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Re: [ATM] Fwd: [Fwd: [atm_free] Effect of pH on polishing fusedsilicawas Re. Session 24...]
Scott,
Yes I have interferometric results and Foucault images
of most of them.
Mark Cowen has a theory that anything you see in phase
contrast is also visible in good Foucault images. At
one time I thought he was wrong. However I have not
found a counter example. What I have found with my
phase contrast setup is that the features are more
defined and easier to see than in a Foucault image.
However, they are still visible even if faint smudges
or lines in a Foucault image. Perhaps my phase
contrast technique is not as high resolution as the
best.
Fourier analysis (like I do) of interferograms can see
very small details but still not all of the fine
detail that a good Foucault image can see. PSI
interferometry can see even smaller and with less
noise but I don't have the equipment to do that. I
use a Bath interferometer that can be made for less
that $50. I think the parts for PSI are much more
complex and expensive.
Fourier analysis resolution is limited by the number
of pixels per fringe and by the linearity of the
camera detector. Fourier analysis needs to use narrow
fringes but that means there are less pixels per
fringe for a given detector. So there is kind of a
natural competing trade off. The more pixels per
fringe the smaller amplitude details you can see.
Fourier analysis also adds noise to the analysis and
so low amplitude features get lost in the noise.
Bottom line is that narrow low amplitude features like
those seen in the phase contrast images hide from my
interferometric analysis.
I can see large (1/2 inch in area) 1/60 wave features
or small (4 pixels wide) 1/2 wave features.
I recently posted an example of the narrow details I
can see on the Yahoo atm_free group in their files
section under Dale
Eason\Foucault\Foucault_comparison.jpg.
It contains three images that show the real Foucault
image, the interferogram, and a simulated Foucault
image generated from the analysis. It is kind of
amazing to see what can be picked up.
There are also posts about using my interferometer
system to figure a mirror along with presentations of
my recent turned edge and the fact it is not showing
up in my simulated foucault images. I am currently
investigating why that is.
Dale Eason
--- Scott Milligan <starzkey@charter.net> wrote:
> Dale:
>
> Great stuff, Dale; by any chance, have you taken
> interferometric results for
> any of the mirrors that you pointed to? I'm curious
> as to whether the small
> amplitude defects captured by phase-contrast will
> show up in interferometric
> results (prior to analysis by Zernike fitting, of
> course, which could be
> expected to smooth such defects out of existence).
> I would also expect that
> an accurate interferometric analysis would need to
> account for the many
> image artifacts introduced by coherent noise in the
> (typical) interferogram.
> I think that several commercially available
> phase-shifting attachments do
> this via a combination of polarization-control
> optics and phase shifting
> either only the test beam or only the reference
> beam.
>
>
> Scott Milligan
>
>
>
> -----Original Message-----
> From: atm-bounces@atmlist.net
> [mailto:atm-bounces@atmlist.net] On Behalf Of
> Dale Eason
> Sent: Saturday, September 01, 2007 4:46 PM
> To: atm@atmlist.net
> Subject: Re: [ATM] Fwd: [Fwd: [atm_free] Effect of
> pH on polishing
> fusedsilicawas Re. Session 24...]
>
> Scott,
> I have now imaged about 8 mirror in phase contrast.
> While I don't know the absolute values I do have a
> good range of relative
> surface roughness including three mirrors that show
> very little roughness in
> phase contrast. I also have high resolution
> Foucault images of the many of
> them.
>
> Two of the smoothest are Carl Zambuto's and Mike
> Lockwoods's.
>
> I have just imaged a mirror of unknown origin that
> may be the smoothest of
> all. Too bad I know nothing else about it nor does
> the owner.
>
> If there is interest I will provide a gallery in the
> Interferometry Wiki
> where I already have examples.
>
> http://starryridge.com/mediawiki-1.9.1/index.php?
> title=Phase_Contrast
>
> My current phase contrast setup has been simplified
> by the use of a super
> bright LED in place of the projector. My slit is
> the same as I use for
> Foucault imaging. The above site shows how to make
> a phase plate using a
> candle. My camera is a Nikon Digital D40 and
> exposures of 20 seconds are
> taken in a dark room.
