Polishing and testing of TMT reference sphere

The Thirty Meter Telescope (TMT) is a joint venture of scientific institutions in Canada, China, India, Japan and the United States to build a 30 m diameter Ritchey-Chretien optical-infrared telescope. The large aperture size and field of view, with its adaptive optics (AO) systems will enable imaging at the diffraction limit, precision astrometry, high contrast imaging and spectroscopy from the ultraviolet through the mid-infrared. Its 30 m primary mirror consists of 492 off-axis aspheric segmented hexagonal mirrors, each having a diameter of 1.44 m across corners. The manufacture of the segments is distributed amongst India, USA, China and Japan. KiwiStar Optics (business unit of Callaghan Innovation), New Zealand manufactured a reference sphere for India TMT Co-Ordination Centre (ITCC). This paper presents polishing, testing procedures and metrology challenges of a 61.7 m long radius of curvature reference sphere. The radius of curvature and surface figure interferometric testing error budget is discussed in detail.