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Kowa Prominar 883 star test pictures (1 Viewer)

kabsetz

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Well-known member
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

Natural history enthusiast.
United States
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

Well-known member
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

Well-known member
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
 

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