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 Å 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.
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BibTeX
@misc{nagarajan2026calibration,
title = {{Calibration of the Next Generation Palomar Spectrograph as a Stellar Speedometer}},
author = {Pranav Nagarajan and Kareem El-Badry},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.05629},
eprint = {2609.05629},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.05629},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Calibration of the Next Generation Palomar Spectrograph as a Stellar Speedometer
AU - Pranav Nagarajan
AU - Kareem El-Badry
PY - 2026
DA - 2026/10/16
PB - arXiv
DO - 10.48550/arxiv.2609.05629
UR - https://arxiv.org/abs/2609.05629
AB - 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 Å 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.
ER -