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. (The bold italic font indicates these are parameters in APT observing templates.)
LRS slit spectroscopy has three limitations:
A narrow slit (relative to the PSF) suffers from throughput variations due to telescope pointing uncertainties and drifts. This is not ideal for high spectrophotometric precision.
A second filter is mounted on the slit mask to mitigate the folding over of the spectrum around 4.5 µm. While this filter helps with the wavelength calibration at short wavelengths, it somewhat reduces the throughput over the full bandpass compared with the slitless mode.
LRS slit observations require full-frame detector readout (i.e., 2.775 s readout time). Given the shortest exposure (i.e., 2 up-the-ramp samples) and the pixel saturation threshold (60% of the full storage capacity), the saturation limit at 5.5 μm is estimated to be only 63 mJy, which may be too low for some observations.
These concerns are specific mostly to time series observations (TSO) (e.g., time-variable brightness of transiting exoplanet systems). For such demanding science cases, single object slitless LRS spectroscopy (and subarray mode) will also be available. This 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 (0.159 s). Figure 1 identifies the location of the slitless spectrum on the detector. The SLITLESSSPRISM subarray, if selected, will allow observers to monitor point sources as bright as 2.9 Jy at 7.5 μm.
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.
Two dither pattern types will be offered:
- POINT SOURCE
- EXTENDED TARGET
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