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wh48gs
Pooh-Bah

Reged: 03/02/07

#5338292 - 07/27/12 11:34 AM

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Poster: MKV

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No matter which resolution criteria we choose - Rayleigh. when two near equal stars are likely to be fully separated, Dawes', when they are likely to be slightly flattened out on the sides facing each other, Sparrow, when the intensity deep between two stars flattens out, or the smallest measurable elongation of the central disc - angularly smaller Airy disc will always allow better angular resolution. As long as it is the limit to resolution of near equally bright stars, smaller central disc means better resolution. Resolving close pairs with wide differences in magnitude is a different story - brigter and larger 1st bright ring makes obstructed apertures inferior. Vla

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I understand, Vla, but with CO there are obviously other factors at play, or so it seems. Angular Airy disc size is proportional to the wavelength (w), and aperture (D), not to central obstruction.

Yes, Mladen, but once we have a non-zero c.obstruction, there is no more Airy disc (which Sir Airy defined for a clear aberration-free circular aperture). What we have is a circle determined by the first diffraction minima which, according to diffraction calculation, shrinks in its linear size (which, with the focal length unchanged, means that it shrinks in angular size as well) as shown here (Table 8).

Vla

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wh48gs
Pooh-Bah

Reged: 03/02/07

#5338307 - 07/27/12 11:41 AM

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If an aperture of given size is used with and without central obstruction, it will resolve closer near equally bright stars with obstruction than without it - assuming level of inherent and induced aberrations low enough to be a non-factor

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Vla, then we can also say that a paraboloid forms a dimensionless image of a star because a spot diagram shows that it does. Well, it doesn't.

Mladen, this is totally unrelated - we are talking diffraction (real world), not ray optics.

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Then why are there no 60% central obstruction masks being advertised for refractors and unobstructed reflectors?

Well, it could be that no one really needs to buy one; they're easy to make. Ignorance of diffraction effects may be a factor too.

Vla

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX
#5338333 - 07/27/12 11:53 AM

I am saddened to see that my thread, opened to get some clear information for those of use who are not interested in arguments regarding optical theory, has now degenerated into the usual back and forth regarding esoterica.

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wh48gs
Pooh-Bah

Reged: 03/02/07

#5338363 - 07/27/12 12:11 PM

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It's negligible if it's not detectable. If you can't detect any difference in performance between a 1/10 wave mirror and a 1/50 wave mirror, then the difference is negligible.

It is about as detectable as with 10% larger aperture and 1/4 wave p-v of spherical aberration added. For near equally bright star pairs, it won't affect the resolution if the larger aperture has less than about 1/2 wave p-v.

But don't forget that this high-frequency contrast advantage of a significant central obstruction (larger than D/4) applies to all high-contrast details smaller than about 2LambdaF (or 2Lambda/D in radians, 412,000Lambda/D in arc seconds). For D=100mm and Lambda=0.00055mm, this is about two arc seconds, and smaller. That would be relevant to quite a few small details on Moon's surface, for instance (although it is not exactly an MTF pattern, so shouldn't apply strictly).

Vla

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wh48gs
Pooh-Bah

Reged: 03/02/07

#5338378 - 07/27/12 12:17 PM

Sorry Brad - it is not "esoterica", but it is totally unrelated to your topic, so I quit on the obstruction subject here.

Vla

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MKV
Carpal Tunnel

Reged: 01/20/11

#5338400 - 07/27/12 12:29 PM

Quote:

I am saddened to see that my thread, opened to get some clear information for those of use who are not interested in arguments regarding optical theory, has now degenerated into the usual back and forth regarding esoterica.

Yes, Brad, indeed, it certainly seems that way, but you can't talk optical quality without theory being part of it. How else do you understand why one test is better than then other, or decide whether you need a 1/10 wave or a 1/50 wave mirror, or whether you need to place an 80% obstruction in front of your refractor to gain better resolution...:)?

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MKV
Carpal Tunnel

Reged: 01/20/11

#5338414 - 07/27/12 12:36 PM

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It's negligible if it's not detectable. If you can't detect any difference in performance between a 1/10 wave mirror and a 1/50 wave mirror, then the difference is negligible.

It is about as detectable as with 10% larger aperture and 1/4 wave p-v of spherical aberration added. For near equally bright star pairs, it won't affect the resolution if the larger aperture has less than about 1/2 wave p-v.

Vla, I will quit here too out of deference to Brad's objection, but let me just say that being able to see any difference between a 1/10 wave and a 1/50 wave mirror at the focus under real observing conditions is pure theory and not reality. So why would anyone lead an ATM to believe he or she should strive to make a 1/50 wave mirror remains a mystery.

