Subject: [interferometry] Digest Number 1913[4 Attachments]
From: interferometry@yahoogroups.com
Date: 1/27/2012, 2:19 AM
To: interferometry@yahoogroups.com

interferometry
interferometry

Messages In This Digest (15 Messages)

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1.1.

Re: Updates in Your Groups, January 22, 2012 [1 Attachment]

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

Thu Jan 26, 2012 4:37 am (PST)

[Attachment(s) from Andrew Aurigema included below]

>
>
> What is actually meant by statements like this:
>
> >Is the take away from this that we should be slightly "outside" of ROC
> when making our fringe patterns for FFT analysis ???
>
> What precisely is outside or inside the ROC?
>
> Regards
> Vladimir.
>

I meant to ask if the test rig should be moved so that fringes are made
just before or just after ROC positioning. I am defining ROC positioning
as when the Z-axis is adjusted so that the bullseye "explodes" into chaos
in the center of the ingram. If I move the rig inward ( inside ROC ) the
bulls-eye reforms and moves off off center as I move the rig left or
right. If I move the rig outward ( outside ROC ) the bulls-eye reforms and
moves off center as I move the rig left or right.

I was just asking if we should be just inside of the Z-axis location that
creates the widest fringes or just outside of that location. I have tried
to image ingrams right at ROC but the information is chaotic so I find
myself on one side or the other. Is there a "best" direction to move the
stage ???

Of course I could be totally wrong about all this. I really do not
understand how any of it works or what parameters should be optimized.

Drew

>
>
>
>

Attachment(s) from Andrew Aurigema

1 of 1 Photo(s)

1.2.

Re: Updates in Your Groups, January 22, 2012 [1 Attachment]

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

Thu Jan 26, 2012 5:55 am (PST)

[Attachment(s) from vladimir galogaza included below]

Andrew,
Thank you for quick response.
Now I understand what means "inside ROC" or "outside of ROC".
As I supposed that is just positive or negative defocus, and zero
defocus, in my opinion, has nothing to do with being at ROC,
This was probably carried from Foucault KE or Ronchi test.
In Bath, what you have described as signature for being behind or
in front of ROC is actually position of test beam final focus in front
or behind final focus of the reference beam.

>I have tried to image ingrams right at ROC

If "right at ROC" equals to defocus = 0, then I think they will be parallel
lines if no other aberrations are present, their number defined by amount of tilt.
Because there is always instrumental aberration from beams separation (astigmatism),
and from measurement from ROC (SA), lines will not be straight.
In original article on Bath interferometer, Karl_Ludwig Bath inserted lot of
interferograms with various shapes, explaining which aberration is responsible
for particular shape. Today, in luxury of computers and OpenFringe, all is served on
plate in form of Zernike coefficients.

Here is simulated image "right at ROC" of the ideal paraboloid, with Bath astigmatism=0.009 and Bath SA=-065 content,
( with tilt = 10, defocus = 0, in orthonormal scaling).

Regards
Vladimir

Attachment(s) from vladimir galogaza

1 of 1 Photo(s)

1.3.

Igrams right at ROC

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

Thu Jan 26, 2012 8:18 am (PST)



Drew,

With a perfect spherical mirror and setup when you have the interferometer as close to the ROC as possible and no tilt you will get just one fringe that covers the whole mirror. That fringe will be very broad and smooth. OpenFringe will not be able to analyze it because it dose not have enough tilt.

On a very bumpy and imperfect mirror you will get very swirly fringes like you are seeing with your mirror. On your mirrors the more fringes you can get the more regular they will look since the mirrors error becomes visually smaller because the fringes are getting more narrow.

Fringes represent the error by moving sideways. The distance between two fringes in your setup is one wave of error, or tilt, or difference from a sphere. As we add more tilt the fringes move closer together because the reference beam and test beam are more tilted to one another. We can still use the fringes as a ruler however it is like we are looking at the ruler from further away and the markings are smaller and closer together. They still represent one wave of error. However on our detector they are less pixels wide and so visually the 1 wave difference looks smaller and for your mirror less swirly.

