Low-resolution spectroscopy is an observing mode for JWST’s Mid-Infrared Instrument (MIRI) that offers slit and slitless spectroscopy from 5 to 14 μm.

See also: MIRI LRS APT Template

MIRI's low-resolution spectrometer (LRS; Kendrew et al. 2015) offers both fixed slit and slitless spectroscopy from 5 to 14 μm using a double prism mounted in the MIRI filter wheel. These optics provide a spectral resolving power (R = λ/Δλ) of ~100 at 7.5 µm, varying from R ~ 40 at 5 μm to R ~ 160 at 10 μm. The LRS forms part of the MIRI imager module, and spectra are imaged onto the imager detector array.

The LRS can be operated to obtain spectra in the slit, or in slitless mode. Slitless spectroscopy is only available for time-series observations of time variable phenomena, such as transiting exoplanets or eclipsing binaries. 

When preparing observations, the observer has control of 3 primary variables: (1) choice of fixed slit vs. slitless, (2) dithering requirements (for fixed slit only), and (3) detector readout mode and exposure settings. 

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Slit vs. slitless spectroscopy

See also: JWST Slit Spectroscopy, JWST Wide Field Slitless Spectroscopy, MIRI LRS TSOs

The LRS can be operated in fixed slit or slitless mode. Incoming light travels the same path for both modes and use the same double prism as the disperser. The modes are very similar apart from the target placement in the field.  For fixed slit spectroscopy, the target is placed at the location of the slit, and for slitless it is positioned at a specific pointing location in a dedicated detector subarray. Figure 1 shows the relevant detector locations in the imager focal plane layout. The choice between slit and slitless operation for LRS is dictated solely by the need for high precision spectrophotometric performance and the conditions

The slit and slitless modes have some operational differences.

• The LRS slit mode is suited to a broad range of spectroscopic observations and therefore supports a wider range of operational choices.

  The single slit is 4.7" long (3.18 mm; 42.7 pixels) and 0.51" wide (0.33 mm; 4.6 pixels). The projected location of the slit on the focal plane array lies between the imager field of view and the coronagraphy regions on the imager detector; its location is fixed. There is no subarray choice for LRS slit: for these observations, the entire imager array is read out (APT Subarray parameter set to FULL). The structure containing the slit blocks a large portion of the background close to the source from the detector, providing better sensitivity than the slitless mode. 
  
  

• Slitless LRS is a mode dedicated to time-series observations (TSOs), and operation is optimized for high precision spectrophotometry, e.g. of exoplanet transits.

  This mode uses a dedicated subarray region on the detector (APT Subarray parameter set to SLITLESSPRISM); its location is shown in Figure 1. The use of a smaller subarray provides faster read times and thus a greater dynamical range. The saturation limit is several magnitudes brighter than for LRS slit. The absence of the slit, however, allows more background radiation to be dispersed over the science spectrum, reducing the sensitivity by around an order of magnitude approximately. While LRS slitless is ideal for high-precision spectrophotometric observations of bright point-source targets, the slit is expected to give better performance for faint targets.

As both modes use the same dispersing element, the dispersion profile is, in principle, the same for both. The nominal spectral range of 5 to 14 μm is dispersed over approximately 390 pixels. The dispersion profile however folds over below 4.5 µm (where the prism throughput is very low), superimposing two parts of the spectrum onto each other. A dedicated filter is mounted over the slit to block radiation shortward of 4.5 µm to avoid this contamination in the slit. This effect is not mitigated for LRS in slitless mode, causing some spectral contamination at the shortest wavelengths.  The calibration at wavelengths past 12 µm is still a work in progress, due to limited signal/noise in the available calibration data, but this situation will improve with time.

Table 1 below shows the main differences between the slit and slitless modes of LRS.

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Dithering

See also: MIRI LRS Dithering, MIRI Dithering

LRS Slit

Dithering is available and mandatory for observations in the LRS slit. Dithering mitigates for the effects of bad pixels, provides subpixel sampling, and provides observations of the background for background subtraction purposes.

Two dither options are offered in the MIRI LRS Template in APT:

1. ALONG SLIT NOD
2. MAPPING

 The ALONG SLIT NOD option places the target alternately at 30% and 70% along the slit length—a separation of approximately 1.9". This should be the default choice for slit spectroscopy of point or compact sources. 

The MAPPING option can be chosen to map a wider field. Selecting this option then asks the user to define the number of steps in the spatial and spectral directions, and the step size in each direction (in arcseconds). Although the maximum dither size is set by avoiding the need to acquire new guide stars, a useful guideline is that dithers larger than 20" will be much slower than ones smaller than this limit. When choosing the MAPPING option, the user does not control the order in which the exposures will be executed.

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References

Kendrew, S., et al. 2015, PASP, 127, 623 
The Mid-Infrared Instrument for the James Webb Space Telescope, IV: The Low-Resolution Spectrometer

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