Impact of instrumental systematic errors on fine-structure constant measurements with quasar spectra

Understand

We present a new `supercalibration' technique for measuring systematic distortions in the wavelength scales of high resolution spectrographs.

• By comparing spectra of `solar twin' stars or asteroids with a reference laboratory solar spectrum, distortions in the standard thorium--argon calibration can be tracked with $\sim$10 m s$^{-1}$ precision over the entire optical wavelength range on scales of both echelle orders ($\sim$50--100 \AA) and entire spectrographs arms ($\sim$1000--3000 \AA).
• Using archival spectra from the past 20 years we have probed the supercalibration history of the VLT--UVES and Keck--HIRES spectrographs.
• We find that systematic errors in their wavelength scales are ubiquitous and substantial, with long-range distortions varying between typically $\pm$200 m s$^{-1}$ per 1000 \AA.