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Sounds like you've got a Lumicon Giant Easy Guider. From memory this has a large rectangular fixed prism built into the body and the "adjustment" of the guide pick off position is achieved by moving the guide camera tube forward and backwards along the body. Is this correct?
I'm also assuming that the unit is fitted to the rear cell with the guide camera at the 12 oclock position ie vertically upwards. That would mean that the long face of the prism is orientated along the RA axis and the "adjustment" is moving in Dec. Again, if the guide camera is central and square (?) to the prism, the the orientation must be at 90 degrees to the OAG body ( ie camera cable outlet to the side, not facing away from the rear cell) this would give the RA axis parallel to the RHS of the camera. If the camera cable is pointing to the LHS of the rear cell, then the extreme RHS of the CCD chip would be at the outer edge of the FOV. If all this makes sense, and is correct, then the RHS of the guide chip is see the extreme off axis image and as the OAG body is about 3" diameter, this is much further off axis than the Canon and may indeed have significant coma.
The coma should reduce as you move the pick-off closer to the centre of the FOV, this means moving the guide camera slightly further out the guide tube..... Worth a trial to see if this is what's happening, it then gives an option... If PHD is happy with the star image it sees and the guiding is OK, then all this is a bit academic; just keep using what you've got!!
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