Low-resolution spectroscopy is an observing mode for JWST’s Mid-Infrared Instrument (MIRI) that offers slit and slitless spectroscopy from 5 to 12 μm.
MIRI's low-resolution spectrometer (LRS; Kendrew et al. 2015) offers both slit and slitless spectroscopy from 5 to 12 μm using a double prism mounted in the MIRI filter wheel, designed to provide a spectral resolving power of R = 40 at 5 μm, and R = 160 at 10 μm for compact sources (<2"). The long-wavelength limit for this mode is determined by the combined throughput of the prisms and the slit mask, which drops off steeply from a peak of around 80% at 8–9 µm to just 25% at 12 µm. Point source sensitivity will be nearly a factor of 10× better when using the slit.
Observers will be able to select parameters for 3 primary characteristics of the low-resolution spectroscopy mode: 1) presence of a slit, 2) dithering pattern, and 3) detector read out mode and exposure time (via the number of frames and integrations).
Slit vs. slitless spectroscopy
The LRS can be operated in slit (FULL) or slitless (SLITLESSPRISM) mode. Figure 1 shows the LRS focal plane layout on the MIRI imaging detector. The single slit is 4.7" long (3.18 mm; 42.7 pixels) and 0.51" wide (0.33 mm; 4.6 pixels). The nominal spectral range of 5–12 µm is dispersed over approximately ~370 pixels.
Strengths and weaknesses of slit and slitless LRS
The relative strengths and weaknesses of LRS in slit and slitless mode are shown in the table below.
Sensitivity (10x better than slitless)
Narrow slit (relative to the PSF) makes it more sensitive to pointing uncertainties and drifts
Slit mask filter to mitigate spectral foldover at short wavelengths reduces throughput
Saturation limit relatively faint
Better saturation limits due to short read time
Allows > 10,000s exposures
No slit losses
Sensitivity (10x worse than slit)
Spectral foldover around 4.5 µm affects calibration
The limitations of LRS in slit mode present particular concerns for time series observations (TSO) (e.g., time-variable brightness of transiting exoplanet systems). The slitless mode is specifially designed for such observations and overcomes the limitations posed by the presence of the slit. This slitless mode, which uses the SLITLESSPRISM subarray, currently only supports TSO proposals. The mode does not use dithers and is optimized for spectrophotometric precision and fast readout times. Figure 1 identifies the location of the slitless spectrum on the detector. In slitless mode, MIRI can perform low-resolution spectroscopy on stars that are several magnitudes brighter than with the slit. Note that the absence of the slit allows more background radiation to be dispersed over the science spectrum, reducing the sensitivity by around an order of magnitude. Whilst 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.
Note on wide-field slitless spectroscopy while using the slit
There is no shutter or way to block light from entering the imager FOV (see Figure 2) when taking a low-resolution spectrum. Point sources in the imaging field will therefore appear as slitless spectra on the Imager FOV. Some stray light from sources in the Imager FOV may be scattered into the LRS spectral region, so observers should consider this when planning their observations. Furthermore, the broad bandpass of the LRS prisms can easily cause the detector to saturate if bright and extended sources are present in the imager portion of the array. Such saturation can affect the detector behavior over the entire array, including calibration of the spectrum even if the spectrum itself is not saturated. Observers should take care to avoid saturation in the full array. If a point source were to lie in the Lyot coronagraph field, however, the instrument has been designed so that this point source spectrum will not overlap with any source in the slit.
While this setup technically allows users to obtain wide field slitless spectroscopy, this mode is not recommended or supported by STScI.
Dither patterns with LRS
MIRI operations offer support for LRS slit dithers. In general, dithering with LRS slit observations can mitigate the effects of bad pixels and obtain subpixel sampling and observations of the background. The majority of science observations will benefit from dithering, although some observations where relative pixel response is important (e.g., extrasolar transiting planets) may benefit from no dithering.
Three dither options will be offered:
- POINT SOURCE
- EXTENDED TARGET
When using LRS in slitless mode, NONE is the only availble option. 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.
JWST User Documentation Home
Mid-Infrared Instrument, MIRI
MIRI Bright Source Limits
MIRI Detector Readout Overview
MIRI Spectroscopic Elements
MIRI Filters and Dispersers
MIRI Low Resolution Spectroscopy Template APT Guide
MIRI Detector Readout Patterns
MIRI Detector Readout Fast
MIRI Detector Readout Slow
MIRI SensitivityJWST Astronomers Proposal Tool, APT
JWST APT website
JWST Exposure Time Calculator
JWST ETC website