From Richard Schwartz:
There are two new control parameters that allow you to control the position
of the roll angle bins.
Set USE_REFERENCE_POSITION_ANGLE to enable this feature. It is a parameter for the calibrated_eventlist class so it can be accessed from an image or visibility class as well.
The specific value is set with REFERENCE_POSITION_ANGLE measured in radians from solar north
For example, for an image object obj
obj->Set, /USE_REFERENCE_POSITION_ANGLE, REFERENCE_POSITION_ANGLE=-0.18
Here's a fun little demo to show it in action
The figure produced by the demo shows all 9 grid responses lined up on the reference_position_angle.
The demo script, hsi_reference_position_angle_demo, is in the file hsi_phz_stacker.pro. If you examine that code you'll see that there is a stacked eventlist roll_angle bin such that it matches the reference_position_angle exactly.