Parameter Files

Parameter files can be used to specify parameter values when running a pipeline or individual steps. For JWST, parameter files are retrieved from CRDS, just as with other reference files. If there is no match between a step, the input data, and CRDS, the coded defaults are used. These values can be overridden either by the command line options and/or a local parameter file. See Parameter Precedence for a full description of how a parameter gets its final value.


Retrieval of Step parameters from CRDS can be completely disabled by using the --disable-crds-steppars command-line switch, or setting the environment variable STPIPE_DISABLE_CRDS_STEPPARS to true.

A parameter file should be used when there are parameters a user wishes to change from the default/CRDS version for a custom run of the step. To create a parameter file add --save-parameters <filename.asdf> to the command:

$ strun <step.class> <required-input-files> --save-parameters <filename.asdf>

For example, to save the parameters used for a run of the calwebb_image2 pipeline, use:

$ strun calwebb_image2 jw82500001003_02101_00001_NRCALONG_rate.fits --save-parameters my_image2.asdf

Once saved, the file can be edited, removing parameters that should be left at their default/CRDS values, and setting the remaining parameters to the desired values. Once modified, the new parameter file can be used:

$ strun my_image2.asdf jw82500001003_02101_00001_NRCALONG_rate.fits

Note that the parameter values will reflect whatever was set on the command-line, through a specified local parameter file, and what was retrieved from CRDS. In short, the values will be those actually used in the running of the step.

For more information about and editing of parameter files, see ASDF Parameter Files. Note that the older Configuration (CFG) Files format is still an option, understanding that this format will be deprecated.

More information on parameter files can be found in the stpipe User’s Guide at For Users.