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Equipment Discussions >> Reflectors

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: 1/10 wave? new [Re: nicknacknock]
      #5557319 - 12/06/12 11:27 AM

1) the surface is rough on a fine scale.
2) the Strehl is unbelievable. To me it merely indicates an inadequate number of points selected.
3) It would be interesting to see this mirror in a Lyot test.
I suspect it would show to be very rough.


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dpwoos
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Reged: 10/18/06

Loc: United States
Re: 1/10 wave? new [Re: nicknacknock]
      #5557426 - 12/06/12 12:19 PM Attachment (25 downloads)

My knowledge of interferometry and interferograms is very limited, but the premiere mirror maker in our club has made and regularly uses his own Bath interferometer, and he says that it is very easy to convince yourself that you are seeing a level of detail that the test simply doesn't support. I have been told the same thing by Bryan Greer (ProtoStar) in a discussion about claimed secondary quality. So, I would be hesitant to ascribe a great deal of meaning to the apparent fine detail portrayed. Of course, others with more Zygo experience might say something different.

Here are two images (using the Bath) that depict a before and after of a mirror from a club scope, where the image on the right depicts the mirror at around Strehl .94 and the image on the left depicts the mirror when it was awful (including obvious astigmatism). BTW, I was the one who first checked out the mirror (actually Saturn was the first target) and the astigmatism was obvious. I think that the fellow who did both the testing and the refiguring believes that even this level of detail is a bit unrealistic, but he knows his rig and so knows what he is seeing, and how to address it.

Edited by dpwoos (12/06/12 04:51 PM)


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: 1/10 wave? new [Re: Asbytec]
      #5557505 - 12/06/12 12:59 PM

Quote:

Some minor misunderstandings, though. By radii, I didn't mean radius of curvature or that of a reference sphere - transverse and longitudinal error. I meant the radius of the diffraction rings and Airy disc, trying to describe the diffracted and aberrant pattern comes to focus, phase, etc. I just don't see how the first minimum can grow in brightness (and the Airy disc get larger) with interference cancelling at that point in the image space, if that is indeed what happens. Both of those are set by aperture and wave length. I'm almost positive there is no other variable that affects them.




Aberrations will affect the radii you're talking about. The Airy disc retains the same angular size pretty much (it's least affected by aberrations until it vanishes entirely into the haze) but interference effects change the maxima and minima of the resultant ancillary rings. Download "aberrator" and experiment with the star image displays while changing the wavefront errors to see this.

Best,
Mark


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: 1/10 wave? new [Re: Starman1]
      #5557515 - 12/06/12 01:06 PM

Quote:

1) the surface is rough on a fine scale.




Not necessarily from that graph - note the vertical scale. This probably amplifies residual noise in the system.

Best,
Mark


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: 1/10 wave? new [Re: mark cowan]
      #5557547 - 12/06/12 01:25 PM

Well, maybe I am wrong or maybe we're saying the same thing talking past each other. For example, when you say the Airy disc is least affected by aberration, I immediately think of the energy that is robbed from it by aberration. Then you state correctly, "until it vanishes into the haze" meaning, I think, it fades but retains it's diameter set by the first minimum.

By the way, ever see the Poisson spot darken at the very center, forming kind of a hole, as you scroll through focus? I think that's interference alternating in and out of phase. So, the Airy disc should actually contain the sum of all the noise that comprises it. If so, then it seems to be more affected by phase differences caused by greater surface error.

It's been a while since I played with Aberrator. If memory serves, the rings stayed the same radius but the relative brightness changed. So, yes, aberration changes the maxima in relative brightness, at least. Maybe I need to look closer at the minima. CLICK! Yea, the contrast changes, as well. Hmmm...

Still, how can the minima not be zero? There must be some points along the diffraction pattern that fall to zero. It's interference. There have to be points that cancel, I believe. If not, then the Airy disc would not be constrained to a given size as the Airy disc vanishes, "into the haze." It would have no minima at that point. Or would it?

