Fringing solution for the mid-infrared instrument on-board the James Webb Space Telescope

Abstract

The Mid-Infrared Instrument (MIRI) on-board the James Webb Space Telescope (JWST) performs medium resolution spectroscopy in the 5 to 28.5micron wavelength range. The Medium-Resolution Spectrometer (MRS) of MIRI uses two Si:As impurity band conduction detector arrays. Coherent reflection of infrared light within the MIRI MRS detectors results in fringing; the detector layers act as efficient Fabry-Perot etalons. In this paper we present three methods to calibrate out the fringes, as part of the MIRI data reduction pipeline. The methods are presented in the context of the investigations on the fringing seen in the MIRI flight model ground test data. The investigations show that the detector fringe transmission depends on the illumination pattern of the observed source on the detector. Optical stimuli of different spatial extents and position in the field-of-view yield different fringe patterns in their extracted spectra. An optical model of the MIRI detectors is hence proposed. By solving the Fresnel equations across the model optical layers, a source-specific fringe correction is derived.