MIRI Filters and Dispersers

The MIRI filter wheel has 10 filters for imaging, 4 filter-diaphragm sets for coronagraphy, and one double prism assembly for low-resolution spectroscopy. 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

See also: MIRI Imaging, MIRI Coronagraphic Imaging, MIRI Low Resolution Spectroscopy

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

Figure 1. MIRI imager filter wheel

MIRI imager filter wheel

Locations of filters in the filter imager wheel. © Wright et al. 2015 (annotated figure).

Imaging filters

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


Table 1. Imaging filter properties

Filter nameλ0 (μm)Δλ (μm)FWHM* (arcsec)Comments

 F560W

5.6

1.2

0.207

Broadband Imaging

 F770W

7.7

2.2

0.269

PAH, broadband imaging

F1000W

10.0

2.0

0.328

Silicate, broadband imaging

F1130W

11.3

0.7

0.375

PAH, broadband imaging

F1280W12.82.4

0.420

Broadband imaging

F1500W15.03.0

0.488

Broadband imaging
F1800W18.03.0

0.591

Silicate, broadband imaging
F2100W21.05.0

0.674

Broadband imaging
F2550W25.54.0

0.803

Broadband imaging

F2550WR

25.5

4.0

0.803

Redundant filter, risk reduction

FND

~1310--For bright target acquisition
Opaqueblackened blankN/AN/ADarks

* FWHM refers to the PSF


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

FilterCoronagraphPupil mask transmission (%)†Central wavelength (μm)Bandwidth‡ (μm)IWA§ (arcsec)Rejection* (on-axis)
F1065C4QPM16210.5750.750.33260
F1140C4QPM26211.300.80.36285
F1550C4QPM36215.500.90.49310
F2300CLyot spot◊7222.755.52.16850


† 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.

Figure 3. MIRI coronagraphic imaging filter throughputs


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

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The page: .MIRI Numerical Repository Data v1.2 was not found. Please check/update the page name used in the 'multiexcerpt-include' macro. 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. 

Figure 4. LRS prism dispersion profile and LRS photon conversion efficiency

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)

See also: MIRI Medium Resolution Spectroscopy

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 × 3.7210.1760.196

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 × 4.8170.2770.196

SHORT (A)

MEDIUM (B)

LONG (C)

7.51–8.77

8.67–10.13

10.02–11.70

2,990–3,110

2,750–3,170

2,860–3,300 

Channel 3
11.55–17.98

5.2 × 6.2160.3870.245

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 × 7.7120.6450.273

SHORT (A)

MEDIUM (B)

LONG (C)

17.70–20.95

20.69–24.48

24.19–27.90

1,460–1,930

1,680–1,770

1,630–1,330

Figure 5. MIRI MRS IFU channels filter throughputs

Figure credit: STScI MIRI team

References

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


Latest updates
  •  
    Updated for Cycle 2 based on commissioning


  • Table 3 was updated.
Originally published