Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | (show all)
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Zambuto Criteria #4 on Strehl
      #6129012 - 10/10/13 12:38 PM

There is extremely interesting information posted on Carl Zambuto's website here on how a high strehl can be misleading:

ZOC#4

I assume since professional institutions rely on the interferometer, that if the testing interferometer is set up correctly and sensitive enough, it could measure all scales of ripple quite accurately. However, is Zambuto on to some deeper point here about the limit of the interferometer, or that perhaps most interferometers aren't able to measure all scales of ripple?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6129231 - 10/10/13 02:29 PM

Quote:

There is extremely interesting information posted on Carl Zambuto's website here on how a high strehl can be misleading:

ZOC#4

I assume since professional institutions rely on the interferometer, that if the testing interferometer is set up correctly and sensitive enough, it could measure all scales of ripple quite accurately. However, is Zambuto on to some deeper point here about the limit of the interferometer, or that perhaps most interferometers aren't able to measure all scales of ripple?




IIRC, interferometers take measurements at a finite number of points on the mirror. The larger the number of points, the more accurate the overall figure is revealed. But, to get to the really micro-ripple CZ talks about, a prodigiously larger number of points has to be sampled or another test performed. There is an inherent limitation here in that the errors measured have to take into account the errors in the testing equipment and astigmatism introduced by the testing conditions.
Plus, how many points per square inch need to be taken into account to reveal the type of error you're talking about? 100? 1000? 10,000?

So other tests, such as the Foucault (pronounced Foo-Ko), or the Lyot (pronounced Lee-Oh) phase-contrast test, which can reveal ripple down to the near-molecular level (even very smooth mirrors appear bumpy in this test) are typically performed. Or the Ronchi (Ron-kee) test.
A combination of tests can reveal in slightly greater detail what the interferogram shows.

Questions arise:
--are what other tests (like the Lyot) reveal significant if the interferogram shows a good figure? CZ would argue yes.
--How can a mirror with a high Strehl ratio not be a smooth mirror?
--Why doesn't the interferogram reveal all we need to know about the mirror?

It's interesting to note that Wolfgang Rohr, in his mirror tests, displays results from Foucault, Ronchi, Lyot, and interferogram tests to reveal the truth about the tested mirrors. Carl's Seven Criteria reveal his more stringent approach to mirror making.


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6129315 - 10/10/13 03:12 PM

If you tested a mirror with Rhonchi, Foucault, and Lyot and all three looked excellent, would an interferometer add anything then?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6129346 - 10/10/13 03:27 PM

An average telescope mirror is figured to a precision many times finer than what the most capable of equipment can consistently measure. That is why you have to use a variety of tests to ascertain the true quality of the optic. Numbers on this scale fail.

That article takes things a step further by showing an extreme case where the optic tricked the interferometer into a high reading. Considering most mirror makers wouldn't rely solely on an interferometer in fact most would use Foucault and Ronchi with star testing, all of which would indicate machine ripple well before you would toss the mirror under an interferometer for final testing.

Unfortunately having one number as a tell-all is a perfect marketing tool and many of us fall victim to paying based on strehl alone.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6129414 - 10/10/13 04:16 PM

Quote:

If you tested a mirror with Rhonchi, Foucault, and Lyot and all three looked excellent, would an interferometer add anything then?




Yes, if done correctly it would add a full surface estimate of the error (that an interferometer can see). Rhonchi isn't that accurate quantitatively, Lyot isn't really quantitative, and Foucault although capable of high accuracy across single diameters at a time, is a sparse sample and quantitatively blind to gross errors of revolution like astigmatism.

A mirror that tests well with IF and shows a smooth microsurface under more sensitive tests is likely to perform better in practice. Its almost impossible to quantify what that "better" is though - although plenty of experience tells me that smoother surfaces perform noticeably better in practice, and this makes it worthwhile to produce those surfaces and test for them.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6129496 - 10/10/13 05:03 PM

Ripple below 4-5mm in width generally are not worth measuring, except for some specialized instruments. The p-v/RMS error scales roughly with their size, and microripple (up to 1-2mm) have worst-case scenario of about 100 wave RMS on the wavefront. Plug that in to the Strehl formula and it gives you the corresponding worst-case Strehl degradation factor 0.996.

If roughness on the first mirror is, say, 1/30 wave RMS, and I wouldn't expect it is more (it's kind of hard to tell, since that photo seems to be manipulated differently than the one at right, and blurred up quite a bit), that still degrades Strehl by 0.96, or so. Still at the very limit of perception for general observing.

The other mirror has the entire roughness range within less than 1/100 wave p-v (1/300-1-400 wave RMS). That really does nothing.

Similarly, 1/4 wave p-v turned edge at 95% radius will reduce the Strehl by 0.96. Half as wide ege, by 0.98; half as much p-v by 0.99. Sure, 90%+ of mirrors are not as good as claimed, and none is perfect. But both these mirrors will perform very well.

BTW Strehl is not "the measure of large scale surface". For one, it is an indicator of wavefront quality, which may and may not be proportional to the surface error. Second, how "small" it gets depends only on the number of sampling points. Given sufficient number, it is as good as wavefront map used for its calculation.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6129501 - 10/10/13 05:09 PM

Thanks Mark, as usual - very informative.

Since we don't get these four types of test results from our optician, for those of us interested in understanding how well an optic performs and who don't have the equipment Herr Rohr has, I suppose it would have to be through star testing.

I wonder how they test for smoothness on small scales in large mirrors like the 8 meter mirrors at Steward if you can't do it with an interferometer?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6129511 - 10/10/13 05:17 PM

every type of test has its own merit. you don't use a micrometer to build a house, or a yardstick to adjust a spark gap.
foucault can also fool you if used improperly, even about smoothness.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6129667 - 10/10/13 06:39 PM

Yeah, it's far too easy to be fooled, or to fool yourself. Foucault works well for smooth mirrors. IF works well for smooth mirrors. Star test works well for everything and should of course agree with other tests if you're doing them right.

I think one of the problems that arises here is how smooth is smooth, or more exactly, how regular and consistent is the smoothness? How free is the mirror from small scale regular patterning (aka ripple) that could scatter significant light? I suspect that simply by producing them so that they don't show any visible roughness under grazing illumination and a KE you get the benefit of removing all the doubt. But I don't know exactly what roles certain sorts of errors play in practice, except in the broad strokes. Vla makes the good argument that below some level roughness can't matter. So long as it's not periodic that has to be true.

The mirror in the first example is a good illustration. Yes, it looks kind of horrible, but after you've seen enough examples of machine made mirrors you get to recognize the symptoms. It would have worked, and probably pretty well, given the overall correction from the IF - although you can just detect some of that roughness there as well. It might not satisfy a critical user. That's usually why you end up seeing examples like this anyway.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6129861 - 10/10/13 08:57 PM

Quote:

Its almost impossible to quantify what that "better" is though



Good point. "functionally non-existant, as in nearly undetectable to the eye" are not quantifiable terms whereas the interferometer shows the roughness and quantifies it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6130079 - 10/10/13 10:41 PM

Quote:

There is extremely interesting information posted on Carl Zambuto's website here on how a high strehl [sic] can be misleading...



It all depends on how the software is used to manipulate the data. Software reduction programs allow you to remove surface "noise" (artificial smoothing) and thereby increase the Strehl ratio.

You can also artificially block off a desired amount of the edge which explains how come synthetic i-grams show no edge and Focualt does. Effectively blocking off most of the TDE will also increase the Strehl score.

Likewise, you can increase the qualy of your results by running Fast Fourier Transformation (FFT) analysis instead of the regular fringe analysis which has many time fewer points and less accuracy.

Mike Lockwood also has a section about commercial interferometry (IF) reports, which he summs up rather spot on that the results of an interferometer are only as honest as the optician/supplier allows them to be.

But, data manipulation and artificially improving the results is not limited to operators of IF reports only. The same can be said of many who use other tests, such as the Ross Null, the Double-Pass Autocollimcaiton Test (DPACT), and not to tlak about the fantastic Foucault measurements (remember that repeatability is not accuracy, but precision, and that just because your micrometer reads 0.002 it doesn't mean it's 0.002 without proper calibration of the measuring tool).

That's why everything defaults to, and hinges on a single factor: honesty (honest mistakes notwithstanding)! Some people have agendas to sell or promote their product and wish to make it look better than it is. Zambuto's and Lockwood's sites are full of such examples of other manufacturers.

Vla made a good point as regards the old adage: that just because you can measure something it doesn't always mean it's significant. Besides, what's the point of having a 0.997 Strehl ratio primary if the secondary is going to be 1/5 wave PV? Or what's the point of miniscule micro ripple when the atmosphere is less than optimal?

Zambuto is right, however, in that reports should be honest, and that's the point of his comment. But, as Vla also observed, the quality difference of images provided for his mirrors vs the "bad" one is so blatant that one gets the impression Zambuto is doing the very thing he is accusing others of doing!

regards,
Mladen

Edited by MKV (10/10/13 10:53 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6130438 - 10/11/13 06:12 AM

And you can even fiddle with Foucault to smoothen the image. Small scale surface errors usually show up only when you pinch off the returning light or use a small pinhole or slit lightsource.

Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6130520 - 10/11/13 08:06 AM

Carl is trust worthy and gives you his best work . That is what matters . No worries on the optic . Might be over kill but a mirror lasts many lifetimes .When the seeing is perfect perfect optics shine .

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6130676 - 10/11/13 09:56 AM

Quote:

And you can even fiddle with Foucault to smoothen the image. Small scale surface errors usually show up only when you pinch off the returning light or use a small pinhole or slit lightsource.



Yes, exactly! Unfortunately, the slitless testers, which have become the "new" ATM norm relatively recently, are notorious in the "fiddle" department because the "slit" is not maintained mechanically but by the position of the eye, and the easiest one is when the "slit" is wide open.

Anyone who's ever done a Ritchey-Common test would know that the smaller the light source is the more sensitive the test becomes. The same holds true for the Foucault test. Ideally, your light source/pinhole should be close to the Airy disc diameter, which is a few microns. The slitless LED source is 5 mm!

Mladen

Edited by MKV (10/11/13 10:14 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6130701 - 10/11/13 10:08 AM

Quote:

Carl is trust worthy and gives you his best work . That is what matters . No worries on the optic . Might be over kill but a mirror lasts many lifetimes .When the seeing is perfect perfect optics shine .



People who are willing to spend the kind of money on Carl's mirrors are also likely to spend money on good accessories and will not cut corners in that department. They will buy top-notch secondary mirrors, and often have parts profesisonaly manufactured for them, whereas those with limited means may try to improvise and cut corners. Clealry the two groups will not have the same quality telescopes for reasons other than just Zambuto's mirrors.

Two mirrors can objectively be compared only if all other factors, parts, accessories, etc. are the same and the images of both are observed side by side simultaneously.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6131262 - 10/11/13 02:29 PM

Quote:

Carl is trust worthy and gives you his best work.



Sorry, didn't mean disrespect to Carl Zambuto!


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6131335 - 10/11/13 03:00 PM

Quote:

But, as Vla also observed, the quality difference of images provided for his mirrors vs the "bad" one is so blatant that one gets the impression Zambuto is doing the very thing he is accusing others of doing!



I can't comment on the vendor, but I can comment on the images in the example - as I see them, the images for both mirrors are about as equal in quality, exposure, contrast, etc. as they could possibly and reasonably be. Therefore I think that your statement/accusation above is completely wrong and groundless.

Quote:

Yes, exactly! Unfortunately, the slitless testers, which have become the "new" ATM norm relatively recently, are notorious in the "fiddle" department because the "slit" is not maintained mechanically but by the position of the eye, and the easiest one is when the "slit" is wide open.



That's completely wrong, too - the slit is formed when the reversed image of half of the light source passes by the knife edge, resulting in a virtual slit. Eye position has nothing to do with it whatsoever, it is only a function of knife and source position.

As a bonus, the virtual slit is always aligned perfectly with the knife edge, so there is no "fiddling" with the slit to get it parallel to the knife. Even better, the nasty diffraction effects from the narrow slit itself are pretty much non-existent.

I consider it an improvement over classic Foucault with a slit, for those and other reasons.

Quote:

Ideally, your light source/pinhole should be close to the Airy disc diameter, which is a few microns. The slitless LED source is 5 mm!



Completely wrong three. What passes the knife is very narrow, and results in shadows with good contrast that show roughness readily.

Quote:

And you can even fiddle with Foucault to smoothen the image. Small scale surface errors usually show up only when you pinch off the returning light or use a small pinhole or slit lightsource.



Well, here are some "un-fiddled" images from my slitless tester (which I use daily for figuring mirrors), showing surface roughness and even cleaning residue quite easily with good contrast:
Slitless Foucault images, 18"
Slitless Foucault images, 16"

Please, before throwing a useful, accurate, good test under the bus, get your facts straight.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6131338 - 10/11/13 03:02 PM

It's not the test used; whether IF, AC, Foucault, etc.

IT'S THE REP(UTATION) AND ABILITY OF THE MAKER!!!!

And I GUARANTEE any side-by-side comparison of any of the mirrors with RIPPLE will prove out to be far deficient in sharp focusing ability.
******
Another thing to ponder-
Numbers arent everything. SEE-ing is.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6131358 - 10/11/13 03:11 PM

I also agree with Mike on this one.(since I just missed reading his post) Regardless what the math says STREHL should be taken with a grain of salt, period--- as what the actual Criteria#4 was stating in substance, if read to the end.
****
This is a classic instance of subjectivity missing the actual truth of the matter!
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6131453 - 10/11/13 03:52 PM

Quote:

.When the seeing is perfect perfect optics shine .




Amen, brother. Welcome to the church of the holy hyperboloid.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: Mike Lockwood]
      #6131585 - 10/11/13 05:02 PM

Post deleted by MKV

Edited by MKV (10/11/13 10:16 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6132558 - 10/12/13 07:42 AM

Mladen, I wish you would
-STOP-

the never-ceasing arguementative attitude you have displayed every time someone who has vast experience and knowledge corrects your misconceptions.
PLEASE!
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6132654 - 10/12/13 09:30 AM

There you go, Mark, I deleted my post. See, I value your opinion. But I am still looking for answers, so please answer these three simple questions:

(1) do you think Vla's statement was "wrong and groundless"?
(2) do you know how wide is a virtual "slit" in a slitless tester (and therefore how sensitive is the test), and how do you determine that width?
(3) do you think it's okay to throw interferometry under the bus?

Thak you.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6132718 - 10/12/13 10:14 AM

I read this on the zambutogroup from cal's doc file

Quote:

Criteria for a GOOD mirror
The M-L envelope that was described in other
postings would be the applicable criteria for most cases, and
that is for accuracy, only. That covers the four items you have
mentioned above. An estimate on the factors above given the
mirror is in the envelope and reasonably smooth would be
something such as the PV wavefront better than 1/8 wave, the
RMS better than 1/30, the Strehl ratio in the mid-90's or better,
and the RTA at less than 1.0.

But we can't just stop there. We must also consider the quality of
the polish, the edge condition, the surface roughness, whether it
has zones, or "rings" that don't necessarily affect the
measurement, and how much astigmatism is present. All these
factors would be evaluated and communicated with the
customer to determine their level of need. Based on that the
recommendation would be made.

I have measured commercial mirrors that are accurate by the
numbers, but had rings, or a turned edge, or a rough surface, or
incomplete polish. Those would require a judgment call
combined with what the customer is after. But for accuracy, if it
falls within that envelope, that is typically accurate enough to
where further work will have diminishing returns. Then only if it
had other problems would it be reworked.





Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6133001 - 10/12/13 12:34 PM

Quote:

(3) do you think it's okay to throw interferometry under the bus?



Hardly. The phase shifting interferometer did a good job of quantifying the surface roughness (as a good case of measles on the wavefront map) where the KE couldn't. They might have also done a surface profile to show the roughness. I'd rather have the original Igram than synthetic fringes however. Carl would be well served switching to IF but it's rather costly, after all he uses IF to test his flats and the same rules apply there.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6133087 - 10/12/13 01:17 PM

Quote:

Quote:

(3) do you think it's okay to throw interferometry under the bus?



Hardly. The phase shifting interferometer did a good job of quantifying the surface roughness (as a good case of measles on the wavefront map) where the KE couldn't. They might have also done a surface profile to show the roughness. I'd rather have the original Igram than synthetic fringes however. Carl would be well served switching to IF but it's rather costly, after all he uses IF to test his flats and the same rules apply there.



Thank you Ed. Your answer is well appreciated, and noteworthy.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6133113 - 10/12/13 01:37 PM

Quote:

Carl would be well served switching to IF but it's rather costly.





Lockwood and Zambuto have about the happiest customers of any mirror producers out there. Whichever way they test their mirrors, it works for them. Why would they want to change. They are both producing high quality, ultra smooth, diffraction limited mirrors with their current test methods.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6133145 - 10/12/13 02:03 PM Attachment (13 downloads)

Quote:

(2) do you know how wide is a virtual "slit" in a slitless tester (and therefore how sensitive is the test), and how do you determine that width?




All Foucault setups (fixed slit or moving slit AKA "slitless") with a slit aperture beyond the effective virtual slit width operate beneath that aperture in actual use. For simple cases (say a sphere) the math shouldn't be that complicated as the geometry is straightforward. For complicated cases (an asphere with zones of error) the answer is complicated due to the longitudinal component.

Perhaps you could just work it out?

Best,
Mark

PS This topic (slitted vs slitless tester) is covered in some detail in the 2nd edition of Dave Harbour's excellent book "Understanding Foucault". I was approached for permission to use an illustration of mine (below) that combined a slitless and slitted source foucogram of the same rough mirror, showing no difference in sensitivity between the two, for the very reason I describe above.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6133186 - 10/12/13 02:25 PM

Quote:

There is extremely interesting information posted on Carl Zambuto's website here on how a high strehl can be misleading:

ZOC#4




Honestly reporting results is at the very core of this topic, and both Zambuto and Lockwood are right in showing that some vendors are not totally honest.

Data, just like astro images, can be manipulated for a desired effect. It's kind of a "makeup" that hides as much as it enhances.

For example, here is how easily "official" reports can be manipulated. The images below represent and OpenFringe analysis of a 6-inch f/8 mirror of dubious quality and a rather pronounced TDE.

The first image show raw surface results based on the Fast Fourier Transformation (FFT) analysis. The results show a mirror with an estimated Strehl ratio of 0.72. RMS error of only 1/10 wave (about 1/3 wave P-V), and a fairly close conic to a paraboloid of -1.155.



Because every igram will have some 'noise' and artefacts, thes programs can subtract them form the results, which in theory reveales only the mirror surface. The operator has perfect control over how much 'nose' to remove, unless one settles for the default (most common) correction.

Thus, when the "smoothing" function is activated you get this:



Now, miraculously, the Strehl ratio is 0.934 (!), and the RMS is 1/24 wave (about 1/7 wave P-V)! Suddenly, this mirror isn't all that poor, but it's obvious that the reason why it couldn't be higher is the TDE.

OF also allows removal of a desired amount of outer edge in order to get an insight just how injurious it is to the overall image quality (which is a very useful function).

Removing 3/16" off the outer edge we get the final result which looks like this:



which shows the Strehl of 0.952 and RMS of 1/28.3 waves (about 1/8 wave P-V). So, in only two steps the mirror improved from a Strehl of 0.72 to 0.952 and RMS from 1/10 to 1/28 wave.

Now, Zambuto's site correctly notices that the synthetic igram doesn't show the TDE otherwise visible in mirror he analyzed.This may suggest that it was "ignored" in the final analysis. If this is the case, then the report is indeed dihonest.

But here is a problem with such examples. Number one, the identity of the vendor/supplier/manufacturer is not revealed. So, the consumers gain nothing by this knowledge. Such info doesn't help them confront or avoid the vendor. Number two, the person reporting dishonesty is also in the vendor business and is thus promoting himself as being honest, which is obviously in his business interest. All of which may be true, but it does lead one to conclude that the only mirrors worth the buck are Zambuto's.

