Identifying Potential JWST Duplicate Observations

Investigators preparing JWST proposals can use the MAST Data Discovery Portal to identify potential duplications between targets they wish to observe and previously approved JWST observations

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See also:  JWST Duplicate Observations Policy
Relevant links outside of JDox: 
MAST Data Discovery Portal and JWST Program Information Look-up

When preparing a JWST observing proposal, investigators are obliged to check their targets against existing or planned JWST observations for potential duplications. You may use the MAST Data Discovery Portal to discover planned (and eventually archived) JWST observations of most targets, and to visualize the approximate footprints of such observations on the sky. The portal will simultaneously provide important, but incomplete, ancillary information for the matched observations, such as the instrument(s) and some information about the observing configuration(s). If potential duplications are identified, you will need to inspect the public APT file of the accepted programs to get detailed exposure information to judge whether a duplication is real. The portal can also display the footprints of archived data from other hosted missions, such as HST images, which may be useful to plan your observations in detail.

Portal target search

The check for potential duplicate observations begins with searching the MAST Data Discovery Portal for your targets, with a radius that is appropriate for the instrument configuration you intend to use (see Table 1).

Table 1. Areal Observations—default duplication search radii

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General search procedures for observations of astronomical targets are described in Data Exploration with the MAST Portal. Exposures that overlap the selected target (or coordinates) within the search radius will be listed in a table, and the approximate footprints of those exposures will appear against a background image of the sky at the target location.

Potential vs. Actual Duplications

The MAST Portal capabilities described here will help you identify potential duplications between your intended observations and those that are planned or that have already executed. Note, however, that the complete footprint of dithered or mosaicked observations is not accurately represented (see Evaluating Potential Duplications below). You must evaluate the details of the planned observations by using the accepted program's APT file (and/or the Aladin display in APT, as appropriate) to determine if the potential duplications are genuine in the context of the JWST Duplicate Observations Policy.

Fixed targets

You may search for fixed targets by entering the target coordinates and a search radius (see below). The appropriate search radius to use for duplication checking is summarized in Table 1, and depends upon the instrument and observing configuration. Figure 1 shows a search in a region near 30 Dor, which will be observed in JWST program GTO-1226.

Figure 1. Portal search results for 30 Dor

Portal search results for 30 Dor using a search radius of 180" (dashed red circle) appropriate to a NIRSpec spectrum, showing the JWST exposures listed in a table (left, with orange background), and the spatial footprint of the initial dither position (blue rectangle) superimposed on the Digital Sky Survey at this location (right). Also shown are the footprints of HST observations with ACS/F658N (green rectangles) and the F814W filter with multiple HST instruments (orange rectangles).

Primary search

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

Navigate to the MAST Portal, and enter the coordinates or name of your intended target and the search radius in the dialog box labelled and enter target, then click the Search button:

To search 30 Dor for potential duplications, enter 30 Dor r=180s

After the search results are displayed, in the Filters panel (far left), find the Mission filter and select the JWST checkbox.

  • Optionally, dismiss the filter panel to see more of the results table and the AstroView panel.
  • Optionally change the color of the displayed footprints using the color selector pull-down menu.

The result will be the footprint of the initial dither/mosaic position of planned JWST observations for this target (but see the caveat below). While you can edit the columns that appear in the List View table, be sure to display at least the following to see important metadata:

  • Instrument
  • Filters
  • Proposal ID
  • Distance

The Proposal ID text in each row is a link to the corresponding program information page, which contains a link to the program summary (.pdf) and the APT program definition (.aptx) file. You may need to slide the horizontal scroll bar to the right to see the Proposal ID column.

Supplemental searches

You may optionally show extant observations with other observatories by selecting additional missions beyond JWST. Alternatively you can conduct additional searches and (easily) display footprints for other missions in a different color:

  • Enter the same coordinates (or name) and radius used in the first search and click the search button
  • When the results appear, check the boxes that correspond to the mission data/instrument/filter of interest
  • Modify the color of the displayed footprints using the color selector at the far right.

The AstroView panel displays all footprints from all search tabs. 

If the observations for other missions overlap your intended target, they may prove useful for detailed observation planning (or as a pre-image).

