Re: [ATM] Spherometers and sphericity

[Mike Lockwood <melockwo@uiuc.edu>]

Ken,

Sorry I hijacked the sharpie test, Ken.  I'll refer to what I do as 
the "sharpie even wear test".

kreig wrote:
> A sphereometer is used to determine the radius of curvature, only.  Dragging
> a sphereometer around on ground glass is a sin and will quickly wear the
> contact points making the sphereometer inaccurate.

Then I guess I'm a sinner, according to your gospel.  It's more of a 
gentle sliding action rather than dragging.

If one uses a quality indicator and zeros it before every use (on a 
decent optical flat), then any wear is automatically calibrated out 
and has no effect on the measurement.  To get precision to 0.00005" 
(that's a little over two waves) I need to zero it immediately before 
I use it anyway, because temperature and other subtle variables affect 
the measurement.

If I ever wear down the 1/4" ball bearing feet of my spherometer 
basess, then I will simply knock them loose and glue on new ones.

If your mirror and spherometer are clean and you have rounded contact 
points on each (bearings, rounded indicator tip), then you won't get 
any scratches, either.  I have pushed a spherometer around on plenty 
of 5u-ground surfaces and I have never gotten a scratch.

I am done preaching, but I will keep "sinning".  :)

	Mike Lockwood


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