Re: [ATM] Help with stubborn zone on 12.5"

[Mark Suchting <masuch@cia.com.au>]

At 04:57 AM 9/28/2004, Brent Russell wrote:
>  I have tried using the 6" tool on the top of the
>edge zone (about 85% radius) and this has helped a bit but not enough.
>Even after 3 lots of 4 rotations of the 6" tool the edge has only budged
>a little....any ideas?

You don't mention what F# your mirror is.

If you focus your  85% zone high it sounds like your edge is rolled rather 
than you have a hyperbola. By saying that you can control the center well 
with the 6" flat do you mean you can polish  it to focus flat while the 
edge is down ?    The classic hyperbola focuses to  a high 70% zone with 
the edge and center _equally_ down . If your edge is rolled after 85% zone 
by 2.5 waves you've got a lot of polishing to bring it up . Run a 3" 
scalloped lap along  the 85% with alternate branching stroke  either side 
of the 85% and your edge will come back on. A 6 " lap would likely roll the 
edge more.

  However it might be better to run your 10" lap for a spherical stroke and 
bring the whole surface ellipsoid again, at the same time bringing the edge 
back on, and creep up on the parabaloid again with the 6" lap.   Large 
mirrors may often take a lot more figuring  strategy, but smaller mirrors 
you are best to sneak up on the parabaloid with a smooth progression and 
everything under control. Make that approach as many times as it takes.

It might help to post a surface profile somewhere .

Hope this helps.

Mark
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