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When an OAG is setup correctly the distance from the corrector to the plane of the autoguider chip and the distance from the corrector to the imaging chip will be the same.
So that is not the issue.
The issue here is you can often adjust the prism to go down lower into the light stream so it is not right out at the edge.
I use a Planewave CDK17 at 2959mm focal length and a MMOAG with an ST402 guide cam. I find there is a bit of flex with the eyepiece holder arrangement of the guide camera and MMOAG. I sometimes would get very eggy stars. I found the camera would rock slightly in the eyepiece holder and pulling it back (it was a bit stiff) would round out the stars.
Also I got a parfocal ring from Scope Stuff. That locks onto the nosepiece of the ST402 so even if I remove it it is set to the right focus like a stop when I put it into the eyepiece holder of the MMOAG.
Unfortunately though it really needs another screw (it has 2) so it can lock on with a slight amount of tilt. Perhaps drilling a tapping a small hole would improve it. I was considering a screw adapter of the correct length to rid it of any flex. Not sure if it is worth it - perhaps.
Anyway - try lowering the prism lower into the light stream. The MMOAG came with some spacers to do just that.
As Fred says the guide star does not need to be round to still get excellent guiding. The software calculates the centroid of the star to a subpixel accuracy anyway.
A lot of stars are double stars as well. Try another star if you have bad guide errors with one. I often find the simple selecting of another stars can suddenly improve guiding a lot.
Greg.
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