Cementing and testing of the 4MOST WFC/ADC doublets

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.