Re: [ATM] marking the center of a circle (was: Re: UltimateCollimation)

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

> That will give you the center of a circle, but will it give you the
> center of the mirror?
> The center of the mirror must not necessarily coincide with the
> center of the glass.

With machine figured mirrors, well centered during figuring, I believe the 
centers coincide closely in practice. With "hand"-figured mirrors, not quite 
as surely, though I have yet to hear a report of significant difference (a 
center spot being significantly offset from the glass center seems much more 
likely!).

The proof is in testing: star test (at optimum seeing!) to cancel all 
"one-sided" asymmetry of the diffraction pattern (most sensitive at or near 
focus), then check with a Cheshire. The spot should appear centered in the 
Cheshire. If *consistently* not. this would suggest that the centers do in 
fact not coincide, and you should consider moving the spot as appropriate.

I would go so far as to say the purpose of star collimation (if any) is none 
other than to verify the placement of the center spot....

Nils Olof 


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