Carl would have served the community much better with such information if he also provided examples of people who use tests other then interferometry as being somewhat dishonest as well, so that one does not walk away form his article thinking interferometry is something inherently dishonest, while Foucault tests are inherently honest!

I used an interferometry example to show how easy it is to turn an ugly duckling mirror into a 0.952 Srehl "babe" simply because that's the data I have. But I also know, form personal experience, that other tests leave me with just as many options for dishonest reporting as does this method, but I always try to remember that tests are inherently not dishonest; only some operators are.


Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6133230 - 10/12/13 02:55 PM

Thanks Mark Cowan. Unfortunately, your answer as regards the virtual slit width does not show how the beneath the aperture used is established.

Your picture is very informative. It shows that at 150 microns both tests are the same, but this is well above the diffraction limit, and not maximally sensitive.

The sensitivity of the small light source is best demonstrated in the Ritchey-Common test for flats. An insufficiently small light source will show the surface to be "flat" when Newotnian interference test shows it to be decidedly curved. The Ronchi grating will show straight lines even though the surface is not flat. This is why Ritchey recommends the that the light source be derived from a steel ball bearing (essentially an Airy disc). It is only then that you begin to see the critical details of the surface revealed by intereference fringes.

Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6133297 - 10/12/13 03:40 PM

Mladen,

You made good points - why did you delete your post? As for #2 question, the virtual slit is as wide as the exposed portion of the source. Since it is placed at the c.o.c. it is being imaged in its original size in the image plane of the mirror. In fact, you're looking through that image at the mirror, but can't see it because it's way too close.

Ideally, you want to look at the mirror through a point of convergence, but it is not possible, due to diffraction, nor necessary. There will be no difference between a point and a spot whose horizontal extent does not exceed the minimum knife edge slide increment. Obviously, the larger source/image, the less coherent diverging beam, and less accurate/contrasty shadow transcription. Too small a source becomes insufficiently bright, so it is generally the best to keep source size smaller than the diffraction point image. For the zonal opening it is usually fulfilled, since it has very long focal ratio, but for the entire mirror it may call for 0.01mm source size (horizontal extent), or less. Going larger, the brightness gain may offset the definition loss up to a point, but beyond that it becomes a negative.

That's what I gather, anyway.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6133448 - 10/12/13 05:13 PM

Quote:

Your picture is very informative. It shows that at 150 microns both tests are the same, but this is well above the diffraction limit, and not maximally sensitive.




Mladen,

No, it doesn't show that at all. You really should determine for yourself the effective slit width in the Foucault test geometry at the settings which people utilize for actual testing. It may surprise you.

Both of those images are taken with an effective slit width that is the same, which is why they show the same detail. Neither of them are taken at 150micron slit width. The proof is that they are in all respects identical, and the slitless version had no slit at all on the source.

Both types of testers operate at the same effective width, one which is determined by the geometry of the test itself when applied. The wide slit on the fixed unit stems from my discovering this interesting fact many years ago, before which I assiduously maintained a narrow slit, and suffered from unneeded diffraction in reading the mirrors. The unused side of that slit serves mostly to reduce light that otherwise could sneak in from off axis, and slitless testers benefit from having a aperture limiting slit installed over the light source as well.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6133486 - 10/12/13 05:39 PM

Mark, am I correct to notice that the contrast of the slitless version seems higher, at least in the central area?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6133633 - 10/12/13 07:00 PM

Quote:

But here is a problem with such examples. Number one, the identity of the vendor/supplier/manufacturer is not revealed. So, the consumers gain nothing by this knowledge. Such info doesn't help them confront or avoid the vendor. Number two, the person reporting dishonesty is also in the vendor business and is thus promoting himself as being honest, which is obviously in his business interest. All of which may be true, but it does lead one to conclude that the only mirrors worth the buck are Zambuto's.






he talks enough about his own mess ups or returns, so you don't have to be so judgemental.


Quote:

Our next case was a 14.5" where astigmatism in the center of
the mirror was hiding between two axes 90° apart. This was in
our very first run of 14.5's. This run of mirrors represented the
largest and fastest we had done to date, so it was new and they
were a challenge at that time. Prior to this we checked mirrors by
Ronchi grating and measured in two axes, at 90° orientations,
much like the literature suggests. This time Rick caught it. He
sent it back and this never happened again, as we now check
every mirror in 45° increments using both tests. So the customer
never saw this mirror.





Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6133916 - 10/12/13 09:39 PM

Quote:

Mark, am I correct to notice that the contrast of the slitless version seems higher, at least in the central area?




They aren't identical, as it was two different sets of exposures at different light levels, processed through Registax with a contrast stretch at the end. I can easily adjust them so they are virtually the same, but David didn't want that because its not needed for the point, which was resolution of detail. This image has been out on the web for many years now.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6134186 - 10/13/13 01:32 AM

Quote:

Mladen,

You made good points - why did you delete your post?



To "play nice".


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6134193 - 10/13/13 01:38 AM

Quote:

and the slitless version had no slit at all on the source...



Mark, the slitless test forms a virtual" slit. The width of that slit is determined by the lateral position of the k-e.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6134293 - 10/13/13 04:05 AM

Quote:

This image has been out on the web for many years now.



Yeah, I know, but I've always wondered whether a slitless test does something with the perceived contrast.
This just because, although you cut most of it off, the light that does make it past the ke potentially gets brighter at positions farther away from the nulled zone. I expect such effect especially for fast mirrors.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6134314 - 10/13/13 04:30 AM

One thing MVK and Ed should re-evaluate---

The -MEASLES- are plainly visible in both IF and Foucault images. Saying otherwise draws visual acuity of the respondent into question. (Take another look at -ALL- the images!)
I have a question-
How many who are touting the acceptability of such poor mirrors with high strehl as being "acceptable" have actually seen what the imaging capability (specifically, contrast) of such a piece actually is? And can they describe it meaningfully to back up such assessment?
(my personal take on this- if one cannot grasp the most simplest test there has been devised; has no excuse for entertaining the thought of tackling IF--- or debating which is valid to said audience. Sorry for the rant, but subjectivity for the sake of arguement that interferes with actual facts has no place here.)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6134319 - 10/13/13 04:34 AM

And to answer a question on slit width, I don't personally use a slit of any sort (rather, a -PINHOLE-.) I feel they are much more consistent, reliable, and revealing- especially of the ripple detail covered here. So sorry, if you want to stick with slits, you're on your own on that. Maybe some research on your part would be interesting to post here, MVK?
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6134347 - 10/13/13 05:22 AM

Quote:

I have a question-
How many who are touting the acceptability of such poor mirrors with high strehl as being "acceptable" have actually seen what the imaging capability (specifically, contrast) of such a piece actually is? And can they describe it meaningfully to back up such assessment?




Very good question!

Also theoretically: why would a surface roughness of RMS 100nm be worse than, let's say, spherical abberation of the same amount?
The latter will direct more light from Airy disk to the rings surrounding this. The roughness would probably just enlarge the Airy disk itself, but I expect not further than that ring?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6134514 - 10/13/13 08:52 AM

Quote:

he talks enough about his own mess ups or returns, so you don't have to be so judgemental.



Please stop with this sensitivity. I am not being judgmental. When he talks of others, he is suggesting either dishonesty or negligence, or both (which could very well be true), but when he talks of his product then its an accidental discovery just in the nick of time. Apples and oranges.

Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6134534 - 10/13/13 09:10 AM

Quote:

Also theoretically: why would a surface roughness of RMS 100nm be worse than, let's say, spherical abberation of the same amount?
The latter will direct more light from Airy disk to the rings surrounding this. The roughness would probably just enlarge the Airy disk itself, but I expect not further than that ring?




Neither would produce anything close to perceivable effect in general observing. We're talking 0.996 Strehl degradation factor. Small-scale roughness does not affect central maxima. It spreads energy in a circle of the radius approximated vs. Airy disc of the aperture by the inverse size of the irregularity relative to the aperture(it sort of acts like a small aperture creating its own diffraction pattern). So, a 2mm roughness in 200mm mirror would spread (too tiny to notice) energy in a halo about 100 times the Airy disc diameter.

Small scale roughness has direct similarity to the effect of central obstruction in that it gets the blame for everything that is not right with the optics, that the user is not aware of.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6134549 - 10/13/13 09:19 AM

You guys should have a mirror contest .

Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6134586 - 10/13/13 09:42 AM

Mark,
My point is that on the Igram the surface roughness is quantified and in other methods it is not. The KE shows roughness but what is good and what isn't? Carl seems to be throwing IF under the bus so to speak but yet he uses IF to test his flats. Surface roughness on a flat is just as damaging as in the primary. Sure, good mirrors can be made without IF but don't bash IF for the sake of self promotion.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6134641 - 10/13/13 10:06 AM

Quote:

I am not being judgmental.




self righteously judgemental


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6134712 - 10/13/13 10:41 AM

Quote:

You guys should have a mirror contest .



Such a contest would require that all telescope componenets be exactly the same and the images are imaged live, simultaneously next to each other. Yo'd have to have some form of quantitative measure of contrast, and other image quality characteristics.

And even if we can measure the difference, it doesn't mean it's significant.

Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6134721 - 10/13/13 10:46 AM

Quote:

So, a 2mm roughness in 200mm mirror would spread (too tiny to notice) energy in a halo about 100 times the Airy disc diameter.



Did you mean to say 2 nm, not mm?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6134827 - 10/13/13 11:42 AM

Quote:

Mark,
My point is that on the Igram the surface roughness is quantified and in other methods it is not. The KE shows roughness but what is good and what isn't? Carl seems to be throwing IF under the bus so to speak but yet he uses IF to test his flats. Surface roughness on a flat is just as damaging as in the primary. Sure, good mirrors can be made without IF but don't bash IF for the sake of self promotion.



Spot on, Ed.


Post Extras: Print Post   Remind Me!   Notify Moderator  
greenglass
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6134970 - 10/13/13 01:04 PM

are the Zambuto Criteria #1,2,3,5,6,7 clear?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Rusty35
member


Reged: 07/30/13

Re: Zambuto Criteria #4 on Strehl new [Re: greenglass]
      #6135011 - 10/13/13 01:27 PM

Im confused,
I dont know anything about mirror making, or testing.
Was planing on ordering a 14"from Zambuto, but after hearing what MKV is saying it sounds like maybe I would be better of looking elsewhere.
Would Discovery be a better choice, they use interferometer testing?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Zambuto Criteria #4 on Strehl new [Re: Rusty35]
      #6135082 - 10/13/13 01:55 PM

Quote:

Im confused,
I dont know anything about mirror making, or testing.
Was planing on ordering a 14"from Zambuto, but after hearing what MKV is saying it sounds like maybe I would be better of looking elsewhere.
Would Discovery be a better choice, they use interferometer testing?




Now, now - don't equate disagreements and discussions over testing methodology with production issues. Carl has an excellent reputation for consistently producing fine mirrors.

There are a number of excellent mirror makers around today. It is a good time for people who appreciate fine optics.

Clear skies, Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
JimMo
I'd Rather Do It Myself


Reged: 01/08/07

Loc: Under the SE Michigan lightdom...
Re: Zambuto Criteria #4 on Strehl new [Re: Rusty35]
      #6135083 - 10/13/13 01:55 PM

You'd pass on a Zambuto mirror because of a post on the internet?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: Alan French]
      #6135097 - 10/13/13 02:02 PM

Quote:

It is a good time for people who appreciate fine optics.






you don't need IF to know this needs work...



Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: Rusty35]
      #6135104 - 10/13/13 02:06 PM

No Carl make great mirrors and has a good reputation. Are his other Criteria clear? #1 and 2 are straightforward but the rest are rather subjective "exceptionally smooth, functionally negligible levels, levels undetectable at the eyepiece, no evidence of any deviation outside the norm." Sounds good.

OTOH anyone producing a mirror using IF free of surface roughness, turned edge and good 2D curve conformance should expect similar performance.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6135136 - 10/13/13 02:23 PM

Quote:

Quote:

So, a 2mm roughness in 200mm mirror would spread (too tiny to notice) energy in a halo about 100 times the Airy disc diameter.



Did you mean to say 2 nm, not mm?




No, I think 2mm as in (average) lateral dimension. The height indeed should be very small.
Look up the theory here: Amateur Telescope Optics under 4.5 "Fabrication errors".

I should have looked here before asking anyway...


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Rusty35]
      #6135207 - 10/13/13 03:02 PM

Quote:

Im confused,
I dont know anything about mirror making, or testing.
Was planing on ordering a 14"from Zambuto, but after hearing what MKV is saying it sounds like maybe I would be better of looking elsewhere.
Would Discovery be a better choice, they use interferometer testing?



I neither said nor implied any such thing. My objection was and is to Zambuto's characterization of interferometry and not to his workmanship. Zambuto has a top-notch reputation for excellence in mirror making.

Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6135213 - 10/13/13 03:08 PM

Quote:

you don't need IF to know this needs work...




You don't nee the Foucualt test either.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6135229 - 10/13/13 03:18 PM

Quote:

But I am still looking for answers, so please answer these three simple questions



I am tired of having arguments twisted around into questions that are not relevant to the thread topic, and then being asked (or another poster like Mark being asked) to answer them, like it's a quiz or piece of homework. I am under no obgligation to answer them, but since they were posted after my previous post, I will elaborate a bit and comment.

Quote:

(1) do you think Vla's statement was "wrong and groundless"?



If he was looking at the comparison images of the two mirrors as shown in the web page listed by the OP, and as far as I can tell he was, then yes I think he is wrong and there are no grounds for saying one image was better quality than the other. Period.

Quote:

(2) do you know how wide is a virtual "slit" in a slitless tester (and therefore how sensitive is the test), and how do you determine that width?



No I don't. Please measure it and report back to Mark and I.

I thank Mark for his excellent comparison of slit/slitless testing, which I recall had been posted before. Perhaps you missed it, but I remember it.

Quote:

(3) do you think it's okay to throw interferometry under the bus?



I fail to see where this question comes from. A correction of facts about slitless testing does not constitute an attack on interferometry.

Quite frankly I've NEVER thrown interferometry under the bus, though I have been highly unkind to poor testing practices. I use interferometry, and will use it in the future, in far more ways than you know. I even use it for your much-mentioned Ritchey-Common testing, since it quantifies errors.

With proper sampling techniques and phase shifting, most of the roughness in the example should be visible after analysis, though Foucault makes it easily visible with a fraction of the effort/time, and thus for a fraction of the cost.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6135243 - 10/13/13 03:26 PM

Quote:

Quote:

you don't need IF to know this needs work...




You don't nee the Foucualt test either.




but it is a autocollimation image which make these gross errors even grosser-er.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Arjan]
      #6135260 - 10/13/13 03:34 PM

Quote:

Also theoretically: why would a surface roughness of RMS 100nm be worse than, let's say, spherical abberation of the same amount?
The latter will direct more light from Airy disk to the rings surrounding this. The roughness would probably just enlarge the Airy disk itself, but I expect not further than that ring?




I think it's related to something I mentioned earlier, the larger scale structure of small scale errors - primary ripple, for example, has a repeating pattern, rather than being random across the surface, so the slope errors, though individually small, can reinforce. SA of the same overall amount of random surface roughness would be more obviously damaging for the same reason. Random error will scatter randomly, increasing veiling glare, but periodic error scatters preferentially, meaning it's at least more detectable.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6135528 - 10/13/13 06:06 PM

Quote:

Quote:

and the slitless version had no slit at all on the source...



Mark, the slitless test forms a virtual" slit. The width of that slit is determined by the lateral position of the k-e.

regards,
Mladen




Why are you telling me this? Figure out exactly what the effective slit width is for any Foucault apparatus working at maximum resolution and then we can talk some more. Either build one and try it (which is what I did) or work it out on paper, I don't much care which.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
richard7Moderator
Not Quite
*****

Reged: 11/02/07

Loc: Sacramento
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6135679 - 10/13/13 07:25 PM

It's getting closer.


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6136116 - 10/14/13 12:00 AM

Quote:

Mark,
My point is that on the Igram the surface roughness is quantified and in other methods it is not. The KE shows roughness but what is good and what isn't? Carl seems to be throwing IF under the bus so to speak but yet he uses IF to test his flats. Surface roughness on a flat is just as damaging as in the primary. Sure, good mirrors can be made without IF but don't bash IF for the sake of self promotion.




I agree with what Ed has to say here and I want to add a thought. It is interesting that there has been little mentioned about a key difference between interferometry and the other tests. Interferometry provides a direct, quantitative measurement of surface shape with respect to a reference surface. With PSI, that measurement is done over a high resolution, uniform grid over the entire surface. PSI measurements are very precise and with proper calibration, can provide a very high level of accuracy. The KE test is a slope test that provides data that must be integrated to get to surface information--in only one direction. The KE test is indeed qualitatively extremely sensitive to small slope errors but it is very poor at producing direct surface data. While it is possible to analyze the effects of slope errors over different spatial frequencies, the KE test does not give sufficient data for good quantitative analysis. There is no doubt that a smooth mirror will generally be better than a rough mirror, but it's the amplitude as shown in a power spectral density plot that counts. PSI data is probably the best way to get real data for that kind of analysis. Think of it this way: Strehl performance is most affected by low spatial frequency figure errors; while high frequency, low amplitude slope errors generally drives scatter. Clearly, scatter can reduce contrast but not enough to be noticeable for visual instruments with sub-quarter wave amplitudes (in the wavefront,) which are easily seen with a KE test.

When I see marketing statements about perfectly smooth mirrors, I am reminded of the first time I visited a Swiss binocular maker back in the 80's. They actually didn't know much (if anything) about optical testing so their approach was to "just make it perfect". That meant that they had (mostly) very high quality products but they spent so much time (an hence money) producing each unit that they eventually were driven out of business by competitors who understood what "good enough" meant.

The folks at Zambuto may make some very good mirrors but without real data, it's impossible to say how good they really are. They are certainly good at marketing and have a lot of happy customers, which may be all that counts.

John


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: jhayes_tucson]
      #6136200 - 10/14/13 01:41 AM

tests:
http://www.astro-foren.de/showthread.php?14862-Zambuto-Enjoy-Your-Mirror&...
http://www.astro-foren.de/showthread.php?9233-Carl-Zambuto-Newton&p=35325...
Obviously not a significant quantity of Carl's mirrors, but indicative.


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6136281 - 10/14/13 04:40 AM

Quote:



I wonder how they test for smoothness on small scales in large mirrors like the 8 meter mirrors at Steward if you can't do it with an interferometer?





I know someone who was able to answer this question for me, and just got their response. To evaluate micro roughness on a surface - they use a laser fed "point source microscope" that gives a 2D readout in Angstroms RMS. For a special application mirror requiring a superpolished surface - they can make and measure it down to 1 ANGSTROM RMS OR LESS!! I find this to be a staggering achievement.




Would it be correct to say that a 10-20 Angstroms RMS mirror by IF would be basically a functionally perfect mirror for visual observing if it's Strehl were also high( eg >0.96) ?

Edited by alancygnusx2 (10/14/13 01:01 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6136395 - 10/14/13 07:59 AM

The knife edge reading taken and the mirror rotated and a new set taken and they match! Is how it is done .More accurate than a single set of readings for seeing Astig and other things . I think in many ways a better test as no other optic to be used .

Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6136405 - 10/14/13 08:12 AM

No, if you move the mirror you lose the frame of reference. Rotating a mirror is more likely to detect astigmatism however but not quantify it.

Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6136409 - 10/14/13 08:15 AM

Well, Zambuto mirrors are smoother than the others shown, but local irregularities are there, and am sure on better or larger photos more microrughness would show in the null shot as well. Figure is less than perfect. But those two mirrors are certainly good enough to be top performers. And so are those shown as "defective" on Zambuto's site. So what is, really, the difference?

