Calibration of the Next Generation Palomar Spectrograph as a Stellar Speedometer

Abstract

We calibrate the Next Generation Palomar Spectrograph (NGPS) for precision stellar radial-velocity (RV) measurements using repeated observations of 24 Gaia-ESO RV standards spanning $G=11$-$18$. Because NGPS is mounted at the Hale telescope's Cassegrain focus, changes in instrumental flexure produce wavelength-solution shifts of tens of km s$^{-1}$, with substantial time and wavelength dependence. We develop a pipeline that measures these shifts directly from each science exposure, using telluric absorption features in the R and I channels and the [O I] 5577 \r{A} sky-emission line in the G channel. This approach eliminates the need for frequent on-sky arc calibrations. The resulting wavelength-dependent flexure corrections reduce the median absolute discrepancy relative to Gaia-ESO RVs from tens of km s$^{-1}$ to 1.6 km s$^{-1}$. Telluric absorption provides substantially more reliable flexure calibration than sky emission lines, which do not track point-source illumination of the slit. The I channel, which contains both many telluric lines and strong photospheric lines across a range of spectral types, generally yields the most stable RVs. The achieved precision remains above the photon-noise limit, indicating that NGPS RV performance is currently limited by calibration and modeling systematics. These results demonstrate that NGPS is well-suited for accurate and precise stellar RV measurements across a wide range of astrophysical studies.