APT Targets

JWST observing proposals must specify the proposed targets (and as appropriate, coordinates) in the Astronomer's Proposal Tool. Allowed types are: fixed targets, Solar System (moving) targets, or generic targets that have unknown coordinates at the time of the proposal. A special case for closely-spaced targets called target groups is also available.

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An astronomical target for JWST refers to any position on the celestial sphere at which the telescope needs to point. The target may be a physical object, a portion of an object, a background region, or even an unknown target at the time of proposal writing.

The Astronomer's Proposal Tool accepts 3 primary types of targets:

  • Fixed targets, typically associated with known astronomical objects outside the Solar System whose positions can be defined by specific celestial coordinates;
  • Solar System targets (also known as moving targets);
  • Generic targets, typically used for target of opportunity observations or pure parallel proposals where specific targets are not known at the time of the proposal submission.

Note: the links above take you to proposal parameter articles on each target type; these articles contain technical details and legal values for many of the parameters listed in the APT target entry section.

A special and highly efficient use case called target groups may be helpful for users proposing closely spaced targets that need to be observed with identical observation specifications, including science targets that require a nearby background observation (within the visit-splitting distance). The allowed templates for using target groups include MIRI LRS, MIRI MRS, NIRSpec fixed slits, and NIRSpec IFU.

Additionally, APT has the ability to ingest a catalog of targets from an appropriately formatted ASCII file.

Due to the single stream approach for JWST proposing, most proposers are required to submit a full list of targets with coordinates in order to accurately compute the total time allocation request. Exceptions to this rule may be described in JWST Opportunities and Policies for each cycle; such exceptions may include targets of opportunity and NIRSpec multi-object spectroscopy proposals, where it is impossible to know all relevant details at the time of initial proposal submission. 



Fixed targets

See also: proposal parameters article on valid specifications for Fixed Targets.

Words in bold italics are also buttons 
or parameters in GUI tools. Bold 
style represents GUI menus/
panels & data software packages.

Fixed targets may be entered in APT by hand (including the coordinates), found using the Fixed Target Resolver tool, or imported as a formatted ASCII table. Select Targets in the tree editor (in the left column), as shown in Figure 1. The active GUI window on the right displays the options for entering targets.
Figure 1. Target entry menu in APT

The target entry GUI contains options for entering a new fixed target, Solar System target, or generic target. For fixed targets, you can enter them manually by selecting New Fixed Target, use the Fixed Target Resolver tool to search for your targets, and Import Targets to load a list of targets from a file. There is also a special use case called New Target Group that will be discussed further below in this article.
The Fixed Target Resolver tool opens a separate window where you can select a catalog(s) to search for your target, specify a search method, and enter search values based on the search method you specified. A simple example is shown in Figure 2. If more than one catalog entry is returned, you can select the target of interest. If you commit a target from this search to your proposal, a note is automatically generated in the Comments section of the target GUI. (This comment can be edited by the proposer, if desired.)
Figure 2. The Fixed Target Resolver tool GUI

APT's Fixed Target Resolver tool is run in a separate pop-up window. Entering a resolvable target name in the Object Name field (e.g., "SS Cyg" in this example) and selecting Search returns the coordinates from one of several  catalog services (SIMBAD by default, but one or more catalogs can be selected). If a list of targets is returned, select one of the entries and click Select Object as Target to commit that object as a new target in the proposal.

Importing from a bulk target list

See also: JWST Bulk Target Ingest

In the APT Targets form, users can upload a file containing a list of fixed targets into a proposal. Allowed file formats include comma-separated, tab-separated, and whitespace delimited values, as well as a Virtual Observatory format table. Additional details are available in JWST Bulk Target Ingest.

Fixed target form GUI

Each target is described in its own form, listed under Fixed Targets in the tree editor. Each fixed target form has a number of parameters—some are required but many are optional, depending on your use case. 

Required parameters:

  • Number: Each target in APT is assigned a number. These numbers can be edited as desired by the user. They are useful for referencing a particular target in various pull-down menus and formatted listings elsewhere in APT.
  • Name in Proposal: Although this is a required entry, the name can be chosen at the user's discretion, and can help you in organizing your proposal.
  • Category and Description: These keywords are needed by the Archive.
  • J2000 Coordinates: Use J2000 epoch coordinates.   

