External upgrades to improve the RV precision of the APOGEE Spectrographs

Several external hardware upgrades have been developed for the APOGEE Spectrographs as part of the Sloan Digital Sky Survey-V (SDSS-V) to improve their radial velocity (RV) precision from a floor of 100-200 m/sec to approx. 30 m/sec. The upgrades include: (1) Back Pressure Regulator (BPR) systems to stabilize the internal instrument LN2 tank boil-off pressure, lessening induced movement of the APOGEE optical bench; (2) Fabry-Perot Interferometer (FPI) calibration sources to improve wavelength calibration; and (3), the use of octagonal core fiber segments in the fiber train to improve radial scrambling. We discuss the fabrication, commissioning, and early performance of these upgrades.