Backgrounds in JWST observations vary seasonally for a given sky position; the magnitude of that variation depends on the observation wavelength and ecliptic latitude. 

See also: JWST Background-Limited Observations, JWST Backgrounds Tool

JWST observations include background emission from the zodiacal cloud, the Galaxy, and the observatory itself. A component of the background that varies seasonally is the zodiacal emission, due to the changing path length of Solar System dust through which JWST must look and the temperatures of that dust. This means that for a given fixed target, the JWST backgrounds will vary seasonally in a predictable way. The variation is most pronounced for wavelengths ~<15 μm, where zodiacal emission dominates the JWST background, and for targets at low ecliptic latitude (near the ecliptic plane). One consequence, as explained below, is that targets near the ecliptic plane may be observable with a low background for only a small fraction of their observable window. This can be a significant scheduling constraint for observations that are determined to be background limited.

JWST backgrounds vary seasonally for a given sky position. The magnitude of its variation depends on the observation's wavelength and the target's ecliptic latitude. 

APT, the ETC, and the JWST scheduling system calculate these backgrounds, using the same background model. The JWST Backgrounds Tool can be used to calculate and visualize the seasonal variation of backgrounds for a given target.

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Spatial and temporal variability of the background

Some components of the JWST background should be constant with time and telescope pointing, while others vary seasonally.

Thermal self-emission

Thermal self-emission dominates the JWST backgrounds for wavelengths > 12.5 μm. During commissioning, it was observed to be variable over ~week timescales by roughly 6% at F2550W, and less variable at shorter wavelengths.

Spatial variation of in-field emission

The in-field zodiacal emission depends strongly on ecliptic latitude. The in-field Galactic emission depends strongly on Galactic latitude. 

Temporal variation of in-field emission

JWST can observe any specified location on the sky for at least 100 days per calendar year. The length of the visibility window increases with increasing ecliptic latitude. Within a visibility window, the in-field Galactic background will be constant, while the in-field zodiacal background will vary predictably with date as JWST looks through different path lengths of dust, with different temperatures, in our Solar System. Figure 1 shows an example of the temporal variability of the background over one year.

Spatial and temporal variation of stray light

The amount of stray light depends on the pointing and the observatory V3 roll angle. Models predict a strong correlation of the stray light level with the in-field zodiacal and Galactic background level, since half the susceptibility to stray light is predicted to occur within 35 degrees of the boresight (observatory V1 axis).

However, JWST is also susceptible to stray light from sources located far from the boresight. The JWST Exposure Time Calculator (ETC) uses maps that predict the susceptibility of JWST to stray light, given material properties and expected levels of dust contamination. For a given celestial position and date (which determines the nominal observatory V3 roll angle), the ETC projects the susceptibility maps onto the astronomical sky, to predict the amount of stray light at each instrument focal plane.

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How seasonal variation of the background affects schedulability

The number of days per year that a target is observable to JWST is a simple function of ecliptic latitude, as illustrated by Figure 2. The JWST backgrounds are also a function of ecliptic latitude—both the minimum background over a year, and the shape of the seasonal variability curve. This has important consequences for target observability.

Figure 3 illustrates how the seasonal variation of the JWST background is a predictable function of wavelength and ecliptic latitude. Targets at high ecliptic latitude and high Galactic latitude will have lower background than targets near the ecliptic plane or the Galactic plane. Figure 3 illustrates this: the background is lower for the 2 fields at high ecliptic latitude, compared to the COSMOS field which is located near the ecliptic plane. Moreover, ecliptic latitude controls how steeply the background rises with time: at high ecliptic latitude the background-versus-time curves are shallow, whereas near the ecliptic plane, the curves rise steeply. As a result, targets near the ecliptic may be observable with low background at 5–10 μm for only a few days per year. Figure 4 expands these results for all ecliptic latitudes. Figure 4 plots the median number of low background days, as a function of ecliptic latitude, for the central wavelengths of the JWST broadband filters. Figure 4 shows that targets with low ecliptic latitude (−40° to +40°) are observable for only a few days per year with low background. For example, targets on the ecliptic have only 14 days/year with background <10% above the minimum at 5 μm. 

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Advice for proposers

See also: JWST Background-Limited Observations

The JWST Background-Limited Observations article describes a simple method, using the JWST ETC, to determine whether the signal-to-noise ratio of a particular observation is sensitive to the time-variable background. Observations that are background sensitive should use the Background Limited special requirement in APT, so that the observation will be scheduled when the background is relatively low. This special requirement affects the schedulability of an observation. 

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