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6136418 - 10/14/13 08:27 AM

Quote:

The first mirror interferogram given in Carl Zambuto's site has an rms of .008 wave, and the wavelength used is 632.8 nm, so it looks like the surface roughness is 50.6 Angstroms RMS - correct?

Would it be correct to say that a 10-20 Angstroms RMS mirror by IF would be basically a functionally perfect mirror for visual observing if it's Strehl were also high( eg >0.96) ?




What is the source for those figures? It is hard to tell from the photo on Zambuto's site, because it is all blurred, but for this level of RMS error the average linear extent of the ripple structure shouldn't be larger than a couple of mm. Which could as well be. If so, this terrible roughness degrades the Strehl by 0.996 ratio, or so. Going down to 10-20 Angstroms wouldn't make no difference whatsoever.

Zambuto says the magnitude of microrriple cannot be measured - which is not correct - but at the same time feels free to imply that it significantly degrades mirror performance. A bit presumptuous?

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Zambuto Criteria #4 on Strehl new [Re: Rusty35]
      #6136490 - 10/14/13 09:18 AM

Quote:

Im confused,
I dont know anything about mirror making, or testing.
Was planing on ordering a 14"from Zambuto, but after hearing what MKV is saying it sounds like maybe I would be better of looking elsewhere.
Would Discovery be a better choice, they use interferometer testing?




There have been a few thousand Zambuto mirrors sold thus far over many years.

There have been many, many opportunities for people to have their Zambuto mirrors tested, and given the price, the cost and/or effort of doing so is incrementally insignificant.

See if you can find a reference to a single Z-mirror testing badly. I have never heard of one.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6136523 - 10/14/13 09:47 AM

It's very simple. Those of you who think Zambuto is exaggerating or lying about his mirrors, or if his test methods are antiquated or unreliable, buy one from somewhere else. We're beating a dead horse here. People who enjoy his mirrors most likely don't care about the fact that he didn't use a interferometer. All they know is they are among the best mirrors they have ever owned. Call him, I'm sure he will take the time to discuss in detail every claim he has ever made. He is that kind of guy.

Post Extras: Print Post   Remind Me!   Notify Moderator  
marcosbaun
member


Reged: 01/01/12

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6136586 - 10/14/13 10:28 AM

Here in Brazil there is a saying : Speak good or bad but talk about me. Zambutto company should be happy for such propaganda. Ok, they deserve.The question is: To what level you need to polish a mirror so that it provides images of the highest quality? To what extent is advantage we pay much more dearly for a mirror that level if another mirror too and just well polished, can provide images as good and almost indistinguishable in quality compared to the super polished, often for a price much more advantageous? Well, you are the one who decides.

Marcos


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6136587 - 10/14/13 10:28 AM

This topic was never about Zambuto's mirrors, but mostly about his claim that (1) a certified 0.996 Strehl 1/15 wave P-V mirror is a piece of junk you wouldn't want in your telescope and (2) that the primary ripple is highly injurious to the image quality.

Some very seasoned individuals (Ed Jones, Vla, John, etc), with various degrees and years of educational, theoretical, professional and practical background and experience, beg to disagree.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6136659 - 10/14/13 11:01 AM

I don't see anywhere where the mirror in question is called a pc of junk by Zambuto. To answer the second question, I would not want a mirror with that amount of primary ripple in my scope if I the had the choice of getting one that had a smooth surface. In actual use, I have found that the Zambuto mirror I owned showed pitch black sky around the moon without any haze or glare that I have seen with other mirrors. Same with planets. I saw a difference. Have you ever compared one of his mirrors with a mass produced mirror in actual use?.

Post Extras: Print Post   Remind Me!   Notify Moderator  
marcosbaun
member


Reged: 01/01/12

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6136679 - 10/14/13 11:11 AM

Ok, but my question remains. :Then ask someone with a solid theoretical and practical experience in optics, what level necessary to consider polishing a mirror like quality. I'm very interested in this issue because I always used the Lyot test.Would test Lyot sufficient to evaluate next to these indices certificates?

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6136870 - 10/14/13 12:34 PM

Quote:

I don't see anywhere where the mirror in question is called a pc of junk by Zambuto.



If you reread Carl's article, you will seen that he says right there in big red letters: "Now the question- would you want this mirror in your telescope?"

Ed Jones, Vla and John apparently think such a mirror would do just fine. Ed is an experienced designer, and maker of optics with professional experience. Vla is solidly familiar with theory ( see his site here ) and John is an optical engineer with 30 years of experience in optical industry, a designer and user of lasers in optics, etc.

Between them you have a huge amount of experience, expertise and hands-on practice, so I think their opinion carries some weight worthy of consideration, don't you?

You also speak of "mass produced" mirrors. Neither of the two mirrors Calr used in his example were "mass produced". He specifically fabricated a mirror similar to the one he tested. Hardly mass-produced. Also, the "bad" mirror might have been one that slipped quality control. Only a random sample of several such mirrors could establish some sort of fabrication pattern, not an isolated mirror.

Zambuto himself says that the claims of Strehl of 0.996 and residual wave error of 1/15 waves PV is correct. The only thing he concentrates on is the idea that primary ripple will be visible by an observer. This seems to be the stumbling block.

Carl offers Suiter's statement that the light will be spread out due to primary ripple to a finite distance, and that this "can be a bad problem because it is condensed enough to easily see". But Suiter doesn't state how severe the primary ripple must be, not does Zambuto offer a quantitative analaysis of the ripple's magnitude to back up his claim.

So, the whole thing with Zambuto's article (which is the topic) is that he reduces everything to primary ripple assumed magnitude as being the reason why only his mirrors should be in your telescope.

This is clearly promoting his product, which is his right, but it seems intellectually dishonest if there is any truth to the opposing opinion that such ripple will not be visible to the human eye.

Qualitative detection of a surface flaw is not suffucient to to conclude that an otherwise 0.996 Strehl 1/15 wave P-V mirror is deemed unsuitable for your telescope.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: marcosbaun]
      #6136889 - 10/14/13 12:40 PM

Quote:

Ok, but my question remains. :Then ask someone with a solid theoretical and practical experience in optics, what level necessary to consider polishing a mirror like quality. I'm very interested in this issue because I always used the Lyot test.Would test Lyot sufficient to evaluate next to these indices certificates?



Clearly, that depends on the intended use. Human eye is less sensitive than cameras in terms of contrast detection as well as spectral coverage. If it is going to be a primarily visual telescope than a mirror should have errors below the threshold of the human eye and not whether it is detectable by optical tests. Remember that just because you can detect something with instruments it doesn't mean it's a problem if it cannot be seen by the human eye.

I think you will probably find your answers

here

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6136900 - 10/14/13 12:47 PM

I see at this point and like several others before me, debating you is pointless. You have your views and others have theirs. Your not going to change my mind and I'm not going to change yours.
Clear Skies


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6136950 - 10/14/13 01:07 PM

Well, no matter how you slice it, Zambuto's mirrors are well received by those fortunate enough to own them ... so, the man is apparently doing something right. He has listed his criteria for a 'production' mirror before it leaves his shop; primary ripple being #4 on his list. The example was chosen to illustrate his criteria. If you don't believe it is important, then by all means don't worry about. If you would be happy with the mirror on the left, then you can have it; given the choice, I'd take the mirror on the right (the Zambuto).

Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6136994 - 10/14/13 01:19 PM

Hi Vla,

My mistake in coming up with the figure 50.6 - which is why I had posed it as a question. (an error because I am not yet familiar with interpreting interferogram results - I need to spend more time on your excellent site).

I picked 10-20 Angstroms RMS because apparently that's what they shoot for at Steward, so I assumed that it would at that level the surface would be essentially perfect. That level is of smoothness is apparently not needed for our visual purposes.


Post Extras: Print Post   Remind Me!   Notify Moderator  
marcosbaun
member


Reged: 01/01/12

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6137035 - 10/14/13 01:35 PM

Quote:

Quote:

Ok, but my question remains. :Then ask someone with a solid theoretical and practical experience in optics, what level necessary to consider polishing a mirror like quality. I'm very interested in this issue because I always used the Lyot test.Would test Lyot sufficient to evaluate next to these indices certificates?



Clearly, that depends on the intended use. Human eye is less sensitive than cameras in terms of contrast detection as well as spectral coverage. If it is going to be a primarily visual telescope than a mirror should have errors below the threshold of the human eye and not whether it is detectable by optical tests. Remember that just because you can detect something with instruments it doesn't mean it's a problem if it cannot be seen by the human eye.

I think you will probably find your answers

here

regards,
Mladen




Thanks for your reply. I have the book written by Vladimir, already a few years, and whenever I have doubts turn to this magnificent book of optics. However is always some doubt. So it is always great to hear opinions from experienced people like you. Over the years, I have used the Ronchi test, followed by a Foucault test (Couder with the mask), ending with a test Stryoscopia, ending with the Lyot. But when compared with well-polished mirrors of excellent ATMs, a reference to it in Lyot test.

regards,
Marcos


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6137115 - 10/14/13 02:01 PM

Quote:

In actual use, I have found that the Zambuto mirror I owned showed pitch black sky around the moon without any haze or glare that I have seen with other mirrors. Same with planets.




This may indeed be due in a very small part to the surface figure but in most cases, scatter is what will drive the "black sky" effect that you are describing. That is much more driven by the quality of the coatings and the cleanliness of the surfaces. Getting a good coating is not trivial and I've seen a lot of very poorly coated mirrors--particularly in the amateur world. A poor coating might noticeably screw up the performance of a good mirror--particularly when you are looking at very bright objects like the moon or planets. A poor coating can introduce a lot of scatter and can even change the shape of the surface (though that depends a lot on the type of the coating and on the characteristics of the substrate.) The shape and smoothness of the mirror will more directly drive the sharpness of the image (and hence Strehl.) High spatial frequency errors along with poor surface quality (due to coatings or debris) will drive scatter.
John


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6137226 - 10/14/13 02:53 PM

Quote:

The knife edge reading taken and the mirror rotated and a new set taken and they match! Is how it is done .More accurate than a single set of readings for seeing Astig and other things . I think in many ways a better test as no other optic to be used .




Except that this doesn't work. I've had finished mirrors show negligible error of revolution this way only to be rejected and reworked when subjected to correct tests (pinhole at COC).

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6137299 - 10/14/13 03:38 PM

Quote:

I see at this point and like several others before me, debating you is pointless. You have your views and others have theirs. Your not going to change my mind and I'm not going to change yours.



That's a friendly note, indeed/sarcasm/. You said "I don't see anywhere where the mirror in question is called a pc of junk by Zambuto" and I said "If you reread Carl's article, you will see that he says...in big red letters: 'Now the question- would you want this mirror in your telescope?"' If that's not telling you that mirror is junk, I don't know what it would take to see it! But it seems nothing will convince you, not even if it's in plain red-and-white.

As for me, show me evidence other than anecdotal impressions. I have changed my mind on hard evidence countless number of times.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: jhayes_tucson]
      #6137301 - 10/14/13 03:39 PM

Quote:

Quote:

In actual use, I have found that the Zambuto mirror I owned showed pitch black sky around the moon without any haze or glare that I have seen with other mirrors. Same with planets.




This may indeed be due in a very small part to the surface figure but in most cases, scatter is what will drive the "black sky" effect that you are describing. That is much more driven by the quality of the coatings and the cleanliness of the surfaces. Getting a good coating is not trivial and I've seen a lot of very poorly coated mirrors--particularly in the amateur world. A poor coating might noticeably screw up the performance of a good mirror--particularly when you are looking at very bright objects like the moon or planets. A poor coating can introduce a lot of scatter and can even change the shape of the surface (though that depends a lot on the type of the coating and on the characteristics of the substrate.) The shape and smoothness of the mirror will more directly drive the sharpness of the image (and hence Strehl.) High spatial frequency errors along with poor surface quality (due to coatings or debris) will drive scatter.
John




It's interesting you say that. I've taken to letting the amount of light scatter tell me when the mirror needs cleaning. Dust can blow off, but the very fine haze that forms on the mirror over time (organic material? superfine dust particles? smog?) definitely yields a haze around stars and planets that is more uniform and spread out than mere spider vane diffraction.

Articles on reflectivity point out the value of frequent cleaning as well.

When I let the image quality AND the appearance of the mirror tell me when I need to wash the mirrors, it seems to settle on about 3-4 months (30-40 hrs under the stars).

After a cleaning, I always see a noticeable improvement in contrast at the eyepiece, letting me know I probably let it go a little too long. It's like when I change the oil in my car--if I notice the before/after difference, I waited too long to do it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6137313 - 10/14/13 03:49 PM

Quote:

Well, no matter how you slice it, Zambuto's mirrors are well received by those fortunate enough to own them ... so, the man is apparently doing something right. He has listed his criteria for a 'production' mirror before it leaves his shop; primary ripple being #4 on his list. The example was chosen to illustrate his criteria. If you don't believe it is important, then by all means don't worry about. If you would be happy with the mirror on the left, then you can have it; given the choice, I'd take the mirror on the right (the Zambuto).



Like I said earlier (I guess no no one reads all the posts), this is not baout Zambuto's mirrors, but about his argument that a highly rated mirror with primary ripple is not something you want in your telescope. But he doesn't show how much of the primary ripple is "bad" and how can one determines the amplitude of such an error. This is like saying that all detectable vestiges of spherical aberration, astigmatism or coma are unacceptable...

I can't imagine anyone who'd turn down the mirror on the right if one had a choice, even if the ripple is not visually compromising, and there are enough people with sufficient background who think it isn't. If you had to choose between a mirror with 1/16 wave spherical aberration residual and one with 1/20 which one would you choose? I think the answer is obvious, but you know that the one with 1/16 wave would be just fine, and that the difference would not be detectable by the eye.

Again, just because we can detect something doesn't mean it's significant for the purpose intended.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: marcosbaun]
      #6137323 - 10/14/13 03:54 PM

You are most kind, Sir, but I am no expert. The people I mentioned have a lot more to say about this than I do.

So what did you learn from the Lyot method and why don't you start a hread explaining to others how you did it, and post some results? It sure sounds exciting.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6137427 - 10/14/13 04:39 PM

Quote:

After a cleaning, I always see a noticeable improvement in contrast at the eyepiece, letting me know I probably let it go a little too long. It's like when I change the oil in my car--if I notice the before/after difference, I waited too long to do it.




Hmm, maybe a little device with a laser, like a collimator but containing a sensor as well - you point it at the mirror from the COC and it tells you from the return beamspread whether it's time to clean your mirror...

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
marcosbaun
member


Reged: 01/01/12

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6137442 - 10/14/13 04:45 PM

Well, I do not know if there will be interest in my observations in relation to Lyot test, because my comments are far more practical than the theoretical. Anyway, taking then your suggestion, I will open a topic specific (if it does not already exist) for the Lyot test, which primarily show how quickly a great blade phase, which provides a much better result than the phase blade manufactured by photographic method. (well, that was what my observations have shown).I will arrange what I have noted about the Lyot test, and then come back to the topic.

regards,
Marcos


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6137453 - 10/14/13 04:48 PM

Quote:

Quote:

After a cleaning, I always see a noticeable improvement in contrast at the eyepiece, letting me know I probably let it go a little too long. It's like when I change the oil in my car--if I notice the before/after difference, I waited too long to do it.




Hmm, maybe a little device with a laser, like a collimator but containing a sensor as well - you point it at the mirror from the COC and it tells you from the return beamspread whether it's time to clean your mirror...

Best,
Mark



Ooh! That would be a hot item. I think they already have that though, and it's called a photometer. It's not cheap, though........


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: jhayes_tucson]
      #6138457 - 10/15/13 07:45 AM

I beg to differ here. I've made direct side-by-side comparisons of rippled optics as shown near the beginning of this thread against very smooth black rouge finished optics of same size, and focal lengths.
The rippled mirrors, on starfields, show stars as having a lack of bright "sparkly" definition, and lack the inky black background that is characteristic of smooth mirrors.
Don't get me wrong, coatings do have an effect on this contrast characteristic.
Resolution isn't affected with these issues, and I -THINK- this is where the high Strehl ratings persist. But the magnitude reach, and contrast -IS- affected. Fine low contrast detail is simply lost with rippled optics.
*****
(and to put the record straight, I also have worked with PSI---and I -NEVER- threw IF under the bus, as has been assumed here; for the purpose of self promotion.)
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6138581 - 10/15/13 09:02 AM

How do you see rippled surface over a smooth one .The light gets scattered in the test!. Now you say it will not scatter light??Kicking Carl under the bus just because his mirror are better than need .Too smooth .Just ribbing here

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6138727 - 10/15/13 10:26 AM

Quote:

How do you see rippled surface over a smooth one .The light gets scattered in the test!. Now you say it will not scatter light??Kicking Carl under the bus just because his mirror are better than need .Too smooth .Just ribbing here



In order to judge an error as signficant you must know how big it is, not whether it is detectable. The magnitude of that ripple in Carl's example is unknown. If it's unknown, then no one can with certainty conclude that it's significant just because it's visible.

All Carl had to do is state how big the ripple is and what the tolerance is to show whether the mirror is accpetbale or not, not whether it "looks" acceptable or not.

Also, you say Carl's mirrors are better than needed? If they are better than needed, then he's doing a lot of needless work.

reagds,
Mladen

Edited by MKV (10/15/13 10:30 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6138758 - 10/15/13 10:47 AM

Quote:

I beg to differ here. I've made direct side-by-side comparisons of rippled optics as shown near the beginning of this thread against very smooth black rouge finished optics of same size, and focal lengths.
The rippled mirrors, on starfields, show stars as having a lack of bright "sparkly" definition, and lack the inky black background that is characteristic of smooth mirrors.




You wouldn't be the first visual observer to note this:

"In viewing extended deep sky objects maximum image contrast is essential to discern small differences of tone. But any stray light in the optical system (from dirty or imperfect lenses) sky brightness, or artificial light will compress the scale of recognizable shades, by eliminating black and lightening all grays.

Light that does not go into the image goes elsewhere to brighten the field. The result is a washed out image, and near the limit of visibility it may mean difference between seeing or not seeing the subject." - Walter Scott Houston, Sky & Telescope, January 1966


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Jeff Morgan]
      #6138893 - 10/15/13 12:03 PM

Quote:

Light that does not go into the image goes elsewhere to brighten the field. The result is a washed out image, and near the limit of visibility it may mean difference between seeing or not seeing the subject." - Walter Scott Houston, Sky & Telescope, January 1966



Jeff, no one is disputing this. But the magnitude of an error cannot be determined by "looks", which is the reason why Ronchi grating is not sufficient to make an accurate paraboloid (unless you're using autocllimation).

Even the Foucault shadows are not sufficient unless you have longitudinal measurements that go along with it. The same goes for any visible defect seen on the wavefront.

Errors must be quantified to determine if they are within acceptable limits below the threshold of visual detection. This cannot be determined by "looks" any more than you can determine how close to a parabola is this mirror. You need measurements to quanitfy the error.

Unfortunately, Zambuto decided the first mirror was "bad" enough by looks alone and concluded it wan't something you'd want in your telescope.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: marcosbaun]
      #6138900 - 10/15/13 12:08 PM

Quote:

Well, I do not know if there will be interest in my observations in relation to Lyot test, because my comments are far more practical than the theoretical.



Marcos, most of the people on ATM forum are hands-on people, rather than theoretical, so I don't see why they would not be interested in the Lyot test, but I could be wrong. You'd think mirror makers would welcome a thread on a well established test of surface quality. The onyl way to find out is to try it.

But, before you post anything, go through similar posts in CN's search section and see if there's anything new you can add.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6139105 - 10/15/13 01:47 PM

Quote:

Unfortunately, ***** decided the first mirror was "bad" enough by looks alone and concluded it wan't something you'd want in your telescope.



Unfortunately? I think just the opposite.

