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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?


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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


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wh48gs
Carpal Tunnel
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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


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kfrederick
Carpal Tunnel
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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 .

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Ed Jones
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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.


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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


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MKV
Carpal Tunnel
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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


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MKV
Carpal Tunnel
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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?


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MKV
Carpal Tunnel
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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.


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greenglass
scholastic sledgehammer
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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?

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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?


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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


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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?

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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...



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Ed Jones
Carpal Tunnel
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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.


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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...


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MKV
Carpal Tunnel
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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


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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.


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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.


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Pinbout
Postmaster
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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.


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