Re: [ATM] coring a lens or mirror

[Jarvis Krumbein <jkoptic@juno.com>]

Guy, the method I've used waits until the rough grind is complete.  At
this stage, core the blank from the back until the cut is between 1/8"
and 1/4" from the front surface.  Fill the cut out area with plaster and
seal with wax.  Finish fine grinding, polishing and figuring.  Coat the
front surface of the mirror with wax or some other protective coating,
some paints work well but should be easy to remove.  Finish coring from
the front surface.  When the core is removed, very carefully bevel the
edge of the cored hole.  I use a diamond cylinder sleeve cutter at a 45
degree angle for this.  The bevel does not need to be deep and I usually
end up with about 1/16".  Remove the protective coating and you are
finished.  Since the hole is usually shadowed by the secondary, it's not
necessary to worry about the Twyman effect.  Save the core.  If you ever
decide to redo the mirror just cement the core back in place using
plaster and seal both sides with wax.  It's a lot easier than trying to
redo a mirror with a big hole.

Jarvis Krumbein


On Thu, 17 Feb 2005 15:44:45 -0800 (PST) Guy Brandenburg
<gfbrandenburg@yahoo.com> writes:
> I have heard different opinions on when one should
> drill the hore in the center of a lens or mirror (eg
> Cassegrain or Lurie-Houghton). 
> 
> Some sources recommend drilling and beveling before
> almost any grinding takes place, and then either
> gluing or plastering the glass cylinder back in. I
> have heard that this can cause serious edge problems
> around the drilled hole.
> 
> Other sources recommend waiting until all of the
> grinding, polishing and figuring is all done, and then
> spray-painting g the entire surface with rustoleum,
> drilling out the core carefully, and then removing the
> rustoleum with a bath of hot water, which causes the
> paint to come off in one piece.
> 
> What methods have the assembled experts tried? Which
> do you think is better?
> 
> Guy Brandenburg
> 
> =====
> Guy  Brandenburg
> Washington, DC
> My home page:
> http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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