Indexing requires highly precise knowledge of pixel positions in laboratory space. Due to the physics of the CS-PAD detector and the brevity of the incident XFEL pulses, a parallelized integrating detector was necessary to build to be able to operate at the framing rates available at the LCLS. 64 asics are arranged in a tetragonal pattern, 2 asics per sensor, 8 sensors per quadrant and 4 quadrants per detector. The placement of these tiles in physical space is measured by the beamline operators optically using electron microscopy. The four quadrants are movable at CXI, necessitating pixel-level adjustments to individual quadrants. cctbx.xfel then refines tile positions to whole pixel, and then sub-pixel precision by calculating spot positions on a highly diffracting dataset and refining tile locations versus actual spot positions for each tile. This creates four levels of metrology information and corrections. These levels and the method of measuring and refining them are detailed below.
Of the operations of cctbx.xfel, calculating and refining metrology is the operation most suited to beam line staff and the authors of the software. Please don't hesitate to ask for assistance regarding any aspect of this procedure.
LCLS provides initial tile placements. Periodically the detector is disassembled and re-built, necessitating new calibration information. The optical measurements are included in the cctbx.xfel in the source code and are reference by your pyana config file. See [preparatory steps] for more information.