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1/10 wave?

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#76 Asbytec

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Posted 06 December 2012 - 11:01 PM

Thanks, Mark and Jerad, your explanation is appreciated. I'm sure the short answer is "yes," too. It's fascinating to understand why that would be.

#77 ausastronomer

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Posted 06 December 2012 - 11:15 PM


Gentlemen - I had the scope, I had the mirror refigured, and I can swear to the results. There was a visible difference. Clusters resolved more clearly. In the Trapezium it was easier to see detail and e and f stars. Planets had different levels of detail.


I am not disputing the quality of the refigured mirror or the fact you noticed a visible difference. What I am disputing is the quality of the mirror you started with before refigure. ie was it a genuine 1/4 wave mirror that was smooth, free of astigmatsm, with a good edge correction and it's only aberation was 1/4 wave of correction error?

I would almost be prepared to bet money that what you started with prior to refigure was a pretty ordinary mirror.




Cheers,

#78 ausastronomer

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Posted 06 December 2012 - 11:18 PM

Jupiter through aberator with 1/4 wave SA and no other aberrations. 10" newtonian 15% CO

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#79 ausastronomer

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Posted 06 December 2012 - 11:21 PM

Jupiter through aberator with 1/10th wave SA, 1/10th wave stig and 1/10th wave air turbulence. 10" newtonian 15% CO

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#80 freestar8n

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Posted 07 December 2012 - 02:48 AM

Jupiter through aberator with 1/10th wave SA, 1/10th wave stig and 1/10th wave air turbulence.



It is also common in these discussions to provide simulations of wavefront error in the presence of turbulence - but that ignores the statistical nature of the realtime imaging process I describe above. I don't know what model is used for the turbulence in aberrator, but it is not trivial to do it properly - and either way it does not capture the realtime fluctuations in seeing that combine with the optical system to provide moments of clarity. This is where both the nature of the surface figure error cannot be captured by a single number (either P-V or RMS) and the atmospheric turbulence cannot be represented by a single static wavefront used to calculate a single PSF.

So - I think simulations like this have a pedagogical value for conveying the potential impact of aberrations on an image - but when it comes to experienced observers under variable seeing conditions, who are judging based on moments of clarity, they don't tell the whole story - which involves statistical optics in addition to aberration theory.

Frank

#81 mark cowan

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Posted 07 December 2012 - 04:07 PM

ie was it a genuine 1/4 wave mirror that was smooth, free of astigmatsm, with a good edge correction and it's only aberation was 1/4 wave of correction error?


Well, it could have astigmatism, a poor edge and not be smooth and still show "genuine" 1/4 wave performance. Errors accumulate.

But in the presence of those sort of errors, low resolution testing might easily rate it as better than it actually was (Foucault being blind to all three for the most part) and so be labeled better; in which case improving it by fixing those errors could produce dramatic results.

Best,
Mark

#82 nicknacknock

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Posted 19 December 2012 - 07:10 AM

Well, I posted my soon-to-be-here-if-the-end-of-the-world-doesn't-come-first 12" Orion Optics 1/10 primary mirror Zygo report and reading this thread I have come to the following conclusions:

1. Mirror quality is a subject that causes friction.
2. People can have diametrically opposite views and everything in between on the subject matter.
3. Zygo reports are to be taken with a pinch of salt.
4. Nothing beats testing the mirror in the field and then experience plays a huge role in understanding if you got a good mirror or a lemon.

I will spy with my little eye and see how my dob performs, paying no heed to the Zygo report even though the test indicates a decent mirror :cool:

Nicos






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