Relative Flux Calibration of the LAMOST Spectroscopic Survey of the Galactic Anti-center

TL;DR

This paper presents an iterative flux calibration algorithm for the LAMOST spectroscopic survey, achieving about 10% accuracy in relative flux calibration across the 4000-9000 Å range, improving spectral data quality.

Contribution

The paper introduces a novel iterative flux calibration method tailored for LAMOST spectra, accounting for individual spectrograph variations and enhancing calibration accuracy.

Findings

Achieved ~10% accuracy in relative flux calibration.

Flux response curves vary up to 30% for individual spectrographs.

Comparison with photometry shows small color offsets.

Abstract

We have developed and implemented an iterative algorithm of flux calibration for the LAMOST Spectroscopic Survey of the Galactic anti-center (LSS-GAC). For a given LSS-GAC plate, the spectra are first processed with a set of nominal spectral response curves (SRCs) and used to derive initial stellar atmospheric parameters (effective temperature $T_{\rm eff}$, surface gravity log\,$g$ and metallicity [Fe/H]) as well as dust reddening $E(B-V)$ of all targeted stars. For each of the sixteen spectrographs, several F-type stars of good signal-to-noise ratios (SNRs) are then selected as flux standard stars for further, iterative spectral flux calibration. Comparison of spectrophotometric colours, deduced from the flux-calibrated spectra, with the photometric measurements yields average differences of 0.02$\pm$0.07 and $-$0.04$\pm$0.09\,mag for the $(g-r)$ and $(g-i)$, respectively. The relatively large negative offset in $(g-i)$ is due to the fact that we have opted not to correct for the telluric bands, most notably the atmospheric A-band in the wavelength range of $i$-band. Comparison of LSS-GAC multi-epoch observations of duplicate targets indicates that the algorithm has achieved an accuracy of about 10 per cent in relative flux calibration for the wavelength range 4000 -- 9000\,\AA. The shapes of SRC deduced for the individual LAMOST spectrographs are found to vary by up to 30 per cent for a given night, and larger for different nights, indicating that the derivation of SRCs for the individual plates is essential in order to achieve accurate flux calibration for the LAMOST spectra.