[ATM] Was Aluminum Mirror, now Spherometer

[Jeff Lowe <lowe@nanotechsys.com>]

From: Richard <cnc@cncservo.co.uk>
Subject: Re: [ATM] Aluminum Mirrors [was 'newbie question']
To: Jeff Lowe <atm@atmlist.net>
Message-ID: <107210896.20070917144707@cncservo.co.uk>
Content-Type: text/plain; charset=us-ascii

Hi Jeff,

JL> I would have just
JL> measured the spacing between the the feet and let Gauss and Pythagoras
JL> give me the radius.

 >You're assuming :
 >1) The interfoot distances are all the same (they're not)

No I am not making any such assumption. In fact it makes no difference 
what the relative spacing between the points is. The general equation of 
a circle says that only one circle will pass through any three non 
co-linear points. Measure the spacing between the points, solve for the 
Cartesian coordinates of each point and solve the general equation of a 
circle for the radius of the spherometer.

 >2) You can make those measurements to optical accuracies (there's not
 >a workshop instrument on the planet that can do that)

Micron level measurements are not that difficult to attain with hand 
measuring tools. Center placement is very insensitive. For normal ATM 
type measurements this will result in radii measurements with resolution 
  below a millimeter.

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