From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Poul Linnet" 
>
>
>After reading Nils Olofs excellent article on collimation in S&T Jan 2003,
I began wondering whether a marking at the physical center of the mirror
blank always corresponds to the center of the parabola  within 1 mm as
required for proper collimation?
>
>Poul Linnet


Hi Poul
 One should note that I don't believe Nils Olof was
saying that this would give you a finished collimation, only that it helped
to remove one of the biggest errors when using laser collimators. If you
really wanted to properly mark a mirror, you could do a fine collimation
using a star or artificial star ( see note below ). You could then place
the donut right at the optical center of the mirror.

Note: Artificial Star:
 While one needs to be concerned about the distance and
size of an artificial star while using it to measure such things as
spherical aberration, for collimation purposes, these are somewhat eased.
As long as clear diffraction rings can be seen it should work well for
collimating with. You should not be able to see a center that is much
bigger than the diffraction limited Airy disk. Having spherical aberration,
by having the source too close, can actually be used to advantage while
collimating. It is an aid and not a detriment. Dwight

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