Re: [ATM] Fringe testing Dall Kirkham secondary

["Rod Brackenridge" <rjbr@lycos.com>]

Hi Ken,
 
Thanks for responding.  I must admit that I am finding the fringe testing a little confusing which is embarassing because everything I read tells me it is simple to understand.
 
I am probably not explaining what I am seeing very clearly so I'll try again.  When I press the edge, the bulls eye moves towards the opposite edge and as it does so the number of fringes rapidly increase, particularly at the edge where the pressure is applied.  If, when looking at the bulls eye pattern, I press the centre, fringes move in towards the centre but the number of fringes does not change.  The spacing between them appears equal- when I am looking at the bulls eye pattern centred on the disks.  
 
I am concluding that the edges are touching and that I need to polish the secondary's edge more.  Please tell me if I am misunderstanding.  I don't want to waste time and make the radii diverge further.
 
Rod.

 ----- Original Message -----
> 
> > The thing you need to remember is that the number of
> > fringes will increase as the distance between the
> > surfaces decreases. If you have 2 curved surfaces and
> > the bullseye fringes are closer together in the center
> > then the convex surface is closer to the center of the
> > concave surface. Just opposite of what you said above.
> > 
> > If the centers touch, you'll have an lot of very close
> > rings at the center increasing in spacing as you move
> > out. If the edges touch, the rings will be clustered
> > around the edge increasing in spacing as you move in.
> > 
> > Thr rings will move towards the decreasing separation.
> > 
> > Ken Hunter
-- 
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