Optional parameters:

  • Name for the Archive: A standard searchable (resolvable) name should be entered for future use in Archive searches.
  • Uncertainty (for J2000 Coordinates):The required accuracy depends on the observations you plan to obtain. High accuracy is not required for some observations such as imaging, but placing a particular object in a fixed slit for spectroscopy requires higher accuracy. 
  • Proper Motion, Annual Parallax:  Enter these values if they're relevant to your use case. If it's used, you must also specify the Epoch for your values.
  • Extended: Select YES or NO from a pull-down menu (the default is Unknown) to inform the data processing pipeline whether the extraction algorithm should consider the extent of the source or not. This is particularly relevant for spectroscopic observations where the source may or may not fill the aperture.
  • Comments: Enter anything you might find useful for future reference.

Below these parameters is the Background Target section that provides an optional check box where you can tell APT that observations of this target will require a separate background observation for use in data processing. If you have not yet entered the background target(s), the GUI box in this section will be empty. After you have entered the background targets, return to your science target and select the desired background target(s). (This information gets passed downstream to the data processing system.) See Specifying APT Background Targets for more information.

Special fixed target cases: mosaics, multi-object spectroscopic targets, and background targets

See also:  JWST Position Angles, Ranges, and Offsets
See also: JWST Background-Limited Observations

Special target cases include target positions provided for mosaic observations and for NIRSpec multi-object spectroscopy

Mosaics are defined by a single coordinate position (the center of the mosaic) and a pattern of fields around this position. The (X,Y) positions of the individual mosaic fields are described in arcseconds relative to the central coordinates, in the reference frame of the detector used for the mosaic observation. A mosaic pattern rotates around the center coordinates, and so the position of each mosaic tile changes position in RA and Dec coordinates, but the (X,Y) coordinates remain fixed.

For NIRSpec multi-object spectroscopy, the target coordinates in APT serve as a reference position on the celestial sphere for the micro-shutter array. Target coordinates are not intended to indicate the position of a particular science target, pointing, or shutter coordinate. A fixed position angle is assigned prior to planning exactly what pointings and micro-shutters need to be opened to capture the spectra of particular objects in the field. The NIRSpec MSA Planning Tool is used to ingest a catalog of candidate multi-object spectroscopy sources and to define the shutter configurations for each exposure in the observation.

For targets that require a background observation, a separate "background" target position should be defined, and the observation of this target should be linked to the science observation of interest. (It is the responsibility of the observer to verify that the chosen coordinates provide a suitably clear field.) For small aperture observations where a nearby position can be defined (within the visit splitting distance), this can be accomplished with a target group. If the background position must be further away than the visit splitting distance, a separate observation of the background target will need to be specified and linked to the science target observation. This is done using the Timing special requirement Group/Sequence Observations Link, set to Non-interruptible. See Specifying APT Background Targets for more information.

Users should be aware that targets within a few arcminutes of either celestial pole can be problematic. Many precession routines break down in this regime, and any uncertainties in coordinate positions will be  exacerbated. Also, the offset position of the FGS guider with respect to the instrument field of view and rapidly changing V3 PA with position, due to the converging lines of Right Ascension, can cause operational complications. All these issues can be resolved, but careful attention is needed. It is suggested that relevant comments be added in APT for any targets that are within a few arcminutes of the celestial poles.



Solar System targets

See also: Proposal Parameters article on valid specifications for Solar System Targets

How to retrieve minor body orbital elements from Horizons

Solar System targets (moving targets) are specified in APT by selecting New Solar System Target in the Targets page. The APT user has tremendous flexibility to identify specific targets and pointing positions relative to targets using a system of target levels and pick list information provided. Level 1 targets are solar system bodies directly orbiting the Sun (planets, comets. or asteroids), while  Level 2 and Level 3 targets are moons of, or positions on or relative to, the specified Level 1 or Level 2 target.