Dale Eason

2a.

optimal distance from ROC

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

Thu Jan 26, 2012 5:05 am (PST)

[Attachment(s) from vladimir galogaza included below]

There are some interesting things mentioned in the resent mails about
optimization of the distance from the ROC

> It usually means more contrast with a definite boundary of ring structure
> > instead of just a bright blob.
and
> > Steve Koehler also wondered if there was an ideal offset from ROC to use.
> > He did a study and found that the ideal offset was n * SA outside the ROC.
> > I forget what n was but I think it was somewhere around 3.

>I suspect that the half-plane filter would still have some of the edge effect of
>unbalanced SA and defocus,

I could not find relevant postings and will appreciate any information about its location.
I also do not know what is "unbalanced SA and defocus"? Or what is "ideal offset from
ROC", ideal for what? It is probably explained in postings I am missing.

In the meantime I have produced some lobes from simulated interferograms
to clarify what are consequences on the lobe shapes, blurring or else of various
defocus and SA contents.

If "inside" or "outside of ROC" is used to describe positive or negative defocus, simulation shows that there is a little
influence of defocus = 2 or -2 case on lobe shape or blur. If "unbalanced SA" means that SA is not zero it has pronounced effect on
de-blurring of lobes ( taking stem (power) out of them?).

Regards
Vladimir.

Attachment(s) from vladimir galogaza

1 of 1 Photo(s)

2b.

Re: optimal distance from ROC [1 Attachment]

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

Thu Jan 26, 2012 8:13 am (PST)



At 07:05 AM 1/26/2012, vladimir galogaza wrote:
>[
>I could not find relevant postings and will appreciate any
>information about its location.
>I also do not know what is "unbalanced SA and defocus"? Or what is
>"ideal offset from
>ROC", ideal for what? It is probably explained in postings I am missing.

Vladimir:

Steve posted a link to a long ago post yesterday. Here it is again:
<http://tech.groups.yahoo.com/group/interferometry/message/2319>

The claim was that a specific ratio of defocus to SA produces
sidelobes that are optimally "sharp" in some sense. That ratio in the
scaling convention I use is -3*sqrt(5/3). So for example with an SA
coefficient of -.65 try adding defocus of +3*sqrt(5/3)*0.65 to your
synthetic interferograms and see what happens.

I just tried it and it does improve results a little bit over a
defocus of 0 using either "traditional" FFT analysis or vortex.

Of course since you know how to do phase shifting interferometry you
should ignore this entire line of thought. What you really want to do
is minimize the potential for instrumental errors, and for that I
think a defocus of 0 should be optimal.

Mike P.

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

3a.

Re: Expensive Digital DSLR camera needed?

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

Thu Jan 26, 2012 5:07 am (PST)



I agree with you Dale.

I steeped up to the canon rebel XS body and EF 85mm lens ( with 0.45x
telecompressor ) for imaging my mirrors. it is a great combo. I ( think )
I am taking pretty good ingrams of the 18" f/2.5 mirrors. I have to
compress the file size after taking it off the camera so I can get more
than 2 ingrams in OF to open, but I don't see the weird edge mess problem I
had before.

The canon was about a grand in all with the lens and compressor and battery
replacement module but it totally solved the problem of getting a good
image off the BATH

ps...... the zoom lenses dont seem to work as well as the fixed lens. I
borrowed a 100mm macro and it was really really sweet but a grand by
itself.

Drew in sunny FL

----------------------------------------------------------

On Thu, Jan 26, 2012 at 12:04 AM, atmpob <atmpob@yahoo.com> wrote:

