Subject: [interferometry] Digest Number 1649[1 Attachment]
From: interferometry@yahoogroups.com
Date: 12/9/2010, 3:24 AM
To: interferometry@yahoogroups.com

interferometry
interferometry

Messages In This Digest (10 Messages)

1a.
Re: Recent Openfringe releases From: Andrew Aurigema
1b.
Re: Recent Openfringe releases From: atmpob
2a.
Re: culing radius From: Vladimir Galogaza
3a.
beam alignment From: Stephen Koehler
3b.
Re: beam alignment From: Vladimir Galogaza
3c.
Re: beam alignment From: Stephen Koehler
3d.
Re: beam alignment From: atmpob
3e.
Bath Astig and OpenFringe From: atmpob
4a.
Re: Beam seperation Bath astig and OpenFringe From: Michael Peck
4b.
Re: Beam seperation Bath astig and OpenFringe From: atmpob
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Messages

1a.

Re: Recent Openfringe releases

Posted by: "Andrew Aurigema" eosraptor@gmail.com   eosraptor1

Wed Dec 8, 2010 5:05 am (PST)

[Attachment(s) from Andrew Aurigema included below]

Dale,

I got some help offline and it turned out that I was not setting all the
mirror parameters correctly before I was asking the software to process the
fringes. With the correct mirror parameters the fringes were accepted and a
report was created.

I have not figured out how to make the 3-D wire grid into a solid looking
surface yet but I will keep looking.

Under Simulations and Graphs I found what I think is a Ronchi and knife edge
test simulation. No matter what I set as initial conditions the output is a
half white half black circle. I was trying to get the output to look like a
ronchi test. On an 18" f/4 parabolic mirror, I have set the lpi to 100 and
the grid position to (-) 0.25". I was expecting to see 16 kind of curved
vertical lines but it is still just a half white half black circle. Is
there a setting I need to select to turn on the ronchi grid ???

thanks for help

Andrew in record cold FLA ( must be global warming )

On Tue, Dec 7, 2010 at 3:44 PM, atmpob <atmpob@yahoo.com> wrote:

>
>
> I'm not sure what you mean by grey circle screen or by "do the fringes".
>
> Maybe you can capture the screen image and post it or email it to me. I
> have not tried doing a fringe tracing so when I get home maybe that is all I
> need to try and it will happen for me too.
>
> Dale Eason
>
>

Attachment(s) from Andrew Aurigema

1 of 1 Photo(s)

1b.

Re: Recent Openfringe releases

Posted by: "atmpob" atmpob@yahoo.com   atmpob

Wed Dec 8, 2010 11:27 am (PST)



I think I show how to do that on the intro video at youtube. Did you see that?

Dale Eason

--- In interferometry@yahoogroups.com, Andrew Aurigema <eosraptor@...> wrote:
>
> Dale,
>
>
> I have not figured out how to make the 3-D wire grid into a solid looking
> surface yet but I will keep looking.
>

2a.

Re: culing radius

Posted by: "Vladimir Galogaza" vladimir.galogaza1@zg.t-com.hr   vgalogaza

Wed Dec 8, 2010 6:28 am (PST)



Stephen,

>We had a mystery a while ago where you were getting much more astigmatism
>than could be explained by your beam separation. It turned out that you were
>not aligning the beams to be parallel or, alternatively, to hit the same spot on
>the mirror. Once you aligned the beams the problem went away.

This would mean that only parallel or centered beams provide correct
astigmatism prediction (ast. from beam separation).
I am not sure that this was conclusion, but admittedly I do not remember well.
I doubt that such conclusion could be right one.

Regards
Vladimir.

3a.

beam alignment

Posted by: "Stephen Koehler" s.c.koehler@gmail.com   steve_koehler

Wed Dec 8, 2010 7:31 am (PST)



Vladimir,

On Wed, Dec 8, 2010 at 8:28 AM, Vladimir Galogaza <
vladimir.galogaza1@zg.t-com.hr> wrote:

>
>
> Stephen,
>
> >We had a mystery a while ago where you were getting much more astigmatism
>
> >than could be explained by your beam separation. It turned out that you
> were
> >not aligning the beams to be parallel or, alternatively, to hit the same
> spot on
> >the mirror. Once you aligned the beams the problem went away.
>
> This would mean that only parallel or centered beams provide correct
> astigmatism prediction (ast. from beam separation).
> I am not sure that this was conclusion, but admittedly I do not remember
> well.
> I doubt that such conclusion could be right one.
>

Are you saying that beam alignment is not an important Bath setup?

Our conclusion from the previous discussion was that it was critical.
Non-aligned beams led to a significant amount of extra astigmatism that
could not be explained by the beam separation.

