Re: [ATM] Astig. due to gravity

[Jim Burrows <burrjaw@earthlink.net>]

At 2008-03-05 20:01 -0500, Russell Jocoy wrote:

>    30 years ago mirror thickness was kind of standard, you know the 
> thicker the better. This also eliminated complicated supports.  Now 
> we are looking at thinner disk's propped up as the old thick 
> disk's...what do you get, droop of the glass, ie, Astigmatism.  Why 
> not support the disk as if you had mounted it in a scope.. Lean the 
> mirror back with enough support to come close to PLOP.... Then 
> test..I believe you will see another animal...Just my thought...Russ

I've pushed the 2D webcam Hartmann test as a thin mirror + PLOP 
support test.  I.e., put the mask on the mirror, do a usual 
horizontal test, then tilt the mirror + support back by a sequence of 
measured angles and see how the surface changes.

A cute example of this was a thin mirror with a 3-point support that 
Carl Zambuto lent me for testing.  I built a test tower and did the 
test out in the garage where there was enough vertical clearance with 
the mirror lying comfortably on the horizontal support.  Neat - the 
test showed 3 little ~ 20 nm bumps right above the support points.

         -- Jim Burrows
         -- http://home.earthlink.net/~burrjaw
         -- mailto:burrjaw@earthlink.net
         -- Seattle N47.4723 W122.3662 (WGS84)

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