Identifying Potential Duplicate Observations

Investigators who are preparing JWST proposals can use the MAST Data Discovery Portal to identify potential duplications between observations of their desired targets and existing or accepted JWST observations

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Main article: JWST Duplication Checking  
See also:  JWST Duplicate Observations Policy,  MAST Data Discovery Portal, JWST Program Information Look-up

When preparing an observing proposal for JWST, investigators are obliged to check their targets against existing or planned JWST observations for potential duplications (see JWST Duplication Checking). You may use the MAST Data Discovery Portal to discover archived and planned 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). 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

Checking for potentially duplicate observations begins with searching the MAST Data Discovery Portal for your targets, with a radius in the Table below that is appropriate for the instrument configuration you intend to use (see JWST Duplicate Observations Policy for details).

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The general procedure for searching for observations of astronomical targets is described in the article on 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 fully developed user tool for JWST duplication checking is not yet available. Until then, 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 mosaiced observations is not accurately represented (see Evaluating Potential Duplications below). You must evaluate the particulars of the planned observations in the context of the JWST Duplicate Observations Policy (using the accepted program's APT file and/or the Aladin display in APT, as appropriate) to determine if the potential duplications are genuine.

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 above, 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 arcsec (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 HST observations with ACS/F658N (green rectangles) and the F814W filter with multiple HST instruments (orange rectangles), the results of additional searches.

Primary search

Navigate to the MAST Portal, and do the following:

  • 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), scroll down to 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.

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 icon.

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 Portal 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 below.

Figure 3. Moving target selection by name

The Keyword/Text filter for selecting a target name in the MAST Portal. The filter will match any occurrence of the text including, for example, "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 filter may not be complete. But since only about 1100 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

You may instead use the MAST Application Programming Interface (API) to search the archive for planned and archived observations, the mechanics for which are described in the article 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.

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 all the metadata about planned observations that are necessary to determine whether your intended observation would duplicate. 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 Program ID link in the Portal (or use the JWST Program Search Tool) to bring up the Program Planning page for that ID
  2. Click the link to download the APT file
    1. Optionally download the Observation Summary (Public PDF) file
  3. Open the APT file (and be sure to select this program in the menu at left) or the PDF summmary
  4. Compare the instrument used and its configuration (imaging/spectroscopy, filters, dispersers, masks) to your intended observation
    1. Visualize the footprint of dithered or mosaiced observations by clicking the View in Aladin button in APT

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

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
  • Include a sound justification for the duplication (e.g., observations of a time-variable source) in the appropriate section of your proposal's Scientific Justification

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.



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