Here are some snippets from an e-mail exchange from January 8 last year.

you said:

*It seems that Stephen's observation about correct beam distance to be used
in formula,*
* is sufficient for quite accurate numerical astigmatism correction,
providing that other*
* sources of instrumental astigmatism are not introduced (like non parallel,
widely separated*
* beams).*

I responded:

*It's nice to see that the results are finally matching theory.*

*So, all that large extra astigmatism was due to not starting with parallel*
*beams, right? That's a good lesson to learn.*

--
Steve Koehler
3b.

Re: beam alignment

Posted by: "Vladimir Galogaza" vladimir.galogaza1@zg.t-com.hr   vgalogaza

Wed Dec 8, 2010 9:01 am (PST)



Stephen,

>Are you saying that beam alignment is not an important Bath setup?

I am not that courageous. Frankly I do not now. There was
indication how thay should be aligned but not why. I always understood
alignment procedure in Wiki as advice how to get fringes. In my experience
fringes can be obtained with reference spot anywhere on the mirror under
test. If particular alignment (parallel or centric) is mandatory, there is no
explanation for it. specifically what negative consequences misalignment can
bring.

>It seems that Stephen's observation about correct beam distance to be
>used in formula, is sufficient for quite accurate numerical astigmatism correction,
>providing that other sources of instrumental astigmatism are not introduced
>(like non parallel, widely separated beams).

At that time we were primarily concerned about your doubts about "correct beam distance".
according to Wiki, beam distance is measured at the splitter output. David's derivations of beam
distance astigmatism was based on lateral distance of the diverging lens focus
and image of this focus made by mirror under test. This is not the same as
the beam distance usually measured at the splitter output, and depends on
the beams alignment. Our experiment included measurement of the true beam distance.
I think that what was found is that with parallel beams the true and usually
measured distances are close enough while with non parallel widely separated
beams this is not so. This does not mean that such beams introduce
more or less astigmatism but that calculation is based on wrongly measured
distances and therefore fails (does not match theory).
This is as much as I can remember. In case that If this is so, than

>So, all that large extra astigmatism was due to not starting with parallel
>beams, right? That's a good lesson to learn.

Is misleading because it will be correct to say that extra astigmatism
was due because of using non parallel beams without using correctly
measured beam distance.

By the way somebody should include in Wiki all reasons why parallel beams
are recommended.

It will be nice if somebody can check it in order to have second
independent experimental proof of concept.

Regards
Vladimir.

3c.

Re: beam alignment

Posted by: "Stephen Koehler" s.c.koehler@gmail.com   steve_koehler

Wed Dec 8, 2010 9:27 am (PST)



Vladimir,

I now understand how we interpreted these results differently.

You were thinking that the unaligned beams made the measurement of beam
separation inaccurate.

I was thinking that the unaligned beams caused an additional astigmatism
unrelated to the beam separation astigmatism.

I think my interpretation is the correct one. The derivation of the beam
separation astigmatism has to do with the lateral separation of the foci
near the cube and lens. Moderate misalignment won't throw off the
measurement of beam separation all that much. I think the astig is coming
from the added shear from misalignment.

I suppose this could be tested by leaving the lens and diagonal mirror in
the same locations (fix the beam separation), then deliberately misalign the
beams by varying amounts. This would require the lens to be moved and
replaced multiple times, so it would be good to use a stop to make sure it
goes back in the same place each time. You could measure the beam
separation the normal way for each misalignment, and see how much that
changes due to the misalignment. Then, see how astig changes, and whether
that can be explained by mismeasurement of the beam separation.

--
Steve Koehler
3d.

Re: beam alignment

Posted by: "atmpob" atmpob@yahoo.com   atmpob

Wed Dec 8, 2010 12:18 pm (PST)



I did part of that experiment by accident years ago. I forgot to align the beams "somewhat parallel" and did not check them. They diverged by more than the width of the mirror at the mirror. Of course when I tried to setup the interferometer I could not get fringes because the expanding beam was not hitting the mirror at all. As a quick fix I moved the small diverging lens until the mirror was illuminated by that beam. I got large amounts of astig in the analysis. Then I figured out why and readjusted the beams to be "somewhat parallel". Astig was gone. I think I reported about that before.

