Design of the MROI delay line optical path compensator

The delay lines for the Magdalena Ridge Observatory Interferometer in New Mexico are required to provide up to 380m optical path delay with an OPD jitter of better than 15nm, in vacuum, using a single adjustable stroke. In order to meet these demanding requirements in a cost-effective manner a unique combination of techniques has been used in the design and construction of the delay line trolley which operates continuously within 190m of evacuated pipe. These features include contactless delivery of power and control signals, active control of the cat's eye optics and the use of composite materials to achieve good thermal stability. A full-size prototype trolley has been built and fully tested and the first production trolley is under construction. We describe the system's key design features and review the construction and alignment of the delay line trolley. Results obtained with the trolley operating in an evacuated 20m-long test rig under the full range of conditions required for successful astronomical observations are presented. An OPD jitter of typically 10nm is achieved over the total tracking velocity range from 0 to 15mm/s.