Now you got me thinking how that can be so. Anyway, thank you, I am beyond my knowledge at this point. But that's okay, it spurs inquiry.


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Jarad
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Reged: 04/28/03

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Re: 1/10 wave? new [Re: Asbytec]
      #5557559 - 12/06/12 01:31 PM

There has to be cancellation, but not necessarily to 0. If you have multiple waves overlapping, it is quite easy to end up with no point where they add up to exactly 0. The minimum could be >0.

Jarad


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: 1/10 wave? new [Re: Asbytec]
      #5557638 - 12/06/12 02:21 PM Attachment (12 downloads)

Quote:

For example, when you say the Airy disc is least affected by aberration, I immediately think of the energy that is robbed from it by aberration.




I mean it retains its shape the longest in the face of aberration, nothing more.

Quote:

If memory serves, the rings stayed the same radius but the relative brightness changed.




Uhm, no. Crank up the diameter and power, add some CO to induce more rings, then add a wave or so of various aberrations. Measure the radii and you'll see the changes, as well as seeing how the central peak can vanish completely under sufficient wavefront error. Here's a simple example, showing a contrast stretched zero error + 20% CO against a wave each of astig, primary spherical, and 5th order SA. It's just an extreme example but it illustrates what I'm talking about...

Best,
Mark


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: 1/10 wave? new [Re: mark cowan]
      #5557664 - 12/06/12 02:43 PM Attachment (26 downloads)

Possibly more germane, here's 1/5th wave of defocus. Mush.

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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: mark cowan]
      #5557974 - 12/06/12 05:48 PM

Link to test results for 10"/F6 Zambuto mirror

The above link will take you to an independent interferometer test of a 10"/F6 Zambuto mirror by Wolfgang Rohr in Germany.

The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?

I rest my case.

Cheers


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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: 1/10 wave? new [Re: ausastronomer]
      #5558040 - 12/06/12 06:34 PM

Quote:

I rest my case.




I don't follow you - what case does this post about the Zambuto mirror support? I don't read anything that bears on whether or not it is possible to differentiate between this mirror and a significantly more accurate one. If you think that the "Zambuto" name alone is going to end the discussion, then I think you are mistaken.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: 1/10 wave? new [Re: ausastronomer]
      #5558047 - 12/06/12 06:40 PM

Quote:

The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror



Please note that another set of tests gave a value of less than 1/8th wave, and the error has a completely different form.

Also note that the peak values occurred near the top and bottom of the mirror, almost certainly due to gravity distorting it, or possibly an artifact of fringe tracing on an aspheric mirror. These raised areas are small, so they have a small effect on the Strehl calculated by the software. (A high area like this all the way around a zone would have lowered the Strehl more.)

(Just to be clear, I'm commenting on the issues of optical test results and procedures, not another vendor's product, so by the TOS I should be allowed to do so.)

However, let's not allow the separate issue of test errors and indiosyncrasies cloud the original question, which was the performance of mirrors of various qualities.

Quote:

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?
I rest my case.



Well, since with the effects of gravity removed it's probably 1/10th wave or better anyway, the specific question is probably not valid, but this goes back to the original question - can better mirrors be distinguished from poorer ones?

The short answer is yes. However, it must be realized that one number/test is not enough to completely characterize a mirror or its performance.

I advocate testing telescopes side-by-side, under the sky, over multiple observing session. You'd think that it would be possible to find the good one in the bunch, right? Well, yes, the better scope should rise to the top after allowances are made for equilibration and other factors.

However, in my experience, the larger the mirror, the less likely one is to find a truly good mirror, properly supported, to compare others too, so it is possible that you might spend nights comparing mediocre optics to each other without knowing it.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: 1/10 wave? new [Re: Mike Lockwood]
      #5558121 - 12/06/12 07:22 PM

The importance of optical quality comes up often, and comparison tests are cited - but I think there are several factors that get ignored. First, it's one thing to be able to tell one from another at the eyepiece, but it's much harder to demonstrate that one actually performs better than another. There may be a softer "snap to focus" - but the real issue in terms of image quality is - how well does it convey the information of the object to the observer. I don't know of a single empirical test with observers that focused on realized performance rather than simply "telling them apart." In an extreme example, you can tell a refractor from a reflector by the secondary shadow and spider vanes - but that has nothing to do with the image quality. Of course - if people cannot tell them apart at all, even with defocus, then that does point to it not mattering - for those observers and those conditions anyway.

