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Equipment Discussions >> Reflectors

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stevew
Now I've done it


Reged: 03/03/06

Loc: British Columbia Canada
Re: 1/10 wave? new [Re: ausastronomer]
      #5550978 - 12/02/12 08:21 PM

Quote:



I can't quite agree with that comment entirely.

With smaller aperture telescopes (under say 12") the differences are fairly noticeable to a skilled observer under good observing conditions between say a diffraction limited mirror (1/4 wave) and a 1/10th wave mirror. However without star testing under excellent conditions it gets quite difficult to tell the difference between say a 1/7th or 1/8th wave mirror and a mirror which is 1/10th wave or better, for the simple reason a mirror that is a genuine 1/7th or 1/8th wave mirror is an exceedingly good mirror in any case. Any differences between mirrors at this level are very subtle at best and only detectable under the very best conditions.

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave). Larger aperture scopes are more noticeably affected by the subtleties of seeing and thermal equilibrium than smaller scopes. More often than not with a scope of this aperture you are limited by seeing and thermal equilibrium not optical quality, provided the mirror is better than diffraction limited, smooth and free of astigmatism.

What you will invariably find with these larger mirrors, from whichever premium maker you care to choose, is that most of them are no better than 1/6th or 1/7th wave across the full face of the mirror at the worst point. This of course still correlates to a true strehl in the very high 90's and will deliver excellent views.

Cheers,




My 16 inch Meade was tested at 1/4 wave. I believe 1/4 wave is the lower limit for a diffraction limited performance. Certainly not the best optic I own, however over the 18 years I have owned it, I must say that it has given me the best views of Jupiter and Saturn that I have ever seen, but only on rare occasions when the atmosphere was not the limiting factor.
Unfortunately around my part of the world, the atmosphere rarely lets the 16 inch operate at it's theoretical limit of resolution.

Steve


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: 1/10 wave? new [Re: Starman1]
      #5552338 - 12/03/12 04:01 PM

Quote:

the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.




I have found this to be true. Often, the seeing moves in and out from average, then for a few seconds sharpens up to excellent, then back to mediocre.


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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]
      #5552659 - 12/03/12 07:30 PM

Quote:

It's more complex than you give it credit. I can't agree entirely with your comments. Your comments really only apply to areas that do not experience dramatic temperature changes from day to night.



No, it's not more complex - please re-read my original statement - I specifically stated my assumption that the mirror was equilibrated.

I was not considering the case of rapidly falling temperatures. I was trying to address the original question that started the thread, which made no reference to cooling.

I have seen mirrors with relatively low P-V error that had significant slope/transverse error. After rework, the P-V error was slightly lower, but the transverse error was significantly lower. Stars were perceived to be "tighter" after the rework, in a variety of seeing conditions. Of course the effect will be more noticeable when the seeing is good.

Quote:

You also need to keep in mind that in regards to the number of 18" plus aperture mirrors running around the world, there are an infinitely greater number of them 2" thick at minimum, than there are < 2" thick. For that matter there are also a lot more mirrors in the 12" to 18" aperture class mirrors which are 2" think running around the world than there are which are less than 2" thick. 2" thick was the norm with 10" to 20" mirrors for decades. At one time it was even a selling point to have a "full thickness mirror".



That's true, and a bit unfortunate.

Quote:

You need to remember that I am more than capable of properly star testing a telescope to confirm its optical quality. In addition, a very skilled optician is a friend of mine and I have seen some of these mirrors on the test stand, under bench test conditions.



Since I was not replying to any of your comments, so I am not sure why you are telling me what I should remember and keep in mind. Have we corresponded before?

Quote:

Mike, when you say larger "spot", you are not referring to the airy disc?
To me that's a gray area I don't fully comprehend. It's easy enough to see how at least one ray could miss the Airy disc or some target "spot", but really the Airy disc forms when the entire wave comes to focus. And if one ray is not cooperating (the wavefront P-V), then it get's sent to the diffraction rings when the diffraction pattern comes to focus. Right?



Yes, I'm talking about the Airy disc. The Airy disc is the smallest image that a perfect mirror can form. This is due to the size of the mirror and the geometry of image formation.

