JWST observations detect background emission from multiple sources: the zodiacal cloud, Milky Way Galaxy, and thermal self-emission from the JWST Observatory itself. Both in-field and scattered emission are important contributors to the JWST background.  

See also: JWST Background Variability, JWST Background-Limited Observations, JWST Backgrounds Tool
See also (external link): How dark the sky: the JWST backgrounds, by Rigby et al. (2022)

Several components of infrared background emission contribute to JWST observations.  Some of these backgrounds are variable with position in the sky and over time. Many observations with JWST are background limited, meaning that the noise will be dominated by the level of background emission, and not by photon noise from the target or detector read noise. The JWST proposal planning system (PPS) calculates these background levels for both planning and scheduling purposes. This page summarizes the sources of background emission that are important for JWST and their relative contribution as a function of wavelength.

A Python-based background tool for JWST is available to compute the backgrounds for a given target and set of assumptions.

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Components of the background emission

Several components contribute to JWST background emission. The primary in-field sources of this background are the zodiacal cloud of the Solar System, and the Milky Way Galaxy. In the thermal infrared (longward of ~12 μm), the background is dominated by thermal self-emission, mostly from the JWST primary mirror segments, as well as scattered thermal emission from the sun shield and other spacecraft components. Since JWST does not have a traditional optical baffle (an encircling tube), light from the out-of-field sky can also gain access to the focal planes through scattering—this additional source of background is called "stray light."

Figure 1 illustrates the relative contributions of these components to the JWST background for a benchmark pointing. This pointing (ecliptic Long, Lat = 266.3°, −50.0°; RA, Dec [J2000] = 17^h26^m44^s, −73°19'56") has a zodiacal emission that is 20% higher than the celestial minimum. It was chosen as a benchmark because it is a stressing case for stray light. The backgrounds are expressed as equivalent units of uniform sky radiance (megaJanskys per steradian, or MJy/sr) at the JWST focal planes. Figure 1 shows that, in general, in-field zodiacal emission and stray light are the main sources of background at wavelengths less than 4 μm; in-field zodiacal emission dominates from 4 to 15 μm, and thermal self-emission dominates at wavelengths longward of 15 μm. At 4–8 μm, the thermal emission from the zodiacal dust is particularly steeply rising, with the surface brightness well described by the Wien approximation. As a result, NIRCam imaging observations at 4–5 μm are background limited and medium filter (F410M, F430M, F460M, F480M) observations are more sensitive than wide filter (F444W) observations.

Stray light, which is out-of-field emission scattered into the field of view, is primarily due to the zodiacal cloud and the Milky Way. In the example shown in Figure 1, this stray light is less than but comparable to the in-field zodiacal emission from 1 to 4 μm. The amount of stray light depends on ecliptic latitude (pointings toward the ecliptic poles will have lower stray light) and the orientation of the Milky Way with respect to JWST for a given pointing. Since the benchmark pointing used in Figure 1 was chosen to be a stressing case for stray light, most extragalactic deep fields should have a lower level of stray light. As one example, the stray light level in the Hubble UltraDeep Field (HUDF) should be about half that indicated by the benchmark pointing.

At wavelengths greater than 12.5 μm, the background seen by JWST is dominated by thermal emission from JWST itself. Figure 2 shows the spectrum of this thermal self-emission, as measured during commissioning, and fits it with a multi-component model based on extensive thermal modeling.  Figure 2 shows that this thermal emission dominates the background at wavelengths longer than 12.5 μm.  Emission from the primary mirror is expected to be the primary component of this emission at wavelengths greater than 20 μm; scattered thermal emission from the sunshield and other parts of the observatory is expected to dominate at shorter wavelengths.  As discussed above and shown in Figure 1, at wavelengths shorter than 12.5 μm, the in-field zodiacal background dominates over thermal emission at the benchmark pointing. The thermal curve plotted in Figure 2 is derived from commissioning results, informed by extensive reverse ray-tracing of integrated thermal models.

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Uncertainty in background levels

In addition to the variability of the backgrounds, there is intrinsic uncertainty in the models used. The ETC calculates the in-field zodiacal and Galactic backgrounds using a model based on Cosmic Background Explorer (COBE) data (Kelsall et al. 1998; Reach et al. 1997), that was developed and used operationally for the Spitzer Space Telescope, with the Galactic stellar contribution refined using data from the Wide Field Infrared Survey Explorer (WISE) survey. This model agrees with the Spitzer Infrared Array Camera (IRAC) measurements at the few percent level (Krick et al. 2012). As such, the in-field backgrounds predicted by the ETC should be very reliable.

Before launch, the predicted level of stray light was uncertain at the level of +30%, −20%. The stray light has now been measured to be about 80% as strong as was predicted before launch. This measurement will be refined through additional measurements of archival data.

Before launch, the thermal background spectrum was estimated to be uncertain at the level of +30%, −20%. From commissioning, it was measured well at wavelengths above 20 μm, with the uncertainty dominated by the MIRI imaging flux calibration. From commissioning, the 10 μm the thermal background was inferred with an uncertainty of about ~+/-50% level; the measurement will be improved during Cycle 1. Users are cautioned that exposure time calculations for background–limited observations at 9–14 μm may carry this extra uncertainty.

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Background levels in the ETC and JIST

See also: JWST Exposure Time Calculator Overview

The JWST Exposure Time Calculator (ETC) will calculate backgrounds for a given celestial position. If the user specifies a date, the ETC will give the best estimate of background on that date. Otherwise, the user can choose a low background (10th percentile), a medium background (50th percentile), or high background (90th percentile), all calculated for the selected celestial position over the period of visibility at 3.5 μm (selected as a typical NIR wavelength of interest, noting that the time-dependent component of the background dominates only in the NIR).

Figure 3 shows how to input this background information into the ETC. The computed background spectrum can be downloaded as a FITS file, and the total background can be found in the "background" section of the input.json file included in the ETC downloads, as described in the JWST ETC Outputs Overview page.

The quick-look JWST Interactive Sensitivity Tool (JIST) makes a simplifying assumption for background: JIST assumes background of 1.2 times the minimum zodiacal background.

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Background Levels in the APT

See also: JWST Astronomers Proposal Tool Overview, APT Special Requirements

If your observation(s) are not limited by background, nothing has to be done when specifying the observation is APT. However, if your experiments with the ETC convince you that your observation is background limited, you can set a Background Limited special requirement on the relevant observation, which can restrict scheduling to periods of lowest background.

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References

Kelsall et al. 1998, ApJ, 508, 44  
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. II. Model of the Interplanetary Dust Cloud

Krick J. E. et al., 2012, ApJ, 754, 53   
A Spitzer/IRAC Measure of the Zodiacal Light

Reach, W. T., Franz, B. A., & Weiland, J. L, 1997, Icarus, 127, 461  
The Three-Dimensional Structure of the Zodiacal Dust Bands

Rigby, J. R. et al., 2022,  arXiv:2211.09890
How dark the sky: the JWST backgrounds

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