>
> Dale Eason
>
> --- Scott Milligan <starzkey@charter.net> wrote:
>
> > Interesting stuff here, guys; thanks for posting
> the link to the
> > Applied Optics paper. Does anyone here have a
> low-cost way to monitor
> > surface micro-roughness in a quasi-quantitative
> way? My own thoughts
> > on this
> > subject:
> >
> > 1. you have a friend that has access to an AFM or
> optical
> > profilometer.
> > 2. you implement some form of the Zernike
> phase-contrast method, and
> > benchmark the patterns against some known
> reference standard.
> > 3. you obtain a short-wave laser (wavelength < 450
> nm), focus the
> > output through a spatial filter to clean up the
> speckle, place the
> > filtered output focal point approximately at the
> focal point of
> > whatever mirror you are interested in, and in a
> dark room, observed
> > the expanded,r eflected bean projected onto a
> white, matte background
> > (white painted wall for instance).
> > Rougher mirrors will show more distinct speckle
> pattern in the far
> > field (try it, it really works if you can find all
> of the pieces).
> >
> >
> >
> > Scott Milligan
> >
> >
> >
> > -----Original Message-----
> > From: atm-bounces@atmlist.net
> > [mailto:atm-bounces@atmlist.net] On Behalf Of Guy
> Brandenburg
> > Sent: Thursday, August 30, 2007 10:04 PM
> > To: atm@atmlist.net
> > Subject: [ATM] Fwd: [Fwd: [atm_free] Effect of pH
> on polishing fused
> > silicawas Re. Session 24...]
> >
> > Interesting comments on pH levels and polishing
> with
> > CeO2
> >
> > Alan Bromborsky <brombo@comcast.net> wrote: Date:
> > Thu, 30 Aug 2007 17:37:14
> > -0400
> > From: Alan Bromborsky <brombo@comcast.net>
> > To: Guy Brandenburg <gfbrandenburg@yahoo.com>
> > Subject: [Fwd: [atm_free] Effect of pH on
> polishing fused silica was
> > Re.
> > Session 24...]
> >
> >
> > To: atm_free@yahoogroups.com
> > From: "Mark Cowan" <toolontop@yahoo.com>
> > Date: Thu, 30 Aug 2007 19:47:54 -0000
> > Subject: [atm_free] Effect of pH on polishing
> fused silica was Re.
> > Session 24...
> >
> > I was looking
> through the paper I cited
> > in the last post and noted the following about
> hydration layers and
> > silicate
> > redeposition:
> >
> > Buffering the CeO2 slurry to pH 4 increased the
> drag greatly, but
> > according to the paper and the assorted
> micrographic and AFM imagery
> > the resulting surface is much smoother - /because/
> (they
> > say) the redeposition
> > from the slurry is greatly reduced. Thus the
> polishing action is
> > surface removal primarily. They also say the lap
> lasts much longer
> > because the "ceria particles are suspended in the
> slurry rather than
> > embedded in the lap." The lack of the redeposited
> layer means that
> > the final surface is made from the pure substrate,
> and so might bond
> > to the coating better, improving coating life.
> >
> > Perhaps you need to start with a fresh lap for
> this experiment. I
> > plan to test some of this with the robotic
> figuring machine, which is
> > much more capable than I am of maintaining
> constant lap speed against
> > high resistance.
> >
> > Best,
> > Mark
> >
> > > Here's an interesting experiment that
> demonstrates that.
> > >
> > > H. Highstone set me a copy of an Applied Optics
> paper from '92 (pp
> > > 7164-7172, Vol 31, no 34) that explored the use
> of various agents >
> > (CeO2, ZrO2, Al2O3, Y2O3, YF3) on fused silica at
> varying pH ratios.
> > > Al2O3 has by far the highest roughness of any
> of these compounds.
> > > Anyway, lowering the pH from 7 to 4 with CeO2
> on fused silica
> > lowers > the roughness by a factor of almost 4x
> under controlled
> > conditions.
> > > You can use citric acid to buffer the pH of a
> slurry
=== message truncated ===
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