Commissioning a laser metrology truss for active optics on the Large Binocular Telescope

LBTO, in partnership with GMTO, has been developing a laser-truss-based metrology system for the active alignment of telescope main optical components. Positive initial results convinced LBTO to commence to develop a "pathfinder" integrated operational active-optics system at prime focus, utilizing this technological approach. The prime-focus active-optics system benefits LBTO directly in improved system performance and is also very useful for GMTO in developing and gaining experience with a critical technical component of the GMT Telescope Metrology System. This paper describes the current system, which is now commissioned and operates in support of regular scientific observing. Technical aspects unique to direct laser truss metrology, such as system stability, the effects of correlated and uncorrelated noise, and the benefits of channel redundancy, will be discussed. Commissioning results and general system performance will also be reported. The paper will conclude with a section discussing some of the unexpected insights and improvements that the TMS has brought about at LBT by enabling the measurement of “clean” aberration data for aberrations arising from shape change on the borosilicate primary mirrors.