> **
>
>
> I meant to say
> What works well is a camera that has a large diameter lens like that of
> the 35mm cameras and can zoom from 100 to 200 mm in focus.
>
>
> --- In interferometry@yahoogroups.com, "atmpob" <atmpob@...> wrote:
> >
> > Expensive is relative.
> >
> > What works well is a camera that has a large diameter lens that of the
> 35mm cameras and can zoom from 100 to 200 mm in focus. Older Nikon DSLR
> like my Nikon D40 8 megapixel work well since they can use the older 35 mm
> film camera lenses.
> >
> > Others have had good success with the Cannon digital rebels and their
> stock zoom lens.
> >
> > The 32" size probably will not be possible at F2.5. 12" at f2.5 will be
> ok.
> >
> > Dale Eason
> >
> > --- In interferometry@yahoogroups.com, "greg wilhite" <gregandjenni@>
> wrote:
> > >
> > > Hi All,
> > > My 1st post to the Group..
> > >
> > > I'm building a Bath tester and I am looking into camera's for it.
> > > I just started learning about this method of testing, so I know and
> understand very little... Please keep that in mind... :O)
> > >
> > > I will be testing mirror from 12" to 32" in F/Ratio's of between F2.5
> to F 3.5.
> > >
> > > I am of course looking to keep cost down but will buy whatever I
> need...
> > >
> > > So ... Do I need an expensive Digital DSLR camera?
> > >
> > > Thanks
> > > GregW
> > >
> >
>
>
>
4a.

Re: Example of side lobes at different positions relative to ROC

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

Thu Jan 26, 2012 6:41 am (PST)



At 11:11 PM 1/25/2012, atmpob wrote:

>http://groups.yahoo.com/group/interferometry/photos/album/442489808/pic/2020475972/view
>
>http://groups.yahoo.com/group/interferometry/photos/album/442489808/pic/830698386/view
>
>There are in the photo section under Dale Eason
>

Two more fine examples of multiple harmonics of the fundamental
fringe frequency in the 2D power spectrum.

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

4b.

Re: Example of side lobes at different positions relative to ROC

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

Thu Jan 26, 2012 8:00 am (PST)



So, Vladimir do you now understand what I mean by less distinct side lobe when compring those taken from inside ROC compared to outside ROC.

The amount of defocus is directly proportional to where the interferometer is placed in relation to the Mirror's ROC. Most of the time on real mirrors (not simulation) the side lobes have a much easier to see ring structure outside of ROC. Sometimes at a glance I can see mirror deffects just looking at them. I can also tell from them if the illumination fell off too much at the side to get a good analsysis.

I also think I get a less noisy surface analysis when using FT vortex on those outside of focus, but I have not done a detailed study to see if that is true.

5a.

should this be a good ingram

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

Thu Jan 26, 2012 6:50 am (PST)

[Attachment(s) from Andrew Aurigema included below]

I am not understanding why the ingram attached is not yielding a good FFT
analysis. It seems to have a good number of fringes. They are even. Do I
need more fringes ??? Less fringes ??? It seems that I get better results
with less fringes and more spacing between them. But these wider spaced
ingrams are way more swirly looking.

457 mm dia x 2266 mm ROC spherical ( I think this mirror has low zones at
4 & 10 oclock near the outer edge and a high stig ridge down the center
from 1 to 7 oclock)

Drew, confused as usual in sunny Florida

Attachment(s) from Andrew Aurigema

1 of 1 Photo(s)

5b.

Re: should this be a good ingram [1 Attachment]

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

Thu Jan 26, 2012 7:25 am (PST)



Drew,

I am not understanding why the ingram attached is not yielding a good FFT
> analysis. It seems to have a good number of fringes. They are even. Do I
> need more fringes ??? Less fringes ??? It seems that I get better results
> with less fringes and more spacing between them. But these wider spaced
> ingrams are way more swirly looking.
>

The number of fringes is fine, but the quality of the fringes is very
poor. Also, the fringes are not clear at the edge of the optic, so I'm not
sure you are focusing the interferogram correctly. The fringes should stop
cleanly at the edge, and not go beyond the edge.

If you want to see some very good quality fringes of the sort you should
strive for, look in the first few folders in the Photos section of the
group. The first and fourth folders contain examples of green light
interferograms of very high quality.

--
Steve Koehler
5c.

Re: should this be a good ingram [1 Attachment]

Posted by: "Jan van Gastel" jhm.vangastel@ziggo.nl   janvangastel

Thu Jan 26, 2012 7:49 am (PST)



I thnk there's not enough contrast everywhere on the igram, especially not on parts of the edge.

