Spectroscopic data require accurate wavelength solutions to fully unlock their scientific potential. While there are many tools for fitting wavelength solutions, there are no tools tailored specifically for solar data. In this talk we discuss the development of a new Python package called solar-wavelength-calibration, maintained by the Daniel K. Inouye Solar Telescope's Data Center and designed to accurately fit wavelength solutions to spectroscopic observations of the Sun.

The novel features of this package include the use of a solar spectral atlas as opposed to line lists (no need for arc-lamp data!) and the explicit treatment of telluric lines with varying intensity and an optional Doppler offset. Additionally, the parametrization of the wavelength solution does not use polynomials, as is common in other tools, but rather relies on a physically motivated model, with fit parameters corresponding to the optical characteristics of the spectrograph. As a result, the final fit is fully compliant with the FITS parametrization of non-linear wavelength axes for grating data (AWAV-GRA), eliminating the need to interpolate data onto a linear grid.

solar-wavelength-calibration is available on pypi and published in JOSS. This talk will give a brief overview of how this package is used in the wild, but will focus mostly on lessons learned and the process of turning a collection of functions that work on specific data into a general purpose tool for astronomers everywhere.