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James Webb Space Telescope
MIRI Low Resolution Spectrometer

The LRS has been designed specifically for single slit spectroscopic observations of compact targets. It uses an entrance slit shown in the figure below, which is 5.5" long and 0.6" wide, and requires the P750L double prism to be selected in the filter wheel. This prism disperses the 5 to 10 μm spectral band of light from the target over approximately 140 detector pixels. There is no shutter or way to block off light going into the slit while imaging nor is there a way to block off light onto the imaging plane when taking a low resolution spectrum. If there are other point sources in the imaging field at the time of an LRS observation, these will appear as slitless spectra on the imaging fields. If a point source were to lie in the Lyot coronagraph field, the instrument has been designed so that this point source spectrum will not overlap with a source in the slit.