And, btw, just like ray optics, diffraction is also theory. As you your self note above: "once we have a non-zero c.obstruction, there is no more Airy disc (which Sir Airy defined for a clear aberration-free circular aperture)."

Regards,

Edited by MKV (07/27/12 01:42 PM)

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX
#5338425 - 07/27/12 12:49 PM

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For we the untutored, what are we supposed to do if we want to fab a nice pedestrian smooth diffraction limited mirror?

Regards,

Obviously, there is value in discussing and debating theory. I am not someone who feels that they "own" a thread and I am not one to try to direct and control an open conversation on a public forum. At the same time, I posted the above question in my original post and I think that I did a good job of succinctly expressing what I was interested in seeing discussed and, I suspect, what many, many others are interested in seeing discussed.

Many of us need a starting point and a roadmap to a successful first attempt at fabricating optics. I am pretty sure that many who have become fine mirror makers did not have a grasp of all the theory before they made a mirror. I would bet that they fought their way through the process using the tools that they could manage to use and became progressively more sophisticated in both theory and practice.

Regards,

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MKV
Carpal Tunnel

Reged: 01/20/11

#5338442 - 07/27/12 01:05 PM

Brad, there is no agreement what is smooth enough, what is diffraction-limited, and which test is 'best'. Just look what happened to the Hubble Space Telescope mirror. You want a smooth, 'nice' mirror? Make a 6-inch f/10 sphere, make sure it has no zones, and its edge is not turned down, test it at its radius of curvature with a Foucualt tester until you get razor straight Ronchi rating at 133 lpi, and you can pretty much be sure you'll have a 'nice' mirror. As to what size secondary for maximum resolution, well, that's another story...

If you want an honest, reasonable, realistic approach to mirror quality read Mike Lockwood's criteria.

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX
#5338462 - 07/27/12 01:25 PM

Quote:

Brad, there is no agreement what is smooth enough, what is diffraction-limited, and which test is 'best'. Just look what happened to the Hubble Space Telescope mirror. You want a smooth, 'nice' mirror? Make a 6-inch f/10 sphere, make sure it has no zones, and its edge is not turned down, test it at its radius of curvature with a Foucualt tester until you get razor straight Ronchi rating at 133 lpi, and you can pretty much be sure you'll have a 'nice' mirror. As to what size secondary for maximum resolution, well, that's another story...

If you want an honest, reasonable, realistic approach to mirror quality read Mike Lockwood's criteria.

The Hubble mirror was a GROSS error caused by human error in assembling optics for some kind of null test, was it not? The bad figure would have been easily spotted by a Foucault test. I am not sure that trotting out Hubble really bolsters your arguments as it seems to have been an object lesson regarding the fact that complexity is not foolproof.

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MKV
Carpal Tunnel

Reged: 01/20/11

#5338510 - 07/27/12 01:58 PM

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I am not sure that trotting out Hubble really bolsters your arguments as it seems to have been an object lesson regarding the fact that complexity is not foolproof.

I understand that it may seem that way to you , but I used it as an example to show that people confuse theory with reality and precision with accuracy on all levels. It's the mindset that relies on virtual reality and not on real tests in assessing optics.

Regards,

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amicus sidera
Carpal Tunnel

Reged: 10/14/11

Loc: East of the Sun, West of the M...
#5338587 - 07/27/12 03:06 PM

I've found this thread to be extremely edifying as regards optical theory, and saved it to disk for future reference. While it did get a bit off-track from the original intent, there's much of value in it.

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EJN
Pooh-Bah

Reged: 11/01/05

Loc: Highway 61
#5338701 - 07/27/12 04:11 PM

Quote:

I am saddened to see that my thread, opened to get some clear information for those of use who are not interested in arguments regarding optical theory, has now degenerated into the usual back and forth regarding esoterica.

I agree. How many photons can tiptoe on the head of a pin?

How much wood can a woodchuck chuck if he is using a Foucault test?

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MKV
Carpal Tunnel

Reged: 01/20/11

#5338797 - 07/27/12 05:33 PM

Quote:

Quote:

I am saddened to see that my thread, opened to get some clear information for those of use who are not interested in arguments regarding optical theory, has now degenerated into the usual back and forth regarding esoterica.

I agree. How many photons can tiptoe on the head of a pin?

How much wood can a woodchuck chuck if he is using a Foucault test?

