The Instrument Configuration
The geometric model in hexrd consists of 5 fundamental coordinate systems interconnected by affine transformations.
The propagation direction of the incident X-ray beam can be specified several ways in the GUI:
- Using spherical coordinates;
- By specifying the X-ray source location; or
- By specifying the propagation vector directly.
On the hexrd.instrument.HEDMInstrument class, the beam_vector is stored as the unit propagation vector, with source_distance defaulting to np.inf (i.e., perfect collimation).
Figure 1: Schematic of the X-ray source position and spherical angle definition in hexrd. Before you get angry, be aware that the convention was chosen so that most configurations were far from the $\psi=0$ degeneracy, which causes numerical issues in an optimization context.
The sample frame is primarily used in the X-ray microscopy setting, although there are occasions where specifying a non-trivial sample orientation is advantageous (e.g., reflection geometry, gas gun experiments). {#firstchap}