[ATM] Fixing oblate sphere (fix oblate spheroid)

[Mike Lockwood <melockwo@uiuc.edu>]

Charles,

Note the changed subject line.  Just FYI, "oblate sphere" and "oblate 
spheroid" can be searched for in the archives, and you'll turn up a 
web page I wrote because of the large number of questions on the list 
concerning oblate spheres.

Here are a couple of my earlier posts that searches turned up:
  http://astro.umsystem.edu/atm/ARCHIVES/JUN06/msg00343.html
  http://astro.umsystem.edu/atm/ARCHIVES/APR06/msg00042.html

j h wrote:
> I have a 12" project going on.  Tested it on Ronchi tonight and it seems 
> that I have a oblate I believe.  Inside COC the lines bow in, outside, they 
> bow out.  Kinda the reverse of what I am hoping to achive.  I have read that 
> I need to remove more of the center.  Can anybody comment on this or point 
> me in the correct stroke to fix this?  I have been polishing MOT, completely 
> polished out, using short COC 1/8 - 1/4 strokes.  I am thinking of switching 
> to MOT, "W" hang over 2 inches top bottom and same on the sides.  Please let 
> me know if this will make a mess of things or if this will help.

Well, if the ronchi image is indeed inside focus, then you have a 
longer ROC in the center than at the edge, which is indeed an oblate 
sphere.  There are a few ways to fix the condition.  Once again, I'll 
post my web page for fixing oblate spheres:

http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/oblate/oblate.html

I just updated it a bit and tried to make the language easier to 
understand.

	Mike Lockwood

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