Re: [ATM] flat query

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Thomas Janstrom wrote:
> OK, with respect to B+C, try to decentre the rings such that the
> remaining segments of arc appear line like, the easiest way to do this
> is to apply some pressure to the edge of the two flats while they are in
> contact. You will see the line zoom of to one side, when you have about
> 5-10 fringes (the lower number is better) take a straight edge and lay
> it over the center of the flat, such that you find the "mean" straight
> path that crosses the fewest fringes.

That's a good method, but it may be tricky with 4" flats when there 
are 9 fringes of difference between the surfaces.  It is much easier 
when you get the difference down to a few fringes.

> For the plus or minus value move your head to or from the flats and see
> which way the fringes move, if you're using ATM Vol 1 by Ingals then he
> explains which way they should move if the surface is concave. I don't
> recall of the top of my head what should happen, sorry

I don't recall either - that's why I find it easier just to press on 
the disks.  ;)

> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of shyam rathod
> Sent: Friday, 30 December 2005 2:09 PM
> 
>  was under the process of producing 4"flat  i have some problrm with
> these
>   A+B= -9(concave
> rings)-----------------------------------------------------------------1
>   B+C=-5(concave rings)and +3 (convex rings) like 5 rings inside are---2
> behaving like concave and outside 3 rings behave as convex thus when i
> put pressure on the bullseye the ineer rings travel inside while outer 3
> rings travel outside.
>   C+A=

Shyam, the problem is that you are assuming the entire surface of all 
three flats are spherical, but they are not.  If there are three 
fringes of convexity inside a particular radius and two fringes of 
concavity outside of it, you have a zone on one or both of the flats B 
& C.  You probably have a depressed zone on one or both B and C, which 
is common if your polishing stroke is too regular.  Press the lap well 
and try varying the stroke length.

What you need to do is determine the difference in distance between 
the combinations of surfaces as a function of radius, that is, solve 
the system of three equations for several different radii, for 
example, solve it for 0", 0.5", 1", 1.5", 2".  Then you can plot the 
shape of all three mirrors as a function of radius.

      Mike Lockwood

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