JWST Background-Limited Observations
Observers can assess if their observations are sensitive to background levels using the JWST Exposure Time Calculator (ETC), and then apply the Background Limited special requirement in APT to limit scheduling to when the background is relatively low. This special requirement affects the schedulability of observations.
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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 noise. For more detailed information, the JWST Background Model article describes the origins of the background emission and its spectral shape, while JWST Background Variability describes time variable aspects of the various background components.
There are multiple components to the JWST background. The one that varies seasonally is the zodiacal emission, due to the changing path length of Solar System dust through which JWST must observe, as well as the temperature and temperature range of that dust. As a result, for mid-infrared wavelengths where the zodiacal emission peaks, a target may have a low background for only a fraction of all the days when the target is technically observable. For an overview of the backgrounds at a given sky position, consider using the JWST Backgrounds Tool to visualize the time variability.
The Background Limited special requirement in APT
The ground system implements the Background Limited special requirement in the following way: software used by APT looks at the total background variation over the year at a given position and wavelength, and then calculates the percentile levels. By default, the lower 10% of this range (the 10th percentile) becomes the limit applied. Of course, restricting the background level has a corresponding effect on limiting the visibility window as well, so there is a trade off involved in using this special requirement. If more scheduling flexibility is needed, a less restrictive percentile limit can be selected.
It is often not obvious whether a particular observation will benefit from the Background Limited special requirement. Here are 2 non-intuitive examples:
- MIRI imaging at 25.5 μm is background limited, but that background is dominated by thermal emission from the primary mirrors (assumed by the ETC to be time independent), not by the time variable zodiacal emission. As a result, all observable dates for a target may have a background level that is <10% above the minimum background.
- While sufficiently deep NIRCam imaging at 4.4 μm will be dominated by the time variable zodiacal emission, for shallower observations the SNR will instead be dominated by detector effects. The integration time where that transition occurs is a function of wavelength, brightness, ecliptic latitude of the target, and the chosen filter.
The bottom line here is that users are responsible for identifying to the scheduling system those observations that are sensitive to the variable background level, by placing the Background Limited special requirement on the relevant observation specifications in APT. Users should not try to guess whether the Background Limited special requirement is needed for their observation, but rather use the ETC to calculate whether it is needed, using the method described below. Appropriate text justification should be included in your proposal.
How to determine if an observation needs the Background Limited special requirement in ETC
See also: JWST Exposure Time Calculator Overview
- In the ETC Backgrounds tab, specify the RA and Dec of the target (see Figure 1).
- Select Low background. Click Calculate (bottom right of the panel, not visible in Figure 1) and make note of the signal-to-noise ratio (SNR).
- Select High background. Click Calculate and, again, note the SNR.
- On your own, calculate the fractional change in the SNR, X, where
X = [SNR(low bkg) − SNR(high bkg)] / SNR(low bkg)(Equation 1)
- If X > 0.05, then the SNR of this observation depends by more than 5% on the variable background level. You should use the Background Limited special requirement in APT, at the default value of <10% above the minimum background, unless you have a good reason not to do it.
- If X < 0.05, then your observation is not sensitive to the time variable background. You should not use the Background Limited special requirement in APT.
The threshold value of X = 0.05 was chosen for consistency with the ground system requirements. Proposers are encouraged to mention the results of this calculation in their technical justification, especially if they select the Background Limited special requirement.
How to apply the Background Limited special requirement in APT
The Background Limited special requirement has a variable: the maximum permitted background level (percentile above minimum) for the RA, Dec, and wavelength of an observation. The default value is a level below the 10th percentile above the minimum background for that RA, Dec, and wavelength. Higher values may be chosen to increase schedulability when there are competing constraints. Refer to the Background Limited special requirement description in the APT Special Requirements article for details.
The Background Limited special requirement affects schedulability
Since the Background Limited special requirement limits the schedulability of an observation to times when the background is relatively low, it is effectively a scheduling constraint. For targets at low ecliptic latitude, the Background Limited special requirement may restrict the schedulable window to 10–20 days per year. Thus, users should apply the Background Limited special requirement only if scientifically justified.
Applying both the Background Limited special requirement and another scheduling requirement (for example, one of the aperture position angle special requirements) may make an observation impossible to schedule. Users will have to decide which constraints are more important to the science goals, and relax constraints until the observation becomes schedulable. Within the Background Limited special requirement, the user may select an acceptable background level higher than the default (of 10th percentile above the minimum) to improve schedulability.
Users should assess whether the signal-to-noise estimates of their observations are sensitive to the level of the time variable background, using the ETC method described above. If yes, then the Background Limited special requirement should be added to the observation, to request scheduling when the background is relatively low.
- Proposers should not use the Background Limited special requirement when it is not justified. Doing so will decrease schedulability while negligibly improving the signal-to-noise ratio.
- Users should exercise caution when adding any additional scheduling requirements to an observation that has the Background Limited special requirement. This is especially true for targets at low ecliptic latitude. Multiple scheduling requirements may make the observation impossible to schedule.
- Users should be aware that a constraint on the aperture position angle can be incompatible with the Background Limited special requirement since both are effectively scheduling constraints.