ATM Secondary de-center

[Clive Milne <milnecd@yahoo.com>]

Dwight Elvey wrote (on the subject of collimation)
> 2. Secondary tilt, centering, offset:
>  These all effect vignetting. 

    And whilst not related to image sharpness,
collimation of a scope without the correct de-center
of the secondary, effectively means that the optical
axis is no longer parallel to the mechanical axis of
the tube assembly. Without then ensuring that the
optical axis is orthogonal with respect to the mount,
pointing errors will result.  Consider the case of a
small Alt Az, say 12" f5 with a 4mm offset (from
desired). At that focal length, 1 degree is approx' 1
inch at the focal plane. 4mm therefore, translates to
+/-10 minutes of arc. A significant figure if you are
slewing around the sky trying to acquire dim galaxies
on your Kaf0400.
     Perhaps worth keeping in mind if you are making
an imaging machine out of your trusty Dob...  
(perish the thought)
       Clive.

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