1. The sensitivity in the slitless mode is around a factor 10× lower than in the fixed slit mode. The absence of the slit leads to a higher background level across the slitless subarray, effectively reducing the signal to noise. 
   
   
2. The dispersion profile of the LRS turns over below 4.5 µm—for a limited wavelength range around 4.2-4.5 µm, multiple wavelengths are dispersed onto the same detector pixels. As a result, the wavelength and absolute flux calibrations in this region are not as reliable as for LRS slit observations, and caution is warranted when analyzing spectra obtained in slitless mode at wavelengths below 5 µm. For fixed slit observations, a dedicated filter is mounted on the slit mask structure to block the radiation below 4.5 µm. 

Sensitivity and saturation limits

See also: MIRI Performance, JWST Time-Series Observations Roadmap, MIRI TSO Recommended Strategies

The performance of LRS in slitless mode is outlined in the "See also" pages listed above. Users should refer to the JWST Time-Series Observations Roadmap and MIRI TSO Recommended Strategies article for advice on setting up observations and calculating exposure times.

Subarrays

See also: MIRI Detector Subarrays, MIRI Low Resolution Spectroscopy, MIRI TSO Recommended Strategies

TSOs with the MIRI LRS are currently only supported in slitless mode, which uses the SLITLESSPRISM subarray. The location and size of the subarray can be seen on the above pages. The shorter frame read time for this smaller subarray provides additional dynamic range compared to full array exposures. The location of the subarray at the left edge of the MIRI imager detector array enables the fast readout time. However, the left-most 9–10 columns (which includes the 4 columns of reference pixels) are not usable for science. The nominal pointing location for targets is near the middle of the subarray, excluding the 4 reference pixel columns. 

An important aspect to note regarding the LRS slitless mode is that the SLITLESSPRISM subarray straddles regions of the detector with different illumination patterns. One portion of the subarray lies behind the focal plane masking structure; these pixels are only illuminated when the double prism is in place in the filter wheel. In some TSOs, different settling response behavior has been observed in this region compared to the rest of the subarray.

LRS TSOs in APT

See also: MIRI LRS APT Template, JWST Time-Series Observations Roadmap, MIRI TSO Recommended Strategies

Time-series observations of bright targets require fast read times to avoid detector saturation. To this end, observations may be carried out with fewer than the recommended N_groups = 5. The minimum N_groups required is 2. However, experience from early cycles shows that the quality of the calibration is sub-optimal for small numbers of groups. Integrations with 2–5 groups are very difficult to calibrate accurately, due to the presence of persistence effects on the detector. Therefore, integrations should ideally use >5 or even >10 groups. 

Slitless spectroscopy mode is selected from the MIRI LRS template in the Astronomer's Proposal Tool (APT) by choosing the SLITLESSPRISM subarray. LRS TSOs must use the slitless option. Selecting the SLITLESSPRISM subarray for LRS will automatically add Time Series Observation and No Parallel to the Special Requirements pane in APT. This creates a waiver for the usual exposure time limit of 10,000 s, to enable longer time-series monitoring, and disables dithering. 

Further advice on how to prepare TSOs with the MIRI LRS is provided in the articles listed above.