A 3D metrology system for the GMT

The Giant Magellan Telescope (GMT) 1 is a 25 m telescope composed of seven 8.4 m “unit telescopes”, on a common mount. Each primary and conjugated secondary mirror segment will feed a common instrument interface, their focal planes co-aligned and co-phased. During telescope operation, the alignment of the optical components will deflect due to variations in thermal environment and gravity induced structural flexure of the mount. The ultimate co-alignment and co-phasing of the telescope is achieved by a combination of the Acquisition Guiding and Wavefront Sensing system and two segment edge-sensing systems 2 . An analysis of the capture range of the wavefront sensing system indicates that it is unlikely that that system will operate efficiently or reliably with initial mirror positions provided by open-loop corrections alone 3 . The project is developing a Telescope Metrology System (TMS) which incorporates a large number of absolute distance measuring interferometers. The system will align optical components of the telescope to the instrument interface to (well) within the capture range of the active optics wavefront sensing systems. The advantages offered by this technological approach to a TMS, over a network of laser trackers, are discussed. Initial investigations of the Etalon Absolute Multiline Technology™ by Etalon Ag 4 show that a metrology network based on this product is capable of meeting requirements. A conceptual design of the system is presented and expected performance is discussed.