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Keep or send back? Under the night sky. Star test 883 + TE-11WZ. (1 Viewer)

mskb

Well-known member
Everyone,

Finally managed to image a star test with an 883 + TE-11WZ . Definitely not sure if Kowa cares about astigmatism as much as we do around here. :) ( My 663M also has it. )

Oh, and the prism edge of course.

Could you comment on what else you see in the attached pics? Any thoughts on whether to keep it / sending it back? Haven't had a chance to do a proper resolution test.

I have had a chance to look through a couple of 883 + TE-11WZ so far. This is the "best" of the bunch.

Thanks!
 

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henry link

Well-known member
It would be better to photograph less defocus, 3 or 4 rings show astigmatism and coma more clearly. This one looks pretty well corrected for spherical aberration.
 

BoldenEagle

Well-known member
Everyone,

Finally managed to image a star test with an 883 + TE-11WZ . Definitely not sure if Kowa cares about astigmatism as much as we do around here. :) ( My 663M also has it. )

Oh, and the prism edge of course.

Could you comment on what else you see in the attached pics? Any thoughts on whether to keep it / sending it back? Haven't had a chance to do a proper resolution test.

I have had a chance to look through a couple of 883 + TE-11WZ so far. This is the "best" of the bunch.

Thanks!
Many thanks for sharing these pictures!

SA seems to be corrected clearly better than in any sample I have tested (4).

Astigmatism seems about as bad as the worst sample I saw. In my current sample there is some stig but if defocused to about 8 rings as these ones, it seems to dissappear.

I have had difficulties to see coma because many times there is not very clear or bright central spot. Instead of defocusing to 4-5 rings, I think I can see it better with just 2-3 rings because if comatic, the spaces between rings show it more clearly as the bright rings seem to touch each other on some side of the circle (is this correct way to detect coma, Henry?). So I'm not sure if your sample has as bad coma as it at first look seems by these pictures.

Hey what's that yellow line in the center, like I would have seen it somewhere before...Ah, oh yeah: every sample I have tested showed that...

Have you compared the image quality to some other scopes, do you think this sample delivers nice and sharp image? If I would have to say something of my sample, it feels quite sharp and particulary contrasty. But if I put it in direct comparison with better corrected sample of Zeiss Diascope 85, the Zeiss is sharper and resolves better. It's not a big difference but I can see it every time I compare them. Yet the Diascope has poor contrast compared to the Kowa.

Regards,

Juhani
 

jring

Well-known member
Hi,

yes, this example seems quite good in regards to spherical and probably coma too. There is some astigmatism visible - another image with only 3 or 4 rings of defocus would be better as has been noted.

The stig could be due to too tight lens mounting and might get better or even go away by slightly loosening the retention ring before the front element - but of course this needs the proper tool and experience to not ruin the notches or worse, the front element.

Joachim
 

kabsetz

Well-known member
Agree that it would be good to see images closer to focus. Could you also give a verbal description of what you see in the best focus Airy disc? It does look like the levels of SA are low enough not to be a concern.

Also, astigmatism at very close to best focus/best focus is useful to know. My experience is that prism lines - although ideally they should not be visible at all - are more or less okay if the best focus Airy disc/central spot does not show any spiking.

- Kimmo
 

Tringa45

Well-known member
Europe
Is there confusion here between astigmatism as an optical aberration in an instrument and pinching of an objective? The latter would be similar to astigmatism of the eye.
If these are central images they would not show astigmatism or coma, which are only visible near the field edge.
Here, https://www.birdforum.net/threads/astigmatism-as-an-aberration-in-optics.285031/ is an old thread I started six years ago, in which I attempted to explain astigmatism as an optical aberration.
A slight elongation of a central image could perhaps be attributed to a deviation from the 90° angle between the roof faces, but AFAIK this is held to a tolerance of under 1 arcsecond for instruments of similar quality to the 883.

