APOGEE LN₂ Tank Back Pressure Regulator System

The APOGEE fiber-fed, near-infrared, wide-field, high resolution spectrographs at Las Campanas Observatory (LCO) and Apache Point Observatory (APO) presently have radial velocity precision floors of 100 — 200 m/sec. Efforts are under way for the Sloan Digital Sky Survey-V (SDSS-V) to improve this accuracy to roughly 30 m/sec by installing a back-pressure regulator (BPR), a segment of octagonal fiber within the fiber train, and Fabry-Perot interferometer calibration sources. Here we present the BPR system, including its valve control system, passive heating design, construction, and laboratory testresults. The BPR system controls the exhaust gas pressure of the instrument’s internal LN2 tank to correct for the approximate 50 m/sec seasonal spectral displacement caused by atmospheric pressure variations.