APT Targets

JWST observing proposals must specify the proposed targets (and as appropriate, coordinates) with the Astronomer's Proposal Tool.   Allowed target types are fixed targets, solar system (moving) targets, or generic targets within unknown coordinates at the time of the proposal. A special case called target groups is also available.

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An astronomical target for JWST refers to any position on the celestial sphere to 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 three 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 the specific targets are not known at the time of the proposal submission.

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

The links above take you to the 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 use case called Target Groups may be useful for users proposing closely spaced targets that need to be observed with very similar observation specifications.  (For cycle 1, the allowed templates include MIRI LRS and MRS, and NIRSpec fixed slits and IFU.)

Motivated by the single stream approach to JWST proposing, most JWST proposals will be required to submit a full list of proposed targets with coordinates in order to accurately compute the total time allocation request.  Exceptions to this rule may be described in the relevant Call for Proposal policies for each cycle, and include (but may not be limited to) Targets of Opportunity and NIRSpec Multi-Object Spectroscopy proposals, where it is impossible to know all relevant details at the time of initial submission. 



Fixed targets

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

Fixed targets may be entered into APT by hand (including the coordinates), looked up using a Fixed Target Resolver Tool within APT, or imported as a formatted ASCII table.  When first opening a JWST APT proposal, selecting the "Target" option from the Tree editor at left as shown in Figure 1.  The active GUI window to the right changes to show 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, options to use the Fixed Target Resolver or to import a list of targets from a file are also available. The special case of adding a Target Group will be discussed further below in this article.
Fixed Target Resolver pops up a separate window and allows the user to type in a common catalog name for a given target, select a catalog source to search for the object, and search for the catalog coordinate. A simple example is shown in Figure 2.  If more than one catalog entry is returned, you can select the option of interest.  Then you can commit the target into your proposal. If selected in this manner, a note is automatically generated and placed in the Comments section of the target GUI for future reference. (This comment can be edited by the proposer if desired.)
Figure 2. The Fixed Target Resolver tool GUI

The Fixed Target Resolver tool runs in a separate pop-up window in APT. Entering a resolvable target name (e.g. "SS Cyg" in this example) and selecting "Search" returns the coordinates from one of several optional catalog services (SIMBAD by default, but any of the listed sources can be selected). In the event a list of possible targets is returned. selecting one from the list and clicking "Select Object as Target" will commit that object into a new target record in the proposal.

Importing from a Bulk Target List

See also: JWST Bulk Target Ingest

The JWST APT Targets form also permits the user to select a file for inserting a bulk listing of fixed targets into a proposal. Allowed file formats include Comma-separated. tab-separated, whitespace delimited, or a Virtual Observatory format table.  JWST Bulk Target Ingest provides the details of this process.

If you update your targets after ingesting, you can also export your APT target list for future reference.  Go to File Menu →  Export →  Export Fixed Target list. Please note that APT exports all coordinate uncertainties in arcsec, regardless of the units set in APT, and only supports CSV format output.

The APT Fixed Target GUI

In the APT Fixed Target GUI itself, a number of parameters are listed; some are required but many of them are optional, depending on your use case. 

Required Parameters:

  • Target Number:  Each target in APT is assigned a target number; these numbers can be edited as desired by the user, but they are useful for referencing a particular target in various pull-down menus and formatted listings elsewhere in APT.
  • Name in Proposal: Although a required entry, this name is at the user's discretion, and can help you in organizing your proposal.
  • Category (and then Description) keywords: needed by the archive.
  • J2000 Coordinates: Coordinates are expected to be provided in J2000 epoch.   

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 somewhat on the observations you plan to obtain.  High accuracy is not required for positioning imaging fields of view, for example, but placing a particular object in a fixed slit for spectroscopy requires higher accuracy. 
  • Extended:  a pull down menu allows selection of YES or NO (defaults to UNKNOWN), as a way of indicating to 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 spectroscopic aperture.
  • Proper Motion, Parallax;  enter if relevant for your use case.
  • Comments: again, enter anything you might find useful for future reference.
  • Finally, the Background Target section 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 or targets, 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 or targets. (This information also gets passed downstream to the data processing system.)

Special fixed target cases: Mosaics and multi-object spectroscopic targets

See also:  JWST Position Angles, Ranges, and Offsets

Special 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 arc seconds relative to the central coordinate, in the reference frame of the detector being used for the mosaic observation.  A mosaic pattern rotates around the center coordinate, and so the position of each mosaic tile changes position in RA and Dec coordinate, but the (X,Y) coordinates remain fixed.

Similarly, for NIRSpec Multi-Object Spectroscopy, the APT target coordinate provided places the reference position of the NIRSpec micro-shutter array at a position on the celestial sphere, but this is not intended to indicate the position of a particular science target or shutter coordinate.  The position angle must be fixed prior to planning exactly which micro-shutters need to be opened to capture the spectra of particular objects in the field.   The NIRSpec MSA Planning Tool is used to read in a catalog of potential multi-object spectroscopy targets and to define the shutter configurations for each desired observation.



Solar System targets

See also: Proposal Parameters article on valid specifications for Solar System Targets
See also:  Helpful video instructions on using Horizons

Solar System Targets (a.k.a. "moving targets") for JWST are created by selecting "New Solar System Target" from the main Target GUI menu. 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 between 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, Keywords, 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.



The Special Case of 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 (within one guide visit-splitting distance) fixed targets 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.  Note that no changes in the template are allowed for the individual targets if using this configuration.

Because of the specific functionality of the target group concept, it is only applicable for use with four 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 very specific use case, but excellent gains in efficiency and a tremendous reduction in mechanism motions can be garnered when that use case applies. For a set of targets needing identical observation and exposure specifications, and are within the visit splitting distance on the sky so that the targets can be observed sequentially in the same visit, unnecessary guide star acquisitions can be avoided and the ordering of activities can be adjusted for both efficiency and to save wear and tear on the filter/grating wheels of the affected instrument.

An example might be a grouping of pre-main sequence stars, all to be observed with the NIRSpec fixed slit or IFU.  To the extent that such a grouping lies within the visit splitting distance for a given place on the sky (typically 40 - 80 arcsec or so; reported in APT for reference), 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 moving the mechanism to the next grating/filter combination, instead of completing all gratings/filters on the first tile before moving to the next.  For a 5x5 IFU mosaic, this reduces 25 mechanism motions down to one for each grating/filter combination.




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Latest updates

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    Updated Figure 1