The 1.5 inch SMR is carried by a pu

The 1.5 inch SMR is carried by a puck made of plastic and rubber. The initial version of the puck uses a durable form of teflon that can slide across the glass surface without significant wear. In this version the SMR is separated from the glass by a thin layer of teflon, which maintains its thickness to a small fraction of a micron during the course of a measurement. We are using this version to measure the ground surface, but it poses a risk of scratching a polished surface. We are developing a new puck with three small flexible rubber air bearings that will be used on the polished surface. In this version the SMR rests on the glass while tracker data are recorded. Air pressure is applied to lift the puck and SMR a few mm above the surface while the air bearings remain nearly in contact with the glass. The puck slides to the next position with minimal force, and slowly lowers the SMR to the surface as the air bleeds out.The puck is attached to four strings that control its position, as shown in Figure 10. The strings are controlled by motorized winches, two with position control and two holding constant tension. The winches and associated pulleys are mounted on steel beams about 3 m above floor level (slightly above the mirror surface) where they can be left permanently without interfering with traffic in the lab. Limit switches constrain the range of string positions so they cannot pull the SMR off the mirror or contact the four retroreflector/PSD assemblies at the edge of the mirror.The SMR follows a pattern programmed into the control computer, pausing at each sample point long enough for the laser tracker and external reference system to make measurements before moving on to the next point. We aim to control the position to an accuracy of a few cm. The precise sample locations do not matter because the position is measured by the laser tracker.