Flexure effects within GMOS can result in wavelength zeropoint shifts if the on-sky science data are wavelength-calibrated using daytime CuAr arc exposures. At this time (2024A), all GMOS baseline arcs are taken during daytime, except for a number of short-wavelength configurations that do not cover any sky lines. 

If an accurate wavelength zeropoint is critical, it is recommended to inspect sky emission lines in the wavelength-calibrated science data to detect possible zeropoint shifts and apply a correction as needed. A new tutorial page - including a customized DRAGONS recipe for an interactive sky-line-based correction for long-slit data - is available here.

Please note that from 2024B all GMOS baseline arcs will be taken on sky mixed with the science observations to minimize flexure effects. Further details can be found in the 2024B Call for Proposals.