JWST associations are produced by combining science exposures using a set of predefined rules that depend on the type of data and observation.
See also: Understanding Data Files
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 product and provide relevant information about those exposures, the JWST science processing pipeline first generates an association pool. These are astropy tables 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 Stage 2 associations or Stage 3 associations in JSON format. Based on the association pool content, the calibration software creates the associated data products.
The basic association products 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 file can contain several 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 file for each program and each target within a program. The list of data that goes into constructing an association file, within a specific program, is contained in what we call association pool. This 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 the JWST science processing pipeline, association files are created by the association generator which, based on rules, classifies the data into one or more of these files. When all of the exposures for an observation or set of observations have been collected, the association generator will determine which exposures are needed for the and data products, and will output an association file with the list of all these files. Multiple association files 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 files that accompanies the JWST data.
An association file is a JSON formatted file that includes the list of all related data that might be combined into a single image, as shown in the example stage 3 association.
Types of associations
The following tables provide a list of data types that might be included in an association file. Some of these files, if they exist for that type of observation, might be or not be used to create the association products. Any background or PSF observation can be part to more than one association product.
Table 1. List of data types that can be included in an association file
|Type of data||Association|
|Astrometric confirmation images||x||x||x|
|Single 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|