SSTRED: A data- and metadata-processing pipeline for CHROMIS and CRISP

Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing that require detailed knowledge about the instrumentation. Space-based solar telescopes provide science-ready data, easier to work with for researchers whose expertise is in the interpretation of data. Recently, data processing pipelines for ground based instruments have been constructed. We aim to provide observers with a user-friendly data pipeline for data from the Swedish 1-meter Solar Telescope (SST), that delivers science-ready data together with the metadata needed for proper interpretation and archiving. We describe CHROMIS and upgrades CRISP. We summarize the processing steps, from raw data to science-ready data cubes in FITS files. We go into detail about calibrations and compensations for data imperfections. Misalignment of Ca ii data is identified, characterized, and compensated for. We describe calibrations that reduce filter transmission profiles and solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration, which uses projective transforms for the registration of multiple detectors. We describe image restoration for CRISP and CHROMIS. Science-ready output is in FITS files with metadata compliant with SOLARNET recommendations. Data cube coordinates are specified within the WCS. Cavity errors are included as distortions of the wavelength coordinate in an extension of WCS. We specify the reference system for Stokes vectors with reference to WCS coordinate directions. CRISPEX accepts SSTRED output with SOLARNET metadata. SSTRED is a mature data processing pipeline for imaging instruments, developed and used for CHROMIS and CRISP, delivers well-characterized, science-ready, archival quality FITS files with well-defined metadata. The code is freely available through git repositories.