[ATM] Auto Collimation Flat - not cored

[Shane LaPierre]

I've got a good 8" optical flat and would like to use it 
as an autocollimator.  It is not coated, and is not cored. 
 I had thought about using it as a reference to make my 
own flat, but is there a way to use it and not aluminize 
it in the center, or remove the coating in the center so I 
can place a knife-edge @ ronchi in the center?  I've also 
thought about getting a good diagonal mirror and just 
moving it 90-degrees off - as in a newtonian.  Let me know 
your thoughts.

Thanks,
Shane LaPierre