Re: [ATM] lurie-houghton progress

[Aplanatic@aol.com]

Hi Guy,
 
Take a photograph of the fringe pattern using a digital 
camera and analyze the fringes with FringeXP, 
which is available from the following URL:
 
_www.ceravolo.com/fringe/FringeXP/FringeXP.htm_ 
(http://www.ceravolo.com/fringe/FringeXP/FringeXP.htm) 
 
Or join the yahoo interferometry group and use 
Dale Eason's fringe analysis program available 
from the Files folder.
 
-- Dave
 
 
 
In a message dated 8/1/2005 7:28:32 P.M. Pacific Daylight Time,  
gfbrandenburg@yahoo.com writes:

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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