[ATM] Two mirrors small size difference

["Paul Bradley" <paul@paul-bradley.com>]

Hi all,

As I've posted before I'm grinding 2 mirrors for a pair of 8" binoculars on the same tool.

I'm now in 120, and I've just noticed whilst re-bevelling that one mirror is ever so slightly 
smaller (it was sent as the tool in a mirror kit) - it look like it's outer edge is at the inner 
edge of the mirror bevel, since the bevel is 3/32" it looks to me like one blank is 8" and the 
other is 7 13/16" (7.8125"). 

Checking the sagitta it looks like the smaller disk has a slightly smaller sagitta, I'm aiming for
about F7 and at the moment the larger blank has a sagitta of 0.73" and the smaller about 0.72",
so the focal lengths will be about the same if my measurements are accurate. 

I guess what I'm really asking is will grinding these 2 slightly different sized mirrors on the same
tool give the same focal length for both mirrors? I'd do the sunlight test but it's been cloudy for
days, and I'm not even sure if I'd get a sharp focus in 120 grit.

Hopefully I can just continue on the same tool then stop the slightly larger mirror down to the same
size as the smaller one in the finished scope.

I'm going to grind more and worry less anyway, but confirmation that this is likely to work ok would
be nice.

thanks.

Paul


 
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