Kowa Prominar 883 star test pictures (1 Viewer)

kabsetz · Aug 24, 2020

With the caveat that I haven't analysed a whole lot of scopes with obvious prism defects and therefore don't have enough experience with a certain level of prism problems combined with a certain level of other aberrations, I'd say that a bad prism line tends to produce some spiking in best focus light points. That can look and feel like some amount of astigmatism, although it is not.

For practical purposes, I'd suggest that if a boosted star-test at best focus does not show spiking, it does not matter much if you see a prism line in a defocused diffraction ring pattern. A clean Airy disk with a faint and circular first diffraction ring is the goal, and if you don't go too far from that, the scope is fine.

Kimmo

BoldenEagle · Aug 24, 2020

kabsetz said:
For practical purposes, I'd suggest that if a boosted star-test at best focus does not show spiking, it does not matter much if you see a prism line in a defocused diffraction ring pattern. A clean Airy disk with a faint and circular first diffraction ring is the goal, and if you don't go too far from that, the scope is fine.

Kimmo

I posted some star test pictures on the Zeiss section regarding of one sample of Diascope 85 fl.

In direct comparison I would definitely say the Diascope has better sharpness and resolution than this sample of Kowa Prominar we are dealing here.

Could you Kimmo (or others) tell from these star test pictures, what are the main causes for Diascope to have better image quality than this sample of Prominar 883? Could this prism defect be one factor? I think they both have spherical aberration about the same amount but I'm not totally confident about that.

The difference in sharpness may be small but I think it definitely is there and I'm not imagining it. Sometimes Kowa seems to get very close but it never seems to better than Diascope. Also in a more challenging air conditions the Diascope delivers better image quality.

Juhani

jring · Aug 24, 2020

Hi,

you said it yourself over in the Diascope post... both have some SA but the Kowa has also a bit of stig and coma - which in the sum makes for a slightly worse image at higher magnifications...

The prism defect might play a role too, but if you don't see spikes at best focus on a bright star (or your sun reflection type artificial stars), it can probably be ignored.

Joachim

BoldenEagle · Aug 25, 2020

jring said:
you said it yourself over in the Diascope post... both have some SA but the Kowa has also a bit of stig and coma - which in the sum makes for a slightly worse image at higher magnifications...

I was just making sure I had concluded things right...

I have been curious about how these findings in star tests relate to the differences between scopes when normally viewing through them; which amount of astigmatism, SA etc. will produce the amount of difference I can detect in direct comparison.

Now I have some experience about that.

Juhani

kabsetz · Aug 26, 2020

BoldenEagle said:
I posted some star test pictures on the Zeiss section regarding of one sample of Diascope 85 fl.

The difference in sharpness may be small but I think it definitely is there and I'm not imagining it. Sometimes Kowa seems to get very close but it never seems to better than Diascope. Also in a more challenging air conditions the Diascope delivers better image quality.

Juhani

This difference in image quality in challenging air conditions (in favour of scopes with lower aberrations) is one of the easier ways of side-by-side evaluation of scope quality. You just need to be careful to switch back and forth several times, since challenging conditions are also changeable conditions, and it is easy to jump to conclusions.

When I first noticed this phenomenon, it seemed counterintuitive to me, and there are lots of opinions around that when conditions are poor, it does not matter how good your scope is. However, poor seeing conditions in fact magnify the defects already in the image, and in challenging air conditions the difference between a perfect or almost perfect scope and a poor or mediocre scope can be quite dramatic.

Kimmo

BoldenEagle · Aug 26, 2020

kabsetz said:
However, poor seeing conditions in fact magnify the defects already in the image, and in challenging air conditions the difference between a perfect or almost perfect scope and a poor or mediocre scope can be quite dramatic.

Kimmo

I can see and understand in practise that a better image quality scope can offer better image also in poor seeing conditions.

But it's little hard for me to understand how the difference can be magnified in poor seeing conditions, because the conditions produce the same amount of aberration for both scopes and it would be intuitive to expect that same amount of aberration is just added to the native aberrations of each scope?

Juhani

kabsetz · Aug 27, 2020

Juhani,

I didn't mean that the difference is magnified, but that the aberrations in the image are. This makes the quality difference easier to see, since the better scope gives an image that looks like the air would be more stable. When there are no added aberrations (when air is stable) even a mediocre scope at most magnifications gives an image which is pretty close to what your eye can resolve, but when air is not stable, no scope does, and "sharper" becomes more apparent.

So, you are right when you say: "it would be intuitive to expect that same amount of aberration is just added to the native aberrations of each scope?", but what the sum looks like is in my view easier to see.

