The JWST pipeline contains two steps devoted to the removal of fringes on MIRI MRS images.
The first correction is applied in the
fringe_step in the calwebb_spec2 pipeline
and consists in dividing
detector-level data by a fringe-flat and is described in the fringe step.
Applying the fringe flat should eliminate fringes from spectra of spatially extended sources, however
residual fringes can remain. For spatially unresolved (point) sources or extended sources with structure,
applying the fringe flat will undoubtedly leave residual fringes since these produce different fringe patterns
on the detector than accounted for by the fringe flat. The second step for fringe removal is the
residual_fringe_step. At this time this step can only be run offline; it is not part of the default pipeline.
After thorough testing and possible updates after commissioning,
it will be determined if this step should be run as part of calwebb_spec3 pipeline.
To run this offline it is best if it is applied after the
mrs_imatch_step, but before the
residual_fringe step can accept several different forms of input data, including:
a single file containing a 2-D IFU image
a data model (IFUImageModel) containing a 2-D IFU image
an association table (in json format) containing a list of input files
a model container with several 2-D IFU data models
This step only works on MIRI MRS data.
It is assumed that the calwebb_spec2 pipeline has been run on the data. In addition, the detection of residual fringes
are better determined if the
mrs_imatch step has also been applied to the data.
Fringe Background Information¶
As is typical for spectrometers, the MIRI MRS detectors are affected by fringes. These are periodic gain modulations caused by standing waves between parallel surfaces in the optical path, acting as a slow-finesse Fabry-Pérot etalons. In the MRS, the principal fringe sources are the detector layers. A detailed detailed discussion on these fringe components can be found in Argyriou, I., Wells, M., Glasse, A., et al. 2020, A&A, 641, A150 and Wells, M., Pel, J.-W., Glasse, A., et al. 2015, PASP, 127, 646.
The primary MRS fringe, observed in all MRS bands, is caused by the etalons between the anti-reflection coating and lower layers, encompassing the detector substrate and the infrared-active layer. Since the thickness of the substrate is not the same in the SW and LW detectors, the fringe frequency will differ in the two detectors. Up to 16 microns, this fringe is produced by the anti-reflection coating and pixel metalization etalons, whereas above 16 microns it is produced by the anti-reflection coating and bottom contact etalon, resulting in a different fringe frequency. The information in the fringe frequency reference file is used to determine, for each MRS band, the frequencies to fit to this main fringe component. The residual fringes are corrected for by fitting and removing sinusoidal gain to the detector level data.