Stage 3 Associations: Technical Specifications

Logical Structure

Independent of the actual format, all Stage 3 associations have the following structure. As in Stage 2 Associations: Technical Specifications, the structure is defined and enforced by the Stage 3 schema.

Informational Meta Keywords

List of products, each consisting of

Output product name

List of exposure members, each consisting of

Filename of the exposure that is a member of this association

Type of exposure

If present, information about errors from the observatory log

If present, the Association Candidates this exposure belongs to

Example Association

The following example will be used to explain the contents of an association:

{
    "degraded_status": "No known degraded exposures in association.",
    "version_id": "20160826t131159",
    "asn_type": "image3",
    "asn_id": "c3001",
    "constraints": "Constraints:\n    opt_elem2: CLEAR\n    detector: (?!NULL).+\n    target_name: 1\n    exp_type: NRC_IMAGE\n    wfsvisit: NULL\n    instrument: NIRCAM\n    opt_elem: F090W\n    program: 99009",
    "asn_pool": "mega_pool",
    "asn_rule": "Asn_Image",
    "target": "1",
    "program": "99009",
    "products": [
        {
            "name": "jw99009-a3001_t001_nircam_f090w",
            "members": [
                {
                    "exposerr": null,
                    "expname": "jw_00001_cal.fits",
                    "asn_candidate": "[('o001', 'observation')]",
                    "exptype": "science"
                },
                {
                    "exposerr": null,
                    "expname": "jw_00002_cal.fits",
                    "asn_candidate": "[('o001', 'observation')]",
                    "exptype": "science"
                }
            ]
        }
    ]
}

Association Meta Keywords

The following are the informational, meta keywords of an association.

asn_id (required): The association ID. The ID is what appears in the Association Naming.
asn_pool (required): Association pool from which this association was created.
asn_rule (optional): Name of the association rule which created this association.
asn_type (optional): The type of association represented. See Association Types.
code_version (optional): The version of the generator that created this association. Typically this is the version of the jwst Python package.
constraints (optional): List of constraints used by the association generator to create this association. Format and contents are determined by the defining rule.
degraded_status (optional): If any of the included members have an actual issue, as reported by the exposerr keyword, degraded_status will say One or more members have an error associated with them. If no errors are indicated, it will be No known degraded exposures in association.
program (optional): Program number for which this association was created.
target (optional): Target ID to which this association refers. JWST currently uses the TARGETID header keyword in the Stage 3 exposure files, but there are no formal restrictions on value.
version_id (optional): Version identifier. DMS uses a time stamp with the format yyyymmddthhmmss but this can also be None or NULL.

`products` Keyword

Association products have two components:

name (optional)

The string template to be used by Stage 3 processing tasks to create the output file names. The product name, in general, is a prefix on which the individual pipeline and step modules will append whatever suffix information is needed.

If not specified, the Stage 3 processing modules will create a name root.

members (required)

This is a list of the exposures to be used by the Stage 3 processing tasks. This keyword is explained in detail in members Keyword.

`members` Keyword

members is a list of dictionaries, one for each member exposure in the association. Each member has the following keywords.

expname (required)

The exposure file name.

It is recommended that this value be set to a filename only, with no path. In this case, the file must be in the same directory as the association file, so the path is not needed. If valid absolute or relative path data is included, the input file is still found and used by the pipeline, but a warning is raised: the association file in this case may be less portable than if the path data is not included.

exptype (required)

Type of information represented by the exposure. Possible values are

science: The primary science exposures. There is usually more than one since Stage 3 calibration involves combining multiple science exposures. However, at least one exposure in an association needs to be science.
background: Exposures used for background subtraction.
psf: Exposures that should be considered PSF references for coronagraphic and AMI calibration.

exposerr (optional)

If there was some issue the occurred on the observatory that may have affected this exposure, that condition is listed here. Otherwise the value is null.

asn_candidate (optional)

Contains the list of association candidates this exposure belongs to.

Editing the member list

As discussed previously, a member is made up of a number of keywords, formatted as follows:

{
    "expname": "jw_00003_cal.fits",
    "exptype": "science",
    "exposerr": null,
    "asn_candidate": "[('o001', 'observation')]"
}

To remove a member, delete its corresponding set.

To add a member, one need to only specify the two required keywords:

{
    "expname": "jw_00003_cal.fits",
    "exptype": "science"
}