Moving targets

Searching for moving targets (mostly Solar System bodies or their satellites) is possible. Rather than entering celestial coordinates (which may not be correct until the observations are scheduled), click the Advanced Search link under the Portal dialog box. When the search panel appears,

  • click JWST in the Mission filter
  • check 1 in the Moving Target filter

as shown in Figure 2 below, then click the Search button at the top left of the page.

Figure 2. Portal moving target search

Select the desired columns in the Portal Advanced Search panel (left). Use the Mission filter (upper right) and the Moving Target filter (lower right) to initiate a search for moving targets.

Once the results appear, you may further narrow the search by entering the common target name in the Keyword/Text filter, as shown in Figure 3.

Figure 3. Moving target selection by name

The Keyword/Text filter for selecting a target name in the MAST Portal. In this example, where charon was entered, the filter will match any occurrence of the text including "CHARON" and "PLUTO+CHARON".

Some footprint details of moving target observations, such as the coordinates or orientations, are subject to change until the observations have actually been scheduled for execution. The AstroView panel is unlikely to be useful in these cases.

Brute force approach

Many moving target names in the planning database are non-standard, so a search on the target name using the Advanced Search field may not be complete. But for cycle 1, since only about 1,100 observations of moving targets are planned in the GTO and ERS programs, a simple search for JWST moving targets with no other selection criteria may suffice.

MAST API target search

As an alternative, you may use the MAST application programming interface (API) to search the archive for planned and archived observations, the mechanics for which are described in the MAST documentation Programmatic Interfaces (see also the more general MAST API Tutorial). The API allows any search that can be specified with the portal; the results will be textual rather than visual. This approach is most useful for identifying potential duplications with a large number of targets. But if planned observations are identified for fixed targets within your search radius, you will want to use the MAST Portal to visualize those results in detail. A Jupyter Notebook with example queries of the database is available.

Download notebook:

Use and adapt the examples to your specific needs. 

Evaluating potential duplications

If the results of one of the above types of MAST Portal searches shows that accepted JWST observations are at or near your specified target position, there is a potential duplication. You should go on to the next step, which is to look at the accepted program's detailed information to assess the situation in detail.

The portal does not provide the full metadata about planned observations that are necessary to determine whether your intended observation is a duplication. The portal also displays only the primary footprint for an observation, but not those for associated dither positions or mosaic tiles. Use the process below to visualize the full footprint.

Check the APT file

You will need to examine the specifications and exact overlap of planned observing programs in order to determine in detail the nature of a potentially duplicating observation. To do this, you will need to fetch the relevant APT file(s) for the specific JWST program(s): 

  1. Click the Proposal ID link in the portal listing (you may need to slide the horizontal scroll bar to the right to see the Proposal ID column).  Or if you prefer, use the JWST Program Search Tool) and enter the Proposal ID number to bring up the program planning page for that ID.
  2. Click the link provided there to download the APT file to your local disk. Optionally, you may download the Observation Summary (Public PDF) file, which is a formatted readable version of the proposal.
  3. Open APT and load the .aptx file like you would with any other APT file.  Or alternatively, open the PDF summary file to view its contents.
  4. Compare the instrument used and its configuration (imaging/spectroscopy, filters, dispersers, masks) to your intended observation. If the observation is a mosaic, visualize the footprint of dithered or mosaicked observations in APT by clicking the View in Aladin button.

Finally, review the JWST Duplicate Observations Policy carefully to determine if an apparent duplication is genuine.

 For help with APT or Aladin, see the relevant tutorial videos on the JWST Observer YouTube channel.  

Resolving Duplications

If there is a genuine duplication, there are a few choices for resolving it:

  • Select a different target.
  • Change your observation in a way that does not duplicate those already planned.
  • Indicate the duplication and include a sound justification for it (e.g., observations of a time variable source) in the appropriate section of your proposal's Scientific Justification, which is part of the PDF file that must be attached to your APT Proposal Information page prior to final submission of your proposal.

Note: all observations obtained in DD/ERS programs, and observations in select GTO programs, will be publicly available as soon as they have been archived, with zero exclusive access period. Archival proposals for these data are part of the solicitation in cycle 1. See the Call for Proposals for details.

Latest updates
    Article updated for 2020 cycle 1 use.
Originally published