Large optical lenses for use in astronomical survey telescopes

The new wide-field infrared camera (WFCAM) for the 3.8 in United Kingdom Infrared Telescope (UKIRT) and the Visible and Infrared Survey Telescope for Astronomy (VISTA) are two future facilities to provide sky survey data to complement the latest 8m-class telescopes. Both use large lenses (with a diameter greater than 500mm) made of fused silica. The cryostat window used for the IR channel in VISTA is 950mm in diameter. The specification and cryogenic mounting of these components, some of which with aspherical surfaces, are an opto-mechanical challenge and are described in this paper. We also discuss various aspects of the manufacturing and testing.