Jan
http://members.ziggo.nl/jhm.vangastel/Astronomy/

----- Original Message -----
From: Andrew Aurigema
To: interferometry@yahoogroups.com
Sent: Thursday, January 26, 2012 15:50
Subject: [interferometry] should this be a good ingram [1 Attachment]

[Attachment(s) from Andrew Aurigema included below]

I am not understanding why the ingram attached is not yielding a good FFT analysis. It seems to have a good number of fringes. They are even. Do I need more fringes ??? Less fringes ??? It seems that I get better results with less fringes and more spacing between them. But these wider spaced ingrams are way more swirly looking.

457 mm dia x 2266 mm ROC spherical ( I think this mirror has low zones at 4 & 10 oclock near the outer edge and a high stig ridge down the center from 1 to 7 oclock)

Drew, confused as usual in sunny Florida

Attachment(s) from Andrew Aurigema

1 of 1 Photo(s)

IMG_0055.JPG

5d.

Re: should this be a good ingram

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

Thu Jan 26, 2012 8:41 am (PST)



Drew,
I think I know what is happening but I don't have time now to explain how to test my theory. I think you are not getting laser to cover the mirror under test. Perhaps it has become unaligned after it fell the other day, or perhaps it never was aligned as best it could or, perhaps that is just too much tilt for the coverage it can have.

I will tell you how to figure that out later.

Dale

--- In interferometry@yahoogroups.com, Andrew Aurigema <eosraptor@...> wrote:
>
> I am not understanding why the ingram attached is not yielding a good FFT
> analysis. It seems to have a good number of fringes. They are even. Do I
> need more fringes ??? Less fringes ??? It seems that I get better results
> with less fringes and more spacing between them. But these wider spaced
> ingrams are way more swirly looking.
>
> 457 mm dia x 2266 mm ROC spherical ( I think this mirror has low zones at
> 4 & 10 oclock near the outer edge and a high stig ridge down the center
> from 1 to 7 oclock)
>
> Drew, confused as usual in sunny Florida
>

5e.

Re: should this be a good ingram

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

Thu Jan 26, 2012 4:33 pm (PST)



Drew,
As promised this morning I now have more time to help you explore the problem.

As I stated earlier perhaps the mirror is not evenly illuminated by the laser when you get a lot of fringes. The other thing may be that your laser needs to be rotated. First try rotating the laser to get the best contrast fringes you can.

Next project the Bath output onto a white card like I did in the You Tube alignment video. Try to get some fringes to appear and then try to get many fringes. Notice where the igram is in relation to the background illumination on the projection. If the igram is close to and over the edge of the background then you probably need to adjust the lens back and forth until the igram is more in the middle or just use less tilt. Large tilts that we use to get many fringes will always put the igram close to the side so try to center it when there are a small number of fringes.

Another way is in a darkened room look to see how the mirror under test is situated in the illumination from the expanding beam. You should be able to see the expanded beam around the perimiter of the mirror under test.

Hope that helps.
Dale Eason

6.1.

Re: Comment on what Mark Whitaker's said of some of his testing

Posted by: "Jeremy Royston" jroyston@gmx.de   jerryrst

Thu Jan 26, 2012 8:30 am (PST)



Dear Vladimir,
You asked about increasing the exposure time in the Hartmann test
to 10sec. This would have the same effect as a 10 sec long film, but
you'd probably have to introduce a filter of suitable density to prevent
over exposure.
Best wishes, Jerry

7.

Example of Ghosts caused by laser mode

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

Thu Jan 26, 2012 8:48 am (PST)



http://groups.yahoo.com/group/interferometry/photos/album/442489808/pic/2051885239/view

That is an example of the extra ghosts side lobes that I usually get if I have the laser turned up into lase mode. The orginial igram probably is also a little over exposed which may add to the problem.

Notice the extra rows of side lobes.

Later I can add the Igram to the photo section if people want.

Dale Eason

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