Quote:

The Airy disc is determined by aperture, not figure.



The minimum possible size of the disc is determined by aperture. The pattern and energy distribution are determined by the central obstruction and figure.

Quote:

A poor 6" figure does not make an Airy "spot" larger than 0.92" arc, it redistributes light throughout the pattern. Is not the result of a poor optic a normal Airy pattern with more light spread into the rings? Or does that offending ray actually show as blurring somewhere in the first minimum?



As the spot loses amplitude, the rings lose or gain amplitude depending on how the energy is moved around, and the various minima grow less minimal. So, yes, the blurring does move into the minima, effectively enlarging the spot and making it less distinct from the surrounding energy.


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bartine
Pooh-Bah
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Reged: 10/03/07

Loc: Potomac, MD
Re: 1/10 wave? new [Re: ed_turco]
      #5552667 - 12/03/12 07:35 PM

For me there is a difference.

I had heavily light polluted skies in Atlanta. Often very turbulent. Had a Meade Starfinder 16.

Scope was OK for viewing, but not great. Re-figured the mirror from 1/4 wave to 1/9. The difference was striking.

Period. Regardless of conditions.


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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: bartine]
      #5552847 - 12/03/12 09:13 PM

Quote:

For me there is a difference.

I had heavily light polluted skies in Atlanta. Often very turbulent. Had a Meade Starfinder 16.

Scope was OK for viewing, but not great. Re-figured the mirror from 1/4 wave to 1/9. The difference was striking.

Period. Regardless of conditions.




If that's the case I would have no hesitation in saying that the original mirror wasn't even close to a 1/4 wave, in reality it was probably 1/3rd to 1/2 wave, with a poor level of polish, despite what it might have been advertised or sold as. I have seen a couple of those 16"/F4.5 Starfinder mirrors that aren't nearly a genuine diffraction limited mirror. On the other hand of course some of those 16" Starfinder mirrors are 1/2 decent and certainly 1/4 wave.

Further, the refigured mirror will have a much higher degree of polish than the Meade original, irrespective of its P to V wavefront error, thus reducing light scatter dramatically and giving better images each and every night.

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]
      #5553069 - 12/03/12 11:11 PM

You seem to know a lot about a mirror that you have never been near? Really, no anecdotal evidence is going to change your mind, and in fact that is the only kind of evidence that we have! I think that this discussion reaffirms what I have come to realize: when all is said and done everybody will form opinions based on their experiences - climate, seeing, optics, eyesight, observing experience, etc. One person's absolute certainty is another person's no way in a million years, and given all of the variables it may be that both are equally correct. Which is why the best advice is, finally, to get out and observe with friends and club members, and see what is what for oneself. Personally, I have not been dissuaded from my belief that a substantially better (than diffraction limited) optic results in (frequently) substantially better views, though I have found your explanations thought-provoking.

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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: dpwoos]
      #5553087 - 12/03/12 11:24 PM

Quote:

You seem to know a lot about a mirror that you have never been near?




Because I know that if it was a genuine 1/4 wave mirror he would not see a difference each and every night, having regard to the fact those Meade mirrors are 2" thick and take plenty of cooling because of the closed tube, the cell design and no fan in most cases. I am very confident what he started with was a pretty average mirror, despite what it might have been sold as.

I have an 18" mirror that is just better than 1/4 wave at it's worst point, so it is a "genuine" 1/4 wave mirror (across the full face of the mirror) and very smooth and the times it can be picked apart from other mirrors with a higher P to V wave rating you can count on one hand each decade.

Cheers,


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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: dpwoos]
      #5553125 - 12/03/12 11:46 PM

Quote:

Personally, I have not been dissuaded from my belief that a substantially better (than diffraction limited) optic results in (frequently) substantially better views, though I have found your explanations thought-provoking.




Some part of the gain you are attributing to a better wavefront error, will in fact be attributable to the fact the mirror is smoother, has a better polish and reduced microripple. This reduces light scatter which in every case will offer an improved view.

However, a better polished mirror generally goes hand in hand with a mirror having a better wavefront surface accuracy. Both stem from the skill and time that goes into the mirror.

