Development and Results for Stressed-lap Polishing of Large Telescope Mirrors

The University of Arizona Mirror Laboratory polishing program relies on the stressed-lap polishing tool. Its implementation is discussed for recent mirrors including the 8.4m off-axis Giant Magellan Telescope (GMT) and Large Synoptic Survey Telescope (LSST) mirrors. The stressed lap has brought 12 telescope primary mirrors to a successful conclusion. The mirror diameters have spanned 1.8m to 8.4m with aspheric departures ranging from 456 μm (Vatican 1.8m f/1.0) to 14,000 μm (GMT 8.4m off-axis segment). Each has a final surface figure error near 25 nm rms. The strengths of the stressed lap include rapid low order figuring with removal rates near 20 μm/hr / (m/s) / psi over a 1.2m pitch diameter and efficient smoothing of high frequency ripples. The stressed lap is a very stiff large tool with the ability to change its optical shape as it moves over the mirror surface. The lap struggles near the mirror edge and for smoothing at 5-10cm spatial scales. The lap also suffered from erratic drag variations during polishing of the first 8.4m GMT off-axis segment, and required several small rigid conformal non-Newtonian laps for support. The hardware and software implementation of the stressed lap is discussed including the methods used to calibrate its shape and moment control system. Future changes to the stressed lap system for the next GMT off-axis mirrors are summarized.