The other issue is the role of statistics in getting a brief, clear view of an object through the atmosphere. This is usually viewed in terms of having a perfect optical system and waiting for moments of perfect atmosphere. But just as good would be to have an imperfect optical system, and wait for a brief time when the atmosphere exactly cancels the imperfections of the optical system. This may seem crazy or pedantic - but for small errors in the optical system that roughly match the power spectrum of turbulence, a moment of perfect atmosphere is not a lot less likely than a moment of exactly complementary atmosphere. As the error in the optical system increases the probability of a match goes down - but in all this it is a matter of statistics rather than - you need perfect optics and perfect atmosphere to get a perfect image.

Frank


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Pinbout
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Reged: 02/22/10

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Re: 1/10 wave? new [Re: ausastronomer]
      #5558184 - 12/06/12 07:57 PM Attachment (6 downloads)

Quote:

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror






when I modeled that mirror up in Jim Burrow's Diffract program his ronchigram looks better than 1/14th~ since autocollimation doubles the error.

for the model i used 1/7~ error undercorrected sphere to get the 1/14th~ overcorrection parabolic in autocollimation.


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maknewtnut
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Reged: 10/08/06

Re: 1/10 wave? new [Re: ausastronomer]
      #5558264 - 12/06/12 08:56 PM


The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?

I rest my case.

Cheers




That's a great way to substantiate one of the biggest differences of opinion in this thread. Namely where one person implied that CZ mirrors are by and large, much better than 1/10 wave P-V. Uhh...no. Carl's work is consistently fantastic, but one has to learn more about differences in testing methods and results before jumping to that conclusion (which many do).

From personal experience, there are also a considerable number of folks who will argue that a 1/6 wave system (which is different from a 1/6 wave primary mirror)isn't worth their money when they can pay more for a 1/8 wave system...or haggle about receiving a refractor with a maker's test result indicating .96 Strehl rather than their buddy's which is .97.

1/10 wave optics are rare. What many involved in this thread seemed to have overlooked is the term 'casual observer' used in the Stellafane test. It was even added that the seasoned observer CAN see the difference, so why the debate?


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bartine
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Reged: 10/03/07

Loc: Potomac, MD
Re: 1/10 wave? new [Re: maknewtnut]
      #5558402 - 12/06/12 10:34 PM

Interesting discussion.

Nothing like this forum to have someone
1. who doesn't have a clue who you are
2. knows nothing about your level of observing experience and who
3. never looked through your telescope ONCE

tell you you are wrong.

Geez.

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.

In this world, you generally get what you pay for. Astronomy and optical precision is no different. 1/4 wave and a high strehl ratio will not compete with 1/10 wave and a low strehl ratio.

Otherwise, I guess all those guys who pony up the big bucks for high quality mirrors are nuts.

Why do people swear about the views through a zambuto mirror? Why do refractor nuts swear about views through their AP Physics or Televue refractors? Because they are better. Better oprically, mechanically and visually.

Some nights will be better than others, but side by side the better scope will always win out.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: 1/10 wave? new [Re: maknewtnut]
      #5558435 - 12/06/12 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.

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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: bartine]
      #5558455 - 12/06/12 11:15 PM

Quote:



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,


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: ausastronomer]
      #5558461 - 12/06/12 11:18 PM Attachment (24 downloads)

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

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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

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Re: 1/10 wave? new [Re: ausastronomer]
      #5558466 - 12/06/12 11:21 PM Attachment (24 downloads)

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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freestar8n
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Reged: 10/12/07

Re: 1/10 wave? [Re: ausastronomer]
      #5558639 - 12/07/12 02:48 AM

Quote:

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


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