Modelling and measurement of polishing tool influence functions for edge control

We present progress on our development of edge control for fabrication of mirror segments potentially for the European Extremely Large Telescope (E-ELT). Zeeko’s Bonnet polishing technology has been adopted to achieve the form correction target. Like other Computer Numerical Controlled (CNC) polishing, accurate and stable tool Influence Functions (IFs) are important. Particular challenges arisen when polishing up to edges where the geometry of the IFs created by a bonnet changes. We described a model based on measured IFs data that allowed us accurately to predict the edge profile. To obtain IFs at the edge, data from interferometers and profilometers have been stitched. Numerical models with empirical Ifs as input data have been used to predict edge profiles with some preliminary success.