Re: [ATM] Two mirrors small size difference

["Thomas Janstrom" <tomjan@ozemail.com.au>]

Paul,

They should still get you to the same focal length, for the sun test at
this grit try some cooking oil spray. You want to get a uniform coating
so it looks smooth, it won't be optically perfect by any stretch of the
imagination, but will allow the light to come to a focus for this
purpose.

Clear Skies, Thomas.
www.tjanstrom.com
www.norsewines.com.au 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Paul Bradley
Sent: Tuesday, 11 October 2005 5:43 AM
To: ATM Mailing List
Subject: [ATM] Two mirrors small size difference


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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