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


Extra information
2 registered and 22 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 9743

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics