Subaru Telescope improved pointing accuracy in open-loop and Az rail flatness

The Subaru telescope had its astronomical first light in January 1999 and has been stably operated since the common use started in December 2000. The telescope is mounted on an alt-azimuth structure. The structure of 550 tons is supported by six hydrostatic oil pads which lift the structure by 50 microns. The azimuth (Az) and elevation (El) axes are driven by direct-drive linear motors, ensuring very smooth pointing and tracking operations. The Az rail consists of eight circular arc pieces. They were installed in January 1997 with a peak-to-peak level of within 0.1mm. However at a later time, vertical undulations of the Az rail were found to be more than 0.2 mm peak-to-peak at some locations where the telescope structure in the rest position applies load. Open-loop tracking accuracy of the telescope, which was about 2 arcsec RMS on the sky, was found to be due to the undulations of the Az rail. We made a table to correct telescope pointings due to the undulations. It has made open-loop tracking accuracy better than 0.2arcsec RMS. Since then, we have been monitoring the flatness of the Az rail. So far the undulations have not changed.