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.
The imager has a single filter wheel that holds all the elements necessary for MIRI's three 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.
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.
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
The MIRI imaging mode allows users to select amongst 10 filters for observations.
Table 1. Imaging filter properties
|Filter name||λ0 (μm)||Δλ (μm)||FWHM1 (arcsec)||Comments|
PAH, broadband imaging
Silicate, broadband imaging
PAH, broadband imaging
|F1800W||18.0||3.0||0.58||Silicate, broadband imaging|
|0.82||Redundant filter, risk reduction|
|~13||10||--||For bright target acquisition|
1 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
|Filter||Coronagraph||Pupil mask transmission (%)1||Central wavelength (μm)||Bandwidth2 (μm)||IWA3 (arcsec)||Rejection4 (on-axis)|
|F2300C||Lyot spot 5||72||22.75||5.5||2.16||850|
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.
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.
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
|3.3 × 3.7|
|4.2 × 4.8|
|5.6 × 6.2|
|7.2 × 7.9|
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)