Another APT distinction in types of moving targets is between standard targets and minor bodies (asteroids and comets). For standard targets, ephemerides can be computed using information directly accessible to and maintained by APT. For the minor bodies, the user must supply orbital elements, either manually or by retrieving them from the JPL Horizons system using the built-in APT function

Note that many of the high level APT entries for moving targets are analogous to the fixed target case (e.g., Name in Proposal, Name for the Archive, Description, and so forth). Observers familiar with specifying moving targets for HST observations will see that the functionality for JWST is identical.

For many more details about specifying solar system targets in APT, see the Solar System Targets proposal parameters article. Also, information is available on special requirements needed for many moving target observations.



Generic targets

See also: Proposal Parameters article on valid specifications for Generic Targets.

Generic targets are targets that can only be described in terms of astronomical characteristics or general location in the sky at the time of the initial proposal. This category is used for targets of opportunity programs (i.e., where the details of the target or targets are not known until something happens at a later date). This category is available for both fixed and Solar System targets. Another use case will be for pure parallel proposals where the parallel observation slots can only be defined on the characteristics of the type of primary observation they may be attached to after acceptance. Target of opportunity observations are automatically On Hold and do not need the special requirement, but any other observations using generic targets should have an On Hold special requirement placed on them, to prevent the observations from being considered by the long range planning system until details are known. When the target is fully specified, the On Hold is removed and the proposed observation then flows into the scheduling system.

Because of the nature of generic targets, the APT fields available in the target GUI are adjusted somewhat. The Name in the Proposal field is still present (and required), but an entry for Criteria is provided. This is where you provide the criteria for the unknown target selection. You may also wish to use the Comments box provided to specify further details about the target selection.

At such time that your proposal is accepted and a given target's details are known, the target information will need to be added into the appropriate fixed target (or Solar System target) form in APT, including filling out keywords and all other relevant information in those forms.

For full details regarding specifying generic targets, refer to the Generic Targets proposal parameters article.



Target groups  

For JWST, APT supports the creation of a special target case known as a target group. A target group is a set of closely-spaced fixed targets (within the visit-splitting distance, typically 40"–80"; reported in APT for reference)) that are intended to be observed together and for which the observation specification is intended to be identical. By placing targets in a target group, the user can specify the observational details once in the relevant observation template and then select the target group as the "target" of the observation. The observational details will then be applied to all of the individual targets automatically, all within the context of a single observation template. Note that no changes in the template are allowed for the individual targets if using this feature.

Because of the specific functionality of the target group concept, it is only applicable for use with 4 observation templates that involve small fields of view or apertures: the MIRI LRS, MIRI MRS, NIRSpec fixed slit, and NIRSpec IFU templates.

Target groups were created for a specific use case, but provide excellent gains in efficiency and a tremendous reduction in mechanism motions when this use case applies. For a set of closely spaced targets within the visit splitting distance on the sky, the targets can be observed sequentially in the same visit, assuming they can be observed with identical exposure specifications. In this case, unnecessary guide star acquisitions can be avoided and the ordering of activities can be adjusted to save wear and tear on the filter/grating wheels of the affected instrument.  

Example: let's say you wanted to observe a grouping of pre-main sequence stars with the NIRSpec fixed slit. To the extent that such a grouping lies within the visit-splitting distance, the observations can be accomplished very efficiently, since no new guide star acquisition is required for the individual targets. Furthermore, let's assume a use case where the NIRSpec IFU is being used to create a small mosaic using multiple gratings/filters. The target group concept allows the entire set of mosaic tiles to be observed with one grating/filter combination prior to advancing to the next grating/filter combination, instead of completing all gratings/filters on the first tile before moving to the next. For a 5 × 5 IFU mosaic, this reduces 25 mechanism motions down to one for each grating/filter combination. This is advantageous for both you the observer and for the observatory as a whole.




Latest updates
  •  
    Added warning note about targets near celestial poles.
  •  
    Minor text updates and clarifications; plus added links to "Specifying Background Targets" article.
  •  
    Article updated for Cycle 1 Call for Proposals release. 
Originally published