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Thanks for the replies. I checked the collimation by comparing the spot from my Glatter Laser with and without a star diagonal in my ED150. The GSO diagonal put the spot about 40mm off centre, the Skywatcher one put it about 15mm off centre. With no diagonal the spot was centred. Can't afford the expensive diagonals, so will have to shim :-( The GSO has 4 tapped holes in the base, but putting screws in these will press directly on the back of the mirror (held in place by the back plate pressing on a thin layer of foam). Almost certainly will cause astigmatism...
Gary
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