Subject: [interferometry] Digest Number 1294
From: interferometry@yahoogroups.com
Date: 3/2/2009, 8:41 AM
To: interferometry@yahoogroups.com

interferometry
interferometry

Messages In This Digest (6 Messages)

1a.
Re: Bath interferometer and 1M f:3.6 From: Tony Gondola
1b.
Re: Bath interferometer and 1M f:3.6 From: Stephen Koehler
1c.
Re: Bath interferometer and 1M f:3.6 From: Tony Gondola
1d.
Re: Bath interferometer and 1M f:3.6 From: steve_follett
1e.
Re: Bath interferometer and 1M f:3.6 From: Vladimir Galogaza
1f.
Re: Bath interferometer and 1M f:3.6 From: Michael Koch
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Messages

1a.

Re: Bath interferometer and 1M f:3.6

Posted by: "Tony Gondola" acgna@comcast.net   sailguy28

Sun Mar 1, 2009 9:04 am (PST)

I've used it down to 12.5" F/3.6 with no issues. I don't think you'll have a problem. I believe the limiting issue is the ability of the lens to spread the beam enough to cover the surface under test.

Tony

----- Original Message -----
From: steve_follett
To: interferometry@yahoogroups.com
Sent: Saturday, February 28, 2009 2:23 PM
Subject: [interferometry] Bath interferometer and 1M f:3.6

I am presently working on a 1 meter F:3.6 mirror. I was
wondering if a bath interferometer would work for a mirror
this fast and this big.

Thanks,
Steve

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1b.

Re: Bath interferometer and 1M f:3.6

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

Sun Mar 1, 2009 10:52 am (PST)

Tony,

> I've used it down to 12.5" F/3.6 with no issues. I don't think you'll have a
> problem. I believe the limiting issue is the ability of the lens to spread
> the beam enough to cover the surface under test.

As you leave the focal ratio fixed, but increase the diameter, you
wind up with a situation where you get more and more curved fringes.
I think this is a much more difficult problem to solve than just
getting the beam to cover the entirety of an optic with short focal
ratio. I haven't done it yet, but you could come up with a graph that
showed the limits for fringe tracing, FT analysis without vortex, and
Vortex. 1M f/3.6 is doable only with Vortex (I think), because you
can't get away without closed fringes.

--
Steve Koehler

1c.

Re: Bath interferometer and 1M f:3.6

Posted by: "Tony Gondola" acgna@comcast.net   sailguy28

Sun Mar 1, 2009 11:29 am (PST)

That's interesting Steve as it does go along with what I've seen in the limited number of igrams I've done so far, I just hadn't connected the dots. A set of charts showing the analysis limits would be very useful indeed.

Tony

----- Original Message -----
From: Stephen Koehler
To: interferometry@yahoogroups.com
Sent: Sunday, March 01, 2009 10:52 AM
Subject: Re: [interferometry] Bath interferometer and 1M f:3.6

Tony,

> I've used it down to 12.5" F/3.6 with no issues. I don't think you'll have a
> problem. I believe the limiting issue is the ability of the lens to spread
> the beam enough to cover the surface under test.

As you leave the focal ratio fixed, but increase the diameter, you
wind up with a situation where you get more and more curved fringes.
I think this is a much more difficult problem to solve than just
getting the beam to cover the entirety of an optic with short focal
ratio. I haven't done it yet, but you could come up with a graph that
showed the limits for fringe tracing, FT analysis without vortex, and
Vortex. 1M f/3.6 is doable only with Vortex (I think), because you
can't get away without closed fringes.

--
Steve Koehler

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1d.

Re: Bath interferometer and 1M f:3.6

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

Sun Mar 1, 2009 3:01 pm (PST)

--- In interferometry@yahoogroups.com, Stephen Koehler
<s.c.koehler@...> wrote:
>
> Tony,
>
> > I've used it down to 12.5" F/3.6 with no issues. I don't think
you'll have a
> > problem. I believe the limiting issue is the ability of the lens
to spread
> > the beam enough to cover the surface under test.
>
> As you leave the focal ratio fixed, but increase the diameter, you
> wind up with a situation where you get more and more curved fringes.
> I think this is a much more difficult problem to solve than just
> getting the beam to cover the entirety of an optic with short focal
> ratio. I haven't done it yet, but you could come up with a graph that
> showed the limits for fringe tracing, FT analysis without vortex, and
> Vortex. 1M f/3.6 is doable only with Vortex (I think), because you
> can't get away without closed fringes.
>
> --
> Steve Koehler
>

Hi everyone,

I've got a couple of questions:

I had assumed that the test got more difficult because of the absolute
deviation from a sphere (tens of waves in this case) rather than the
f-ratio per se. Is a 1-meter F:3.6 harder than a .5 meter F:3.6?