John
 

jring

Well-known member
Hi John,

there is no confusion - pinching lenses can lead to astigmatism. Other things can too - like not randomly rotating your mirror blank often enough when grinding or rotating elements away from their marked optimal position found by the optician.

In case of pinching it can often be removed by relaxing, rotated elements can be aligned to the marked position - or by trial and error during a long star testing session and when you ground an aberration into your mirror, you are sol - or you are NASA and can build a corrector.

Joachim
 

mskb

Well-known member
Thanks all! I will get the 3-4 diffraction ring images + the best focus images soon.

@BoldenEagle, I have compared it to an S2+20-70X and the sharpness at 60X is very similar. The S2 has comparable levels of astigmatism, SA correction as the one shown here, not as good as Kowa distortion control and noticeably lower brightness, but it was (a)much better focus-snap-wise from either side of focus and (b) much better ease of view and eye placement. The other two 883 copies had the mix of aberrations that Henry had noticed in his review, to varying degrees - coma + astigmatism + spherical aberration. Prism lines in all were basically a given. The first one's best focus was a smear! Imagine that at a >$2K price point! If only the S2 had Kowa's colors + objective diameter, my decision would be very easy. I could live with the barrel distortion very noticeable with the 20-70X and its lower FoV. As I am writing this, I almost feel like I should just give up on the 883 and move on with an S2 for now.

@kabsetz , I am failing to recollect the best focus image at 60X. I will try to image the best focus point I could get and share a picture.

Thank you all for your help! You all enable better, more informed purchases. The rest of us are grateful!
 
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Tringa45

Well-known member
Europe
there is no confusion - pinching lenses can lead to astigmatism.
Joachim,

I'm not disputing that pinching can lead to astigmatism but interchanging the terms IS confusing.
Astigmatism as an optical aberration is inherent in the design of some objectives and eyepieces. See my link or Rutten ("Telescope Optics"), page 29.
It is best evaluated by viewing the edge of a grid pattern. The combination of Kowa 883 and TE-11WZ fares very well, as does my ATM 65HD with 30xW.
Amongst binoculars, Swarovski EL SVs are excellent and Zeiss FLs pretty awful.

John
 

jring

Well-known member
Hi,

you are correct that astigmatism can also be caused by the design - usally as a tradeoff to minimze other aberrations far off the optical axis. But when we are star testing a telescope on axis and see astigmatism, we can be quite sure that it is not inherent to the design.

Astigmatism is exactly what we see in the image shown by mksb. Compare to the example image shown on the start test page of Vlads e-book. Or look it up in the Suiter if you want it from the bible.

As for probable sources, I have listed a few but my money is on pinched optics.

Joachim, who has done this on a few scopes... and still searches for a way to loosen the super tight retaining ring on an example of Kowa 613 with a bit of stig...
 

Tringa45

Well-known member
Europe
To quote Rutten: "Astigmatism is probably the most difficult aberration to understand."
When one hears the term, one thinks of irregular curvature of the cornea and its analogue in a telescope of pinched or decentred lenses.
To describe these defects as astigmatism is misleading and an indication that one has not understood astigmatism as an aberration.
Remember the term "Anastigmat" for some old camera lenses? An objective, which is perfectly symmetrical about its optical axis can still show astigmatism.
Pinching is pinching and astigmatism is the discrepancy in the focal points of tangential and sagittal rays from the same source outside the optical axis.

John
 

jring

Well-known member
To quote Rutten: "Astigmatism is probably the most difficult aberration to understand."
When one hears the term, one thinks of irregular curvature of the cornea and its analogue in a telescope of pinched or decentred lenses.
To describe these defects as astigmatism is misleading and an indication that one has not understood astigmatism as an aberration.
Remember the term "Anastigmat" for some old camera lenses? An objective, which is perfectly symmetrical about its optical axis can still show astigmatism.
Pinching is pinching and astigmatism is the discrepancy in the focal points of tangential and sagittal rays from the same source outside the optical axis.

