The MIRI filter wheel has 10 filters for imaging, 4 filter-diaphragm sets for coronagraphy, and one double prism assembly for low-resolution spectroscopy (there is also a lens element that was used for ground-testing). The medium-resolution spectrometer has 2 wheels for controlling gratings and dichroics positions.

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For the Mid-Infrared Instrument (MIRI), both the imager (MIRIM) and medium-resolution spectrometer (MRS) channels are fed from a single pick-off mirror (POM).

The imager has a single filter wheel that holds all the elements necessary for MIRI's 3 functional areas: imager, coronagraph, and low-resolution spectrometer (LRS). For low-resolution spectroscopy, a double prism is used to disperse the light; it is mounted in the imager filter wheel, with its position designated P750L.

The MRS has its own dichroic filter/grating wheels that move gratings and dichroics simultaneously to allow for a specific wavelength coverage.

Filter throughput curves

When calculating sensitivities, we encourage you to use both the JWST Exposure Time Calculator (ETC) and the ETC engine, named Pandeia. The sensitivities depend on a variety of telescope contributions. Details can be found on the JWST ETC Instrument Throughputs page.

Imager

The MIRI imager filter wheel includes:

10 filters for imaging (F2550WR is redundant)
4 filter-diaphragm combinations for coronagraphy
one neutral density filter
one ZnS-Ge double prism for the LRS mode (P750L)
one opaque position for darks
one lens for ground testing purposes

Imaging filters

The MIRI imaging mode allows users to select among 10 filters for observations.

Table 1. Imaging filter properties

Filter name	λ₀(μm)	Pivot^* λ (μm)	BW^† Δλ (μm)	Effective^‡ response	Blue^§ λ- (µm)	Red^§ λ+ (µm)	FWHM¹ (arcsec)	Comment
F560W	5.6	5.635	1.00	0.245	5.054	6.171	0.207	Broadband Imaging
F770W	7.7	7.639	1.95	0.355	6.581	8.687	0.269	PAH, broadband imaging
F1000W	10.0	9.953	1.80	0.466	9.023	10.891	0.328	Silicate, broadband imaging
F1130W	11.3	11.309	0.73	0.412	10.953	11.667	0.375	PAH, broadband imaging
F1280W	12.8	12.810	2.47	0.384	11.588	14.115	0.420	Broadband imaging
F1500W	15.0	15.064	2.92	0.442	13.527	16.640	0.488	Broadband imaging
F1800W	18.0	17.984	2.95	0.447	16.519	19.502	0.591	Silicate, broadband imaging
F2100W	21.0	20.795	4.58	0.352	18.477	23.159	0.674	Broadband imaging
F2550W	25.5	25.365	3.67	0.269	23.301	26.733	0.803	Broadband imaging
F2550WR	25.5	25.365	3.67	0.269	23.301	26.733	0.803	Redundant filter, risk reduction
FND	~13	12.900	6.73	0.000764	8.456	15.473	--	For bright target acquisition
Opaque	blackened blank	N/A	N/A	N/A	N/A	N/A	N/A	Darks

^* The pivot wavelength satisfies the equation $\begin{array}{l}F_\lambda \lambda_{pivot}^2 = F_\nu c\end{array}$ , relating the flux measured in wavelength versus frequency units ( $\begin{array}{l}F_\lambda d \lambda = F_\nu d \nu\end{array}$ ). It is calculated as $\begin{array}{l}\lambda_{pivot} = \sqrt{\frac{\int d\lambda T \lambda}{\int d\lambda T / \lambda}}\end{array}$ , where $\begin{array}{l}T\end{array}$ is the transmission. See Tokunaga & Vacca 2005.

^† Bandwidth is the integral of the normalized transmission curve: $\begin{array}{l}BW = \frac{\int d\lambda T}{T_{max}}\end{array}$ . See equation 1 in appendix E of Rieke, G. H. et al. 2008.

^‡ Effective response is the mean transmission value over the wavelength range of $\begin{array}{l}\lambda_{pivot} \pm BW ~ / ~ 2\end{array}$ .
^§ The half power wavelengths of a passband are the wavelengths at which the transmission falls to 50% of its peak value.

¹ FWHM refers to the PSF

Plot generated for Cycle 4. Figure credit: STScI MIRI Team.

Coronagraphic imaging filters

These filters are associated directly with each coronagraph and are not interchangeable. Selecting the filter selects the coronagraph.

Table 2. Coronagraph filter-mask combinations

Filter	Coronagraph	Pupil mask transmission (%)^†	Central wavelength (μm)	Bandwidth^‡ (μm)	IWA^§ (arcsec)	Rejection^* (on-axis)
F1065C^¶	4QPM1	62	10.575	0.75	0.33	260
F1140C	4QPM2	62	11.30	0.8	0.36	285
F1550C	4QPM3	62	15.50	0.9	0.49	310
F2300C	Lyot spot^◊	72	22.75	5.5	2.16	850

^† Coronagraph filters are paired with pupil masks to reduce diffracted light from both the telescope pupil and the coronagraph, but at the expense of some loss of total intensity.

