To: "Jeff Anderson-Lee" ,
,
        
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


>From: jandersonlee{at}sbcglobal.net

>From: 
>> You are projecting laser spots on surfaces.  On that same surface are
>> other marks (center ring on mirror, center spot on laser collimator's
>> face.  Because the spots and marks are all on one 'two dimensional'
>> surface...tilting that surface should not affect how 'centered' the
>> spots/marks appear.
>
>Actually if it gets tilted too much and is close enough to you,
perspective
>kicks in and what appears to be center visually is off-center (hence
>secondary offset).  But for small tilts of the primary mirror that should
>not enter in.

Good point.  Thanks for keeping me honest.

However, for most of the distances/angles/tilts we are dealing with...this
is probably not often a serious problem.  (But I have not run any numbers
on this, so this is just off the cuff.)

One way to avoid this is to make sure your centering marks for laser spots
are small, not large.  You can center dots in small circles more accurately
by eyeball than you can center dots in large circles.  And small circles
are less prone to perspective/distortion problems for a given/constant
distance.

Tom Krajci
Tashkent, Uzbekistan

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