JWST associations are produced by combining science exposures using a set of predefined rules that depend on the type of data and observation.
Relationships between multiple exposures are captured in an association, which is a means of identifying a set of exposures that belong together and may be dependent upon one another. The association concept permits exposures to be calibrated, archived, retrieved, and reprocessed as a set rather than as individual objects.
In order to capture a list of exposures that could potentially form an association and provide relevant information about those exposures, the Data Management System (DMS) first generates an association pool. These are simple ASCII files that contain the metadata for all the data in a given proposal. These pools are then used by the association generator that runs within software to generate association definitions in JSON format. Based on the association table content, the calibration software creates the associated data products.
The basic association that the pipeline generates are combinations of mosaics or dithers for a single observation. Higher level products are built by associating data outside the routine science data processing pipeline; these might include multiple observations from a single target in a program or large mosaics similar to HLA for HST data. An association can contain science data products, related files that support the science data (e.g., jitter data or target acquisition images), and contemporaneous calibration files used to calibrate the science data.
There is a separate association for each program and each target within a program, if it can be determined that multiple targets in a program are not related. Each association will follow the same format. The pool of data that is considered to construct an association within a specific program is called association pool, and it will include:
- All observations from the same target in a given program
- Observations of a given target from multiple science instruments within the same program
- Different filters for the same target within the same program
- Exposures from linked observations within the same program
- Calibration exposures, which can be members of more than one association pool
In DMS, associations are created by the association generator which, based on rules, classifies the data into one or more associations. When all of the exposures for an observation or set of observations have been collected, an association generator will determine which exposures are needed for the and data products, and will output an association table that documents the content of the association. Multiple association tables can be generated from a single association pool. Observers should not need to run the generator; instead, it is expected they will edit the existing association that accompanies the JWST data.
Types of associations
The following tables provide a list of data types that might belong to an association if they exist for that type of observation. Any background or PSF observation can belong to more than one association.
Table 1. How data is associated
|Type of data||Association|
|Astrometric confirmation images||x||x||x|
|Singe target observation||All dither points||x||x||x||x|
|All nodding points||x||x|
|All mosaic tiles within an observation folder||x||x|
|All mosaic tiles in different observation folders||x||x|
In different orientation ( target grouped via special requirements sequence observation non-interruptable)
|Autowave calibration observations||x|
|Autoflat calibration observations||x|
|MSA Plan sources catalog||x|
|PSF observation associated with the science target||x|
JWST-STScI-004078 " Design of Imaging Associations "