Manufacture and verification of M6C mirrors the pre-focal stations of the ESO-extremely large telescope

The M6C mirror of the Extremely Large Telescope is a Zerodur elliptical flat with wavefront specification of 60 nm RMS at 45 degrees angle of incidence over an optical clear area of 694 x 504 mm. The mirror is being processed at Glyndwr Innovations Ltd via small tool CNC polishing, with a sub-aperture interferometric stitching test being used to measure the surface error. Here, the design and performance of the interferometric stitching test is discussed, complete with a full uncertainty budget to ensure conformance to the specification. Current work on the stitching test has found measurements of the M6C to be repeatable to 4 nm RMS surface error, and an overall uncertainty of measurement of 12.2 nm RMS. Using this test the optic will be polished to a measured surface error of less than 17.5 nm RMS.