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


Here's a fun little demo to show it in action

.r hsi_image_script_4refposangletest
hsi_reference_position_angle_demo, obj,REFERENCE_POSITION_ANGLE=-0.18

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.