Re: [ATM] [atm_free] RE: data and musings on thin mirror

["Jerry" <wa4guu@verizon.net>]

Steve

That is neat. I like it. I will have to play with your method a bit more to
make sure I understand 100% correctly. It looks good so far. And for sure
the line number "difference" (and f# and grating line density) is the
answer. For a given f# and grating line density, the number of lines showing
indicates the distance to the COC. The line number "difference" indicates
the distance between COCs. In the case of the 16 inch ultra-thin each line
is worth .068 inch from COC.

I was using ratio because I started with the .15" COC to grating distance
that Dale gave. And then I figured the difference of center and edge COC by
ratio. I just quickly judged a 2:1 ratio and that meant .075" correction. 

Of the various estimates that I made, the closest was the one where I
calculated the distance the grating should have been for the number of lines
that the cone would illuminate on the grating. Using diameter 16" and radius
of curve 110.5" and a 100 lpi grating, I figured for Dale's image the
grating was .413 from COC. Using the 3 1/2 band to 6 band ratio (2.5 lines
difference), the one I took more care in counting, the correction then would
be .172 or 30%. And the 3 to 6 would be .207 or 35%.

That seems to match Dale's robo-foucault very well, but then not so well
with the interferometry. The mirror has astigmatism so there may be a
diameter that reasonably matches both interferometer and Foucault.

This particular mirror makes judging the center easy because it is very
spherical in the center. Just slightly oblate. A smoothly corrected mirror
would not be quite so easy in the center. The outer part is hard to judge on
this one and a smooth curve would probably be easier.

This also should work to determine the correction between any two locations
on the curve.

That is enough (too much) for one post. 

Jerry
 


Jerry,

what do you think of this way of performing the test?  I think it does
work to determine the focal shift without knowing the stage position. 
The trick is to use the line count DIFFERENCE, instead of the RATIO.

--
Steve Koehler
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