The MIRI Medium Resolution Spectrometer (MRS) observes primarily first-order light dispersed by a given grating. However, the dichroic filters have a small spectral leak which causes a few percent of the second-order light from 6.1 µm (Channel 1B) to leak into the 12.2 µm (Channel 3A) bandpass. As a result, sources with an extremely blue spectral shape may show a corresponding emission-like artifact, about 0.4 µm wide, around 12.2 µm.

In Build 10.1 of the JWST Science Calibration Pipeline (released on February 28, 2024), a new step was added to the pipeline to correct for this spectral leak at 12.2 µm using a scaled version of the 6.1 µm spectrum. This correction is automatically applied to one-dimensional spectra extracted from observations of point sources and allows the pipeline to better recover the intrinsic spectrum of such sources (see Figure 1 below).

However, no such correction is available for the three-dimensional IFU data cubes or for extracted spectra of extended sources. This is because the Channel 1B field of view is smaller than the Channel 3A field and, therefore, the shorter wavelength data cannot be used to correct the 12 µm data throughout the field of view. Additional information can be found on JDox.

Users seeking more information about the MRS spectral leak and its possible impact on their observations are encouraged to reach out to the JWST Help Desk using the MIRI card.