Detailed design and first test results of the ELT prefocal station hosted metrology

The ELT prefocal stations provide wavefront sensing to support the active control of the telescope during observations; they also include mirrors to distribute the telescope optical beam to the scientific instrument or diagnostic tool that finally uses the light collected by the ELT. Built in to the prefocal stations is a hosted metrology positioning system where metrology measuring instruments including a laser tracker and alignment telescope will be installed. This metrology will be used during coarse alignment of the ELT, to maintain the internal alignment of the prefocal stations, and to locate them with respect to their surroundings. The detailed design and application of these instruments is described here, together with a first set of test results.