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Hi Graffy
It is impossible to achieve perfect collimation. The secondary mirror is approximately centred on the spider vanes and then as seen in the focuser draw tube. The secondary mirror minor axis is then roughly aligned at right angles to the focuser draw tube axis, which itself is roughly at right angles to the scope axis. Added to this, it is assumed the primary mirror is exactly centred to the scope tube.
Use the laser collimator as a rough alignment then the cheshire collimator to finish off. Often lasers are themselves only approximately collimated. Or because of some slack when in the focuser, will indicate a false off-centre alignment. Rotate it when in the focuser to see if the laser beam stays to a fixed point or inscribes a circle.
A cheshire collimator will, as your eye can see, return focus to centre of the eyepiece and essentially create an alignment which adjusts for all the other approximations. It is very forgiving and give you about as good as you're going to get.
Regards, Rob
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