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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To reach the highest spectrophotometric precision for long-duration observations of time-variable targets (such as transiting exoplanets) with the MIRI LRS is to operate the LRS in a slitless mode, which prevents pointing-induced throughput variations. The LRS slitless subarray (SLITLESSPRISM) further extends the dynamic range of the LRS with its shorter readout time of 0.159 s. 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.

Slit vs. slitless spectroscopy

See also: 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. Two important differences to consider for slitless LRS are:

  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. 

  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.

Sensitivity and saturation limits

See also: MIRI PerformanceJWST Time-Series Observations Roadmap

The performance of LRS in slitless mode is outlined in the "See also" pages mentioned above. Users should refer to the JWST Time-Series Observations Roadmap article for advice on setup and calculating exposure time.


See also: MIRI Detector SubarraysMIRI Low Resolution Spectroscopy

TSOs with the MIRI LRS are 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.

Exposure time limitations

See also: MIRI LRS APT Template

Regular (non-TSO) MIRI observations have an exposure time limit of 10,000 s for a single exposure. For TSOs, this limitation can be waived to allow observations of long time-variable phenomena in a single exposure, which is optimal for stability and photometric precision. Selecting Time Series Observation in the Special Requirements pane in APT enables this waiver. 


See also: MIRI LRS APT TemplateJWST Time-Series Observations RoadmapMIRI Example Science ProgramsJWST Example Science Programs

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, and disables dithering. 

Further advice on how to prepare TSOs with the MIRI LRS is provided in the articles mentioned above and example science cases.

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