Dale Eason

--- In interferometry@yahoogroups.com, Stephen Koehler <s.c.koehler@...> wrote:
>
> Vladimir,
>
> I now understand how we interpreted these results differently.
>
> You were thinking that the unaligned beams made the measurement of beam
> separation inaccurate.
>
> I was thinking that the unaligned beams caused an additional astigmatism
> unrelated to the beam separation astigmatism.
>
> I think my interpretation is the correct one. The derivation of the beam
> separation astigmatism has to do with the lateral separation of the foci
> near the cube and lens. Moderate misalignment won't throw off the
> measurement of beam separation all that much. I think the astig is coming
> from the added shear from misalignment.
>
> I suppose this could be tested by leaving the lens and diagonal mirror in
> the same locations (fix the beam separation), then deliberately misalign the
> beams by varying amounts. This would require the lens to be moved and
> replaced multiple times, so it would be good to use a stop to make sure it
> goes back in the same place each time. You could measure the beam
> separation the normal way for each misalignment, and see how much that
> changes due to the misalignment. Then, see how astig changes, and whether
> that can be explained by mismeasurement of the beam separation.
>
> --
> Steve Koehler
>

3e.

Bath Astig and OpenFringe

Posted by: "atmpob" atmpob@yahoo.com   atmpob

Wed Dec 8, 2010 4:46 pm (PST)



Vladimir,

Send me an Igram that you believe has your Bath astig present and the parameters of the mirror and the Bath beam seperation. I want to try out the removal feature.

It would be nice if that is the only astig but that is probably not possible.

Dale Eason

4a.

Re: Beam seperation Bath astig and OpenFringe

Posted by: "Michael Peck" mpeck1@ix.netcom.com   mikepeck5440

Wed Dec 8, 2010 4:50 pm (PST)



At 10:36 PM 12/7/2010, atmpob wrote:

>I will add the ability to remove the Bath astig. What I need to know
>is how to use the value that Dave Rowe calculated in the Wiki for Z4.
>
>In it he states to be careful and use the correct sign. Should it
>be positive.
>
>Second what is the scaling on his value. Should I use it as is or
>multiply it by the sqrt of 6.
>

Just in case Steve didn't answer privately, if you're looking at the
section titled "Aberrations in the Bath Interferometer" it's the
value given in equation 9 for the Zernike polynomial scaling you use
(the coefficient is half the OPD for astigmatism). The sign should be positive.

Assuming the X axis of the image is parallel to the axis defined by
the separation of the beams it's Z4 that is affected.

Mike P.

------
Michael Peck
mpeck1@ix.netcom.com
http://wildlife-pix.com

4b.

Re: Beam seperation Bath astig and OpenFringe

Posted by: "atmpob" atmpob@yahoo.com   atmpob

Wed Dec 8, 2010 7:45 pm (PST)



Thanks Mike,

Well now as I try to implement this I come upon the problems that probably caused me to resist it in the first place.

It gets very complicated. If anyone is interested here are the issues.

If OpenFringe only delt with zernike based wavefronts it would be easy. Just remove the Bath astig part from the calculated Z4.

But....
Surfaces can be derived from two methods, FFT and Fringe Tracing.
It has to support FFT analysis that are not based on Zernike values but produces a set of Zernike values that the zernike based routines think came from fringe analysis. That way the user can display the surface data either as a FFT wavefront or Zernike Based wavefront.

Here is the problem. The FFT produced wavefront needs to have the Bath astig removed from it and that is not hard. However the zernike values it then passes on to the Zernike based routines will now not have that Bath astig in it. Those routines do not know that and will subtract the Bath astig once more. Thus adding a negative version of the Bath astig into the Zernike based surface.

For various reasons there a many places in the code that convert wavefronts into zernike values. The code becomes way too complex to try to untangle it all.

The other way out is to subtract it only from the FFT based wavefronts. That means the people who do fringe tracing will never get that option.

The Bath astig only becomes objectionable for small fast mirrors. Most ATM's are going bigger and not smaller. It is an obscure option for the most part and I'm not sure that it is worth the trouble. Those that want it try and convince me.

Dale Eason

--- In interferometry@yahoogroups.com, Michael Peck <mpeck1@...> wrote:
>
> At 10:36 PM 12/7/2010, atmpob wrote:
>
> >I will add the ability to remove the Bath astig. What I need to know
> >is how to use the value that Dave Rowe calculated in the Wiki for Z4.
> >
> >In it he states to be careful and use the correct sign. Should it
> >be positive.
> >
> >Second what is the scaling on his value. Should I use it as is or
> >multiply it by the sqrt of 6.
> >
>
> Just in case Steve didn't answer privately, if you're looking at the
> section titled "Aberrations in the Bath Interferometer" it's the
> value given in equation 9 for the Zernike polynomial scaling you use
> (the coefficient is half the OPD for astigmatism). The sign should be positive.
>
> Assuming the X axis of the image is parallel to the axis defined by
> the separation of the beams it's Z4 that is affected.
>
> Mike P.
>
>
> ------
> Michael Peck
> mpeck1@...
> http://wildlife-pix.com
>

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