Time-series observations (TSOs), which are typically very precise spectro-photometric observations of time-variable objects, are allowed for the JWST MIRI low-resolution spectrometer (LRS).

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A typical problem with spectroscopic observations using a narrow (relative to the PSF) slit is throughput variations due to telescope pointing uncertainties and drifts. The natural solution, therefore, to reach higher spectrophotometric precision is to operate the MIRILRS in a slitless mode, thus preventing any pointing-induced throughput variations. The LRS slitless subarray (SLITLESSPRISM 1) further offers the optimal solution between the requirement for fast readout times and a large enough detector area to get a proper spectral image. This mode will be particularly useful for characterizing transiting exoplanets, but may also be used for other sources such as eclipsing binaries and cataclysmic variables.

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Slit vs. slitless spectroscopy

Main article: MIRI Low-Resolution Spectroscopy

The expected light loss from the slit mask is significant when compared with slitless spectroscopy, particularly at longer wavelengths (λ > 9 μm).  In addition, there are considerable increases in achievable precision over a long baseline. There are, however, two main limitations to slitless spectroscopy:

  1. The sensitivity is around a factor 10× lower than the LRS in slit mode. The absence of the slit leads to a higher background in the slitless subarray, effectively reducing the signal to noise. Observers should keep this in mind when considering fainter targets.
  2. The dispersion profile of the LRS turns over below 4.5 µm—for a limited wavelength range around 4.5–5 µm, different 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. For slit observations, a dedicated filter is mounted on the slit mask structure to block the radiation below 4.5 µm.

TSO mode in APT

See also: MIRI LRS APT Template

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 Ngroups = 5. The minimum Ngroups required is 2. However ground testing indicates that the accuracy of the detector calibration worsens significantly as read out ramps are shortened. Observations for which absolute calibration is important are advised against selecting Ngroups < 5.

Slitless spectroscopy mode is selected from the MIRI LRS template in the Astronomer's Proposal Tool (APT), by choosing the SLITLESSPRISM  subarray. Low-resolution spectroscopic TSO observations must use the slitless option. Selecting the slitless mode 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 allow longer time-series monitoring.

Selection of slitless LRS spectroscopy results in several unique settings:

  • Disables dithering for optimal stability and repeatability
  • Defaults to FAST read mode
  • Waives the 10,000 s exposure time limit
  • Makes target acquisition mandatory


Users should always refer to the JWST Exposure Time Calculator for the most up to date sensitivity numbers and bright limits.

Articles in the MIRI performance section provide information about the sensitivities and bright limits for the MIRI LRS modes.



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