Description

Class:

jwst.residual_fringe.ResidualFringeStep

Alias:

residual_fringe

The JWST pipeline contains multiple steps to mitigate the impact of fringing on science spectra, which generally suffice to reduce the fringe signal to below a few percent of the target flux.

The first correction is applied by default in the fringe step in the calwebb_spec2 pipeline and consists of dividing the uncalibrated “rate” image by a static fringe flat constructed from observations of a bright source that fills the entire MRS field of view. For more details see the fringe step. This step generally does a good job of removing the strongest fringes from an astronomical scene, particularly for nearly-uniform extended sources. Since the fringe signal is different for point sources, however, and varies as a function of the location of a point source within the FOV, the static fringe flat cannot fully correct such objects and the default high level data products will therefore still show appreciable fringes.

The pipeline also includes two optional residual fringe correction steps whose purpose is to find and remove signals whose periodicity is consistent with known fringe frequencies (set by the optical thickness of the detectors and dichroics) using a Lomb-Scargle periodogram. The number of fringe components to be removed is governed by a Bayesian evidence calculation. The first of these residual fringe correction steps is a 2-D correction that can be applied to the flux-calibrated detector data in the residual_fringe step. This step is part of the calwebb_spec2 pipeline, but currently it is skipped by default. To apply this step set the step parameter, --skip = False. This step is applied after photom, but before cube_build.

The residual_fringe step can accept several different forms of input data, including:

  1. a single file containing a 2-D IFU image

  2. a data model (IFUImageModel) containing a 2-D IFU image

  3. an association table (in json format) containing a single input file

The second of the residual fringe correction steps is a 1-D correction that can be applied to one-dimensional spectra extracted from MRS data cubes by setting the optional parameter extract_1d.ifu_rfcorr = True in the extract_1d step. Empirically, the 1-D correction step has been found to work better than the 2-D correction step if it is applied to per-band spectra. For more details on this step see extract_1d step.

Assumptions

This step only works on MIRI MRS 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. While the fringe frequencies are well known, amplitudes can vary due to beating between the different fringe components and additionally are sensitive to the detailed location and intensity of objects within a given astronomical scene. Fringing thus cannot be corrected in its entirety for an arbitrary astronomical scene without forward modeling.