A new set of MIRI Imager and Low-Resolution Spectroscopy (LRS) subarrays, designed to eliminate the primary source of electromagnetic interference (EMI) and facilitate instrument enhancements, will be available for Cycle 6 proposers.

The MIRI subarrays SUB128, SUB64, and SLITLESSPRISM are affected by EMI noise dominated by a 390 Hz signal, caused by electronic frequencies out of phase with pixel read timing. Since build 10.1 (May 2024) the JWST calibration pipeline has included the emicorr step to fit and remove this noise pattern (see Brandt, Regan & Bergeron, 2025). The new subarray design eliminates the 390 Hz component, while a lower‑level 10 Hz signal is not removable and will continue to be handled by emicorr.

The new imaging subarrays (SUB128_IP, SUB64_IP, where _IP stands for “In Phase”) match the field of view and readout time of their current counterparts to within 5% (See Fig. 1). The new LRS SLITLESSPRISM_IP subarray is 10% smaller and has a 2% faster group time than SLITLESSPRISM, achieved by removing unused rows at the bluest end of the subarray. Moving SLITLESSPRISM_IP up on the detector also pushes the “shadowed region” sometimes affecting time series observations further into the infrared, moving it from about 10.6 µm to about 12.2 µm.

In addition, two new LRS subarrays will be offered: SLITLESSPRISM_IPS (SLITLESSPRISM, In Phase, Short) and SUBSLIT. SLITLESSPRISM_IPS is a smaller, 24% faster subarray than SLITLESSPRISM_IP, providing additional dynamic range for bright targets. Its reduced size eliminates the “shadowed region” affecting time-series observations at the expense of wavelength coverage beyond 12.5 µm. The SUBSLIT subarray enables Low-Resolution fixed-slit spectroscopy with a factor 9 higher dynamic range. 

The coronagraphic subarrays are also affected by the 390 Hz EMI noise; however, due to the precision alignment requirement on the 4-quadrant phase masks, these will not be replaced at this time. 
Both the current and new Imaging and LRS subarrays will be offered in Cycle 6 to provide a transition period for users. The MIRI team can advise on subarray selection via the JWST Help Desk.