Anyone who's seen, used, and refigured enough mirrors, and who has heard feedback from the telescope owners, would know at a glance that that mirror is unsuitable for the highest quality telescopic applications. Period.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Jeff Morgan]
      #6139136 - 10/15/13 01:59 PM

Quote:

Quote:

I beg to differ here. I've made direct side-by-side comparisons of rippled optics as shown near the beginning of this thread against very smooth black rouge finished optics of same size, and focal lengths.
The rippled mirrors, on starfields, show stars as having a lack of bright "sparkly" definition, and lack the inky black background that is characteristic of smooth mirrors.




You wouldn't be the first visual observer to note this:

"In viewing extended deep sky objects maximum image contrast is essential to discern small differences of tone. But any stray light in the optical system (from dirty or imperfect lenses) sky brightness, or artificial light will compress the scale of recognizable shades, by eliminating black and lightening all grays.

Light that does not go into the image goes elsewhere to brighten the field. The result is a washed out image, and near the limit of visibility it may mean difference between seeing or not seeing the subject." - Walter Scott Houston, Sky & Telescope, January 1966




Indeed. No amount of rationalization changes this.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: Mike Lockwood]
      #6139299 - 10/15/13 03:26 PM

Quote:

Quote:

Unfortunately, ***** decided the first mirror was "bad" enough by looks alone and concluded it wan't something you'd want in your telescope.



Unfortunately? I think just the opposite.

Anyone who's seen, used, and refigured enough mirrors, and who has heard feedback from the telescope owners, would know at a glance that that mirror is unsuitable for the highest quality telescopic applications. Period.




Hi Mike,

Thanks for weighing in. In terms of micro roughness - can you give us a recommendation of how many angstroms RMS ( ie at that height or lower) you would like to see over most of the surface on a high performing visual mirror?

Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6139407 - 10/15/13 04:28 PM

The REAL issue to resolve in this thread that Mike and Mark bring up is, why does putting your knife edge on the left turn your focogram blue, while putting it on the right turns it red?
Jonzee


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: Mike I. Jones]
      #6139419 - 10/15/13 04:38 PM

Blue... mirror has cooled.
Red.... hot off the tool.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6139449 - 10/15/13 04:58 PM

Mikey you are back with a smile . The eye is not as sensitive to red as other colors I think

Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6139492 - 10/15/13 05:29 PM Attachment (16 downloads)

Actually, all my images are black and white, and my tester LEDs are green. I made it red because it's...pretty. And now it's almost time for Halloween, so it turns into a pumpkin.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mike Lockwood]
      #6139948 - 10/15/13 10:36 PM

Quote:

Anyone who's seen, used, and refigured enough mirrors, and who has heard feedback from the telescope owners, would know at a glance that that mirror is unsuitable for the highest quality telescopic applications. Period.



Just curious, can you also tell how good a parabola is, based solely on the "looks", that is -- on the shadows alone, without the SA measurements? Or, for that matter, just based on a Ronchi iamge? You seem to be saying that you can judge the amplitude of the primary ripple based on how it looks; no measurements needed. This is really getting interesting.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6140071 - 10/15/13 11:43 PM

I was responding to your last post when it seems you deleted it. You keep questioning Mike Lockwood and Carl Zambuto's ability to judge the amplitude of primary ripple just by looking at it. I was wondering how many mirrors you have made and what are your credentials as far being able to make high quality optics as Lockwood and Zambuto do. Why do these two profesionals have to prove anything to you?. If you take exception to what Zambuto is saying, why not invite him to respond here. He has in the past. This would be far better than calling him out on a forum where he has no idea of what your saying about him.

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6140121 - 10/16/13 12:20 AM

I deleted the post because it was essentially saying the same thing as the previous. My experience is nowhere near Carl's or Mike's. That's not a disqualification to ask questions. You don't need medical education and practice experience to ask doctors legitimate questions.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6140357 - 10/16/13 07:24 AM

You aren't asking as many questions as you should; but rather being quite arguementative, and disrespectable to accomplished individuals here, and elsewhere without their being aware....
****
In thinking about a question on what -DEGREE- (ie, quantity in angstroms, or NMs) that ripple or microfinish should be, I aproach it much more simply.
ANYTHING DETECTABLE, IS UNACCEPTABLE; regardless of the number. A good knife edge test can, and does show the very fine, light sleeks from polishing with cerium. These artifacts have a very high slope change over a miniscule dimension, but are virtually infinite in number. Therefore, they do have a deletrious effect on imaging, regardless what kind of Strehl the optic has otherwise.
****
Ripple has the same effect, with characteristically less slope, but covering much more area than the particulate sleeks mentioned above. I would postulate the scatter attributable to this type of defect would alter the scatter to a different degree- but still would have an effect on overall magnitude reach and contrast.
PSI smoothing function essentially removes these- and is perhaps the only thing that can be subjectively abused with interferometry that can be construed as a negative with this test.
So now we have almost gone full circle. IF is a valuable tool, as long as the one doing the testing is honest. All that's needed is to review the reps of the opticians, and to look at customer feedbacks, etc. These guys will not only know how to polish an ACCURATE surface, they will also know how to polish a SMOOTH surface. This comes from understanding cause/effect of polishing, and pitch characteristics.
This can only come with experience. To have an intelligent discussion, it would also be beneficial to have some of this, so that the RIGHT questions can be discussed without having somewhat heated debate about misconceptions.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Chuck Hards
Post Laureate
*****

Reged: 05/03/10

Loc: The Great Basin
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6140478 - 10/16/13 09:17 AM

This has been a very interesting and enlightening thread.

I've known a few professional mirror makers in my life, though not Mr. Zambuto. They all strive to produce the best optics possible while still making a modest profit. At some point they have to say "good enough". And if the maker says he'd use a mirror in his own scope, I take that as meaning he's willing to sell it without reservations.

Someone once said that an amateur can often produce a much better mirror than a professional, simply because he or she can put in the time required to bring it to as close to perfection as the tooling and available testing methods allow. The professional is always working under the constraint of being able to turn a profit and can't put as much time into it. The successful ones, from a business standpoint, are the ones that can get close-enough to perfection in a reasonable amount of time. It seems that Mr. Zambuto is one of them, based on his reputation- which is based on his products in the hands of knowledgeable customers.

I'd love to own a Zambuto mirror. My mirror-grinding days were 35-40 years ago, before I decided that there were people much faster and better at it than I was. I'd like to try it again when I retire. The knowledge base will be much greater then than it was back when I was walking-around-the barrel. In the meantime, it's good to know what pros are producing the best stuff.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Chuck Hards]
      #6140500 - 10/16/13 09:27 AM

I have a Zambuto 17inch.F8 Hyperbolic -1.43 Before it was coated Mark H and I tested it . Sure looked good on the rack .Marks numbers said it is better than 1/20 wave .smooth and edge you could shave on . Maybe over kill

Post Extras: Print Post   Remind Me!   Notify Moderator  
RodgerHouTex
Pooh-Bah
*****

Reged: 06/02/09

Loc: Houston, Texas, USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6140600 - 10/16/13 10:25 AM

By exceeding a customers expectations, as Carl does routinely, he produces high quality mirrors. If he just did what was necessary, he would produce good quality mirrors similar to what can be had from large commercial manufacturers.

Owning a 16 inch Zambuto and a 10 inch Celestron Astrograph, it is easy to see the difference in quality. The Celestron mirror is very good, the Zambuto is unbelieveably excellent. Nothing but darkness where the light shouldn't be.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: RodgerHouTex]
      #6140717 - 10/16/13 11:34 AM

Gentlemen, please, no one on this thread has said anything about Carl's mirrors except that they are excellent products. In fact, the only other lab results I have seen that seem to be of the same quality are of the Takahashi reflectors.

They hardly need defending. This discussion is not about Carl's mirrors, but optical tolerances.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6140747 - 10/16/13 11:51 AM

Mark, my comments and quesitons are of conceptual, not personal nature, so there's absolutely no disrespect from me towards anyone.

As for your comment that everything detectable is unacceptable, it sounds great but is, well -- unaccptable! In reality, all optics have detectable residuals. Obviously, not all detectable residuals are unacceptable. If they were, optical production would be an exercise in futility.

This is why optical science created tolerances. Depending on what the criteria are, tolerances may be tighter or looser, but their main purpose is to provide an objective, not subjective, measure of acceptable quality.

Tolerances, by definition, are based on measurements, not "guestimates" or approximations. In objective analysis you don't have "good" and "bad" parameters but only measured quantities that are either inside or outside the tolerance envelope.

In the case of a primary ripple only an accurate measure of its size and amplitude can with certainty determine whether a particular surface is inside or outside tolerance limits.

Thus, just as a parabola is never deemed (in)adequate solely by its Foucault shadows, without any measurements, by the same principle the primary ripple slope should not be judged by its looks alone, because there is considerable variety of the size and slope in this category.

regards,
Mladen

Edited by MKV (10/16/13 11:54 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6140901 - 10/16/13 01:14 PM

Quote:

Maybe over kill




And even if so, how in the world is that a bad thing? You know what kind of performance mirrors like that can deliver. Obviously some people don't, and they can't imagine why anybody would bother to go the extra mile to get there, nor will they bother. So, maybe best to just let them remain that way.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6141062 - 10/16/13 02:43 PM

Quote:

Quote:

Maybe over kill



And even if so, how in the world is that a bad thing? You know what kind of performance mirrors like that can deliver.




If it's truly an overkill then its unnecessary. An overkill, by definition, is creating a distinction without a difference.

But, Zambuto's mirrors are not the topic; establishing tolerances without measurements is.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joe G
scholastic sledgehammer


Reged: 01/10/07

Loc: Southern California
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6141202 - 10/16/13 03:52 PM

Mladen,

Let's take the case of a craftsman that builds furniture, say a table. He uses various grit sandpaper to get to a desired level of smoothness to meet a certain quality standard for his market. He then finishes the table with coatings that give a certain look. He can choose to sand it exceptionally smooth. He can choose to apply many coats of finish that will enhance the end product. Obviously he needs to base his decisions on what market he is trying to sell into and what that market may be willing to bare.

He knows from his experience how much sanding is required and sands to the point where it stops impacting the finished product. How he chooses to coat the table impacts the end quality as well and might aid in determining how much to sand. He bases these decisions on his experiences and his perceptions, i.e. his eyesight, his touch, etc.

He doesn't need to get the microscope out to measure the wood grain and how the individual grains vary from one another. He simply rubs his hand over the wood and looks at it to make a decision regarding the quality objective he strives to meet.

When Carl, Mike, and the Marks make mirrors, they use tools to "guesstimate" surface ripple. They continue to work their mirrors based upon their experience to the point where they deem the mirror meets their standards. Maybe it would be great if you could precisely measure this with certainty. Maybe it would be nice if you knew the detection point where the user could visually detect a difference. But from experience, given the tools they use, they work the mirror until they deem it meets their standards. They are making "guesstimates" based upon their process and experience and the tools they choose to use. While they may not have a quantitative measurement, it would seem that their customers believe their process works such that they are willing to pay a premium.

You seem to imply that their experience is inadequate because they don't have a specific measurement for it. They seem to differ.

Given all the problems involved with quantifying mirror standards (which you have pointed out again and again), reputation, consistency and process seem to satisfy their customers. This is to some extent why mirror making approaches an art.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6141343 - 10/16/13 05:26 PM

Quote:

In terms of micro roughness - can you give us a recommendation of how many angstroms RMS ( ie at that height or lower) you would like to see over most of the surface on a high performing visual mirror?



You can't put a number on it, and I wouldn't know what number to assign, either. It depends on the size of the defects, because that together with their height, determines their slope.

If I see roughness on something that I'm working on, using my normal techniques, it's a sign that either the glass is quite soft (true of cheap, imported mirrors that often have anneal issues, too), or that something else is wrong (very hard pitch, etc.). I'm simply going to do my best to make a surface that doesn't show any roughness in my testing. I do not do Lyot testing.

Quote:

Mark, my comments and quesitons are of conceptual, not personal nature, so there's absolutely no disrespect from me towards anyone.



Since you don't seem to get it, I'm going to spell it out for you. I said that looking at a Foucault image was enough to judge roughness. You asked:
Quote:

Just curious, can you also tell how good a parabola is, based solely on the "looks", that is -- on the shadows alone, without the SA measurements? Or, for that matter, just based on a Ronchi iamge?



That may have been a rhetorical question, but it is clealy a dumb question. Asking me to answer it is insulting. It subtly implies that I'm being careless, especially to those who don't know the subject. That simultaneously insulting, irritating, condescending, and disrespectful.

You then called another person's answer "unacceptable".... after they took time out of their schedule to voluntarily post something for the benefit of others, including yourself. You're not a moderator, so you have no right to use that term to describe anyone's post.

So, from my perspective, what you are basically saying to more than one person is: "I don't like your answers, and I don't care that you are trying to help people understand things while you could be doing something else with your valuable time. Please answer my questions - I'd rather you do all of the work while I sit here and critique your responses, even though I admittedly have less optical experience than you."

Quote:

But, ***** mirrors are not the topic;



Have you read the thread title? (Yes, that is sarcasm.)

Quote:

...establishing tolerances without measurements is.



That's what you have steered it to by overwhelming the thread and irritating two or three professional opticians.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6141354 - 10/16/13 05:34 PM

Quote:

But, Zambuto's mirrors are not the topic; establishing tolerances without measurements is.




That appears to be your only topic here...

BTW, I'm still waiting for you to answer a relatively simple question that you ignored.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6141390 - 10/16/13 05:50 PM

Quote:

Quote:

Quote:

Maybe over kill



And even if so, how in the world is that a bad thing? You know what kind of performance mirrors like that can deliver.




If it's truly an overkill then its unnecessary. An overkill, by definition, is creating a distinction without a difference.

But, Zambuto's mirrors are not the topic; establishing tolerances without measurements is.




So you have a Ferrari and a Yugo. The Ferrari is still just a car but it has been refined to the highest hand made standards. Why you ask. Because there is a market for such a car created buy people who can afford one. You are basically saying that a Ferrari shouldn't be made when a Yugo will get you around. This may be true to some extent but if you can afford the Ferrari, well....


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6141444 - 10/16/13 06:10 PM

I am not a mirror maker. But I have witnessed testing of mirrors using Ronchi and Foucault testing and I have seen a dozen or so mirrors on the test bench while they were being made (at Parks).

It is easy to see small bumps in a surface with the Foucault test, and it is insanely easy to see edge issues, center hills, and zone problems using both of those tests.

I have seen some "finished mirrors" that displayed no noticeable bumpiness or edge issues in either test--mirrors I would have been glad to own.
A couple of those mirrors I saw in use in the field, and they were pretty good. I am picky and I would have been quite satisfied.

Those mirrors were not tested using a Lyot test or interferometry, though one of the mirrors I saw (a 12.5") showed an absolutely beautiful finish on the bench and a perfect Ronchi test. Later, the mirror was tested with IF at another lab, and the mirror was, to make a long story short, "the best mirror we've ever seen on the test bench". The guys at Parks called it the best mirror they'd ever made, too. The key, they told me, wasn't the correction, but the incredible smoothness they had achieved on the fine-annealed piece of Pyrex.

What became obvious to me after seeing those mirrors was that a mirror that tested well with the Foucault and Ronchi tests would also test well when using IF. IF quantified what the eye could see with the other tests, but didn't really reveal more about the mirror other than putting some numbers to what was seen in the other tests. I all-of-a-sudden realized that one needn't have any IF testing done on a mirror to produce a good one.

My own Zambuto is the best mirror I've ever owned, and the only mirror that could handle 50X/inch I've ever owned. I have seen an occasional mirror that rivals or bests it in the circumstances in which it was used, and one of those was a (shudder) mass-produced mirror. When a mirror is well made, there is, through the eyepiece, an almost indefinable improvement of contrast in addition to the sharpness that you would expect. I went from a mirror with enhanced coatings to one with standard coatings, and was afraid of losing the light until I saw the dramatic level of contrast the mirror achieved.

I'm familiar with star testing, though I think it is heavily influenced by conditions in and outside the scope. But that contrast in the image--that can't be faked. When I look at a small planetary at, say, 300X and never feel the need to reach for a filter (or forget if I have one already on there), that's contrast. When a planet breaks up into the tiniest of features you could imagine (and could never draw), that's contrast.

Is it because the mirror is smoother than average? Or simply well-figured? I can't say. But I don't think it is an accident that a lot of really good mirror makers do not rely on IF to decide if their mirror is good enough, i.e. up to their standards. I just appreciate that they took the time to do it right. And, from what I've seen, the Foucault and Ronchi tests are adequate for getting there.

The Lyot test is so revealing that it seems to reveal more about how the mirror was ground and polished (it's easy to see the polishing swirls). The surface issues it reveals may be invisible in the field. Or not. I'll leave that for mirror makers to say.

My 2 cents from an end user. I'll butt out of this discussion now.


Post Extras: Print Post   Remind Me!   Notify Moderator  
marcosbaun
member


Reged: 01/01/12

Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6141639 - 10/16/13 07:33 PM

Quote:


The Lyot test is so revealing that it seems to reveal more about how the mirror was ground and polished (it's easy to see the polishing swirls). The surface issues it reveals may be invisible in the field. Or not. I'll leave that for mirror makers to say.





Excuse my meddling again, and I would say something about the Lyot. What can I say (in my humble knowledge) that have occurred in practice on the test Lyot, is that it is a powerful and simple to use. With it I see accidents on the surface of the mirror impossible to be seen with such perfection by Foucault. Polishing swirls, minor accidents surface, I believe that at the nanoscale, are perfectly seen in great detail. It is regrettable that this test is little practiced. The test Lyot help a lot in making the best mirrors. Logically the masters in the manufacture of mirrors perfectly know the moment your mirror is well polished and can by practicing acquired over time, waive this test if they wish.
regards,
Marcos


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6141852 - 10/16/13 09:33 PM

Quote:

So you have a Ferrari and a Yugo. The Ferrari is still just a car...



So, you think a 1/16 wave PV, 0.996 Strehl mirror with primary ripple of undetermined amplitude is a "Yugo"?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6141856 - 10/16/13 09:36 PM

Quote:

That appears to be your only topic here...




Actually, Mark, it was about the other mirror.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6141882 - 10/16/13 09:49 PM

Quote:

Quote:

So you have a Ferrari and a Yugo. The Ferrari is still just a car...



So, you think a 1/16 wave PV, 0.996 Strehl mirror with primary ripple of undetermined amplitude is a "Yugo"?





Yep, I wouldn't buy it or want it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6141889 - 10/16/13 09:52 PM

Quote:

Quote:

Quote:

So you have a Ferrari and a Yugo. The Ferrari is still just a car...



So, you think a 1/16 wave PV, 0.996 Strehl mirror with primary ripple of undetermined amplitude is a "Yugo"?





Yep, I wouldn't buy it or want it.




And for 3-4x more than a CZ mirror


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6141981 - 10/16/13 10:34 PM

Quote:

And for 3-4x more than a CZ mirror



That's a shame, but it's good marketing skills.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6142021 - 10/16/13 11:01 PM

Quote:

Yep, I wouldn't buy it or want it.



Primary ripple can contribute as little as 1/40 wave RMS. If your "Yugo" turned out to be a 0.996 Strehl mirror with 1/40 wave primary ripple you'd still think it's "junk"?


Post Extras: Print Post   Remind Me!   Notify Moderator  
RodgerHouTex
Pooh-Bah
*****

Reged: 06/02/09

Loc: Houston, Texas, USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6142071 - 10/16/13 11:47 PM

Once again, because of Carl's reputation and uncompromising principles, if I can afford it, I would ALWAYS buy his mirrors.

However I have heard similar qualities associated with Mike Lockwood who has contributed very capably to this thread.

IIRC, in a very old book by Jean Texereau, "How to Make a Telescope" he rails against the damage caused by micro-ripple and the improvement in a mirror that has a smooth surface. And by the way, he uses the Foucalt test to assess this issue qualitatively. You should pick up a copy.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6142275 - 10/17/13 04:41 AM

Quote:

Quote:

Yep, I wouldn't buy it or want it.



