[pmrid (Peter)]

I have toyed with various OAGs over the years but never really mastered their use. Some things never seem to change. My current iteration is a ZWO.

The problem I am having is with star shapes. I have moved the pick-off mirror so it is on the long side of the imaging sensor (an ASI1600MM) and as close at it will go without obscuring it.

And yet the stars are very banana-shaped and ASIair calibration fails.

I am using a lovely SharpStar 61EDPH f5.5 triplet APO with a dedicated 0.8 reducer/flattener so it ought to be providing a flat field to the very outside of the FOV - which it does on the main sensor. But my OAG seemingly has other ideas.

The guide cam (an ASI120MM mini) is in a helical focuser and seems to be nicely secured.

I wonder what I am missing.