Description
- Class:
- Alias:
wavecorr
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.
Upon successful completion of the step, the status keyword “S_WAVCOR” in the primary header is set to “COMPLETE”. For each SCI extension, the “WAVECOR” keyword is set to True if the slit was wavelength corrected (it is a point source) or False if it was not corrected (it is not a point source).
NIRSpec MOS
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 updates the WCS models of slits that contain a point
source to include a wavelength correction. 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, which contains pixel shifts as a function of source position
and wavelength, and can be converted to wavelength shifts with the dispersion.
For each slit, the wavecorr step uses the average wavelengths and
dispersions in a slit (averaged across the cross-dispersion direction) to
calculate corresponding corrected wavelengths. It then uses the average
wavelengths and their corrections to generate a transform that interpolates
between “center of slit” wavelengths and corrected wavelengths. This
transformation is added to the slit WCS after the slit_frame and
produces a new wavelength corrected slit frame, wavecorr_frame.
NIRSpec Fixed Slit (FS)
The source position within the primary slit is estimated based on the target coordinates and aperture reference point during the extract_2d step. This estimated position (in the dispersion direction) is used in the same manner as described above for MOS slitlets to update the slit WCS and compute corrected wavelengths for the primary slit only.