Primary ripple can contribute as little as 1/40 wave RMS. If your "Yugo" turned out to be a 0.996 Strehl mirror with 1/40 wave primary ripple you'd still think it's "junk"?




******************

YES, MLADEN, READ THE DESCRIPTIONS OF PERFORMANCE EARLIER IN THIS THREAD, MADE BY ASTUTE OBSERVERS, AND MANUFACTURERS COMPARING MIRRORS WITH/WITHOUT RIPPLE!!!!!!! Strehl and RMS can in these cases, be totally unreliable!
Seems that somehow, you have never seen the difference in a side by side comparison, or lack the visual acuity to tell the difference.
And your attitude here has been DISRESPECTABLE through practically the whole thread.
In light of these aspects, you have disqualified yourself from making any comments about testing, tolerance, or criteria used; other than to ask sensible questions and pay attention, which you haven't. (Instead, you have trumpeted opinion that only "fits" what YOU think should apply for self agrandisement.)
---
Now enough already!
This is the last comment I'm going to make in this thread. The ATM forum has a disproportionally large amount of this kind of diatribe occurring- more so than in any others, and is attributable to a small handful of individuals. I fail to understand -WHY- it's permitted---.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6142290 - 10/17/13 05:22 AM

Here is a recent reference by Robert E. Parks that discusses many of these issues. Since "RMSD" doesn't capture slope information there is increasing desire to characterize the spatial variation in a manner similar to turbulence, which is with a structure function.

So you wouldn't spec based on a single "RMSD" or corresponding single number "Strehl Ratio" - whether from interferometry or whatever - because it doesn't capture the spatial frequency info. Instead you place limits on the allowed magnitude of the structure function at different frequencies.

As shown in this write up, removing the dominant Zernike terms has little impact on the higher frequency components of the structure function, which themselves correspond to higher slope.

He summarizes with:
Quote:

The customer should have given a specification that covered mid-spatial frequency errors as well as figure and finish. As optics are polished by more and more deterministic methods using small tools or wear functions relative to the size of the optical surface, the likelihood of seeing mid-spatial frequency errors will become greater, thus it becomes more important than ever to specify what is an acceptable phase error in the mid-spatial frequency region. We show it is possible to write a mid-spatial frequency specification relatively simply and show how to measure this error
interferometrically by analyzing the resulting phase data using the structure function.




There are many examples of professional mirrors being spec'd based on structure function, in astronomy and other areas.

The separate question is how well you can map a Foucault image to a quantitative structure function. I don't know of anyone doing that - but it seems like a sensible thing to do and would allow amateur work to follow this trend to use the structure function to characterize a mirror in a more complete and functionally important manner.

To put this in the context of this thread, and to be clear I'm not taking sides on the issue of quality - there is a way to characterize the ripple by this approach, and the Foucault image clearly shows ripple that could be bad. In order to determine how bad it is by this method you would need to quantify it and calculate the structure function to know for sure.

But there is no doubt that some in this thread are comfortable doing the inverse - which is to know from experience that ripple evident from Foucault will impact performance - and therefore is direct evidence the structure function would be out of spec. I'm not taking sides on any of this, but just pointing out it is a known issue that has fairly recently led to a more complete spec on mirror quality.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: freestar8n]
      #6142393 - 10/17/13 07:53 AM

Precisely.

In past threads, MVK has stated that the simplest (Foucault) test was -NOT valid, because -HE- couldn't get repeatability down to a respectable level. (ie, .025"+ @ F/5-6~... I believe the description he used was that Foucault was "hopeless") This ocurred roughly 18-24 months ago in threads dealing with testing/tolerance/criteria levels being discussed. I deem this no violation of PM-TOSS regulations in communications; and as facts as he portrayed by him at the time and it's public knowlewdge posted in past threads!

My point to pose at this time-
Who's point is acceptable-
His, where he cannot verify 1/2 wave PVW in using the simplest test where it gives the most leeway-
Or a professional maker who can verify by D&C conditions where PVW is the max error and reiduals are less than that over the -MAJORITY- of the optic's surface????
This is outlined in Tex's book. Through personal experience in polishing 10K of optical components, this is quite valid in concept,experience, and practice.
I will repeat-
my personal -PRECISION- for those interested, and to clarify a point.
Readings on ROC up to 60" repeatable to .001" and most times to half that value. Up to 120" ROC repeatability readings to .002"; and commonly half that value. MVK has chosen not to reveal his precision.
If this cannot be matched, your (MVK) comments about test criteria and validity are highly suspect, and should be taken with a "grain of salt"- and highly disrespectable to one who -can- match this aspect.

******
I would consider such suspect arguementative diatribe as due the consideration of the validity as compared to the pertaining criteria portrayed by those who actually are familiar with otical theory/performance. They should be given the attention on their merits as should be due!!!!!
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6142496 - 10/17/13 09:14 AM

Quote:

Quote:

Yep, I wouldn't buy it or want it.



Primary ripple can contribute as little as 1/40 wave RMS. If your "Yugo" turned out to be a 0.996 Strehl mirror with 1/40 wave primary ripple you'd still think it's "junk"?




I have already said that I would not buy the mirror in the photo. It would be like buying the Yugo with no steering wheel when right next to it sits a complete car. The primary ripple shown in the example would have been quite easy to fix. What it indicates to me is sloppy workmanship by the company that made it. The company who made it must not have very good quality control. It's puzzling why they would work an optic until it has very good PV error and then not do a thing about the roughness. That degree of roughness had to show up in just about any test they did on the mirror. Veiling glare is not what I'm looking for in an optic.

Edited by ausastronomer (10/18/13 03:29 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6142666 - 10/17/13 10:49 AM

Quote:

The company who made it must not have very good quality control. It's puzzling why they would work an optic until it has very good PV error and then not do a thing about the roughness. That degree of roughness had to show up in just about any test they did on the mirror.



Precisely. Why else would they make such a mirror only to neglect the ripple? Unless, of course, (a) they measured it an found it was inside the tolerance envelope or (b) based on their professional experience and/or visual tests they concluded "that degree" of roughness is not detrimental!

It's a company with very expensive optics. They must know something about optics to charge so much for their product and because they cater to customers who are most likely no novices.

Or, maybe, the mirror Zambuto rejected was a production fluke. Perhaps testing a few more of their mirrors would determine if primary ripple is present in all of them.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6142694 - 10/17/13 11:01 AM

Quote:

Quote:

The company who made it must not have very good quality control. It's puzzling why they would work an optic until it has very good PV error and then not do a thing about the roughness. That degree of roughness had to show up in just about any test they did on the mirror.



Precisely. Why else would they make such a mirror only to neglect the ripple? Unless, of course, (a) they measured it an found it was inside the tolerance envelope or (b) based on their professional experience and/or visual tests they concluded "that degree" of roughness is not detrimental!

It's a company with very expensive optics. They must know something about optics to charge so much for their product and because they cater to customers who are most likely no novices.

Or, maybe, the mirror Zambuto rejected was a production fluke. Perhaps testing a few more of their mirrors would determine if primary ripple is present in all of them.

regards,
Mladen




Antares optical claims very high specs on their flats but I know someone who caught them with their pants down. Not providing the spec they claim. How many people buy from them and never check the flats.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6142706 - 10/17/13 11:08 AM

Quote:

Quote:

The company who made it must not have very good quality control. It's puzzling why they would work an optic until it has very good PV error and then not do a thing about the roughness. That degree of roughness had to show up in just about any test they did on the mirror.



Precisely. Why else would they make such a mirror only to neglect the ripple? Unless, of course, (a) they measured it an found it was inside the tolerance envelope or (b) based on their professional experience and/or visual tests they concluded "that degree" of roughness is not detrimental!

It's a company with very expensive optics. They must know something about optics to charge so much for their product and because they cater customers who are probably more than novices.

Or, maybe, the mirror Zambuto rejected was a production fluke. Perhaps testing a few more of their mirrors would determine if primary ripple is present in all of them.

regards,
Mladen





Round and round we go, where we stop no one knows. That amount of roughness is inexcusable in a high priced optic made for astronomy use. It can't possibly contribute anything good to the image. If it was a fluke, then their quality control is lacking as I have mentioned. There is no way anyone who knows anything about optics would consider that mirror finished, let alone someone who is charging top dollar and calling themselves a premium optical shop. I haven't seen many low priced commercial mirrors with that degree of roughness. I personally haven't seen any, only in photos. I can't understand why you would consider that mirror acceptable. Maybe you just like rough surfaces.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6142736 - 10/17/13 11:31 AM

Quote:

I can't understand why you would consider that mirror acceptable.



I don't consider it acceptable or unacceptable. Maybe someone should have contacted the company for answers from the horse's mouth.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: MKV]
      #6142783 - 10/17/13 11:53 AM

Post deleted by ausastronomer

Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: freestar8n]
      #6143145 - 10/17/13 03:08 PM

Frank, thanks for posting the link to this excellent article.

Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: freestar8n]
      #6143167 - 10/17/13 03:18 PM

Quote:

The separate question is how well you can map a Foucault image to a quantitative structure function. I don't know of anyone doing that - but it seems like a sensible thing to do and would allow amateur work to follow this trend to use the structure function to characterize a mirror in a more complete and functionally important manner.

To put this in the context of this thread, and to be clear I'm not taking sides on the issue of quality - there is a way to characterize the ripple by this approach, and the Foucault image clearly shows ripple that could be bad. In order to determine how bad it is by this method you would need to quantify it and calculate the structure function to know for sure.




Yes and no. Foucault imaging can show ripple or other small scale roughness, yes. But it doesn't directly translate to amplitude, period, and spatial content (think Jackson Pollock, where the texture gets finer the smaller the size, but the overall energetic contribution remains similar). The actual amplitude, etc., remains the subject of estimation without some way to calibrate the image.

However, since an asphere has built into it a known amount of SA, using that - or at least some portion of it over a portion of the face - should allow you to estimate more accurately what smaller scale errors amount to in amplitude. This is complicated though, because what Foucault measures (and what it shows) are changes in surface slope, not error amplitude.

I'd favor the current approach (stated by myself and others) to simply produce surfaces that show no small scale error under Foucault testing. If done visually and as part of ordinary production practice this requirement imposes little if any additional processing time, as methods that work to produce such surfaces are simply better methods in fact, not extraordinary methods applied in the face of rough surfaces to improved them. If OTOH one goes to the trouble to do high resolution imaging with Foucault, even a surface that appears impeccable visually under grazing illumination with Foucault (the most revealing method) will show small residual error at a hundredth wave PV or less level. Theses images reveal (for selected parts of the surface of an asphere) surface defects at the same level as Lyot phase contrast does (by direct comparison of the tests some time ago via the ATM List). In other words, no mirror looks perfect if you look hard enough. But that isn't an argument for leaving residual error on the surface.

The argument from experience indicates though, since working mirrors using Foucault and then testing them (both on the bench and under the sky) gives good sensitivity to the levels of ripple and other roughness that are damaging to viewing, and it leads to the same conclusion - which is to work them in such a way as to show no visible roughness under tests that are at least as sensitive as viewing itself.

For mirrors produced through any of a number of small-scale deterministic processes, though, ion beam milling should be capable of removing the last vestiges of "tool marks", shouldn't it?

Key point: A bench test that is as sensitive to error as viewing (under ideal conditions) should serve to indicate the level to which error should be removed in practice to achieve sensibly perfect surfaces. This isn't theoretical, and it can be calibrated by simple trial and error until success is achieved. This has already been done, quite a few times, and the common conclusion is that error that can't be detected under Foucault is sufficiently small to have no adverse effect on the performance of the mirror in practice.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6143218 - 10/17/13 03:50 PM

Quote:

The actual amplitude, etc., remains the subject of estimation without some way to calibrate the image.




Yes I was thinking something based on image analysis of the pixel values at different knife positions and not a normal Foucault test. The goal would be to get a quantitative estimate of just how much phase error is introduced by those ripples over what spatial distance. This would allow quantitative statements about the structure function to supplement the "RMSD" or "Strehl." I put them in quotes because their values also depend on how exactly they are deduced from the measurement of a rough surface.

I think that there is no question a completely smooth test is preferable - but it would certainly be nice to know quantitatively what impact the ripple will have. In other professional studies where the structure function is spec'd, it is used to place limits on the spread of the encircled energy graph. This is obviously important professionally, but also presumably would be noticed visually by amateurs.

As long as experienced mirror makers have a good sense of ripple in the test correlating with the star images, there may not be a need to quantify this stuff. But certainly for multi-million dollar mirrors, they would not want to agree to a spec that was beyond that needed for the intended performance - because it would add to time and cost.

If a mirror does show clear rippling, and if it also has a good interferometric study, then if the interferometry is fairly raw and not smoothed, it may be possible, as Parks says, to calculate the structure function directly. If you know the structure function you can estimate the impact on encircled energy - and then you can assess just how noticeable it would be.

If it is clear that the ripple does have big impact, then everything is simple and ripple should be avoided at all cost. But if there is a range where Foucault shows it clearly, because it is so slope sensitive, but it just shouldn't impact encircled energy - then there would be good reason not to worry about it or pay much more for it.

Note that again I'm not taking sides here. I'm just saying there is a way to quantify this stuff, and it is done professionally - and the unknown is just how sensitive Foucault is to the the relevant parts of the structure function that would impact star appearance.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: freestar8n]
      #6143333 - 10/17/13 05:08 PM Attachment (14 downloads)

Quote:

But if there is a range where Foucault shows it clearly, because it is so slope sensitive, but it just shouldn't impact encircled energy - then there would be good reason not to worry about it or pay much more for it.




In my experience this isn't true. There's a continuum of improved performance up to the point at which roughness at all scales vanishes from visual Foucault. And not to make too fine a point, but the experience of the operator, the quality of the Foucault apparatus, the way in which the apparatus is deployed, all matter greatly in determining roughness visibility. Smooth mirrors with good figures of revolution are the only ones Foucault can measure accurately, so there's more reason to produce smooth surfaces.

Stepping away from Foucault, PSI is a better choice and captures ripple well. I've seen images from software-nulled IF analyzed using FFT transforms that can resolve ripple easily as well, and there you could obtain quantitative data easily. Shack-Hartman tests can be run with minimal automation effort and sample full mirror surfaces at a resolution I've estimated to be no more than a few mm - that should do the job, since it determines the surface tilt directly.

Here's a Focogram (full resolution version) of a commercially produced mirror that shall remain nameless. It's a good example of the kind of errors that Foucault has no trouble at all revealing. Are they damaging?

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6143386 - 10/17/13 05:36 PM

Quote:

Antares optical claims very high specs on their flats but I know someone who caught them with their pants down. Not providing the spec they claim. How many people buy from them and never check the flats.



You're absolutely right, Danny. But in this case we don't know whose mirror Zambuto tested, nor do we know if this is a fluke, or the other company's run-of-the-mill product.

Mladen

Edited by MKV (10/17/13 06:17 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6143454 - 10/17/13 06:20 PM

Quote:

Quote:

Antares optical claims very high specs on their flats but I know someone who caught them with their pants down. Not providing the spec they claim. How many people buy from them and never check the flats.



You're absolutely right, Danny. But in this case we don't know whose mirror Zambuto tested, nor do we know if this is a fluke, or their run-of-the-mill product.

Mladen




I don't need to know cause their clients are way up there and I'm way down here. I can't afford a cz mirror. I definitely can't afford those mil specs.


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6143526 - 10/17/13 07:00 PM

"Is it damaging?" - based on all the discussions here - the answer is yes!

On the subject of testing methods, a Shack Cube interferometer has been mentioned(on other threads) as one that could be built by amateurs without tremendous difficulty. I assume you could get useful quantitative info about the mirror surface from that, but at what level?

Edited by alancygnusx2 (10/17/13 07:17 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
professor emeritus


Reged: 04/24/13

Loc: columbia, sc
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6143628 - 10/17/13 08:00 PM

Mladen:

I saw your worked in the US Navy before you retired. Your points on the tolerance and specs applied very well in mass production industries, be it automobile or HDTV. I used to work in a company that sells to DOD and aerospace companies, so I understood fully where you came from. I have several threads asking the price/performance of the mirrors in the past, and did not come away with satisfactory answer. Here is what I reconciled: This is a hobby industry without a lot of resources to develop any test protocols or ASTM methods (some tests are available but not standardized). In other words, mirror making is still very much an art, and a few master mirror makers have created quite a lot devoted followers. Therefore, criticism about Carl's approach to his rule #4 example, though valid, does not sit well with his clients and others, who prefer perfection (or near perfection ?) in a mirror. There is nothing wrong with that altitude as long as they can afford it. If someone does not feel that way from practical viewpoint, then just do not buy it. No offense here, different strokes for different folks.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: schang]
      #6144027 - 10/18/13 12:11 AM

Shien, your points are well taken. Contrary to numerous insinuations, accusations, and even personal insults, I really have nothing critical to say about Zambuto's mirrors: There is not a glitch on them! So what's there to critique?!

I understand that Carl's personal tolerance standard is zero detectable ripple of nay kind.So, his rejection of the mirror made by another company was a no brainer.

My beef is with the undertone of his article that one can somehow know that a mirror is outside of the established scientific tolerance envelope just by looking at shadows, and that interferometry is essentially ineffective, even unnecessary, when compared to the Foucault test. It is no secret that that optical tolerances are based on quantitative, not qualitative data (null tests may be somewhat of an exception), and that interferometry is the tool of choice because it quantifies errors like no other method does.

I don't know what strategy the company whose mirror Zambuto tested uses, but if they can sell their mirrors at prices several times over Zambuto's, they are doing something right business-wise! It would be really interesting to hear their side of this story.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: schang]
      #6144037 - 10/18/13 12:22 AM

I have been searching for quantitative data on Primary ripple and think I hit the jackpot.

Take a look at Suiter's Star Testing book, 2nd edition, page 268, figure 13.3

Its a graph of the effect of primary ripple on MTF. You can see that if primary ripple is reduced to or below 1/40 th wavelength RMS on the wavefront, its effect is almost negligible compared to the perfect MTF curve.

Looks like for microripple 1/100th wavelength RMS is indistinguishable from perfect. That's given later on in the same chapter.

Edited by alancygnusx2 (10/18/13 12:23 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6144038 - 10/18/13 12:22 AM

Quote:

Here's a Focogram (full resolution version) of a commercially produced mirror that shall remain nameless. It's a good example of the kind of errors that Foucault has no trouble at all revealing. Are they damaging?



Good quesiton! So, what did you find out? Did you ever use that mirror? Did you have it tested?

I'd love to know if those "dimples" translate into something detectable by the eye at the focus, and by how much!

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6144078 - 10/18/13 12:57 AM

Quote:

I have been searching for quantitative data on Primary ripple and think I hit the jackpot.

Take a look at Suiter's Star Testing book, 2nd edition, page 268, figure 13.3

Its a graph of the effect of primary ripple on MTF. You can see that if primary ripple is reduced to or below 1/40 th wavelength RMS on the wavefront, its effect is almost negligible compared to the perfect MTF curve.

Looks like for microripple 1/100th wavelength RMS is indistinguishable from perfect. That's given later on in the same chapter.



This is in agreement with what you can find in Vla's internet presentation. But Carl Zambuto ( Criteria #3 ) justifies removing not jut the primary ripple (medium roughnss), regardless how small, but the microripple (small roughness) as well.

Strangely, he's backing up his claim with none other than Dick Suiter (page 237, of Suiters first edition):

“...the diffraction pattern of scattered light from microripple is a shattered dim glow, quite similar to the aura that occurs with a turned down edge.”

Seems like there is a bit of a contradiction here.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6144081 - 10/18/13 01:00 AM

This mirror was in for testing, which was difficult due to the amount of roughness, and showed about 1/6th wave PVW error in SA. The contribution of the roughness as depicted would drop its performance markedly. It was presented by the owner as basically a dog.

