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#51 sirchz

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Posted 04 December 2012 - 01:35 PM

As I understand it, the distance to the first ring stays the same even as the star image grows fainter, so the true size of the Airy disc does not diminish with magnitude, only the apparent size of the spurious disc.


First, I may be misunderstanding what you are saying. If so, I apologize in advance.

Having said that, I think "in theory" that as the central obstruction increases, the radius of the first minimum decreases, the intensity of the central peak decreases and the intensity of the rings increases.

I think the calculation is actually done (to some extent) in Arfken, Mathematical Methods for Physicists (and many other places as well).

I'll look it up when I get home and either provide more details or apologize for a bad memory. :grin:

#52 Asbytec

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Posted 04 December 2012 - 08:11 PM

Don, thank you. It's interesting topic. IME, a difference can be noted, but why? Is it the inherent smoothness or is it the transverse aberration?

Yes, I suspect the Airy disc remains the same diameter (pending any article by Sirchz), but does it change appearance other than being dimmer? Does it get mushy and surrounded by faint light at lessor correction and more tight at better correction. If so, is this responsible for the improvement we can see? No reason to doubt that, really, but is a mushy appearance the reason?

The diffracted wavefront coming to focus is full of interference, some points on the disc are canceled while others are augmented (CO aside for the moment.) If the optic is perfect, then it adds no more disruption to the phase nor the pattern of interference. However, if the optic does not produce a spherical wavefront, then the greater optical path from different rays changes the pattern of interference.

But, the radii of the Airy disc, first minimum and second maximum, etc., should remain constant and based solely on the wavelength and aperture. So, as I understand it, the first minimum is a point of maximum interference cancelling all energy from that point (or at that radii) regardless of the figure. So, the Airy disc will still fall to zero at a set first minimum distance even as the peak intensity falls off. The PSF curve at the very edge of the Airy disc remains very steep, hence the Airy disc well defined. (I was wrong above thinking the slope toward minimum would be more shallow.)

So, I am not sure transverse error has a role in giving the Airy disc a mushy appearance. This diffracted and aberrated wavefront still cancels energy at the first minimum, just the pattern (at set radii determined by wavelength and aperture change in relative brightness.) However, it does redistribute the light across the pattern as the total energy must remain the same.

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected. In other words, what's left to affect the average is the overall wavefront deviation (back to transverse ray's again :lol:) But, I think at focus ray's do not define the pattern seen, they are simply geometrical representations of self interfering wavefront and not a portion of the wave providing energy into the first minimum. As I can understand it, anyway.

So, what would give the in focus Airy pattern that washed out look and is this what makes the difference between a better optic and a lessor one? Or is it the redistribution of light across the pattern through interference induced by diffraction, surface deviation (phase), and the CO? I dunno, but I suspect it's the redistribution of light into the rings that makes the difference. CO induced diffraction would simply add to the wavefront's diffraction making the redistribution more pronounced (and a tiny bit more so if the secondary is not perfectly flat?)

#53 BillFerris

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Posted 04 December 2012 - 09:28 PM

Really, no anecdotal evidence is going to change your mind, and in fact that is the only kind of evidence that we have!


All the evidence isn't anecdotal. Ceravolo, Dickinson and George conducted an experiment in which one person figured four 6-inch, f/8 mirrors, respectively, to 1-, 1/2-, 1/4- and 1/10-wave accuracy. The figures were confirmed with an interferometer.

The finished paraboloids were installed in identical optical tube assemblies and Dobsonian mounts. Then, labelled only A, B, C and D, the four scopes were shipped to Terence Dickinson for multiple nights of testing and evaluation. When Dickinson was done with his evaluation, the scopes were sent to Douglas George, who performed his own independent tests.

Both Dickinson and George knew the four scopes included mirrors figured to different levels of accuracy. Neither knew which was which. They had to rely on their own evaluations of images at the eyepiece to distinguish one from another. In the end, both were in agreement that the 1/10-wave primary could be distinguished from the 1/4-wave primary, but only when conditions were excellent and careful attention was paid to subtle differences in the view.

Ceravolo, Dickinson and George chose to perform their experiment using small mirrors to limit the impact atmospheric seeing would have on the results. It seems to me a similar test could be performed using larger mirrors. And with the substantial number of large aperture Dobsonians in use that also have documentation of the quality of their mirrors, it should be possible for an interested party to organize such a test.

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#54 Starman1

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Posted 04 December 2012 - 11:45 PM

Don, thank you. It's interesting topic. IME, a difference can be noted, but why? Is it the inherent smoothness or is it the transverse aberration?

