Detector coordinate (in pixels) along the direction of
the fast changing array index (of the 2D diffraction
data) for this reflection. This is often the position
where the calculated reflection is predicted to occur.
The fast changing array index of the 2D diffraction
array is often also defined as the detector
X-coordinate, while the slow changing array index is
defined as the Y-coordinate. This can vary depending on
the convention of the detector and the facility,
especially for multi-panel detectors.