ATM 3 inch reflector

[Ray Aldridge <pbwriter@fwb.gulf.net>]

In wandering through the archives, I came across a thread on making a 
reflector from one of the Surplus Shed's 3" spherical mirrors, which are 
pretty cheap.  I'd like to let my sons make one of these.  I had a reflector 
of similar capacity when I was a kid, and it was a lot of fun.  The sticking 
point is the secondary.  It would pain my penny-pinching soul to pay several 
times the cost of the primary for a diagonal this small.  So here's my 
(possible stupid) question:  is there any way to cut one of those surplus 
first surface flat mirrors without destroying the coating?  If it helps, I 
have a small diamond spindle shaper, originally used for touching up stained 
glass shapes.

Thanks much,

Ray