- Kimmo

jring · Aug 27, 2020

Hi,

image degradation from aberrations of the instrument and from bad seeing add up - and sometimes there is those days where you look through your scope, shrug and say "too bad with the seeing today". And then you walk over to some other guy who happens to have a real cherry and look through that and it's still ok.

Reasons can be multiple - smaller aperture scopes often fare better than larger ones in bad seeing as they can enjoy longer periods of no turbulence. Binoscopes or giant binoculars tend to fare better because you have two light paths and the brain helpfully takes the better image.
And there is also the possibility that the sum of aberrations in your scope was already objectionable while the cherry with no or minute own aberrations still offered an ok image.

Joachim

dwatsonbirder · Aug 27, 2020

Sorry, a bit off topic, but I saw a video from Kowa today which shows not just the power, but the incredible image quality possible with this set up + extender, and thought others may be interested. I'm almost tempted to try one against my ED82, but I've a feeling I may end up even more broke than I am currently!

Alexis Powell · Aug 28, 2020

dwatsonbirder said:
Sorry, a bit off topic, but I saw a video from Kowa today which shows not just the power, but the incredible image quality possible with this set up + extender, and thought others may be interested. I'm almost tempted to try one against my ED82, but I've a feeling I may end up even more broke than I am currently!

I think you are being too impressed with the switch from the exaggerated perspective of the smart phone camera (or whatever it is) to the view through the scope. If you were at that site with your 82ED, it would do very nearly as well. The top end of 96x on the Kowa is only 1.3x more than your Nikon at 75x, and a good sample of either scope can have superb image quality. I have the Kowa with extender etc and I have the Nikon 78ED and 82ED. The former has a nice wide zoom with good eye relief, and the larger aperture helps when digiscoping at high magnifications, but I still use my 78ED for most applications without feeling I'm losing anything.

--AP

BoldenEagle · Aug 29, 2020

kabsetz said:
So, you are right when you say: "it would be intuitive to expect that same amount of aberration is just added to the native aberrations of each scope?", but what the sum looks like is in my view easier to see.

Sounds realistic. But it may be also little individual thing how one will see it.

I did little experimenting on the subject today: I was comparing my Kowa once again vs. Diascope on unstable air conditions brought by heat haze. Viewing small text revealed once again Diascope to be slightly sharper. As heat haze became more and more severe, I would say the difference between scopes was obscuring. If there would have been bigger differences between native aberrations of the scopes, maybe there would have been more obvious difference also in severe heat haze? That was basically what my brain told me when I wasn't sure which scope showed the small text better because I was unable to read it with neither of the scopes or at least tell which one is better or worse.

But I think there must be some limit where for example heat haze will be so strong (=many times stronger aberrations than between scopes in good conditions), that you actually can't benefit much from the lower aberration scope anyway?

I may not be enough qualified to draw these conclusions because I'm not having that "real cherry" scope to compare to these I now can test; Kowa and Diascope are really close each other and neither is a cherry.

Juhani

PS. At high magnification, Kowa has markedly better contrast compared to Diascope. Yet Diascope usually resolves little better because it's just little sharper.

BoldenEagle · Sep 3, 2020

I got APM HiFW 12.5mm eyepiece for my Kowa and I compared it side by side with above-mentioned Zeiss Diascope and also with Kowa's 25-60x zoom (not side by side obviously...).

Long story short:

My initial finding is that Kowa + APM is at least equally sharp as Diascope both alone @41x and @65x using 1.6x adapter with corresponding magnifications. It may be even slightly sharper but my resolution chart is not high quality so it needs further confirmation. Contrast and brightness are really outstanding. The overall image quality is awesome and makes the zoom feel little less "sparkling"...

Of course it is prime eyepiece and may have some native optical advantage due to lesser amount of lens to air- surfaces etc. but it also makes me wonder that maybe my TE-11WZ zoom eyepiece is not that perfect sample. Of course these are not related to the optical quality but there is already seen some of the infamous black paint specks when zooming...Guess I have to try another zoom eyepiece sometime and see if it can be sharper.

Juhani

rpg51 · Jun 3, 2021

Interesting stuff as I ponder the purchase of a new scope. Part of me wishes I never saw this thread so I could remain blissfully ignorant.

BoldenEagle · Oct 11, 2021

I finally managed to make resolution measurements for my 883.

