Design of the new SDSS 2.5m telescope wide field corrector for SDSS-V

SDSS-V is the fifth generation of the Sloan Digital Sky Survey and is an ambitious follow-on to a project that has been producing ground-breaking science for two decades. SDSS-V uses two dedicated 2.5m telescopes - the SDSS telescope at Apache Point Observatory in New Mexico, and the du Pont telescope at Las Campanas Observatory in Chile - feeding BOSS and APOGEE spectrographs at each site. These survey machines generate multi-object, all-sky spectroscopy in the optical and near-IR in support of primary science programs. The new wide field corrector for the SDSS 2.5m telescope is one of several major infrastructure upgrades undertaken for SDSS-V, necessitated by the replacement of the legacy fiber plug plate system with a new robotic Fiber Positioning System (FPS), which places different requirements on the focal characteristics of the telescope. The original 2-element corrector produced a focal surface which was non-telecentric and suffered from axial color, throughput, and image quality issues when used in the H-band with the APOGEE spectrograph. We have designed and built a 3-element, all fused silica corrector which addresses the optical shortcomings in relation to the FPS. In addition, the optomechanical design required very minimal changes to the telescope interfaces and also facilitates in-situ axial adjustment of one lens element to fine-tune the as-built spherical radius of the focal surface, to match the nominal design value to which the FPS was built. This paper discusses the optical and optomechanical design details of the new wide field corrector, concluding with a brief summary of recent commissioning results.