Fabrication and testing of 1.4-m convex off-axis aspheric optical surfaces

Abstract

ABSTRACT New developments in fabrication and testing techniques at th e College of Optical Sciences, University of Arizona have allowed successful completion of 1.4-m diameter convex off-axis aspherics. The optics with up to 300 µm aspheric departure were finished using a new method of computer controlled polishing and measured with two new optical tests: the Swingarm Optical CMM (SOC) and a Fi zeau interferometer using a spherical reference surface and CGH correction. This paper shows the methods and equipment used for manufacturing these surfaces. Keywords: Optical fabrication, aspheres, optical testing, large optics 1. INTRODUCTION Fabrication of large optical surfaces is difficult because of the complicated tooling needed for all stages of handling, manufacturing, and measuring. Aspheric surfaces are especia lly difficult because the polishing tools need to conform to a non-spherical shape, and the measurement cannot rely on the symmetry provided by spherical shapes. Off-axis aspheres cannot even use the axisymmetry of more common aspheric shapes, which are figures of revolution. The unique difficulty for convex surfaces arises from the difficulty performing optical tests – reflection from a concave surface causes light to converge, allowing the test system to be smaller than the optic being measures. Reflection from a convex surface causes the light to diverge, requiring auxiliary optics that are larger then the surface being measured. We present the successful completion of such surfaces that are la rge (1.4 meters in diameter), non-symmetric aspheres (off axis portions of a paraboloid or OAP, having up to 300 µm aspheric departure), and convex (requiring new optical test methods). This paper summarizes the manufacturing for these OAPs, emphasizing the techniques developed at University of Arizona that are novel and interesting on their own. The po lishing of the aspherical surf aces was performed with a new class of computer controlled polishing machines that use the swing arm geometry to provide robust motion control. The measurement of the optical surface was performed using two methods: x Swing arm Optical CMM SOC: A new machine was constructed that provide two-dimensional shape information from a series of scans that use swing arm geometry and utilize an interferometric, non-contact measurement. These measuremen ts are accurate to ~6 nm rms.