ATM Collimation

[David Brooks <davidbr@microsoft.com>]

I've run into a quirky collimation problem -- inconsistency between
results obtained by using a sight tube vs a laser.  Would appreciate any
suggestions as to (a) why this is occuring, and (b) how to fix.

Details - this is an 8" f6.  Here's the sequence of steps I usually
follow:

1. confirm focuser is 90 degrees to optical axis
2. center secondary under focuser using sight tube -- i.e., move
secondary until it's circular appearance is centered
3. aim secondary by centering primary in the secondary
4. aim primary by using laser -- i.e., reflect beam off primary back to
center of laser source.

Usually when I follow this sequence on my other scopes the laser hits
the center of the primary (or very close).  In this case it continues to
show up about 1.5" off center (always in the same direction, toward the
focuser).  When I "correct" the aiming of secondary using the laser (and
adjust the primary as well) a star test confirms this is NOT collimated.
When I go back to the original collimation using only the sight tube
things look fine.  Why is the collimation obtained by the laser not
consistent with what I get with the sight tube?  This is driving me
crazy...

David Brooks
(Sunny) Seattle, WA