I would draw attention to comparing that image to the Zerodur mirror on the link in the OP. With the exception of the swirl marks go in different directions, these surfaces are very similar. But the one I showed is not from a "high end, aerospace and industrial optics" source, it was from a company consistently known for such dogs. And it looks almost the same as the "high end" high-dollar product...

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6144086 - 10/18/13 01:05 AM

Thanks Mark. It would be nice if one had an example of a primary ripple at the other end of the spectrum, around 1/13 WPV, and compare the appearance in Focograms to see there is any discernible difference.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6144128 - 10/18/13 01:49 AM

Quote:

Quote:

But if there is a range where Foucault shows it clearly, because it is so slope sensitive, but it just shouldn't impact encircled energy - then there would be good reason not to worry about it or pay much more for it.




In my experience this isn't true. There's a continuum of improved performance up to the point at which roughness at all scales vanishes from visual Foucault. And not to make too fine a point, but the experience of the operator, the quality of the Foucault apparatus, the way in which the apparatus is deployed, all matter greatly in determining roughness visibility. Smooth mirrors with good figures of revolution are the only ones Foucault can measure accurately, so there's more reason to produce smooth surfaces.

Stepping away from Foucault, PSI is a better choice and captures ripple well. I've seen images from software-nulled IF analyzed using FFT transforms that can resolve ripple easily as well, and there you could obtain quantitative data easily. Shack-Hartman tests can be run with minimal automation effort and sample full mirror surfaces at a resolution I've estimated to be no more than a few mm - that should do the job, since it determines the surface tilt directly.

Here's a Focogram (full resolution version) of a commercially produced mirror that shall remain nameless. It's a good example of the kind of errors that Foucault has no trouble at all revealing. Are they damaging?

Best,
Mark




Mark,
This looks like a classic case of what we used to call "orange peel" and no one ever thought that was any good! Still, amplitude and frequency are what count--at some level all mirrors look like this!
John


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: jhayes_tucson]
      #6144134 - 10/18/13 01:56 AM

Quote:

This looks like a classic case of what we used to call "orange peel" and no one ever thought that was any good! Still, amplitude and frequency are what count--at some level all mirrors look like this!



Back in the late 70's that how everyone called it! And your point is spot on.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6144175 - 10/18/13 02:44 AM

Attention All,

1) Play the ball, not the man!

2) Play nice and stay on topic.

3) Be respectful to each other.

There are some very valid arguments here on both sides of the table , debate them like the intelligent gentlemen that I suspect all of you are.

Cheers,


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6144200 - 10/18/13 03:11 AM

Quote:

Quote:

I have been searching for quantitative data on Primary ripple and think I hit the jackpot.

Take a look at Suiter's Star Testing book, 2nd edition, page 268, figure 13.3

Its a graph of the effect of primary ripple on MTF. You can see that if primary ripple is reduced to or below 1/40 th wavelength RMS on the wavefront, its effect is almost negligible compared to the perfect MTF curve.

Looks like for micro-ripple 1/100th wavelength RMS is indistinguishable from perfect. That's given later on in the same chapter.



This is in agreement with what you can find in Vla's internet presentation. But Carl Zambuto ( Criteria #3 ) justifies removing not jut the primary ripple (medium roughnss), regardless how small, but the microripple (small roughness) as well.

Strangely, he's backing up his claim with none other than Dick Suiter (page 237, of Suiters first edition):

“...the diffraction pattern of scattered light from microripple is a shattered dim glow, quite similar to the aura that occurs with a turned down edge.”

Seems like there is a bit of a contradiction here.

regards,
Mladen




Hi Mladen,

I don't think there's anything deceptive on Zambuto's website. Its a complex and nuanced topic to begin with, and that he's presenting a mirror with a near perfect Strehl that's very smooth but stating its a suboptimal performer - naturally very confusing ( and why I put a post out here in the first place), but appears to be correct.

All the opticians are singing a song in unison that slope errors seen on foucault from primary ripple have an impact on visual contrast. I think we can trust them on that.

Suiter's MTF and Vla's MTF graph both show primary ripple has a significant adverse impact on MTF.

The paper on the Structure Function that Frank posted clearly outlines that that primary ripple can adversely affect performance despite good overall figure and fine scale smoothness.

Finally, the proof is in the pudding, Zambuto's mirrors are exceptionally good performers - especially in getting the last word in contrast, so he obviously has mastered both the art and science of making these surfaces as close to perfect as possible. When comparing optics, I have not heard of a mirror of the similar aperture exceeding the performance of a Zambuto on the field.

all the findings are consonant, yes?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6144324 - 10/18/13 07:23 AM

Agreed, +1.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Chuck Hards
Post Laureate
*****

Reged: 05/03/10

Loc: The Great Basin
Re: Zambuto Criteria #4 on Strehl new [Re: jhayes_tucson]
      #6144409 - 10/18/13 08:45 AM

Quote:

This looks like a classic case of what we used to call "orange peel" and no one ever thought that was any good!




In my mirror-making days, we called it "dog biscuit". Long time ago!


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Chuck Hards]
      #6144476 - 10/18/13 09:22 AM

Did not Carl say his coating machine can do ion milling? Any one know how that works? The Keck was milled I read to 1/100

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6144589 - 10/18/13 10:35 AM

Quote:

I don't think there's anything deceptive on Zambuto's website




Hello Alan,

I don't see any deception on his site. So, that thought never even crossed my mind. It seems very clear that Zambuto considers micripple a serious defect, and he's backing this up with a select quote from Suiter's book while ignoring the quote you provide simply because it doesn't fit into his paradigm.

In his second edition Suiter suggests primary ripple has no appreciable effect on MTF if it's below 1/40 wrms, while in his first edition he states that even a microripple (in the order of 1/100 wrms or less) has an effect similar to the TDE!

I doubt that Suiter was comparing microripple's TDE-like effect in terms of severeity, but rather in terms of the nature of the phenomenon. However, it seems that, for some reason, Zambuto treats microripple in terms of severity as a signfificant surface error that must be removed.

reagrds,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6144591 - 10/18/13 10:37 AM

Quote:

When comparing optics, I have not heard of a mirror of the similar aperture exceeding the performance of a Zambuto on the field.



How about matching?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: alancygnusx2]
      #6144608 - 10/18/13 10:47 AM

Quote:

Suiter's MTF and Vla's MTF graph both show primary ripple has a significant adverse impact on MTF.



I don't seeit that way. This is <a href="6129496" target="_blank">what Vla says earlier on this thread</a>:

"Ripple below 4-5mm in width generally are not worth measuring, except for some specialized instruments. The p-v/RMS error scales roughly with their size, and microripple (up to 1-2mm) have worst-case scenario of about 100 wave RMS on the wavefront. Plug that in to the Strehl formula and it gives you the corresponding worst-case Strehl degradation factor 0.996...Similarly, 1/4 wave p-v turned edge at 95% radius will reduce the Strehl by 0.96. Half as wide ege, by 0.98; half as much p-v by 0.99. Sure, 90%+ of mirrors are not as good as claimed, and none is perfect. But both these mirrors [the one with the primary ripple and Zambuto's] will perform very well."


Post Extras: Print Post   Remind Me!   Notify Moderator  
richard7Moderator
Not Quite
*****

Reged: 11/02/07

Loc: Sacramento
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6144688 - 10/18/13 11:35 AM

Seeing that this thread has also run off course too many times it's time.


Post Extras: Print Post   Remind Me!   Notify Moderator  
richard7Moderator
Not Quite
*****

Reged: 11/02/07

Loc: Sacramento
Re: Zambuto Criteria #4 on Strehl new [Re: richard7]
      #6145427 - 10/18/13 07:01 PM

It's unlocked for now but please go easy.

Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6145464 - 10/18/13 07:29 PM

Quote:

Quote:

When comparing optics, I have not heard of a mirror of the similar aperture exceeding the performance of a Zambuto on the field.



How about matching?




That's been my experience, and that's exactly what I would expect to see as well.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6145523 - 10/18/13 08:02 PM

Quote:

"Ripple below 4-5mm in width generally are not worth measuring, except for some specialized instruments. The p-v/RMS error scales roughly with their size, and microripple (up to 1-2mm) have worst-case scenario of about 100 wave RMS on the wavefront. Plug that in to the Strehl formula and it gives you the corresponding worst-case Strehl degradation factor 0.996



I once refigured a mirror that initially had a decent figure, but bad roughness, on the scale of a few milimeters wide. When tested, it literally looked like it had been figured with sandpaper. The client complained of poor contrast and images that weren't sharp.

After refiguring, he said the difference was startling, and the performance was superb.

So, I don't buy the 0.996 Strehl figure as being valid. I think a measurement of scatter would be more appropriate in cases like this.

One last point - the slow, careful work that creates the most accurate optical surfaces is not likely to create roughness. However, the fast, somewhat careless work that produces roughness is unlikely to produce highly accurate surfaces. So, it's unlikely that you're going to find a mirror with a great figure that also is rough, though they do pop up occasionally.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Zambuto Criterion #4 on Strehl new [Re: mark cowan]
      #6145596 - 10/18/13 08:56 PM

It would be interesting to do an experiment similar the the one done by Peter Ceravolo, Doug George, and Terry Dickinson with mirrors with varying degrees of spherical aberration, except with two mirrors similar to the pair illustrated in the link in the first post. Without such a reality check, this could just go on and on and on...

Clear skies, Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
professor emeritus


Reged: 04/24/13

Loc: columbia, sc
Re: Zambuto Criterion #4 on Strehl new [Re: Alan French]
      #6145825 - 10/18/13 11:14 PM

Not gonna happen in the foreseeable future.

Post Extras: Print Post   Remind Me!   Notify Moderator  
svdwal
Vendor (mBrain Software)


Reged: 03/10/13

Loc: Leiden, The Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: schang]
      #6146022 - 10/19/13 03:09 AM

Quote:

Your points on the tolerance and specs applied very well in mass production industries, be it automobile or HDTV. I used to work in a company that sells to DOD and aerospace companies, so I understood fully where you came from. I have several threads asking the price/performance of the mirrors in the past, and did not come away with satisfactory answer. Here is what I reconciled: This is a hobby industry without a lot of resources to develop any test protocols or ASTM methods (some tests are available but not standardized). In other words, mirror making is still very much an art, and a few master mirror makers have created quite a lot devoted followers.




A good point. Testing a mirror costs money too. If a particular quantitive test makes a mirror 10 times as expensive, and if customers are not prepared or able to pay for that, that particular quantitive test will not be made.

I do not agree about the reputation attribution. Zambuto has a high reputation because he has knowledgable customers who value his work highly. The knowledge of his customers is something that becomes apparent if you look at the things they do with their telescopes.

Being called a master is only a compliment if it is made by people who know what they are talking about.


Post Extras: Print Post   Remind Me!   Notify Moderator  
schang
professor emeritus


Reged: 04/24/13

Loc: columbia, sc
Re: Zambuto Criteria #4 on Strehl new [Re: svdwal]
      #6146460 - 10/19/13 11:17 AM

Let me retract my complimentary statement of "master" (maybe skilled ?), because frankly, I do not have sufficient knowledge about mirror making.

Post Extras: Print Post   Remind Me!   Notify Moderator  
svdwal
Vendor (mBrain Software)


Reged: 03/10/13

Loc: Leiden, The Netherlands
Re: Zambuto Criteria #4 on Strehl new [Re: schang]
      #6147033 - 10/19/13 04:58 PM

I think it is possible to call somebody a master if his customers do so, and if you have good reason to believe his customers know what they are talking about.

Anyway, the main point is that the customers are knowledgable enough to distinguish good mirrors from bad ones. Give them any two mirrors and they can say which one is better. Which means that they can order all mirrors from good to bad (a topological ordering). If they can do that reliably and repeatable, then they do not need a test that assigns a number to a specific quality of a mirror. They can do the test themselves and reject a mirror if it is not good enough. The only thing that is needed is to tell the mirror maker that the mirror must be better than a specific mirror in the ordering.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Lamb0
Carpal Tunnel
*****

Reged: 07/25/07

Loc: South Eastern(ish) Nebraska
Re: Zambuto Criteria #4 on Strehl new [Re: svdwal]
      #6149290 - 10/21/13 05:02 AM

Quote:

So, you think a 1/16 wave PV, 0.996 Strehl mirror with primary ripple of undetermined amplitude is a "Yugo"?




Yup, I wouldn't buy it. If someone claims "0.996 Strehl" I do NOT trust it, but look hard and long at any other information provided. One thing to remember is that Strehl as commonly calculated is only an upper limit. Whether there's dog biscuit, orange peel, ripple, or micro ripple... Strehl does NOT commonly tale them into account for the final figure - even if the optician does their best to measure accurately and reports honestly. I most commonly see Strehl as just another item on the list, unfortunately, so do some advertisers. To accurately measure the total figure of the mirror with the repeatable precision required to quantitatively take into account such defects fully would require such time and equipment as to make the optics unaffordable for far too many prospective customers. We do have an advantage... unlike many consumer products, especially the mass market versions - they "eat their own dog food"! I'd rather trust the optics and information from professionals such as CZ, Mike, and Mark are capable of delivering with the quantitative and qualitative tools they choose to utilize - and the experience of their Mark I Eyeballs!


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Lamb0]
      #6149536 - 10/21/13 09:56 AM

Quote:

To accurately measure the total figure of the mirror with the repeatable precision required to quantitatively take into account such defects fully would require such time and equipment as to make the optics unaffordable for far too many prospective customers. We do have an advantage... unlike many consumer products, especially the mass market versions



The particular mirror Carl Zambuto tested was not a mass produced item. The mirror, according to Zambuto's own website, "was fabricated by an optical company that specializes in aerospace and industrial optics". In other words, it was a mirror made to specs of the buyer.

And "According to Carl's measurements, he agreed the spherical aberration is indeed in the range of 1/15 wave with a corresponding Strehl ratio of better than .99. In fact, Carl Zambuto's large scale surface measurements agree very precisely with the interferometer." (from Carl Zambuto's site referenced in the very first post by the OP)

Carl rejected the mirror because it doesn't fit his own criteria of what is "good", not necessarily the criteria required by the byer. And since the company was not contacted for an explanation of the surface roughness, and because the intended purpose of the mirror is unknown, it's impossible to say whether the mirror came out like that becuase the optician was "in a hurry", or for some other reason.

It is not true that to accurately measure quantititative error is too expensive or unreliable. Even a simple Bath interferometer can pretty well approximate not only the figure but also the surface roughness, and the particular software in this case is actually free.

In the particular example shown here , the upper left image is a k-e shadowgram and the upper right image is an igram-based simulation synthesized with inteferometric data. The difference is that the latter offers both qualitative and quantitative information and the former only qualitative.

And, remember, this whole thread was not about Zambuto's mirrors, even though it was made that way, unnecessarily. Zambuto's mirror and personal standard of zero tolerance of surface roughness (using the k-e method) ensures flawless mirrors for all his mirrors, but the industry makes optics according to specs of the buyer, so the quality (and price) vary accordingly.

reagrds,
Mladen

Edited by MKV (10/21/13 01:44 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6150527 - 10/21/13 08:16 PM

Quote:

It is not true that to accurately measure quantititative error is too expensive or unreliable. Even a simple Bath interferometer can pretty well approximate not only the figure but also the surface roughness, and the particular software in this case is actually free.

In the particular example shown here , the upper left image is a k-e shadowgram and the upper right image is an igram-based simulation synthesized with inteferometric data. The difference is that the latter offers both qualitative and quantitative information and the former only qualitative.




No, it doesn't really do that - and I'm familiar with the original images, as I participated in the discussion about Dale Eason's comparison of Foucault vs reconstructed Foucaul via IF analysis with OpenFringe at the time.

If you take a careful look (and you should) you'll see numerous differences between the two images, all of them cases where the IF doesn't actually capture sufficient detail to accurately replicate the Foucault imaging - which is, by definition, what the mirror surface actually looks like, in all of its ignominious lack of quality (on purpose by Dale). So, if the IF doesn't capture it (that is the roughness) than the IF analysis is not going to include the roughness, and it acts as it almost always does, which is to apply a low-pass filter to the actual surface. The conclusion you draw from such an analysis is not in accord with reality - that is to say the actual performance of the mirror, not some theoretical version - and that's really been the issue here all along.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6150707 - 10/21/13 10:07 PM

Mark, if you read the Dale's original article, he makes it abundantly clear that his FFTA method makes the mirror look worse, not better by creating artifacts from digital "noise". So, if anything can be said based on his finding, is that in reality the mirror is somewhat better than Dale's reduction program suggests.

As far as the issue here has been concerned, it is the Zambuto standard of zero-roughness applied to all his mirrors, vs. the industry standard, which varies accordingly (price and customer specs). There are two impressions one gets from reading Zambuto's Criteria 3 and 4: interferometry is no match to the knife-edge method, and any roughness is absolutely detrimental to the human eye threshold; neither of which is true.

We also don't know why an aerospace industrial manufacturer bothered to make a mirror to a high standard, which Zambuto certifies as having 1/16 WPV spherical residuals and a Strehl of 0.99-plus, but left the surface showing primary ripple. Zambuto speculates that the optician must have been "in a hurry", yet he made no effort to contact the company to find out.

The mirror could have just as well been an order that was canceled and the mirror simply tossed out before it was completed. We simply don't know, yet it's being presented as something that was given a "pass". There is a lot to this story we still don't know,yet the industry and interferometry are given a failing grade based on speculation.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6150773 - 10/21/13 10:42 PM

Quote:

Mark, if you read the Dale's original article, he makes it abundantly clear that his FFTA method makes the mirror look worse, not better by creating artifacts from digital "noise". So, if anything can be said based on his finding, is that in reality the mirror is somewhat better than Dale's reduction program suggests.




So, you are saying that Dale claims his reduction program presents a 'worst-case scenario' for the mirror?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6150831 - 10/21/13 11:21 PM

Quote:

Quote:

Mark, if you read the Dale's original article, he makes it abundantly clear that his FFTA method makes the mirror look worse, not better by creating artifacts from digital "noise". So, if anything can be said based on his finding, is that in reality the mirror is somewhat better than Dale's reduction program suggests.




So, you are saying that Dale claims his reduction program presents a 'worst-case scenario' for the mirror?



Correct. Dale states that any "bumps present in the simulated [image] but not in the real [image] are the result of the noise I mentioned." A little later he conlcudes that there "are many more bumps in the simulated image than in the real Foucault image" and that these "are caused by system noise" from FFTA.

However, one must also read Dale's conclusion as regards the Foucault method and non-spherical mirrors, i.e. why IF has an advantage over the k-e method:

"The conclusion I reach is that for my setup it is pretty easy to detect features that are 1/60 wave on the surface of a mirror by examining Foucault images. Many factors like slit width, camera and lens settings, and display type effect the visibility of Foucault shadows. It should be noted that the best way to see fine detail in a Foucault image is when the knife is placed near the focus of that area. That is when the area will have a gray shade that makes variations easy to see. To see the whole mirror this can only happen when it is a sphere. With the parabola it becomes much harder to see the whole mirror at one time."

Keep in mind, however, that Dale is speaking of the mirror surface, and not the wavefront when he says 1/160 wave. The two images show the "twin peaks" which are about 1/30 wave on the wavefront and that these are barely visible. So, any talk of a k-e seeing 1/100 wave is doubtful. Only the Gaviola caustic test -- if properly performed (good luck!) -- in theory can reach that limit. But, unlike the Foucault test, interferometrty can see "deeper" then 1/30 on the wavefront. IIRC, the Zygo is limited at 1/50 WPV on the wavefront.

regards,
Mladen

Edited by MKV (10/21/13 11:38 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6150897 - 10/22/13 12:05 AM

Quote:

he makes it abundantly clear that his FFTA method makes the mirror look worse, not better by creating artifacts from digital "noise". So, if anything can be said based on his finding, is that in reality the mirror is somewhat better than Dale's reduction program suggests.



No, the only thing that can be said is digital noise is camera and interferometer (system) dependent, and this varies with the system.

