MIRI Backgrounds

A brief description, about 3 sentence

The massive gains in sensitivity that MIRI will achieve, compared to current and planned mid-infrared instruments, are primarily due to the large (25 m2) collecting area and the cold (40 K) radiative environment provided by the JWST. When compared to ground-based observatories, the absence of atmospheric absorption bands and thermal emission from both the atmosphere and telescope makes JWST more than competitive even with the 30 and 40-m class telescopes now being planned.

At the short wavelength end of MIRIs spectral range around 5 μm, the remaining radiative background signal is dominated by emission from the zodiacal dust concentrated in the ecliptic plane; longward of 17 μm or so, stray light from the observatory sunshield and the thermal emission of the telescope optics become dominant.

For the purposes of sensitivity modeling, we represent the backgrounds by the sum of six gray-body emission spectra, whose emissivities and effective temperatures are listed in table 1, with spectral energy distributions plotted in Figure 1, where the background spectrum is seen to rise steeply across MIRIs spectral range.

Components A and B in Table 1 are fits to the scattered and emissive components of the zodiacal dust spectrum toward the celestial north pole (Wright 1998), scaled by a factor of 1.2 to be representative of typical pointing scenarios. Components CF are derived from a fit to a detailed straylight model of the observatory background (Lightsey & Wei 2012), such that in- dividual terms should not be identified as being physically representative of any specific observatory subsystem. This particular model gives a total of 188 MJy sr-1 at 20 μm, just under the second-level requirement of 200 MJy sr-1.

To include the thermal background from the telescope, components AF are summed together to give the background spec- trum that is plotted as the thick solid line labeled TOTALin the figure below. As a contingency, we also show the result of doubling the current estimates of facility emission by adding an additional component (G in the table below) with the effect of increasing the back- ground flux at λ = 20 μm to 350 MJy steradian-1.

ComponentEmissivityTemperature (K)
A

4.20 × 10-14

5500
B4.30 × 10-8270
C3.35 × 10-7133.8
D9.70 × 10-571.0
E1.72 × 10-362.0
F1.48 × 10-251.7
G1.31 × 10-486.7


To interpret the figure below, it may be helpful to note that for the MIRI imager pixel scale of 0.11′′, the usable area of the JWST entrance pupil (Atel =25.03 m2) and the nominal transmission of the telescope optics up to the MIRI entrance focal plane (τ =0.88 at the start of the mission), a flux of 0.4 MJy sterad-1at λ =5 μm corresponds to 7.5 photon s-1pixel-1in a 1 μm passband. At λ =20 μm, a flux of 188 MJy sterad-1equates to 890 photon s-1pixel-1μm-1


Background emission spectra used in MIRI sensitivity modeling.  (Glass et al. 2015.)


References






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