^‡ Bandwidth is defined to extend to wavelengths on either side of the central wavelength that correspond to 5%–10% of the transmission efficiency.

^§Inner working angle (IWA) is defined as the 50% transmission radius.

^* Rejection is the total flux attenuation of a star when centered onto the coronagraph. The term is unitless since it is a ratio of two intensities (out of mask / on the mask).

^¶ Band pass useful for NH₃ and silicates.

^◊ The "spot" refers to the circular occulting mask in the Lyot-type coronagraph.

Low resolution spectrometer (LRS)

The LRS mode of MIRI uses a double zinc sulfide/germanium (ZnS/Ge) prism mounted in the MIRI imager filter wheel (see Fig. 1). The LRS provides a spectrum with resolving power R~100 covering 5 to 14 μm in a single exposure; where resolving power R is defined as λ/Δλ. The resolving power increases linearly from R~40 to ~160 from 5 to 10 µm. The dispersion profile of the double prism shows a turnover around 4–4.5 µm, which folds the spectrum back onto itself at short wavelengths. To mitigate this, the slit mask was fitted with a filter that blocks light shortward of ~4.5 µm. Note that for slitless, this spectral foldover remains present. Figure 4 shows the spectral dispersion profile of the LRS in both its modes of operation, as well as the photon conversion efficiency of the LRS in both slit and slitless mode, derived from commissioning data.

Even though the optical path is the same for LRS in slit and slitless modes, due to field-dependent optical distortion the dispersion profile is different for the 2 modes. Each mode therefore has its own wavelength calibration reference file in the JWST calibration pipeline.

Left: Dispersion of the MIRI LRS double prism, showing how wavelengths are mapped onto pixels, for fixed slit and slitless modes. Note the turnover in the dispersion shortward of 4 µm, and the difference between the two profiles due to field-dependent optical distortion. The green dashed line indicates the filter cut-off implemented for LRS slit. Right: the photon conversion efficiencies (PCE) for slit (blue) and slitless (red) modes, in DN/s per Jy/spaxel, which was derived assuming the source flux is integrated over the slit aperture (i.e. "spaxel" corresponds to the slit aperture).

Medium resolution spectrometer (MRS)

The MRS has 4 separate IFUs (called channels 1-4), each covering a separate wavelength range between 4.9 to 27.9 μm. All 4 channels are observed simultaneously, but each exposure can only cover one-third of the available wavelength range in a single configuration.

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

For complete spectral coverage, 3 different spectral settings must be observed, called SHORT (A), MEDIUM (B), and LONG (C). The dichroic filter wheel comprises of 3 working positions to move gratings and dichroics simultaneously. Each is located on separate wheel discs. The 2 wheels feed light into the 4 spectrometer channels inside MIRI. Filters (and associated resolving power) are summarized in both Figure 5 and Table 3.

Table 3. MRS wavelength coverage

FOV name λ-range (μm)	FOV (arcsec)	Number of slices	Slice width (arcsec)	Pixel size (arcsec)	Sub-band name	λ-range (μm)	Resolving power (λ/Δλ)
Channel 1 4.9–7.65	3.2 × 3.7	21	0.176	0.196	SHORT (A) MEDIUM (B) LONG (C)	4.90–5.74 5.66–6.63 6.53–7.65	3,320–3,710 3,190–3,750 3,100–3,610
Channel 2 7.51–11.7	4.0 × 4.8	17	0.277	0.196	SHORT (A) MEDIUM (B) LONG (C)	7.51–8.77 8.67–10.13 10.01–11.70	2,990–3,110 2,750–3,170 2,860–3,300
Channel 3 11.55–17.98	5.2 × 6.2	16	0.387	0.245	SHORT (A) MEDIUM (B) LONG (C)	11.55–13.47 13.34–15.57 15.41–17.98	2,530–2,880 1,790–2,640 1,980–2,790
Channel 4 17.7–27.9	6.6 × 7.7	12	0.645	0.273	SHORT (A) MEDIUM (B) LONG (C)	17.70–20.95 20.69–24.48 24.40–27.90	1,460–1,930 1,680–1,770 1,630–1,330

References

Wright, G. et al. 2015, PASP, 127, 595
The Mid-Infrared Instrument for the James Webb Space Telescope, II: Design and Build

Notable updates	16 Jul 2024 Updated PCE plots (Figures 2, 3, 5) for Cycle 4. 11 Aug 2023 Updated MRS PCE plot (Fig. 5) 27 Nov 2022 Updated for Cycle 2 based on commissioning 31 Mar 2017 Table 3 was updated.
Originally published	29 Dec 2016

JWST User Documentation

MIRI Filters and Dispersers