Lightweighted secondary mirror for the United Kingdom Infrared Telescope

The paper describes the manufacture and testing of a lightweighted Zerodur secondary mirror for the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. The 313 mm diameter mirror is mounted on a Piezo platform for fast tip/tilt corrections. Therefore, the mirror mass has to be minimized to achieve high dynamic properties of the adaptive tip/tilt platform. The goal was to test the convex secondary without large auxiliary optics (Hindle sphere). We measured the mirror through the back surface using a small null lens system. A special transparent and highly homogeneous Zerodur was used for this purpose. We demonstrate that grinding a honeycomb structure and acid-etching of the back side of the mirror does not affect the figure of the polished convex surface.