Yes, I suspect the Airy disc remains the same diameter (pending any article by Sirchz), but does it change appearance other than being dimmer? Does it get mushy and surrounded by faint light at lessor correction and more tight at better correction. If so, is this responsible for the improvement we can see? No reason to doubt that, really, but is a mushy appearance the reason?

The diffracted wavefront coming to focus is full of interference, some points on the disc are canceled while others are augmented (CO aside for the moment.) If the optic is perfect, then it adds no more disruption to the phase nor the pattern of interference. However, if the optic does not produce a spherical wavefront, then the greater optical path from different rays changes the pattern of interference.

But, the radii of the Airy disc, first minimum and second maximum, etc., should remain constant and based solely on the wavelength and aperture. So, as I understand it, the first minimum is a point of maximum interference cancelling all energy from that point (or at that radii) regardless of the figure. So, the Airy disc will still fall to zero at a set first minimum distance even as the peak intensity falls off. The PSF curve at the very edge of the Airy disc remains very steep, hence the Airy disc well defined. (I was wrong above thinking the slope toward minimum would be more shallow.)

So, I am not sure transverse error has a role in giving the Airy disc a mushy appearance. This diffracted and aberrated wavefront still cancels energy at the first minimum, just the pattern (at set radii determined by wavelength and aperture change in relative brightness.) However, it does redistribute the light across the pattern as the total energy must remain the same.

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected. In other words, what's left to affect the average is the overall wavefront deviation (back to transverse ray's again :lol:) But, I think at focus ray's do not define the pattern seen, they are simply geometrical representations of self interfering wavefront and not a portion of the wave providing energy into the first minimum. As I can understand it, anyway.

So, what would give the in focus Airy pattern that washed out look and is this what makes the difference between a better optic and a lessor one? Or is it the redistribution of light across the pattern through interference induced by diffraction, surface deviation (phase), and the CO? I dunno, but I suspect it's the redistribution of light into the rings that makes the difference. CO induced diffraction would simply add to the wavefront's diffraction making the redistribution more pronounced (and a tiny bit more so if the secondary is not perfectly flat?)

Well, a larger secondary throws more energy into the rings, so lessens contrast. Remember that every extended object is a series of Airy discs surrounded by diffraction rings. The brighter those rings are (the more energy they possess) the lower the contrast in the image (the more damage to the MTF).
Scattered light can create its own wavefront errors, throwing more light into the rings, and even, at worst, creating multiple overlapping Airy discs. Ever see the "hairy" disc created by seeing conditions? It's actually overlapping discs coupled with reflected light that smears the perfect diffraction pattern. It's worse than the problems caused by less-than-perfect optics.
Seeing is a major contributor to optical problems. I've seen nights where each star was a point with a single diffraction ring. And I've seen nights in the same scope where each blobby star was surrounded by a thick retinue of diffraction rings.

[Aside: you are correct to think the diameter of the Airy disc is determined by wavelength and aperture--not magnitude.]

Some time we'll talk about diffraction spikes and how seeing modifies their visibilities, but that's a discussion for another time.

#55 dpwoos

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Posted 05 December 2012 - 12:41 AM

You have cited this test more than once, and I think it is interesting and the results are interesting. However, the most accomplished mirror maker in our club (and a fine observer) participated in the well-known mirror test at Stellafane, and he has a very different story to tell. Also, I and others in our club have experiences that also don't agree with those of Dickinson. You say that the Dickinson test isn't anecdotal, but as much as I respect all of the folks involved to me this is far from a scientific test. Do you think that a paper based on this test would be accepted in a scientific publication? I think not, and I bet that you don't either. Bottom line - it is anecdotal evidence.

But as I have said, no one should accept what I have to say, or what Dickinson or Ceravalo or you has to say. In our club there are plenty of opportunities to observe with all kinds of optics of all kinds of quality, and one can see what one can see and make up ones mind on how to achieve good and great views. I am certain that most other clubs offer the same opportunities. We can (and must) decide for ourselves.

#56 Asbytec

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Posted 05 December 2012 - 01:40 AM

I've seen some pretty bad Airy discs, I suppose they can be called Hairy. :lol:

I wasn't so much interested in seeing effects on the Airy disc, but more trying to understand the difference between micro ripple and transverse ray aberration. To me, both of those contribute to the pattern of interference and the phase of the wavefront - hence light distribution.

But, total RMS should be less affected by micro ripple averaged over the wavefront than large areas of P-V error. And the difference is brighter rings, dimmer disc. That in turn is what differentiates a better figure from a lessor one as opposed to a Hairy disc. Again, in turn, this difference can be seen in contrast resolution supporting the case that 1/10th wave (RMS) can be noticeably better.

