The backbone-residual model. Accurately characterising the instrumental profile of a fibre-fed echelle spectrograph

TL;DR

This paper introduces the backbone-residual model for precise instrumental profile characterization of fibre-fed echelle spectrographs, demonstrating high accuracy on the HRS instrument through spectral reconstruction and Monte Carlo methods.

Contribution

The paper proposes the backbone-residual model combining a bell-shaped backbone with a spline residual for accurate IP characterization in fibre-fed spectrographs.

Findings

IP characterized with 0.006 peak value accuracy

The BR model effectively captures the IP structure

High precision achieved on HRS instrument

Abstract

Context: Instrumental profile (IP) is the basic property of a spectrograph. Accurate IP characterisation is the prerequisite of accurate wavelength solution. It also facilitates new spectral acquisition methods such as the forward modeling and deconvolution. Aims: We investigate an IP modeling method for the fibre-fed echelle spectrograph with the emission lines of the ThAr lamp, and explore the method to evaluate the accuracy of IP characterisation. Methods: The backbone-residual (BR) model is put forward and tested on the fibre-fed High Resolution Spectrograph (HRS) at the Chinese Xinglong 2.16-m Telescope, which is the sum of the backbone function and the residual function. The backbone function is a bell-shaped function to describe the main component and the spatial variation of IP. The residual function, which is expressed as the cubic spline function, accounts for the difference between the bell-shaped function and the actual IP. The method of evaluating the accuracy of IP characterisation is based on the spectral reconstruction and Monte Carlo simulation. Results: The IP of HRS is characterised with the BR model, and the accuracy of the characterised IP reaches 0.006 of the peak value of the backbone function. This result demonstrates that the accurate IP characterisation has been achieved on HRS with the BR model, and the BR model is an excellent choice for accurate IP characterisation of fibre-fed echelle spectrographs.