the light on a wall at close distance).

I placed the light source about 1.5 m high above ground on a pole at my backyard. There was shading trees around it and between it and the scope there was quite high grass/other vegetation.

When taking the picture you need to be able to lock the camera auto exposure and focus. Then you just unfocus the artificial star with the scope on both sides the sharp point and using at least 2 second delay snap the pictures. You may have to underexpose quite a lot. Take multiple shots and check them to get the best results. I had to take quite many shots before I got results that looked about the same I could see with my naked eye. My pictures were heavily cropped afterwards.

Some time ago I tried to take these pictures with my Canon 7D and using another tripod to get it behind the eyepiece...It was impossible to get any good results...

I hope these will be useful.

Regards,

Juhani
 

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henry link

Well-known member
I've seen that same prism defect in a number of scopes and binoculars. Anything that distorts the diffraction pattern can't be a good thing, so it probably degrades the image more than the spherical aberration alone would do. I haven't found anything about poorly made roof prisms in the literature of star-testing. It's just not something that comes up in star-testing astronomical mirrors and objective lenses.

Since this scope is clearly not the best possible specimen nor an outright lemon only you can decide if it's good enough. I've been thinking of trying to find a cherry or near cherry 883 for myself, but I'm daunted by the prospect of being stuck with just this kind of decision.

Henry
 

jring

Well-known member
Hi,

I didn't mention the roof prism edge as I have to admit that I don't have any experience how that is normally showing in a star test or not and thus can't say whether your example is a bad case or not - although I suppose the former.

Most of my star testing happens with astro scopes where roof prisms are usually avoided and the only spotting scopes I have done star tests on have been my Kowas and the Apo Televid 77 of a friend - all porro.
I have seen the roof line in a few not so great pairs of binoculars, but have not been able to star test them due to lack of opportunity and, more importantly, of a good enough magnifier to reach the necessary magnification without introducing other aberrations.

Joachim
 

BoldenEagle

Well-known member
Since this scope is clearly not the best possible specimen nor an outright lemon only you can decide if it's good enough. I've been thinking of trying to find a cherry or near cherry 883 for myself, but I'm daunted by the prospect of being stuck with just this kind of decision.

I'm not thinking of returning this sample or getting some other scope anytime soon. I'm quite pleased with this one and certainly it has already offered an extra performance compared to my previous scope out in the field.

Yesterday I compared again Kowa against Diascope using printed 1951 Usaf resolution chart. I placed the scopes at the same distance and I could read the same element with both of them @60x. I couldn't measure exactly magnification but I tried to zoom Kowa @75x with the extender and compared it to the max. mag. of Diascope, no difference in resolution. Kowa seems to have better contrast because black appears to be blacker than with Diascope, which turns black little gray. With the extender zoomed @96x I could see more details than Diascope @75x. I know that test chart has to be high quality and thats why also this test was anecdotal and I didn't even try to measure real resolution. But I will at some point when I will get my hands on official test chart and have an eyepiece with enough magnification.

If I'm getting a new scope sometime, it won't be Kowa Prominar 883 even if I could find a perfect sample. The reasons are following:

a) The fine focus is too fast, using high mags. you need very fine tuning and easily overfocus, it's of course more severe with the extender. I can't understand why Kowa couldn't get this right. Old Zeiss Diascope and Leica Apo Televid are pretty good examples what would the ideal speed of the fine focus to be.

b) It's not as bright as I expected. I would like to have more aperture and perhaps better lens coatings. Especially with the extender you will end up with quite small exit pupil and dark image at maximun mag.

What I would like to have is a 95-100mm front lens, about 25-70x zoom (+ extender available), dual speed focus system with two knobs and the right gear ratio between them. At the moment there are no such high quality scopes available.

Finally, I still wonder how bad scope samples are out there. I think this sample that I now have, should be about the least quality what one would expect to get when you pay about 2.5k€ for a scope, nothing much less wouldn't be acceptable. Of the three Kowa Prominar 883 samples I have seen, the other two must have been real lemons, because they were both way worse just to look through than this one and one of those two was confirmed to have more than one major defect in a star test (didn't star test the other one but it probably had even more defects because I just couldn't get it sharp @60x or even 50x).

Regards,

Juhani
 

mayoayo

Well-known member
I think if the roof line is affecting the image quality, the obvious effect would be spikes when focusing in the star.. I had a pair of swift eaglet that showed a horizontal line ghosting inside the image at certain angles, quite distracting.. The binoculars showed clear diffraction spikes when pointed at a star
 

giosblue

Well-known member
Unless you the scopes side by side in a direct comparison in exactly the same lighting conditions you simply cannot compare and judge one scope against another.
 

BoldenEagle

Well-known member
Unless you the scopes side by side in a direct comparison in exactly the same lighting conditions you simply cannot compare and judge one scope against another.

I agree 100%.

That's why I had same reference scope when testing both Kowas. By that reference I could see that the other Kowa was PROBABLY even worse than the other, at least certainly not any better, even if I didn't compare those two at the same time.

After all that didn't really matter to me because both of them were anyway so bad copmpared to my reference scope (not very good sample either) Leica Apo Televid82, that I would certainly never have bought one of them.

Juhani
 

BoldenEagle

Well-known member
I think if the roof line is affecting the image quality, the obvious effect would be spikes when focusing in the star.. I had a pair of swift eaglet that showed a horizontal line ghosting inside the image at certain angles, quite distracting.. The binoculars showed clear diffraction spikes when pointed at a star

I can't see any spiking when the star, a real one or an artificial, is sharp focused. Even with the extender the star is a round point which is surrounded by a circular faint glow.

But when I unfocus outside, the outer ring shows spiking, not in any specific direction but the whole outer ring seems spiking and "living" more than the outer ring of the inside focus pattern.

Juhani
 

BoldenEagle

Well-known member
Lately I have been probably watching more of these bright point like light sources than birds with my Kowa, so here are another shots with the extender @96x.

Light source was silver flagpole knob about 40 meters away on a bright sunny day, some heat haze present.

There are some artefacts brought by the camera, can't see them with naked eye.

Juhani
 

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kabsetz

Well-known member
Considering the conditions, I think these look very good except for the prism line, which is too prominent for comfort. In my ATX 95, I have never seen any trace of a prism line, even though it is a normal Scmidt-design.
 

BoldenEagle

Well-known member
Considering the conditions, I think these look very good except for the prism line, which is too prominent for comfort. In my ATX 95, I have never seen any trace of a prism line, even though it is a normal Scmidt-design.

Kimmo, how detrimental do you think these kind of prism defects are for image quality among spotting scopes? Would there be obvious difference if one would compare this sample with a one without that prism defect?

Juhani
 

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