I guess what I am driving at is, image quality depends on how that complex series of interference collapses to focus. That complex diffraction and aberrant interference both augments and destroys energy at specific locations (determined by wavelength and aperture) leaving the radii unchanged and energy simply distributed differently. The result is more dependent on total RMS and less so on "tightness" (or lack of) in the Airy pattern itself.

In other words, if I understand it correctly, worse correction should not turn the Airy pattern into a continuous, washed blob of light. It will still maintain its patterns of dark minima and brighter maxima defined only by wavelength and aperture, not transverse error or any other aberration.

Ah, I am rambling...sorry. Just trying to understand why better mirrors are indeed better.

#57 mark cowan

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Posted 06 December 2012 - 02:13 AM

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected.


No, this is definitely an RMS error. But then, ALL errors are RMS errors. A whole lot of small errors can add up to significant wavefront error, and they don't cancel like you're suggesting. Small scale ripple casts a veiling haze that lowers overall contrast.

I guess what I am driving at is, image quality depends on how that complex series of interference collapses to focus. That complex diffraction and aberrant interference both augments and destroys energy at specific locations (determined by wavelength and aperture) leaving the radii unchanged and energy simply distributed differently. The result is more dependent on total RMS and less so on "tightness" (or lack of) in the Airy pattern itself.


You're leaving out the whole point of minimizing transverse error, which is to tighten the FOCUS. Transverse error describes slope deviations on the glass, and slope deviations translate to errors in focal length, if you want to think about it that way. So no, the radii are not unchanged, and these changes in energy distribution are always detrimental, never favorable.

In general, the edge of the airy disc may remain well defined, but the result of dumping energy into the rings is mush to the eye.

Both Dickinson and George knew the four scopes included mirrors figured to different levels of accuracy. Neither knew which was which. They had to rely on their own evaluations of images at the eyepiece to distinguish one from another. In the end, both were in agreement that the 1/10-wave primary could be distinguished from the 1/4-wave primary, but only when conditions were excellent and careful attention was paid to subtle differences in the view.


Aside from anything else, though, all the mirrors were very well made with the sole deviation of greater SA on the lower rated ones. I.e., they were smooth with well controlled figures. This is not what you normally get for mirrors that don't test well. :shrug:

Best,
Mark

#58 orion61

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Posted 06 December 2012 - 05:31 AM

I saw an article about 20 yrs ago in astronomy mag or was it S&T?? wehere they set up 1/4 1/8 and 1/10th wave systems
even people walking down the sidewalk could clearly see the difference..

#59 Asbytec

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Posted 06 December 2012 - 06:13 AM

Mark, thanks. Its an interesting topic, can one see a difference? I believe so. Then the question is, what properties of a better mirror make that better image happen. How does diffraction limited SA differ from better correction? Yes, less light into the rings and better contrast is the answer.

At L/4 SA has much lower RMS (Strehl ~.0.8) and large transverse errors across the entire surface. At 1/10th (Strehl >/~ 0.95.), an improvement that large in Strehl should be noticeable. So, I argue the positive results are not surprising.

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.

Anyway, it was fun exploring the topic. I appreciate you engaging in good debate.

#60 nicknacknock

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Posted 06 December 2012 - 07:56 AM

Any comments on the attached test?

I do have my opinion about it, but I am interested to hear comments from you folks since the discussion here is really interesting and people lurking in this thread are much more knowledgeable than me...

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#61 Starman1

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Posted 06 December 2012 - 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.

#62 dpwoos

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Posted 06 December 2012 - 12:19 PM

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.

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#63 mark cowan

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Posted 06 December 2012 - 12:59 PM

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

#64 mark cowan

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Posted 06 December 2012 - 01:06 PM

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

#65 Asbytec

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Posted 06 December 2012 - 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.

#66 Jarad

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Posted 06 December 2012 - 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

#67 mark cowan

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Posted 06 December 2012 - 02:21 PM

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.

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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#68 mark cowan

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Posted 06 December 2012 - 02:43 PM

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

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#69 ausastronomer

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Posted 06 December 2012 - 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

#70 dpwoos

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Posted 06 December 2012 - 06:34 PM

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.

#71 Mike Lockwood

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Posted 06 December 2012 - 06:40 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

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.

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. :bangbangbang:

#72 freestar8n

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Posted 06 December 2012 - 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

#73 Pinbout

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Posted 06 December 2012 - 07:57 PM

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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#74 maknewtnut

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Posted 06 December 2012 - 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 [/quote]

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?

#75 bartine

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Posted 06 December 2012 - 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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