Cheers,


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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: Mike Lockwood]
      #5553163 - 12/04/12 12:16 AM

Quote:

So, yes, the blurring does move into the minima, effectively enlarging the spot and making it less distinct from the surrounding energy.




Thank you. I guess that makes sense, but hard to imagine the minima being brighter. Maybe it's a relative thing, set the mimima to zero then measure the fall off in the airy disc from that point.

But, what you're saying is the Airy disc is the same diameter in a given aperture regardless, it's just some of the light encroaches nearer to the first minima that defines it. That's hard to grasp because the peak intensity falls off and so would the size and brightness of the spurious disc. Seems light near the edge of the Airy disc would fall off equally with a more shallow curve onto the first minimum instead of an increase.

Anyway, thanks for replying. Got me thinking...an aberrated and diffracted wave front collapsing to focus is a mysterious thing.

There is little doubt smoothness is critical. One might wonder what a mirror with 1/20th wave micro ripple (pretty smooth, I guess) on the surface would look like so long as the edge was not turned back by L/4 on the wave front.

I dunno, someone said it best above. IME, better optics are just better up to a point, and it becomes increasingly harder to tell beyond that point (all other variables, including experience, held constant and assuming perfect seeing.) Maybe it is the associated smoothness that makes the largest difference. But, certainly a less fuzzy minimum and brighter peak/Dimmer rings are important, too. That's resolution, that can be seen. IME, anyway.


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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: Asbytec]
      #5553712 - 12/04/12 10:34 AM

The Airy disc includes the spurious disc in the center (the point) and out to the middle of the first minimum in between the spurious disc and the first diffraction ring.
Throwing more energy out of the spurious disc reduces its height faster than its diameter. By the time the spurious disc is 10% smaller in diameter, the intensity of the spurious disc has fallen to 50% or less (from the theoretically perfect 84%).
So improved resolution on double stars does not come from greater diminishment of the intensity of the central spike by throwing more intensity into the rings. If it did, double star observers would all use 50% secondaries to throw more light into the rings.
What does diminish the apparent size of the spurious disc is to reduce the height of the central spike and diffraction rings (i.e. view a star of fainter magnitude).
The apparent, visible, width of the spurious disc does diminish as more of the star image (spike and diffraction rings) falls below the threshold of visibility. 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. Since the visible width of the gaps between diffraction rings will seem to grow as the image gets fainter, this could lead to a better resolution of double stars.
Indeed, I have found that I can see closer gaps between double stars in faint doubles than I can in bright doubles.
It should be noted that as the intensity of the central spike diminishes, so does the intensity of the diffraction rings.


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sirchz
super member


Reged: 09/21/09

Re: 1/10 wave? new [Re: Starman1]
      #5554012 - 12/04/12 01:35 PM

Quote:

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.


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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: Starman1]
      #5554564 - 12/04/12 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 ) 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?)


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BillFerris
Post Laureate
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Reged: 07/17/04

Loc: Flagstaff, Arizona, USA
Re: 1/10 wave? new [Re: dpwoos]
      #5554673 - 12/04/12 09:28 PM

Quote:

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.

Bill in Flag


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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: Asbytec]
      #5554869 - 12/04/12 11:45 PM

Quote:

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 ) 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.


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dpwoos
Pooh-Bah
*****

Reged: 10/18/06

Loc: United States
Re: 1/10 wave? new [Re: BillFerris]
      #5554930 - 12/05/12 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.

Edited by dpwoos (12/05/12 12:43 AM)


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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: Starman1]
      #5554982 - 12/05/12 01:40 AM

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

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.

Edited by Asbytec (12/05/12 03:07 AM)


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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]
      #5556807 - 12/06/12 02:13 AM

Quote:

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.

Quote:

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.

Quote:

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.

Best,
Mark


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: 1/10 wave? new [Re: mark cowan]
      #5556904 - 12/06/12 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..


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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]
      #5556928 - 12/06/12 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.

Edited by Asbytec (12/06/12 06:23 AM)


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nicknacknock
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Re: 1/10 wave? new [Re: Asbytec]
      #5557008 - 12/06/12 07:56 AM Attachment (24 downloads)

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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