The wavelength correction (wavecorr) step in the calwebb_spec2 pipeline updates the wavelength assignments for NIRSpec fixed-slit (FS) and MOS point sources that are known to be off center (in the dispersion direction) in their slit.


For NIRSpec MOS exposures (EXP_TYPE=”NRS_MSASPEC”), wavelength assignments created during extract_2d are based on a source that’s perfectly centered in a slitlet. Most sources, however, are not centered in every slitlet in a real observation. The MSA meta data assigned to each slitlet in the extract_2d step includes estimates of the source x (dispersion) and y (cross-dispersion) location within the slitlet. These are recorded in the “SRCXPOS” and “SRCYPOS” keywords in the SCI extension header of each slitlet in a FITS product.

The wavecorr step loops over all slit instances in the input science product and applies a wavelength correction to slits that contain a point source. The point source determination is based on the value of the “SRCTYPE” keyword populated for each slit by the srctype step. The computation of the correction is based on the “SRCXPOS” value. A value of 0.0 indicates a perfectly centered source, and ranges from -0.5 to +0.5 for sources at the extreme edges of a slit. The computation uses calibration data from the WAVECORR reference file. The correction is computed as a 2-D grid of wavelength offsets, which is applied to the original 2-D grid of wavelengths associated with each slit.

NIRSpec Fixed Slit (FS)

Fixed slit data do not have an a priori estimate of the source location within a given slit, so the estimated source location is computed by the wavecorr step. It uses the target coordinates in conjunction with the aperture reference point in V2/V3 space to estimate the fractional location of the source within the given slit. Note that this computation can only be performed for the primary slit in the exposure, which is given in the “FXD_SLIT” keyword. The positions of sources in any additional slits cannot be estimated and therefore the wavelength correction is only applied to the primary slit.

The estimated position of the source within the primary slit (in the dispersion direction) is then used in the same manner as described above for MOS slitlets to compute offsets to be added to the nominal wavelength grid for the primary slit.

Upon successful completion of the step, the status keyword “S_WAVCOR” is set to “COMPLETE”.