Re: [ATM] lurie-houghton progress

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Yes, convex on concave! ;-) I thought that from one
side to the other (7.5 inches) you might cut 2 or 3
fringes if you followed a straight line that would cut
as few as possible, which is what I think you mean.

We have constructed a a 5-sided wooden box with 2
Hg-vapor 'black lights' with the orange and purplish
lines filtered out with theatrical lighting gels, so
we just get the 546.1 nanometer green line. There is a
more-or-less 45-degree pane of glass, one side frosted
with 120 grit, the other side reflective, that we can
look at the fringes with.

With the 2 relatively short-focal-length surfaces
where the concave one was very close to spherical and
the convex one seemed to present rather straight lines
(and this was immediately after a very short period of
polishing), there were something like 20 to 30
interference fringe lines visible at the time. What it
will look like later on - first of all when both of
the pieces of glass have reached thermal equilibrium =
and secondly after we have polished enough for a good
surface - is anybody's guess.

I will try to borrow my wife's digital camera and take
some photos, but that won't be today.

With the float/plate glass, all you need is 2 crossed
polarized plates to see if they are free of
inhomogeneities (if I spelt that correkly) (;-)) and
strain and such. I ought to try that some time. In any
case, we used something else - BAK4 - because we got a
really good price on 2 identical blanks. And we
checked to make sure they had no strain. Or at least I
think (hope) we did.

Oh, the question I had was this: Suppose we saw via
Ronchi test and via Foucault knife-edge that surface
#1 had a beautiful spherical concave figure; and then
we put the matching surface #3 on top of that, in the
monochromatic light source, and we were sure that
everything had reached thermal equilibrium. And
suppose that after we pushed a little bit and released
the pressure, we saw lines curving slightly like the
lines of wood in a quarter-sawn tree. (as shown here:
http://www.woodfloorsonline.com/techtalk/properties.html
)
Then what would one conclude, and what action would
one take? Would one conclude that one of the pieces of
glass has astigmatic cylinder?

Thanks.

Guy
--- Mike Lockwood <melockwo@uiuc.edu> wrote:

> Guy,
> 
> Guy Brandenburg wrote:
> > Once the long-FL concave surface is decent, we'll
> have
> > to do some trial and error to figure out what to
> do
> > with the convex surface to get it to match the
> concave
> > surface, as shown by straight interference fringes
> > under the monochromatic light source. Anybody have
> > practical experience with that?
> 
> Yes, I do.  Your test setup is already constructed,
> right?  What 
> questions do you have?
> 
> > Then we put the convex one (**carefully**) on top
> of
> > the convex one in our monochromatic light box, so
> that
> > we could look at the interference fringes. The
> fringes
> > were almost perfectly straight as well! Again, we
> were
> > very, very pleased.
> 
> I assume that's convex one on top of the conCAVE
> one.  ;)
> Can you take a picture of the fringes and post it? 
> I would be very 
> surprised if you had less than a few fringes after
> polishing, since 
> both surfaces generally change curvature while being
> polished, and 
> usually in different directions.  How many fringes
> were you viewing 
> when you saw nearly straight fringes?
> 
> > The glass for a LH doesn't have to be expensive at
> > all. Plate glass would work fine, as long as it
> > doesn't have strain. The only thing is that it's
> not
> > terribly transparent.
> 
> Is plate glass quite homogenous?
> 
> 	Mike Lockwood
> 
> 
> 


Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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