John

Hi John,

my opinion stands that pinching is not an optical aberration per se but one of the different reasons for astigmatism (Z4 and Z5 for primary astigmatism). It can also lead to trefoil aka elliptical coma (Z9 and Z10).

But maybe we should either agree to disagree or continue on CN as it gets a bit technical for this forum.

Joachim
 

mskb

Well-known member
Hello Everyone,

I have decided to send back the 883. I apologize for not being able to provide the best + near focus images for this 883 copy.

While nothing has come close to satisfying me in colors than Kowa's spotters, nothing has failed in ergonomics for me as much as their scopes. My K663 continues to be a struggle. The 883 retained the same issues. It might just turn out be a burden regardless of how good the deal price ($2.2K) or the quality of view is. I positively look forward to future K883 models whenever they come. I am likely choosing an S2.

I have so far looked through a {Monarch 82ED-A 82mm, a Meopta S2 82mm and the K883}, and {a K663, ATS60 & an Opticron 60}. FWIW, the following are the conclusions, admittedly subjective, I come away with.
(1) 60ish mm scopes offer comfortably bright and sharp views upto 30X. 80ish mm scopes raise the bar to 40ishX. The viewing comfort improvements are in general very iffy beyond that.
(2) Even though the build quality is great, I least preferred the Monarch 82ED owing to its colors.
(3) Beyond 40ishX, I vastly preferred the S2 over the 88: far more comfortable, similar if not better contrasty and sharp views even at 70X, OK colors and snappy focus. I wonder how the colors and brightness of the Meopta 30-60WA match with the Kowa TE-11WZ.
(4) If going the S2 route, the three significant things I would miss relative to the K883 are excellent colors and distortion control, and a noticeable brightness improvement. But I personally would gain enormous comfort, and snappy focus while actually using the scope. How much of this improved experience with the S2 is owing to its robustly maintained higher ER, through the zoom range, I am unsure.

I am extremely thankful to Joachim for helping me out as I went through this process. Also thanks to @Bill Atwood for exchanging notes!

Thanks a lot everyone!
 
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mskb

Well-known member
Everyone,

I couldn't help but get the best and near-focus images for your scrutiny of this 883 copy.

One of the sides of focus was challenging to get (atmosphere/instrument cooling?), so I am attaching a variety. Thanks to the sublime astigmatism, you should be able to tell which images go with each side of focus. The naming should help too - {1,2}: best focus, {3,4,5} and {6,7,8} are the two sides of focus.

Could you see if you could go to town with these?

@kabsetz, on the verbal description of the best focus point, I am unsure how to differentiate between spiky-ness and the stig's cross. Let me know what you feel from the images - hope they are somewhat useful.

Thank you very much!
 

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mskb

Well-known member
My apologies; I noticed I had redundancies in the uploads. I have deleted and reorganized the above pictures: {1,2}: best focus, {3,4,5} and {6,7,8} are the two sides of focus. Looking at these, it feels like the best focus {1,2}, one side of focus {3,4,5}, and other side of focus {6} are usable?
 

mskb

Well-known member
I take it the images aren't usable / confusing. Ignoring the last two, would "moderate" astigmatism, and "slight" miscollimation/coma be a decent enough conclusion? Thanks!
 

jring

Well-known member
Hi,

sorry, when I saw the post yesterday it had quite a lot images more and I had not too much time to sift through all these. Now that they are sorted, I would say noticeable astigmatism - probably a tiny bit less than 0.37 wave example shown in here (which on its own results into 0.8 Strehl or what is called diffraction limited).


I'm not convinced that we really have any significant coma here - some images look a bit non-concentric (45 and 58) but the others don't, so I am inclined to pass it off as caused by the bad seeing...

Joachim
 

mskb

Well-known member
Thank you Joachim. That description is very helpful and closes the topic for me.

Thanks all.
 

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