[ATM] Odd collimation problem

[Jerry]

It would be expected if your diagonal was de-centered. The de-centering
could be accidental or by design. Some de-centering along the plane of the
diagonal surface away from the focuser (toward the fatter part of the light
cone from the primary) is desired. It doesn't amount to much on slow
mirrors, but on large and fast mirrors the de-centering is more significant.

Jerry

-----Original Message-----
From: Chris Dalla Piazza
 
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.