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Paul Drufva
super member

Reged: 06/26/11

Loc: Connecticut
#5338912 - 07/27/12 06:59 PM

I feel for you cheepersleeper! could'nt agree more that the origional thread has degraded into meaningless blather for most of us.So what test is better than foucault for the average atm? I'm still clueless! Perhaps I missed something. By the way Mladen, I don't find your previous comment at all amusing."Why then why bother asking?" Maybe he asked hoping to get a straight answer? I too would like to know, so what other tests are there that don't cost an arm and a leg for the average guy? Foucault is tedious, but has yielded excellent results for me, would be nice if there was something easier without expensive optical flats, null lenses etc. Bottom line is that there is still nothing as easy, inexpensive and accurate as foucault. If there is I would like to hear about it, and am sure others would as well.

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Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair
Re: I have read all the posts about tests [Re: Paul Drufva]
#5338986 - 07/27/12 07:52 PM

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would be nice if there was something easier without expensive optical flats, null lenses etc.

Hey Paul,

as DavidG has pointed out time and time again the ross lens can be had for very little money thru surplus stores or swap meets like stellafane. with typical setups you won't use all of the lens which helps with tolerances since all you have to worry about is the center portion of the lens.

I got an 8" flat off ebay which is at least 1/3w flat with no edge issues and is smooth for \$30.

Edited by Pinbout (07/28/12 07:35 AM)

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wh48gs
Pooh-Bah

Reged: 03/02/07

Re: I have read all the posts about tests *DELETED* [Re: Paul Drufva]
#5339026 - 07/27/12 08:16 PM

Post deleted by wh48gs

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: I have read all the posts about tests [Re: Paul Drufva]

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By the way Mladen, I don't find your previous comment at all amusing."Why [sic] then why bother asking?" Maybe he asked hoping to get a straight answer? I too would like to know, so what other tests are there that don't cost an arm and a leg for the average guy? Foucault is tedious, but has yielded excellent results for me, would be nice if there was something easier without expensive optical flats, null lenses etc. Bottom line is that there is still nothing as easy, inexpensive and accurate as foucault. If there is I would like to hear about it, and am sure others would as well.

Paul, if you look at the title of this thread it says "I have read all all the posts about tests" and if that is true than surely he must have read about other tests that I and others have posted.

So, logically, it looked to me like he knew what tests are available (since he read all the posts about them) but was looking for reasons why some tests were better. Unfortunately, the reasons include the "meaningless blather", as you call it, because there is no consensus on this. Differences of opinon must be debated and sorted out.

As for tests, Ed Jones suggested a modified conjugate null test. This test is no different than testing a Newotnian at its focus, but it can alo be done at or close to the ROC of the mirror (see the enclosed example). Then, there is the Ross test, as Pinbout suggested, and now Vla suggested one using a large pcx lens. And there are many others.

Every test has its pros and cons, which is why one should use more than one.

Sometimes the problem is not som much inn the test but in the results. Sometimes theory is confused with reality, precision with accuracy; sometimes we chase parameters that provide theoretical distinction without difference, etc. There is no simple answer on a silver spoon. And no one is forcing anyone to accept, read or try anything they don't care for.

Regards,

Edited by MKV (07/28/12 01:54 AM)

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cheapersleeper
Carpal Tunnel

Reged: 01/22/10

Loc: Sachse TX
#5339500 - 07/28/12 05:38 AM

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Note that I say I READ them. I don't claim to understand them. The general impression that I get is to kick my Foucault tester down the stairs, grind my Ronchi grating underfoot, shake my fist at Polaris and to spin around rapidly shooting lasers at flats and stuff.

I do not normally quote myself but in this case I will do so for the second time in 24 hours. Note the above. If I strip it of any and all color and rhetorical devices it is:

While I have read the posts about optical testing I do not grasp the principles and implementation of many of them but I get the impression that some of you are claiming that we should not rely on the tests that have been commonly in use by amateurs in favor of other tests.

It was a simple question and to a great extent it was answered almost immediately.

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Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair
#5339560 - 07/28/12 07:48 AM

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Many of us need a starting point and a roadmap to a successful first attempt at fabricating optics.

going to a class is the best "roadmap" to successfully fabricating a parabolic mirror.

People from all over the world travel to delaware in early march to attend the delmarva mirror making seminar to, in 3 days, make a parabolic mirror. there are 3 master there to help out, Dick Parker, Dave Groski and Steve Swayze. They have a Ross Null tester and a autocollimator.

its a great weekend, alot of work, and steve swayze works all night long, literally, to help test and guide everyone to successfully making mirror to 1/8w and if your on the autocollimator you can get it to 1/10 wave.

Plus they feed us very very well.

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