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Put a laser collimator or sight tube with cross hairs in the focuser, measure down to the laser dot or centre of the cross hairs. That gives you the vertical distance from the end of the tube.
Remove the secondary holder, measure the centre of the spider from the tube wall along each vane and centre the spider by adjusting the vanes. Put a length of threaded rod in the spider where the secondary holder fits and hold in place with a nut either side of the centre of the spider. Adjust the focuser to centre the crosshairs or laser dot on the threaded rod. Remove threaded rod. You have now found the lateral position opposite the focuser on the tube wall. Vertical and horizontal lines passing through these points define the point opposite the centre of the focuser.
Adjust focuser to align cross hairs or laser dot where the lines intersect. Mark with a small dot for future reference.
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