I used Edmund Optics USAF 1951 pocket resolution chart and set the distance so that I could use group 1 patterns 4,5,6. Test was done outdoors at quite good seeing without any obvious heat haze or other harmful atmospherics. The test chart was illuminated with Led light and scope was equipped with TE-11WZ- zoom + 1.6x extender resulting in 96x magnification. The standard was to see the smallest pattern which will still show the orientation of the vertical and horizontal lines.

The best result I got was 1.41" so maybe not bad but certainly not perfect either. The professional guys here can compare this result to the star test pictures I took with this sample and tell if these findings sound what would be expected.

I will try to repeat this test and check if I get consistent results. I might get an eyepiece which offers even greater magnification, so if anyone knows which short focal length (about 3.5 -5mm) eyepieces could be used with Kowa 883, please let me know. I'm mostly interested of the inexpensive ones that just will do this job, so no need for ultra wide angle or other "luxury" properties. Decent eye relief for eyeglasses would be wanted though.

Regards, Juhani

Tringa45 · Oct 11, 2021

BoldenEagle said:
The best result I got was 1.41" so maybe not bad but certainly not perfect either.

That's exactly the result I got with my 883 some years ago (group 2,5 @ 23 m using a backlit Edmund glass slide).
I used a 3,5 mm Nagler for 140x and might have made the diffraction limited 1,32" had I gone a little closer for the next element or resolved the 2,5 a little further back.

John

jcwu88 · Oct 13, 2021

mayoayo said:
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....

I totally agree of challenging to take a good photo.
I have worked in optical lab before: to check laser beam quality, we use Etalon to see laser diffraction patterns. The projecting plane must be vertical otherwise you will get elliptical. Same for the star test, to get meaningful photos, the camera/lens position is very critical.

andy79 · Dec 31, 2021

Thank you all for the great gear info here on birdforum, i'm on the market for my first spotting scope, as I reached to the conclusion that a high quality one will cover all my related intererests: nature/terestrial long reach viewing, photography, ocasional astro, a decent amount of mobility. (For the very intensive hiking, I'll stick with a small bino).
Birdwatching is not (yet) smth I see doing too often, but who knows.

I slowly reached the conclusion that the Kowa 883 would satisfy me in the long run, as I expect it to be a lifetime investment.

The topic is a bit concerning, as I really want to avoid crazy expensive lemons or at least sub par copies, for this pricing. The idea of sending back and forth copies is something I dont want to get doing.

My question is how frequent and dramatic is the soft focusing at 60x or other faults? If there is a high chance that my copy will be worse than a different scope at 50-60% of the 883 price, I'll have to reconsider.
Unfortunately I can buy online only, so can't test beforehand.

Thank you

Binastro · Dec 31, 2021

Hi Andy and welcome.

I doubt that anybody knows the variation in quality, except the makers.

Different batches may have different spreads of quality.

All the Kowas should benefit from fluorite crystal, so that chromatic aberration is probably better than other scopes.

Then there is the quality of the observer's eyesight, if necessary corrected by prescription spectacles.

Then what level of perfection will the owner be satisfied with.

I think I would be happy with 80% perfection.

Then statistics and luck.

If I fancied a Kowa scope and could only buy it untested, I would buy it and if it was good enough for my purposes, I would keep it.

If not, I'd try one more.

If this didn't suit, I would probably look elsewhere, say Meopta or Nikon.

It is an individual decision.

Another way would be to buy a known high quality sample secondhand from someone who knows how to test scopes.

Happy New Year to all.

Regards,
B.

andy79 · Dec 31, 2021

Thank you Binastro. The unknown to me is how wide is that variance in image quality with terrestrial and astro, between a good copy and a not so good one with real life cases at higher magnification. I see from the tests posted by other owners(controlled envs), but how these inpact viewing and picture quality, I don't really understand.
I may go with Kowa, eventually, I dont want the buyer's remorse with the 1k cheaper ones.

And a Happy New Year to you too!

Binastro · Dec 31, 2021

With astro, it depends what one is looking at.

If looking at planets, anything below 100x is likely to disappoint.

I would expect to use 120x and 150x on a Kowa 883.

However, an Evostar 90mm f/10 refractor at £210 with tripod and mount would probably outperform a Kowa 883 on planets.

A 200mm Dobsonian would be far better.

With astro, colour fidelity does not matter much, although it does seem to matter for birdwatching.

What surprises me is that owners of excellent or even exceptional samples of spotting scopes, sell them to buy newer possibly far inferior scopes.

A lot of buying and possession seems to be irrational thinking.

I think that the few really exceptional scopes that I have had were either bought secondhand or custom made for me by exceptional makers.

Regards,
B.

Kowa Prominar 883 star test pictures (1 Viewer)

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