# Modeling long-range wavelength distortions in quasar absorption echelle spectra

## TL;DR

This paper addresses the issue of long-range wavelength distortions in UVES spectra, demonstrating that proper modeling and correction are essential for accurate measurements of the fine-structure constant from quasar absorption lines.

## Contribution

It introduces a comprehensive model for long-range wavelength distortions in UVES spectra and shows how to effectively correct for them to improve $eta$ measurements.

## Key findings

- Proper modeling removes systematic bias in $eta$ measurements.
- Ignoring distortions leads to significant errors exceeding statistical uncertainties.
- Correct correction allows accurate recovery of $eta$ from quasar spectra.

## Abstract

Spectra observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the European Southern Observatory's VLT exhibit long-range wavelength distortions. These distortions impose a systematic error on high-precision measurements of the fine-structure constant, $\alpha$, derived from intervening quasar absorption systems. If the distortion is modelled using a model that is too simplistic, the resulting bias in $\Delta\alpha/\alpha$ away from the true value can be larger than the statistical uncertainty on the $\alpha$ measurement. If the effect is ignored altogether, the same is true. If the effect is modelled properly, accounting for the way in which final spectra are generally formed from the co-addition of exposures made at several different instrumental settings, the effect can be accurately removed and the correct $\Delta\alpha/\alpha$ recovered.