A 1 m SWING-ARM PROFILOMETER FOR LARGE TELESCOPE MIRROR ELEMENTS

摘要

Advances in the requirement for large aspheric and free-form ultra-precise surfaces place difficult demands on available manufacturing techniques and the associated metrology. Modern Computer Numerical Control (CNC) polishing machines can produce surfaces to meet demanding specifications but, without complementary fast, accurate and traceable metrology, economic volume production may not be feasible for certain projects.rnA major application for this metrology arises in the manufacture of Extremely Large Telescope (ELT) mirror segments which make up primary mirrors 30-100 m in diameter. One example is the Euro-50 telescope, shown in figure 1, the 50 m diameter primary mirror consisting of 618 hexagonal segments each of 2 m diameter across flats. The aspheric departures of the segments from the overall best fit sphere range from ±3 μm for segments of the inner-most ring to ±300 μm for the edge-segments. Each segment has a form tolerance of 36 nm RMS and the individual radii of curvature must conform and match to 0.5 mm. The segment manufacture will take some years and traceable metrology is required to ensure that the first segment matches the last.