We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic ('Cervit') blank - a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.
|Number of pages||18|
|Early online date||17 Jan 2002|
|Publication status||Published - May 2003|