Specifying APT Background Targets
How to specify a separate background target and observation, and link it to your science target observation, in a JWST proposal using the Astronomer's Proposal Tool.
The need for a separate background target (and observation) must be judged based on your science target brightness, the expected background level, and the selected instrument and mode.
The infrared background varies as a function of position, time, and wavelength. Depending on your source brightness (or surface brightness) and the expected background level, you may decide that a separate background observation is needed at a position near to your science target in order to allow this background to be subtracted. In general, longer IR wavelengths are more impacted by background and background variability.
There are a several different ways to get a background measurement for a science observation:
- There are separate dedicated background targets that are observed as separate (usually linked) observations,
- There are background observations performed as part of a target group, and hence are part of the SAME observation, and
- For completeness, there are also in-scene nods in the NIRSpec IFU and MOS modes that obtain background but don't require a separate target or observation. (These cases are not discussed further in this article.)
Since there are many variables, you should consult the relevant observing strategy articles for your instrument of choice for further guidance.
Creating a Separate Background Target
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(1) In the Targets folder, add a new target with the coordinates for the separate background pointing. (Hint: choose an appropriate name to make it clear this target is for background.)
(2) Go back to the science target, and check the box for Observations of this target require companion background observation(s).
(3) When that box is checked, a new field appears below it containing a list of other targets in your proposal.
(4) Select the just-created background target by its name in the Target selection box. This linking will be fed downstream to the data processing system so that background subtraction can be performed.
It is now possible to create a new observation of the background target with the appropriate settings that will ultimately provide a background data set for subtraction from your science target by the data processing pipeline.
In the observation template for the science trget, you will now see an error if the observation parameters (in this case, the need for a linked background observation) are not as expected. APT has a special requirement called SEQUENCE NON-INTERRUPTIBLE that should be used to link the science observation and the separate background observation so they will be executed together. This strategy will minimize any time variable fluctuations between the science and background observations.
A given background position can be used for multiple science observations of the science target (and even potentially for observations of multiple science targets) if the observational setup is the same. To use different backgrounds for other observations of the same target, different background targets with relevant coordinates will need to be defined in the Targets folder. See the proposal parameters article on this topic for more details.
Note that targets chosen to be background targets cannot also be science targets. If your proposal has targets that serve as both science targets and background targets, you will need to create the target twice with slightly different names. Otherwise, the data processing pipeline will not be able to automatically apply the correct processing.
Creating Backgrounds within Target Groups
If you are using the MIRI LRS or MRS, or the NIRSpec FSS or IFU and your background target is close by (within the visit-splitting distance), you can use a target group to observe the science and background target within one observation (as long as you can use the same exposure parameters for both). Target groups are collections of closely-spaced positions on the sky (within a visit-splitting distance) that can be observed together in a given observation template (e.g., without an additional guide star acquisition). Using this mode is both efficient (reduces overheads) and also reduces mechanism motions, and its use is encouraged when appropriate.
To create and use a target group:
- Go to the Targets folder and create a New Target Group. This is a container within which you can place the targets that are to be observed together. (In this case one or more science targets and one or more background coordinate positions).
- As the Target in your observation, select your new target group by name and provide the observation details. Each target in the target group will be observed with the specifications provided.
APT will guide you to connect the appropriate background target to each science target, as described in the previous section.
Frequently Asked Questions about Background Targets
When do I need to specify a background observation?
In general, subtracting the background adds noise if it is a pixel-by-pixel subtraction, so it should be avoided when the background is negligible compared to the source signal. Use the JWST Exposure Time Calculator to assess your target surface flux density relative to various background levels to understand when it makes sense to subtract background.
Why do I get an error on my other science observation that does not need background?
In the event that you have two observations of the same target, but a background observation is not needed on the "other" observation, the diagnostic that was added in APT to remind you to link your background to science observations ends up causing this error. Simply make a duplicate target in this situation: one target that needs background and one that does not. It is easy to create a duplicate target and give it a slightly different name, but you do need to remember to keep them both up to date if you make changes.