MIRI LRS TSOs
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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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:
- 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.
- 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 Performance, JWST 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 Subarrays, MIRI 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.
LRS TSOs in APT
See also: MIRI LRS APT Template, JWST Time-Series Observations Roadmap, MIRI Example Science Programs, JWST 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.