Re: (ATM) Fergy's Mirror 2

[iolo@mist.demon.co.uk (Iolo Davidson)]

In message <DKw7+qfFYkA@banyan_abq3.aquilagroup.com> 
      kferguson@aquilagroup.com writes:

> I think I will just build a spherometer and be done with it.  I have a good 
> 3/4 travel indicator which aughta be the ticket.  I am thinking of having 
> three pins with rounded points, set on a 1" radius with the indicator in the 
> center. Is there some problem with this design? (too big/small radius??). 

Too small.  I made a ring spherometer a bit bigger than that and 
got very misleading readings on a ten inch mirror.  And very 
different ones on the matching tool.

The method that worked well was to lay a metal ruler across the 
centre of the mirror, on edge, and measure from the top of that 
to the bottom of the dish with a vernier gauge (you can get 
el cheapo ones that are good enough).  Then measure the same way 
on a piece of flat plate glass, and the difference is the 
measurement you are after.  That is the equivalent of using a 
spherometer the size of your whole mirror, and you calculate from 
the measurement in the same way.

-- 
Iolo Davidson