Quote:

...interferometry is no match to the knife-edge method, and any roughness is absolutely detrimental to the human eye threshold; neither of which is true.



It is certainly true in some cases (I have seen them), so saying it's not true is quite wrong.

Quote:

****** speculates that the optician must have been "in a hurry", yet he made no effort to contact the company to find out.



Are you really going to make that call? Let me know how it goes..... and if you even get past the receptionist....

Quote:

There is a lot to this story we still don't know,yet the industry and interferometry are given a failing grade based on speculation.



It not speculation - it's experience. If you want to dismiss extensive experience, that's your prerogative, but it is unwise.

No, certain operators are given a failing grade based on dishonesty or poor practice. I'd give the rough mirror a failing grade too based on my experience, no speculation whatsoever - just experience. Care to dismiss my experience?

What's your experience, by the way?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mike Lockwood]
      #6151034 - 10/22/13 03:02 AM

Mike, based on your experience, can you tell us why would a company that does aerospace contracts produce a mirror which Carl certifies as being 1/16 WPV spherical and with a Strehl rating of 0.99-plus and leaves the surface (apparently) unfinished?

As for Dale's article, he specifically states the noise is caused by the type of (Michael Peck's) FFT analysis routine. If you think it was Dale's equipment you can reach him on the Yahoo interferometry group.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6151233 - 10/22/13 08:32 AM

" and any roughness is absolutely detrimental to the human eye threshold; neither of which is true."
****
It has been said before several times that roughness affects contrast significantly, makes stars soft and fuzzy, and affects the magnitude reach of the optic: and the optic can still have a high strehl/PVW rating. Several makers here have related this information to you, and others who have the ability to discern this. I made a study of such at length when using black rouge mix vs -cerium only- and have a lengthy report. Still own that paricular scope as well, and will keep it for a long time.
There are significant payoffs in using care in polishing smooth accurate surfaces.
I fail to see -WHY-, the total disregard of such first hand knowledge of this fact persists in your posts.
???
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6151302 - 10/22/13 09:32 AM

Quote:

can you tell us why would a company that does aerospace contracts produce a mirror which Carl certifies as being 1/16 WPV spherical and with a Strehl rating of 0.99-plus and leaves the surface (apparently) unfinished?






that's what I would like to know.

All this surface roughness debate, IF vs KE, is a distraction, anyone can see the surface needs work.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6151356 - 10/22/13 10:00 AM

Well, it was one mirror, which was selected specifically to illustrate a point. I wouldn't read too much into this ... it isn't like it was a 'random sample' or even representative of other mirrors produced by the firm.

Carl's point (Criteria #4) is that mirrors produced in his shop will not look like that mirror. Yes, it has a high Strehl in spite of the visible surface roughness (which is the point Mr. Zambuto is making here I think); so if you think it is good enough for your needs, then by all means take it. Me, I'd still prefer the mirror on the right ...

Edited by Dave O (10/22/13 10:02 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6151466 - 10/22/13 10:58 AM

Quote:

Well, it was one mirror, which was selected specifically to illustrate a point. I wouldn't read too much into this ... it isn't like it was a 'random sample' or even representative of other mirrors produced by the firm.




I understand that, but someone paid 4x a CZ mirror for that mirror.

at that price point that surface shouldn't happen.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6151546 - 10/22/13 11:51 AM

Agreed on that. But another point, Strehl, by itself cannot be totally descriptive of quality, nor quantity of error- as all this discussion has shown. As Mike said, there should be some sort of statistic on scatter that would accompany the rest of the test information; whether from microfinish or microripple.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6151566 - 10/22/13 12:05 PM


What find more interesting is that Carl uses a very thin very watery suspension of decanter cerium on his mirrors. The master Texereau always advocated red rouge for the best surfaces and many old timers swore by it. I always got scratches with rouge and hated the red mess but then I didn't mill it before use like some ATMs will do. I now finish mirrors with a fine particle size cerium (Opaline) but never having tried phase contrast testing I'm wondering how much influence polish concentration plays. Maybe this should be another thread.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6151767 - 10/22/13 02:09 PM

Quote:

Zambuto speculates that the optician must have been "in a hurry", yet he made no effort to contact the company to find out.



Actually I just talked to Carl, and in his words they "didn't return his phone call". Big surprise there.

Quote:

Mike, based on your experience, can you tell us why would a company that does aerospace contracts produce a mirror which Carl certifies as being 1/16 WPV spherical and with a Strehl rating of 0.99-plus and leaves the surface (apparently) unfinished?



The one thing that my experience has taught me over the years is to never, never, NEVER try to figure out WHY some person/company did something. You will drive yourself crazy and waste a lot of time.

It's sort of like asking *why* certain people on CN behave the way they do.

However, a good guess of the answer to most questions of "why" involving mirrors is "because they were in a hurry".

Quote:

As for Dale's article, he specifically states the noise is caused by the type of (Michael Peck's) FFT analysis routine. If you think it was Dale's equipment you can reach him on the Yahoo interferometry group.



FFT analysis routines (assuming they are modern, i.e. floating point and done with sufficient precision) do not cause noise. However, they can amplify or reduce it. (BTW I have a masters degree in electrical engineering with an emphasis on signal processing, so I can say that is quite true. I also have a pretty good grasp on interferometric analysis as a result.)

What's your background Mladen?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Thomas Karpf
Carpal Tunnel


Reged: 02/09/09

Loc: Newington, CT
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6151949 - 10/22/13 03:53 PM

Quote:


What find more interesting is that Carl uses a very thin very watery suspension of decanter cerium on his mirrors. The master Texereau always advocated red rouge for the best surfaces and many old timers swore by it. I always got scratches with rouge and hated the red mess but then I didn't mill it before use like some ATMs will do. I now finish mirrors with a fine particle size cerium (Opaline) but never having tried phase contrast testing I'm wondering how much influence polish concentration plays. Maybe this should be another thread.




Given the vast work experiences of Carl Zambuto, Mark Harry, and Mike Lockwood (in no particular order; please don't take offense) and the general lack of poor feedback on any of their mirrors, I'd be more than comfortable also putting them in the category of 'master'. I'd certainly be happy taking anything they say on the subject of creating mirrors for astronomical uses as gospel.

Granted that Jean Texereau has a book published nearly 30 years and Carl, Mark, and Harry are technically unpublished (Yahoo groups and Cloudynights notwithstanding). As Carl Zambuto has experience that is at least 30 years more current than Mr. Texereau, I'm unclear why Carl's practices and procedures should be questioned based on a 30 year old book. Is there any question at all that Carl Zambuto (as well as Mark Harry and Mike Lockwood) knows what he is doing?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: Thomas Karpf]
      #6151991 - 10/22/13 04:24 PM



Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mike Lockwood]
      #6152574 - 10/22/13 10:03 PM

Quote:

FFT analysis routines (assuming they are modern, i.e. floating point and done with sufficient precision) do not cause noise. However, they can amplify or reduce it. (BTW I have a masters degree in electrical engineering with an emphasis on signal processing, so I can say that is quite true. I also have a pretty good grasp on interferometric analysis as a result.)




Look, no one is questioning your credentials here, Mike. I simply relayed what Dale Eason stated:

Quote:

"An undesirable side effect of the FFT technique is that it can add false surface roughness to the analysis".




If you disagree with that, talk to Dale. I am merely the messenger. Which is why my background is irrelevant here.

Also, generally speaking, camera noise (i.e. "salt and pepper" type) is quite unlike what Dale's FFT analysis shows.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Thomas Karpf]
      #6152604 - 10/22/13 10:24 PM

Thomas, no one here is questioning Carl's, Mark's or Mike's experience, nor is anyone questioning the wisdom of Carl's zero-ripple approach -- if it is cost-effective. The industry, on the other hand, makes optics to customers' specs.

The mirror Carl examined was made by an optical company that does industrial and aerospace contracts. Let me say that it is no small matter to be an industrial company with aerospace contracts. It is also no small matter to simply dismiss one bad mirror as indicative of that company's dishonest IF reporting, or alleging that its opticians are in a hurry.

After all, The Hubble Space telescope was made with professional engineers (with advanced degrees I may add!) and master opticians, with tons of experience, and they still made a "boo-boo". It can happen to anyone.

regards,
Mladen

Edited by MKV (10/22/13 11:27 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6152633 - 10/22/13 10:42 PM

Quote:

If you disagree with that, talk to Dale. I am merely the messenger. Which is why my background is irrelevant here.





A 'messenger' ... does Dale even know that you are acting as his 'messenger' on this forum?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6152697 - 10/22/13 11:22 PM

LOL! What Dale wrote is public record. I was merely making everyone aware of what he wrote as regards FFTA added noise.

regards,
Mladen

Edited by MKV (10/22/13 11:29 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6152748 - 10/22/13 11:56 PM

Quote:

The particular mirror Carl Zambuto tested was not a mass produced item. The mirror, according to Zambuto's own website, "was fabricated by an optical company that specializes in aerospace and industrial optics". In other words, it was a mirror made to specs of the buyer.




And later...


Quote:

The industry, on the other hand, makes optics to customers' specs.





Are you suggesting the client when ordering specified 1/15 wave please, oh and I'd like it quite rough thanks?

Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: dave brock]
      #6152813 - 10/23/13 12:43 AM

Well, isn't it obvious that opticians should only ever bother to make mirrors as good as they can get away with in the marketplace? What possible reason could there be to do any better? Besides, poor collimation, thermal effects, secondary error or obstruction, tube currents, coating deterioration, cell support, dirt on the glass, vibration, or the atmosphere will always ruin any alleged perfection.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: dave brock]
      #6152830 - 10/23/13 01:12 AM

Quote:

Are you suggesting the client when ordering specified 1/15 wave please, oh and I'd like it quite rough thanks?



Ha! Well, we don't know how a 1/15 wave PV mirror ended up that way, do we?

I seriously doubt a company that supplies optics for aerospace contracts could get away with such finish -- unless it was specifically requested for experimetnal purposes (very likely), or it was a mistake (less likely), or the optician was in a hurry...and the quality control was asleep (how likely is that?).



Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: MKV]
      #6152899 - 10/23/13 03:06 AM

Post deleted by ausastronomer

Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Zambuto Criteria #4 on Strehl new [Re: dave brock]
      #6152901 - 10/23/13 03:09 AM

Quote:

Are you suggesting the client when ordering specified 1/15 wave please, oh and I'd like it quite rough thanks?




A key message in the reference by Parks I provided earlier is that the customer and the optician agreed to a spec on the finish and the wavefront error - but did not spec the mid-frequency errors. The result did meet the agreed spec, but did not meet the customer's needs because he had not realized the spec they had agreed to was not adequate for the application.

His other point was that with computer driven figuring with small tools, these errors are more likely to happen in the high tech shops that otherwise would be expected to do well. On the other hand, a "craftsman" doing hand figuring with a large tool will tend to avoid these errors - but might not be able to crank out arbitrary figures on large mirrors in a short time - to a specified spec.

It's still hard for me to judge anything here since I don't know the intended application for the mirror, and I don't have quantitative info on the slope errors. I can imagine the problems would show if it were used visually in an a Newtonian - but it's not clear that is the purpose it was originally spec'd for.

A lot of this is moot if the customer does specify the surface error in terms of a structure function. The high tech shop would then need to increase the cost due to the added time - but if they agree to the price at that spec and it meets the spec - everyone should be happy. Would it still look bad in Foucault? That remains somewhat a mystery since Foucault isn't very quantitative - and would depend on how tight the spec was and the intended use for the mirror.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: Dave O]
      #6152948 - 10/23/13 04:39 AM

Post deleted by ausastronomer

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6152959 - 10/23/13 04:52 AM

Brings to mind a coating I got on a customer's mirror. The coater (remaining anonymous) had instructions to forward the piece directly to the customer. Prior to this, they were returned to me for evaluation/inspection, where they looked fine.
That one time, I skipped having inspected the coat directly, customer sent a pic looking -THROUGH- the mirror.
It looked as if it was haphazardly sprayed with silver spraypaint- full of pinholes, perhaps thousands!
****
The coater's reply to my inquiry- I didn't specify
PINHOLE FREE! (*BLEEP*!)
One or two pinholes is one thing. But this was absolutely incredible. The shame of it was, the figure on the glass was really nice and smooth, and accurate.
About all I realized out of this was the cost of the glass, and shpg/ins costs. Needless to say, I haven't made use of that particular coater since.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6153255 - 10/23/13 09:43 AM

Quote:

In other words, you are merely repeating something that you read somewhere; but have no real understanding of it?



Let's not delve into sophism here. Dale discovered that the FFTA routine made the mirror look worse. His images leave no doubt. I think I have a "real" understanding of what he said. End of story. If you disagree, provide counter evidence.

As to whether those extra patterns represent digital noise, I will leave that to those who know more about this. To me, digital noise means "graininess", not additional faux structures. Dale's FFTA-derived simulaiton has no graininess, but shows additional "slope" structures that are not random.

Your expertise input on FFTA noise is welcome any time.

regards,
Mladen

Edited by MKV (10/23/13 09:49 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: freestar8n]
      #6153258 - 10/23/13 09:45 AM

Quote:

It's still hard for me to judge anything here since I don't know the intended application for the mirror, and I don't have quantitative info on the slope errors. I can imagine the problems would show if it were used visually in an a Newtonian - but it's not clear that is the purpose it was originally spec'd for



Bingo!


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6153299 - 10/23/13 10:02 AM

Quote:

After all, The Hubble Space telescope was made with professional engineers (with advanced degrees I may add!) and master opticians, with tons of experience, and they still made a "boo-boo". It can happen to anyone.




Indeed. It might be interesting to look that up for a little perspective. But IIRC, they made a "perfect" mirror, the error was in the spacing of the test apparatus.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6153392 - 10/23/13 10:41 AM

Quote:

Quote:

It's still hard for me to judge anything here since I don't know the intended application for the mirror, and I don't have quantitative info on the slope errors. I can imagine the problems would show if it were used visually in an a Newtonian - but it's not clear that is the purpose it was originally spec'd for



Bingo!




The mirror was parabolic, meant to focus all light rays to a point. It was in the hands of an hobbyist when it was determined it was lacking in performance and needed testing. If the intended application was a precision figure with roughness added by design, then I'm Sylvester Stallone.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6153420 - 10/23/13 10:51 AM

Quote:

Quote:

It's still hard for me to judge anything here since I don't know the intended application for the mirror, and I don't have quantitative info on the slope errors. I can imagine the problems would show if it were used visually in an a Newtonian - but it's not clear that is the purpose it was originally spec'd for



Bingo!




*bold added by me ...

So, it seems that we now can agree that the surface roughness on an otherwise excellent mirror (with great Stehl ratio) may affect performance?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Jeff Morgan]
      #6153479 - 10/23/13 11:21 AM

Quote:

[Indeed. It might be interesting to look that up for a little perspective. But IIRC, they made a "perfect" mirror, the error was in the spacing of the test apparatus.



This is why tool calibration is essential and cannot be overlooked. neither is your accuracy assured simply because you keep getting repeat readings to the nth decimal place unless your measuring apparatus has a calibrated error.

As to why The Hubble fiasco occurred, despite highly qualified and certified people working on the project, is a different story. Concluding the machinists were "too busy" or "in a hurry", or worse, "dishonest", when they produced the nulling apparatus is nothing but empty speculation.

Likewise, we know nothing about the mirror Zambuto tested except that it is a precision optic with a primary ripple. Suggesting it was left that way because the optician was "in a hurry" or "dishonest" is simply not corroborated by facts or even likelihood, given the company makes mirrors for aerospace industry.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6153493 - 10/23/13 11:34 AM

Amateur telescope makers acquire optics in all sorts of ways, including swap tables. If the mirror was advertised as having certified Strehl of 0.996 and 1/15 wave PV spherical residual, I don't see why an amateur would not have acquired such a mirror.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6153506 - 10/23/13 11:44 AM

Quote:

So, it seems that we now can agree that the surface roughness on an otherwise excellent mirror (with great Stehl ratio) may affect performance?



That's irrelevant. The question is if the mirror was made that way purposefully, or because the optician was "dishonest" and "in a hurry".

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dave O
sage
*****

Reged: 12/21/11

Loc: Sri Lanka
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6153551 - 10/23/13 12:12 PM

Quote:

Quote:

So, it seems that we now can agree that the surface roughness on an otherwise excellent mirror (with great Stehl ratio) may affect performance?



That's irrelevant. The question is if the mirror was made that way purposefully, or because the optician was "dishonest" and "in a hurry".

regards,
Mladen




I doubt it was 'made' that way on purpose, rather it was likely 'left' that way because it was 'finished'.

As to 'dishonest'; don't think that was an issue here either. The mirror was likely made to a specification that did not include surface roughness (or lack thereof). The spec probably listed some PV criteria and Strehl ratio. The mirror was made 'to specification'. The methods used were designed to be 'fast', which is why the surface was 'rough' ... 'time is money' as they say ... yes, the optician was 'in a hurry'. Since the mirror met the specification, it was 'done', why would he spend more time on it.

But, how it came to be is really irrelevant and was not the question posed by the OP (the original question was along the lines of how a high Strehl could be misleading).

That is why Mr. Zambuto specifies his Criteria #4 ... his mirrors don't leave his shop unless they are 'smooth'.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6153645 - 10/23/13 12:59 PM

The roughness is detected on the test. Because it scatters light!

Post Extras: Print Post   Remind Me!   Notify Moderator  
pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6153761 - 10/23/13 01:55 PM

Quote:

Quote:

The question is if the mirror was made that way purposefully.

regards,
Mladen




Sylvester Stallone here.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6153784 - 10/23/13 02:09 PM

Quote:

However, is Zambuto on to some deeper point here about the limit of the interferometer, or that perhaps most interferometers aren't able to measure all scales of ripple?






this is the original question and the answer is no.

He's not onto a deeper issue with IF, just that an operator that uses IF can suck like anyone else.


Post Extras: Print Post   Remind Me!   Notify Moderator  
alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6153940 - 10/23/13 03:33 PM

Quote:

Quote:

However, is Zambuto on to some deeper point here about the limit of the interferometer, or that perhaps most interferometers aren't able to measure all scales of ripple?






this is the original question and the answer is no.

He's not onto a deeper issue with IF, just that an operator that uses IF can suck like anyone else.




Well, that's one interpretation.

Mine is that he was essentially correct, and that as the paper on the Structure Function points out, measurements of Strehl and smoothness can miss problems caused by primary ripple.

I suppose if I could ask for one piece of objective data on a mirror, it would be the IF derived wavefront map, that pretty much tells the whole story, presuming its done correctly. As amateurs, of course, we don't get information, and the majority of us don't have an IF set up to do it ourselves.


Best,

Alan


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: pstarr]
      #6154296 - 10/23/13 07:37 PM

Quote:

Sylvester Stallone here



Hello "Sylvester"! If I wanted to study the effects of, say 1/30 wpv primary ripple, as a pure defect, or as compared directly to a similar mirror with no primary ripple, I would contract a company to make an essentially flawless mirror (high Strehl ratio, low spherical residual) with the ripple 'built into' the specs.

I just can't see a company making a living through aerospace contracts, and making poor optics. There is more to this. Surely, someone in the aersopasce industry knows something about optical quality, don't you think?

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Dave O]
      #6154323 - 10/23/13 07:55 PM

Quote:

But, how it came to be is really irrelevant and was not the question posed by the OP (the original question was along the lines of how a high Strehl could be misleading).

That is why Mr. Zambuto specifies his Criteria #4 ... his mirrors don't leave his shop unless they are 'smooth'.




Well, that just it -- Mr. Zambuto didn't need another mirror to state that his mirrors don't leave his shop unless they are 'smooth' Instead, he chose to show that someone else's mirrors do, without telling us the whole story.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6154336 - 10/23/13 08:03 PM

Quote:

Well, that just it -- Mr. Zambuto didn't need another mirror to state that his mirrors don't leave his shop unless they are 'smooth' Instead, he chose to show that someone else's mirrors do, without telling us the whole story.

regards,
Mladen






well you can do like lockwood did and call him up and ask him more about the entire event and report back to us.

just think you can actually talk to Carl himself, kind of like an interview.

