After calibrating with a starndard diagonal, the problem with off axis
guiders is that you invariably spin them arround to find a guide star
somewhere in the vicinity of your target. This means that the star
orientation will change. Even if you rotate the head the guider so that the
stars move east and west along the long axis of the camera, the axis
directions will only be correct with the OAG within 45 degrees of the
vertical position. Further from vertical and the directions sart to swap.

If you must use an OAG (I personally do not care for them). I suggest very
short calibration times to and calibrating for each shot. You will find
that you can center the star and use short time, you should be able to keep
the star in the field. The way to find out how long you can calibrate is to
take a 9mm eyepiece, center a star and using the RA guide speed buttons,
time how long it takes to slide the star out of the eyepiece. Divide this
time by 3 and use it for your calibration time.
John Hoot
SSC

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