The pathloss step calculates and applies corrections that are needed to account for various types of signal loss in spectroscopic data. The motivation behind the correction is different for the MIRI, NIRSpec, and NIRISS observing modes. For MIRI LRS fixed slit, this correction simply accounts for light not passing through the aperture. For NIRSpec, this correction accounts aperture losses as well as losses in the optical system due to light being scattered outside the grating. For NIRISS SOSS data it corrects for the light that falls outside the detector subarray in use.


The correction is applicable to MIRI LRS fixed-slit, NIRSpec IFU, MSA, and FIXEDSLIT, and NIRISS SOSS data. The description of how the NIRSpec reference files were created and how they are to be applied to NIRSpec data is given in ESA-JWST-SCI-NRS-TN-2016-004 (P. Ferruit: The correction of path losses for uniform and point sources). The NIRISS algorithm was provided by Kevin Volk.



This step calculates a 1-D correction array as a function of wavelength by interpolating in the pathloss reference file cube at the position of a point source target. It creates 2 pairs of 1-D arrays, a wavelength array (calculated from the WCS applied to the index of the plane in the wavelength direction) and a pathloss correction array calculated by interpolating each plane of the pathloss cube at the position of the source (which is taken from the datamodel). Pairs of these arrays are computed for both point source and uniform source data types. For the uniform source pathloss calculation, there is no dependence on position in the aperture/slit.

Once the 1-D correction arrays have been computed, both forms of the correction (point and uniform) are interpolated, as a function of wavelength, into the 2-D space of the slit or IFU data and attached to the output data model (extensions “PATHLOSS_PS” and “PATHLOSS_UN”) as a record of what was computed. The form of the 2-D correction (point or uniform) that’s appropriate for the data is divided into the SCI and ERR arrays and propagated into the variance arrays of the science data.


The algorithm for MIRI LRS mode is largely the same as that for NIRSpec described above, with the exception of the format in which the reference data are stored. First, the position of the target on the detector is estimated from the target RA/Dec given in the exposure header (TARG_RA, TARG_DEC keywords). This position is then used to interpolate within the pathloss reference data to compute a 1-D pathloss correction array. The 1-D pathloss correction is then interpolated into the 2-D space of the data being corrected based on the wavelengths of each pixel in the science data. The 2-D correction array is then applied to the science data and stored (as a record of what was applied) in the output datamodel (“PATHLOSS_PS” extension).

If for any reason the source is determined to be outside of the slit, the correction defaults to the center of the slit.

The MIRI LRS correction is only applicable to point source data. The step is skipped if the SRCTYPE of the input data does not indicate a point source.


The correction depends on column number in the science data and on the Pupil Wheel position (keyword PWCPOS). It is provided in the reference file as a FITS image of 3 dimensions (to be compatible with the NIRSpec reference file format). The first dimension is a dummy, while the second gives the dependence with row number, and the third with Pupil Wheel position. For the SUBSTEP96 subarray, the reference file data has shape (1, 2040, 17).

The algorithm calculates the correction for each column by simply interpolating along the Pupil Wheel position dimension of the reference file using linear interpolation. The 1-D vector of correction vs. column number is interpolated, as a function of wavelength, into the 2-D space of the science image and divided into the SCI and ERR arrays and propagated into the variance arrays. The 2-D correction array is also attached to the datamodel (extension “PATHLOSS_PS”) as a record of what was applied.

Error Propagation

As described above, the NIRSpec and NIRISS correction factors are divided into the SCI and ERR arrays of the science data, and the square of the correction is divided into the variance arrays (VAR_RNOISE, VAR_POISSON, VAR_FLAT) if they exist. For MIRI LRS, the correction factors are multiplicative, hence they are multiplied into the SCI and ERR arrays, and the square of the correction is multiplied into the variance arrays.