The 4-meter Multi-Object Spectroscopic Telescope (4MOST) is a wide-ﬁeld, high-multiplex spectroscopic survey facility under development for the Visible and Infrared Survey Telescope for Astronomy (VISTA) of the European Southern Observatory (ESO). The 4MOST Wide Field Corrector and Atmospheric Dispersion Corrector (ADC) lenses have been manufactured and tested at KiwiStar Optics, New Zealand. The two ADC wedged doublets form a Risley Prism Pair; are both 650 mm in diameter and relatively thin; and have been cemented using Sylgard 184 silicone elastomer. The cementing procedure and its challenges are reported here. Interferometric measurements of the single surfaces, glass blanks and the ﬁnished doublets have been compared in order to assess the eﬀect of the cementing process on the ﬁnal surface ﬁgures of the doublets before and after positioning in their cells.
A new prime focus corrector for the WEAVE project for the William Herschel Telescope is being produced. The corrector consists of six lens elements, the largest being 1.1 m in diameter. It also incorporates an Atmospheric Dispersion Corrector. Testing procedures for the WEAVE prime focus corrector lens elements are described here. Critical issues encountered in practice, including the influence of the lens size, wedge and weight on the testing procedure are discussed. Due to large lens dimensions, a dedicated test tower and lens support system has been developed to measure the optical surface form errors of the concave surfaces and the transmitted wavefront of each lens. For some of the lens elements, sub-aperture measurements have been performed using an off-axis Hindle sphere and the resultant OPD maps have been stitched together. The challenge of testing a wedged lens with a combination of a long radius convex surface and a short radius concave surface has been resolved by using another lens from the system as an auxiliary lens. The practice of testing convex surfaces via internal reflection/transmission through the lens element has been avoided entirely in this case and some discussion justifying the choices of metrology approach taken is given. The fabrication and acceptance testing of the lens elements has been completed within the expected time and budget, and all elements have been shown to meet requirements.