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.
Parent page: MIRI Observing Modes
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 LRS forms part of the MIRI Imager, and spectra are imaged onto the imager detector array. The LRS can be operated with slit or in slitless mode; the latter is optimised for time series observations (TSOs). This page describes the differences between the two options.
Observers will be asked to configure 5 aspects of their observations:
- Subarray (which controls whether the observation will be slitless or with slit)
- Target Acquisition
- Dither or mapping pattern
- Detector read mode and exposure settings
- Mosaic settings
Slit vs. slitless spectroscopy
The LRS can be operated in slit or slitless mode. Incoming light travels the same path for both modes, and use the same double prism as disperser. The modes are very similar apart from the target placement in the field: for slit spectroscopy, the target is placed at the location of the slit, for slitless it is positioned at a specific pointing location in a dedicated detector region. Figure 1 shows the relevant detector locations in the imager focal plane layout.
The slit and slitless modes have some operational differences. Slitless LRS is a mode dedicated to time series observations (TSOs), and operation is optimised for high-precision spectrophotometry over long observations, e.g. of exoplanet transits. 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. The structure containing the slit blocks a large portion of the background from the detector, providing better sensitivity than the slitless mode.
Slitless LRS, in contrast, uses a dedicated subarray region on the detector (called 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. 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.
As both modes use the same dispersing element, the dispersion profile is in principle the same for both. The nominal spectral range of 5–12 µm is dispersed over approximately ~370 pixels. The dispersion profile however folds over below 4.5 µm (where the prism throughput is very low), superimposing two parts of the spectrum on each other. A dedicated filter is mounted over the slit to block radiation shortward of 4.5 µm, to avoid this contamination. The effect is not mitigated for LRS in slitless mode.
The strengths and weaknesses of slit and slitless LRS modes are summarised in Table 1 below.
Table 1. Relative strengths and weaknesses of LRS slit and slitless modes
Sensitivity (10 times 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
Brighter saturation limits due to short read time
Allows > 10,000 s exposures
No slit losses
Sensitivity (10 times worse than slit)
Spectral foldover around 4.5 µm affects calibration
Sources in the imager field of view when performing LRS slit spectroscopy
There is no shutter or way to block light from entering the imager field of view (see Figure 1) when taking a low-resolution spectrum in slit mode. Point sources in the imaging field will therefore appear as slitless spectra on the imager FOV. 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. Bright extended sources that are not saturated may cause a small amount of scattered light in the detector pixels below the LRS slit (<< 1%); this may be of concern for observations of very faint targets. Observers should take care to avoid bright targets in the imager portion of the field, to avoid saturation in the full array. This can be checked using the Aladin visualization option in APT. 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.
The spectra of sources coincidentally located in the imager field cannot be processed, nor calibrated.
Target Acquisition (TA) is available for both LRS slit and slitless observations, and in most cases recommended. The blind pointing accuracy of the telescope is not sufficient to place a point source target in the slit with the required accuracy for calibration. For slit spectroscopy, accurate knowledge of the source position in the slit is critical for calibration, in particular for correction of wavelength- and position-dependent slit losses, and for wavelength calibration. For slit spectroscopy of extended sources, the user should decide on the importance of TA given the telescope's pointing accuracy and the nature of the observations (e.g. a single pointing vs mosaic). TA can also be disabled for off-source background observations, which contain no science target.
For low-resolution slitless spectroscopy, which is optimised for time series observations, TA is required for placing the target at the nominal pointing position in the SLITLESSPRISM subarray with sub-pixel accuracy. This is particularly important if multiple observations from different epochs are to be combined to improve the sigal to noise ratio.
Target acquisition uses dedicated "regions of interest" (ROIs) on the detector, measuring 64 x 64 pixels in size. The locations of these ROIs and the TA observations sequence is different for slit and slitless LRS. Further choices should be made when preparing the TA portion of an observing program, for the TA target, filter and exposure duration. We refer to the dedicated articles for further details on TA for both LRS modes.
Dithering is available and recommended for observations with the LRS in slit mode. Dithering can mitigate the effects of bad pixels, provide sub-pixel sampling, and provide observations of the background for background subtraction purposes. The majority of science observations will benefit from dithering.
Three dither options are offered in the MIRI LRS Template in APT:
- ALONG SLIT NOD
- NONE (only for LRS slitless)
The 3rd option, dither = None, is only enabled for LRS slitless obervations; it is not available for LRS slit mode.
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.
In LRS slitless mode, dithering is disabled. This mode is optimised for high-precision spectrophotometry in time series observations; for such observations dithering is not scientifically useful and therefore not supported.
LRS exposure specifications
MIRI LRS slit spectroscopy supports two different detector readout patterns:
The maximum exposure duration for a single exposure with the LRS slit is 10,000 seconds. This limit applies to all JWST instruments and modes that are not time series observations. LRS slitless observations are always marked as time series observations; the 10,000 second limit is therefore waived to allow for lengthy observations of time-variable phenomena.
The LRS can be used to produce mosaics of extended sources with slit spectroscopy, by either specifying a mapping pattern from the MAPPING option in the list of dither patterns or in the "Mosaic" tab. When using the latter, the user can specify the number of required rows and columns, and the percentage overlap between each pointing.
The user is always advised to check the layout of their mosaic using the Aladin visualization capability in APT.
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
This page has no comments.