The Calibration Reference Data System (CRDS) facilitates access to calibration reference and parameter files for use in the Roman STScI Data Pipelines. The Roman exposure-level pipeline utilizes the reference and parameter files stored in CRDS to process the Level 1 uncalibrated data into Level 2 calibrated rate images. In addition, simulation tools such as Roman I-Sim make use of reference files in CRDS.

Selecting Reference Files from CRDS

Most Level 1 (L1) to Level 2 (L2) data processing steps rely on calibration reference files that contain information necessary to remove instrumental effects and aid in the creation of Level 2 calibrated products. The Wide Field Instrument (WFI) reference files may be improved over time as additional data are acquired. The reference files for the WFI imaging mode are created, tested, and validated by the WFI Reference File Pipeline developed by the Science Operation Center (SOC) at STScI, and are available via the Roman Calibration Reference Data System (CRDS). 

There are specific keywords used to match the data being processed in the exposure-level pipeline to the appropriate and most up-to-date reference file to be used in a particular calibration step. The USEAFTER keyword is ubiquitous for all reference files in CRDS and represents the date after which the reference file is applicable, with CRDS matching science data to the nearest preceding USEAFTER date. For instance, if the observation date of the data being processed is 2027-01-01, and there are dark reference files with USEAFTER dates of 2026-12-01 and 2027-01-07, the first file would be selected. Beyond the USEAFTER date, there are additional matching criteria for each reference file, which are listed in the table below.

Additional information on the format and contents of each reference file may be found in the  romancal readthedocs documentation on calibration reference files.

CRDS in Roman Pipelines

The exposure-level pipeline, which calibrates data from L1 to L2, automatically retrieves from the Roman CRDS operations server the appropriate reference files based on the matching criteria described above. The names of the reference files used in the processing pipeline are stored in the metadata of the L2 files and are also recorded in logs. The versions of the reference files used by the exposure-level pipeline are dictated by the CRDS context file (also known as a PMAP file). This context file itself lists for each reference file type the "RMAP" file to be used. The RMAP files themselves contain the mapping of all files of a particular reference file type to the selection criteria. The name of the context file used in calibrated WFI data products is also stored in the ASDF metadata.

Additional information for utilizing this system can be found within CRDS documentation. Command line tools and python packages allow a user to examine the best references and explore the data of a particular reference type.

For additional questions not answered in this article, please contact the Roman Help Desk at STScI.

References

1. Roman CRDS Site
2. CRDS Documentation for JWST