I got the feeling the #4 was like wow, this person paid how much? and had an IF report? and this is what they got?...tsk tsk.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Pinbout]
      #6154718 - 10/24/13 01:15 AM

Danny, as I said, Carl's criteria don't require a bad mirror to be good. His criteria are clear and his Foucograms show that he stands by his own standards -- which is flawless mirrors. I don't see what he gained by introducing someone else's mirror that doesn't come close. By not revealing the name of the company that made that mirror, the readers don't benefit either, so why go through the trouble because they don;t know who the "bad" company is!

If he feels that his mirrors are better, then he should seek aerospace and industrial contracts. His superior work will market itself -- IF it is true that other companies ROUTINELY make bad mirrors!

If it's not true, then cherry picking someone's ugly duckling -- i.e. a mirror that was possibly made specifically below standards for experimental purposes, or is simply a reject picked up by an ATM at some swap table -- seems a bit over the top.

'nuf said.

regards,
Mladen

Edited by MKV (10/24/13 01:19 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: MKV]
      #6155039 - 10/24/13 09:05 AM

Post deleted by ausastronomer

Post Extras: Print Post   Remind Me!   Notify Moderator  
Thomas Karpf
Carpal Tunnel


Reged: 02/09/09

Loc: Newington, CT
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6155401 - 10/24/13 12:04 PM

Quote:

Danny, as I said, Carl's criteria don't require a bad mirror to be good. His criteria are clear and his Foucograms show that he stands by his own standards -- which is flawless mirrors. I don't see what he gained by introducing someone else's mirror that doesn't come close. By not revealing the name of the company that made that mirror, the readers don't benefit either, so why go through the trouble because they don;t know who the "bad" company is!




He showed evidence that criterium #4 matters. Who the manufacturer was is irrelevant other than the fact that it wasn't Carl Zambuto. Mentioning the manufacturer could be considered libel, and could, at least, cost him the time and effort required to remove the image from his website, formally apologize to the careless manufacturer, etc. Just asking his lawyer to defend a case like this costs Carl time and money.

He's not trying to take work away from mirror manufacturer fill-in-the-blank; he's trying to drive work to HIS business.

And succeeding, I'd wager. Certainly I'd believe that this thread has pushed at least one possible customer to check out Carl's website, look at the images, and settle on Carl for their next scope.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman81Moderator
Carpal Tunnel
*****

Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6155744 - 10/24/13 03:11 PM

Well, he does currently have a one year backlog (that I know for a fact) and perhaps he wants to keep limits on the scope of his operation kind of like Astro-Physics does. He is likely giving priority to the amateur astronomy community until he has the capacity to take on more work. He is only man after all and only has maybe one other helper, AFAIK.

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Thomas Karpf]
      #6155760 - 10/24/13 03:20 PM

Justy wheI thought we've covered it all...


Quote:

He showed evidence that criterium #4 matters..



Yes, something like this: "Zero ripple, and, oh by the way, here's the picture of how every one of my mirrors must look -- or your money back!" would have been more than enough.


Quote:

Who the manufacturer was is irrelevant other than the fact that it wasn't Carl Zambuto. Mentioning the manufacturer could be considered libel, and could, at least, cost him the time and effort required to remove the image from his website...



Well, Wolgang Rohr posts images of brand name optics and even copy rights them! And he's still in business. Here is a shot of a Carl Zeiss lens.

See, identifying a company is not really a problem, even if it's a giant like Zeiss. Rohr also posted Zambuto's, Takahashi's, and others' optics with no problems.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Starman81]
      #6155795 - 10/24/13 03:39 PM

Quote:

Well, he does currently have a one year backlog (that I know for a fact) and perhaps he wants to keep limits on the scope of his operation kind of like Astro-Physics does. He is likely giving priority to the amateur astronomy community until he has the capacity to take on more work. He is only man after all and only has maybe one other helper, AFAIK.



That's, of course, his choice, but the industry does pay more, so you can make fewer mirrors and sell them for a lot more. That way you'd work less, have more time for family and fun, and make a lot more money.

The particular mirror he used to show primary ripple was made of Zerodur and cost as much as 2.5 times what Zambuto would have charged ATMs for a similar mirror.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6156034 - 10/24/13 05:57 PM

I think this thread has nearly been done to death guys. It's now going around in circles. If you have something worthwhile to contribute then by all means post it, but keep it on topic and NON personal.

If we have another post that addresses the person not the subject matter, I will lock it.

Cheers


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Zambuto Criteria #4 on Strehl new [Re: ausastronomer]
      #6159186 - 10/26/13 01:30 PM

Quote:

I think this thread has nearly been done to death guys. It's now going around in circles. If you have something worthwhile to contribute then by all means post it, but keep it on topic and NON personal.

If we have another post that addresses the person not the subject matter, I will lock it.

Cheers




Well...I learned something from reading this thread. Now I know why Zambuto doesn't provide any specs for the mirrors he sells!
John


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl *DELETED* new [Re: jhayes_tucson]
      #6159276 - 10/26/13 02:17 PM

Post deleted by Mark Harry

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mike I. Jones
Post Laureate
*****

Reged: 07/02/06

Loc: Fort Worth TX
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6159334 - 10/26/13 02:58 PM

One of the consistent problems I've seen in this thread (and many others in the forum) is the overwhelming volume of short, subjective comments and opinions, but the lack of references to or calculations based on any published, refereed, authoritative work in this area. I recommend this very recent paper, for example:

Harvey J.E., "Parametric Analysis of the Effect of Scattered Light upon the Modulation Transfer Function", Optical Engineering, 52(7), Jul 29, 2013.

It's but a click away online, or a quick find in any decent public or university library. BTW, it's a great read, and specifically addresses scattering in Newtonian primaries.

Why not take the opportunity to actually educate yourselves and have a fact-based discussion, and maybe even make some useful contributions, rather than doing this mindless subjective arguing that ends up getting the thread locked?

This groundless, fact-free arguing is why I have become scarce on the forum. Read a technical paper now and then, folks!
Mike


Post Extras: Print Post   Remind Me!   Notify Moderator  
piaras
scholastic sledgehammer


Reged: 01/26/09

Loc: Niagara Region
Re: Zambuto Criteria #4 on Strehl new [Re: Mike I. Jones]
      #6159340 - 10/26/13 03:03 PM

Mike said it all. I stopped reading after pg 4.
Pierre


Post Extras: Print Post   Remind Me!   Notify Moderator  
jhayes_tucson
professor emeritus
*****

Reged: 08/26/12

Loc: Bend, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Mike I. Jones]
      #6159447 - 10/26/13 04:22 PM

Quote:

One of the consistent problems I've seen in this thread (and many others in the forum) is the overwhelming volume of short, subjective comments and opinions, but the lack of references to or calculations based on any published, refereed, authoritative work in this area. I recommend this very recent paper, for example:

Harvey J.E., "Parametric Analysis of the Effect of Scattered Light upon the Modulation Transfer Function", Optical Engineering, Opt. Eng. 52(7), Jul 29, 2013.

It's but a click away online, or a quick find in any decent public or university library. BTW, it's a great read, and specifically addresses scattering in Newtonian primaries.

Why not take the opportunity to actually educate yourselves and have a fact-based discussion, and maybe even make some useful contributions, rather than doing this mindless subjective arguing that ends up getting the thread locked?

This groundless, fact-free arguing is why I have become scarce on the forum. Read a technical paper now and then, folks!
Mike




Mike,
Agree 100%. That was the underlying basis for my last comment.
John


Post Extras: Print Post   Remind Me!   Notify Moderator  
Howie Glatter
Vendor


Reged: 07/04/06

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6159500 - 10/26/13 04:56 PM

"As to why The Hubble fiasco occurred, despite highly qualified and certified people working on the project, is a different story. Concluding the machinists were "too busy" or "in a hurry", or worse, "dishonest", when they produced the nulling apparatus is nothing but empty speculation."

As I recall from reports at the time, two different null corectors were made for testing the mirror. One had all refractive optics and the other was all reflective. When the reflective corrector was assembled, an element was mis-spaced because its protective cap was left on for a distance measurement using a laser. When the results on the mirror using the two null correctors disagreed (the refractive one was correct, by the way), the responsible individuals did not bother to get to the bottom of why the results disagreed. Instead, they arbitrarily decided to trust the result from the reflective corrector and sweep the other one under the rug.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Howie Glatter]
      #6159559 - 10/26/13 05:42 PM

The knife edge could have told them witch null corrector was wrong .For all the thousands of things they got correct and only this / I say well done .And to fix a telescope in space . We get to see the images .

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: Howie Glatter]
      #6159601 - 10/26/13 06:10 PM

Quote:

When the results on the mirror using the two null correctors disagreed (the refractive one was correct, by the way), the responsible individuals did not bother to get to the bottom of why the results disagreed. Instead, they arbitrarily decided to trust the result from the reflective corrector and sweep the other one under the rug.





when intelligent people make dumb decisions.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6159609 - 10/26/13 06:14 PM

Quote:

And to fix a telescope in space




Doing the right thing vs doing things right


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Mike I. Jones]
      #6159694 - 10/26/13 07:34 PM

THANKS, MIKE!
yOU HAVE SAID SO ELOQUENTLY WHAT I FAIL TO DO WHEN I GET SO -REACTIONARY-.
MAY GOD BLESS YOU IN YOUR FORESIGHT!
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Mike I. Jones]
      #6162522 - 10/28/13 01:24 PM

Quote:

This groundless, fact-free arguing is why I have become scarce on the forum. Read a technical paper now and then, folks!



Mike, needless to say, your input would have been greatly appreciated right from the start. No one disputed that Zambuto's mirriors are excellent workmanship. I think the issue was with the mirror he tested, and whether we can assign tolerances to barely perceptible surface irregularities (roughness) based on shadows alone, and at what point is the the surface roughness below the threshold of human eye perception. The debate became emotional as some perceive any form of inquisitiveness as unacceptable, even offensive, and attach too much ego, in my opinion, to something that is, for the most part a hobby.

As for the lack of "fact"-based knowledge, there were numerous attempts to steer the discussion in that direction, but unfortunately didn't succeed. For example, several references were made to Suiter's book

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/6144037/...

one stating that
Quote:

1/40 wavelength RMS of surface roughness "is almost negligible compared to the perfect MTF curve."




In addition to that, you had Vla's statements based on his site (which is entriely facts-based), which was referenced several times.

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/6133297/...

There was also a reference made to Dale Eason's experiment

http://www.atmsite.org/contrib/Eason/fft

Your own reference to James E. Harvey's article is a good source, and so are various articles by people like David M. Aikens on "Resonable tolerances of surface roughness and the amount of light scatter"

http://www.researchgate.net/publication/241337127_Meaningful_surface_roughnes...

as well as "The cost of tolerancing"

http://www.researchgate.net/publication/229005794_The_cost_of_tolerancing

Jame E. Harvey reveals that even among some professional optical egineers (not amateurs!), there is a perception that MTFs are not significantly degraded by scattered or straight light, which basically hinges on what one considers "significant" and not some facts-based rule!

So, not even the professional community is of one mind on some of these issues. Therefore appealing to experience, and or education is no guarantee that the answer is universally accepted, or that the outcome will be satisfactory (eg. The Hubble Space Telescope fiasco).

In another example, Aikens questions the 30 Ångstrom wavelength RMS as the acceptable tolerance which Harvey seems to favor. So a little openmindedness and humility can go a long way in searching for the truth.

More importantly, Harvey establishes very unequivocally that the human mind chooses higher contrast over resolution, even when systems that have slightly decreased contrast actually show better resolution.

No less important is the consideration of all these graphs as they relate to human vision. A good paper on human eye MTF is shown here

http://www.iovs.org/content/40/1/203.full.pdf

Note that the frequency is listed only up to 100 (c/deg), and not 400 or more, as in some MTF graphs, and that human visual acuity changes (diminishes) as we age, so that the combined population of telescope users does not see the same thing when they use the same telescope, and therefore may not be of the same mind when it comes to deciding which is better.

In conclusion, I agree with you that a "facts"-based, or at last quantitatively defined, discussions would be better than emotional outbursts and short-answer opinionated statements, but in general any such "facts"-based" discussion (I hope everyone understands that the term "fact" here is used only figuratively) is a better approach, but is generally not practiced on this forum.

regards,
Mladen

Edited by MKV (10/29/13 01:10 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Napersky
Carpal Tunnel
*****

Reged: 01/27/10

Loc: Chicagoland
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6209181 - 11/22/13 11:05 AM

Right On Ed

Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6214561 - 11/25/13 08:03 AM

Pretty much agree with what you said, Mladen. The factual route was offered more than once, but it was rejected/ignored if it would threaten to collide with the notion of some that Zambuto's conclusions "cannot be questioned - period". So the absence of the factual aspect here is not the cause of futile arguing - it is the choice.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6215392 - 11/25/13 03:12 PM

So are -YOU questioning him? Do you think he got where he is today by making rediculous claims?
Yes, it -IS- a choice! We all have one, and I believe what my experience has confirmed. (and I won't change my mind, either; regardless of the endless quotes or arguements.)
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ed Jones
Carpal Tunnel
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6215527 - 11/25/13 04:06 PM

I would like to hear about other who have done the Loyt phase contrast test (not having done it myself) and the effect of different polishes and not rehash the KE vs If argument. Carl says he runs with a slurry that looks like water, I wonder why he finds this necessary, does anyone else?

Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6215797 - 11/25/13 06:20 PM

2nd that.

I've done plenty of high-res Foucault imaging, and I know that all mirrors show some surface detail once you get to a fine enough level. I also have seen direct comparisons between high-res Foucault imaging and Lyot testing (this was done by Dale Eason some years ago and posted on the ATM List which is now back online) and it confirms that though nothing escapes the Lyot test, the same nothing generally doesn't escape high-res Foucault, at least over the range where you have good gray-scale (for a paraboloid at ROC with no nulling). Because it's imaging and not visual inspection, you can easily adjust the contrast to bring out surface detail.

I'd like to see a direct comparison done between a rough mirror with otherwise excellent correction (not incompatible) and a flawlessly smooth mirror with an excellent figure, in the field, and available to anyone to check out.

Such a test could be housed in a dual-mirror scope built precisely to test the difference. A bino-scope could be easily used for that. The reason for this is that nothing convinces like an unlabeled A-B test, and that would be as close as you could get. You could switch the mirrors out occasionally for added rigor. Something like a couple of 10" f/5s would work nicely.

I wouldn't expect there to be as much difference for imaging as for visual use, due to contrast transfer differences that the former can overcome easily. But having seen in the field both before and after the difference that correcting roughness brings I'd expect it to be visually obvious.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Zambuto Criteria #4 on Strehl new [Re: Ed Jones]
      #6215802 - 11/25/13 06:23 PM

I used to run a slurry on machine. I find a black rouge mixture (by hand, well diluted) that looks smoother even with KE. The cerium I use is 'liquid 85', which is a darn good high quality polishing compound- useful with a lot of glasses, and is very smooth by itself. I never did the Lyot test either.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6215861 - 11/25/13 06:51 PM

Why the side by side test ? It take no more time to have a smooth surface . What you gain in the test?? Think the smooth mirror would ever be topped by the rough one? Your test would show Maybe?? You could not tell the difference . If it takes little more time to make smooth and it stays that way for ever

Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: wh48gs]
      #6215877 - 11/25/13 06:59 PM

Quote:

So the absence of the factual aspect here is not the cause of futile arguing - it is the choice.



Very well put, Vla. And it looks like it's going to continue in that direction.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Zambuto Criteria #4 on Strehl new [Re: kfrederick]
      #6215881 - 11/25/13 07:00 PM

You're not thinking inside the box, Kevin.

Quote:

What you gain in the test??




For those who think it doesn't matter, it would be an eye opener. As I said, I've already seen the difference refiguring makes, primarily by removing roughness. That's the essential step in improving rough mirrors, at least for others who do that kind of thing commercially. The overall correction I can easily gauge by eye in the field. And by running an A-B blind test over a range of volunteers you "gain" the data to write it up for publication, and take it out of the realm of speculation.

You say both "no more time" and also "little more time" to make smooth surfaces, and basically I agree with either. Why anybody doesn't go to that small amount of trouble escapes me, especially when simple tests easily applied are more than capable of monitoring surface smoothness.

Best,
Mark


Post Extras: Print Post   Remind Me!   Notify Moderator  
wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Zambuto Criteria #4 on Strehl new [Re: Mark Harry]
      #6215919 - 11/25/13 07:20 PM

I am not questioning him, Mark. I disagree with him. And it has nothing to do with how he's got where he is now.

Vla


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6215940 - 11/25/13 07:28 PM

Quote:

I've done plenty of high-res Foucault imaging, and I know that all mirrors show some surface detail once you get to a fine enough level. I also have seen direct comparisons between high-res Foucault imaging and Lyot testing (this was done by Dale Eason some years ago and posted on the ATM List which is now back online) and it confirms that though nothing escapes the Lyot test, the same nothing generally doesn't escape high-res Foucault...




Herbert Highstone demonstrates this, otherwise described in his article on Yahoo's Interferometry Forum.

Clearly, pretty much everything that you see on the Lyot test is seen on the hi-res Foucault imaging. The Lyot test has higher contrast which is favored by the eye. James E. Harvey established that, as I mentioned earlier.

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: mark cowan]
      #6215953 - 11/25/13 07:31 PM

Quote:

And by running an A-B blind test over a range of volunteers you "gain" the data to write it up for publication, and take it out of the realm of speculation.



And how do you control for the visual acuity of the volunteers? What makes you so sure they all see the same thing?

regards,
Mladen


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Zambuto Criteria #4 on Strehl new [Re: MKV]
      #6216041 - 11/25/13 08:34 PM

Well, the difference between rough with a good figure and smooth with a good figure is evident in the star test in good seeing.
But, obviously, the question is whether everything that is visible in the star test, or Lyot, or Foucault, or IF, is visible visually in an in-focus image.

And what would we expect that difference to look like? Would it be in planetary details? Or contrast in the eyepiece? Or sharpness of star images?

I've had sequential ownership of two good 12.5" mirrors, and they differed primarily in roughness, but also in P-V error on the surface.
I could talk about the differences I've seen, but, to be honest, the differences could be due to many factors, so I'm not prepared to say I've seen the results of smoothness as opposed to simply a better figure, coating, cooling, or even the scope used with each mirror. I have, however, seen the differences in the star test and those differences are, according to Suiter, related to smoothness of the figure.

The crux of the matter is whether it is worth it to pay extra for the smoother surface. In my case (a Zambuto mirror), the answer is that I didn't pay extra at all. Other maker's mirrors were either slightly cheaper, the same price, or even more. So if I was going to pay for a premium mirror, why not get a smoother mirror?
There is a great sameness (not a wide range) among prices for premium mirrors at my 12.5" aperture. So is there a debit to having the smoother mirror? Of course not.
We are only talking about the difference between commercial Chinese mirrors and premium mirrors from other companies (and countries) and perhaps among the Premium non-Chinese mirrors and, indirectly, discussing whether it is worth it to pay the extra for a Premium mirror.

Because, once you are paying a premium price, the difference of +/- a couple hundred dollars isn't the primary issue. Why not get a smoother mirror, even if only one night in a thousand is good enough to reveal it (and I think it's more frequent than that)?


Post Extras: Print Post   Remind Me!   Notify Moderator  
MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: Zambuto Criteria #4 on Strehl new [Re: Starman1]
      #6216080 - 11/25/13 08:49 PM

What do you consider "smooth"? Is it anything below 30 Angstroms? What is smooth enough? And how do you determine that?

Mike Jones called this discussion non-facts based "futile arguing". I agree. I would like to see facts-based data.

regards,
Mladen

Edited by MKV (11/25/13 10:04 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
****