Re: [ATM] lurie-houghton progress

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Ah, so you just push and prod enough and you can
change the number of fringes. OK, that helps. And the
idea of rotating one of them 90 degrees to check for
astigmatism also helps. Thanks.

Now, we are not trying to make a flat (though I do
have some reference flats). I understand that that
since we are making 2 curved corrector plates for a
Lurie-Houghton, if we are off by 2 or three waves,
then the entire telescope would probably work out OK!
(Not that I don't think we can do better!) One of our
curves is about 6 feet ROC, and the other one is about
20 feet. But here's the question that we really have:
Assume that our concave surfaces are essentially
perfect. Now, what if you put the convex surface on
the 'perfect' concave surface, and in your light box
you see all sorts of ugly curves like fingerprints or
end-grains of lumber, instead of straight lines. How
do you interpret those curves in such a way as to
guide you when working on the convex surface? Any
thoughts?

Guy

--- Ken Hunter <atm_ken_hunter@yahoo.com> wrote:

> 
> 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...
>  

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