Figure 1: HDA schematic consisting of two concentric hemispheres with radii and , an electrostatic lens at input, and a position sensitive detector at the exit. The HDA voltages are set to pass the reference trajectory (red dashed line) having nominal pass energy and launching angle , with specified entry radius and exit radius . Two additional trajectories (red lines) of the same energy are shown to originate from a point source, but with . Here, the entry is seen to be paracentric with displaced from , the mean radius. The dash-dot line shows the conventional optical axis at for central entry . A general trajectory is also shown (in blue) with pass energy seen to start at entry radius and launching angle , exiting the HDA at radius . The PSD records the exit positions of the trajectories.