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Re: [ATM] lurie-houghton progress


Guy...
 
When you place two nearly identical but opposite curves together and obtain fringes, you push and prod until you get maybe 5 fringes across the disks if possible. This is done by adjusting the thickness of the "air wedge" between the blanks. (The closer to no wedge the more sensitive and fewer fringes). When you have reached this condition, stretch a string from one edge of the disk to the other starting and ending on the same fringe. Looking along this string, count the number of fringes crossed by the string... count them only once. Each fringe crossed represents a half wave DIFFERENCE in curvature between the two curves on the glass. Your goal is to make the string cross NO FRINGES and to actually stay within 1/10th or 1/20th of staying in line with the reference fringe.
 
You'd have to do this test across at least 2 diameters to test for astimatism and need a known accuracy flat to do so.
 
Hope this helps...
 
Ken Hunter
 

Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:
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 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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