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For what its worth Pete, I had the Zwo OAG just recently, and have updgraded to the Player One OAG that bolts to my camera rather than threading on.
With the Zwo OAG, star shapes on the guide sensor were pretty terrible, and on the same scope with the Player One OAG, the stars are pinpoints, so as Joshua said, OAG orthagonality is probably the issue here.
The other factor is this - with the ZWO OAG and terrible star shapes on the guider, I was still guiding at 0.3~0.5" RMS error...
To show how similar my scenario is...
I was using the ZWO OAG, a 4/3" sensor (same size as the 1600mm) the Askar 65PHQ with 0.75x reducer (65mm f/4.8 ~312mm focal length).
This scope with (or without) the reducer, provides pinpoint stars to the edge of a 35mm camera sensor, so the 4/3" sensor is no match for it whatsoever, however, the ZWO OAG always produced very odd stars.
I have not used a guide scope since the first time I got an OAG in 2008/2009, and I can safely say that on most of my scopes over the years, guide star shapes were a bit screwy, if not completely terrible (I'm looking at the newtonians!!) and guiding was always just fine regardless..
The OAG that bolts directly to the camera body is the only one that's ever had guide stars looking as good as the stars on the imaging sensor..
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