MIRI Filters and Dispersers

The MIRI filter wheel has 10 filters for imaging, four filter-diaphragm sets for coronagraphy, and one double prism assembly for low-resolution spectroscopy. The medium-resolution spectrometer has two 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 three functional areas: imagercoronagraph, 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. If, however, you require the filter throughputs, they are currently available on University of Arizona's webpage.  



Imager

Main articles: MIRI ImagingMIRI Coronagraphic ImagingMIRI Low-Resolution Spectroscopy

The MIRI imager filter wheel includes: 

  • 10 filters for imaging (F2550WR is redundant)
  • four 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. (Figure adapted from Wright et al. 2015)

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)FWHM1 (arcsec)Comments

 F560W

5.6

1.2

0.22

Broadband Imaging

 F770W

7.7

2.2

0.25

PAH, broadband imaging

F1000W

10.0

2.0

0.32

Silicate, broadband imaging

F1130W

11.3

0.7

0.36

PAH, broadband imaging

F1280W12.82.40.41

Broadband imaging

F1500W15.03.00.48Broadband imaging
F1800W18.03.00.58Silicate, broadband imaging
F2100W21.05.00.67Broadband imaging
F2550W25.54.00.82Broadband imaging

F2550WR

25.5

4.0

0.82Redundant filter, risk reduction

FND

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

1 FWHM refers to the PSF

Figure 2. MIRI imaging filter throughputs

MIRI imaging filter throughputs


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 (%)1Central wavelength (μm)Bandwidth2 (μm)IWA3 (arcsec)Rejection4 (on-axis)
F1065C4QPM16210.5750.750.33260
F1140C4QPM26211.400.80.36285
F1550C4QPM36215.500.90.49310
F2300CLyot spot 57222.755.52.16850

1 Coronagraph filters are paired with pupil masks to reduce diffracted light from both the telescope pupil and the coronagraphic occulting spot, but at the expense of some loss of total intensity.

2 Bandwidth is defined to extend down to wavelengths that correspond to 5%–10% of the transmission efficiency.

3Inner working angle (IWA) is defined as the 50% transmission radius.

4 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).

5 The spot refers to the occulting mask in the Lyot-type coronagraph.


Figure 3. MIRI coronagraphic imaging filter throughputs

MIRI coronagraphic imaging filter throughputs



Low-Resolution Spectrometer (LRS)

The LRS mode of MIRI uses a double prism mounted in the MIRI Imager filter wheel (see Fig. 1). The LRS provides an R~100 spectrum covering 5 to 12 µm in a single exposure. In practice the spectral resolving power increases approximately linearly from R~40 to ~160 from 5 to 10 µm. The dispersion profile of the double prism shows a turnover around 4 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. 

Fig. 4 shows the as-designed spectral dispersion profile of the LRS, as well as the measured photon conversion efficiency of the LRS in both slit and slitless mode. 

Figure 4. Left: As-designed dispersion of the MIRI LRS double prism, showing how wavelengths are mapped onto pixels. Note the turnover in the dispersion shortward of 4 µm. The green dashed line indicates the filter cut-off implemented for LRS slit. Right: the photon conversion efficiencies 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)

Main article: MIRI Medium Resolution Spectroscopy

The MRS has four separate IFUs (called channels 1-4), each covering a separate wavelength range between 5–28.5 μm. All four channels are observed simultaneously, but each exposure can only cover one-third of the available wavelength range in a single configuration.

For complete spectral coverage, three different spectral settings must be observed, called SHORT (A) 1, MEDIUM (B), and LONG (C). The dichroic filter wheel comprises of three working positions to move gratings and dichroics simultaneously. Each is located on separate wheel discs. The two wheels feed light into the four 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)

Sub-band
name

λ-range
(μm)

Resolving power
(λ/Δλ)

Channel 1
4.89–7.66

 3.3 × 3.7

A
B
C

4.89–5.75
5.65–6.64
6.52–7.66

3,320–3,710 

3,190–3,750 

3,100–3,610 

Channel 2
7.49–11.71

 4.2 × 4.8

A
B
C

7.49–8.78
8.65–10.14
9.99–11.71

2,990–3,110 

2,750–3,170 

2,860–3,300 

Channel 3
11.53–18.05

5.6 × 6.2

A
B
C

11.53–13.48
13.37–15.63
15.44–18.05

2,530–2,880 

1,790–2,640 

1,980–2,790 

Channel 4
17.66–28.45

7.2 × 7.9

A
B
C

17.66–20.92
20.54–24.40
23.95–28.45

1,460–1,930 

1,680–1,770 

1,630–1,330 


Figure 5. MIRI MRS IFU channels filter throughputs

MIRI MRS IFU channels filter throughput


1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.



References

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

MIRI Filter Throughput Curves (External University of Arizona MIRI webpage)




Published

 

Latest updates

  • Table 3 was updated