Is there an kind of interferometer that would be more suited my
testing case?

Is there a good book or introduction to interferometery I should read
to get a working understanding of this process?

Sorry about all these questions. I'm really interested in this
process because I would like to reach a quarter-wave wavefront on this
mirror. All the other tests I am familiar with know about break down
before this size/speed combination and they generally don't detect
astigmatism.

Thanks again for your help.

Steve Follett

1e.

Re: Bath interferometer and 1M f:3.6

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

Mon Mar 2, 2009 12:40 am (PST)

Steve,

>Is a 1-meter F:3.6 harder than a .5 meter F:3.6?

Steve will answer the question, here is my exercise in writing.
It depends on used data reduction method. As Stephen already explained,
with increasing mirror diameter instrumental aberration increases as well.
Increased instrumental aberration is for Bath interferometer used from ROC
inseparable from real one on glass. It is therefore subtracted numerically
from measurement (henceforth its name "numerical nulling"). What has to be
subtracted is calculated from the formula for Instrumental spherical
aberration (ISA):

ISA=(cc*D^4) / ((384*ROC^3*Lambda) (cc is conic constant)

Therefore diameter of the mirror is not a problem per se. It becomes a
problem
for particular method for analysis of measurement data (interferogram).
Some analysis methods have restrictions on the form of the interferogram.
Those restrictions are number of fringes and if they are closed, open very
curved
and so forth. Since mirror diameter enters the picture on fourth potention
rapid rise in ISA is dictating that resulting fringes are numerous and
strongly
curved or even closed. While Stephen mentioned vortex method and its
limitations,
there is another available measurement analysis method which does not object
against
closed fringes. This is Phase Shift Interferometry (PSI) but unfortunately
nobody on this list tried it with big fast mirrors. I would appreciate
comments
on applicability and limitations of PSI to big fast mirrors, at least from
theoretical standpoint.

>Is there an kind of interferometer that would be more
>suited my testing case?

PSI-Bath interferometer is very similar in construction and used components
to conventional Bath . So PSI could be solution for crossing constraints
the big fast mirrors pose to vortex method and classic Bath interferometer.
At least I hope so, until my dreams are dispersed by some physical
law I am not aware of.
PSI data reduction method is written and made available, for free, by
Michael Peck
http://home.earthlink.net/~mlpeck54/astro/astro.html.
Almost nothing prevents you to try it and report how it works on 1m,f/3.6
monster.
It will be great for ATM interferometry.

Regards
Vladimir.

1f.

Re: Bath interferometer and 1M f:3.6

Posted by: "Michael Koch" astroelectronic@t-online.de   astroelectronic

Mon Mar 2, 2009 1:08 am (PST)

Steve,

> I had assumed that the test got more difficult because of the
absolute deviation from a sphere (tens of waves in this case) rather
than the f-ratio per se. Is a 1-meter F:3.6 harder than a .5 meter F:3.6?

Yes, the 1-meter interferogram will have the double fringe density.

> Is there an kind of interferometer that would be more suited my
> testing case?

Yes, but it's extremely expensive. I think the best test is a Fizeau
interferometer. It's a null test exactly on the optical axis, but only
for a ring-shaped zone. By moving the reference sphere along the
optical axis this zone can be moved in or out. Several interferograms
of different zones can later be stitched together by software. Look
for "Verifire Asphere" on the Zygo Website. US patents 6781700,
6879402 and 6972849. The mathematics are described in the paper
"Interferometric Measurement of Rotationally Symmetric Aspheric
Surfaces" by Michael F. Kuechel. I spent a few days with this
mathematical derivation until I fully understood how it works. This
paper is available on the Zygo website.

> Is there a good book or introduction to interferometery I should
read to get a working understanding of this process?

-- Daniel Malacara, Optical Shop Testing
-- Malacara + Servin, Interferogram Analysis for Optical Testing
-- My website is neither a good book nor an introduction. It's more a
collection of ideas, formulas and calculations. May be you find it
helpful: http://www.astro-electronic.de/faq3.htm

Michael

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