SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP

¹ Institute for Solar Physics, Dept. of Astronomy, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
e-mail: mats@astro.su.se
² Institute of Theoretical Astrophysics, University of Oslo, Postboks 1029, Blindern, 0315 Oslo, Norway

Received: 17 May 2021
Accepted: 10 July 2021

Abstract

Context. Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing, which requires detailed knowledge about the instrumentation. Space-based solar telescopes provide science-ready data, which are 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.

Aims. 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.

Methods. We briefly describe the CHROMospheric Imaging Spectrometer (CHROMIS) instrument, including its (pre)filters, as well as recent upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and polarization optics. We summarize the processing steps from raw data to science-ready data cubes in FITS files. We report calibrations and compensations for data imperfections in detail. Misalignment of Ca II data due to wavelength-dependent dispersion is identified, characterized, and compensated for. We describe intensity calibrations that remove or reduce the effects of filter transmission profiles as well as solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration code, with multiple enhancements and new features. It uses projective transforms for the registration of multiple detectors. We describe how image restoration is used with CRISP and CHROMIS data. The science-ready output is delivered in FITS files, with metadata compliant with the SOLARNET recommendations. Data cube coordinates are specified within the World Coordinate System (WCS). Cavity errors are specified as distortions of the WCS wavelength coordinate with an extension of existing WCS notation. We establish notation for specifying the reference system for Stokes vectors with reference to WCS coordinate directions. The CRIsp SPectral EXplorer (CRISPEX) data-cube browser has been extended to accept SSTRED output and to take advantage of the SOLARNET metadata.

Results. SSTRED is a mature data-processing pipeline for imaging instruments, developed and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP spectropolarimeter. SSTRED delivers well-characterized, science-ready, archival-quality FITS files with well-defined metadata. The SSTRED code, as well as REDUX and CRISPEX, is freely available through git repositories.