JWST Background Model
JWST observations will detect infrared 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.
Several components of infrared background emission will contribute to JWST observations, and these backgrounds are variable with position in the sky and over time. Furthermore, some components of the background are modeled based on pre-mission expectations, which are uncertain at some level. Many observations with JWST will be 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 for providing a visualization of the various background components for a given target and set of assumptions.
Components of the background emission
Several components contribute to the background emission that JWST will detect. 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 ~15 μm), the background is dominated by thermal self-emission, mostly from the JWST primary mirror segments, as well as scattered thermal emission from the sunshield. Since JWST does not have a traditional optical baffle, 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 expected contributions of these components to the JWST background for a benchmark pointing. This pointing (ecliptic Long, Lat = 266.3°, −50.0°; RA, Dec [J2000] = 17h26m44s, −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 scattered 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.
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.
By contrast, the predicted levels of stray light and of thermal self-emission carry considerable intrinsic uncertainties. These estimates depend on extensive modeling of the scattering properties of observatory materials, estimates of the amount and properties of contaminating dust particles, knowledge of the deployed observatory configuration, and thermal models incorporating material emissivities which result in temperature estimates of all surfaces which can act as sources of thermal background.
The stray light estimates are thought to have uncertainties of order (+30%, −20%). Proposers should use the ETC to understand the extent to which stray light contributes to the total background of their observations, and bear in mind these uncertainties on the stray light predictions.
The thermal background curve (thermal self-emission in Figures 1 and 2) is a conservative best estimate based on detailed thermal modeling. It is neither a worst nor best case scenario, and is also uncertain at the (+30%, −20%) level. Users are cautioned that, for cycle 1, exposure time estimates will be highly uncertain for background-limited observations at wavelengths longer than 15 μm until actual performance can be determined on orbit.
Both the thermal and stray light backgrounds will be measured during the commissioning of JWST; results will be disseminated to users prior to the cycle 2 proposal deadline.
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 microns (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.
Background Levels in the APT
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.
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