[ATM] Odd collimation problem

["Chris Dalla Piazza" <dalchri@hotmail.com>]

With my CCD rig, I collimate my f/4.5 Newtonian scope by defocusing and
using the donut pattern created by the shadow of the secondary mirror.  I
usually try to pick a region of the Milky Way so I'll have lots of stars.
Then I look for the part of the picture where the secondary mirror shadows
all seem to point.  I take this point to be the optical axis. Then I use the
collimation bolts to center this point on the CCD chip.

 

What I have noticed for about a year or more is that this point where all
the secondary shadows point is in two different places on either side of
focus.  One on side of focus, I might center this point but then when I go
to the other side of focus, the shadows of the secondary mirror seem to
point to a place about 20' away in my FOV.

 

The stars do not shift in position as I focus so I don't think this would be
misalignment of the focuser motion with the optical axis.

 

This condition has persisted between mirror cleanings which indicates to me
that the orientation of the primary mirror probably isn't a factor if this
were astigmatism of the primary (unless I accidentally managed to get the
orientation the same).

 

Does anyone know what could cause this?

 

Is it a collimation issue or an optical issue?

 

Perhaps a deformed secondary?

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