Source Association

DIA object association

DiaSources are associated with known solar system objects and stationary objects using an association radius of 1 arcsecond.

Solar System objects are stored in the SSObject catalog and stationary objects are stored in the DiaObject catalog. DiaSources not associated with either will create new DiaObjects. DiaSources associated with stationary objects will contain a non-null diaObjectId, while DiaObjects associated with solar system objects will include a non-null ssObjectId. Each associated DiaSource will either have a non-null diaObjectId or ssObjectId but will not have both fields populated.

The time (in MJD, TAI) when a DiaSource was reassociated from a DiaObject to SSObject (if such reassociation happens) will be included in the DiaSource catalog.

Solar System object association

Alerts for DiaSources with known solar system objects predicted to be within the 1-arcsecond association radius of the DiaSource detection include the ssSource records for each solar system object (SSO). The ssSource table contains information derived from the known orbit of the associated solar system object and the current observation of the DiaSource.

Because a single known moving object predicted position may fall within the 1-arcsecond association radius for multiple nearby DiaSources, a diaDistanceRank field is included in the ssSource table with the rank of the diaSourceId -identified source in terms of its closeness to the predicted SSO position. If the diaSourceId is the nearest DiaSource to this SSO prediction, diaDistanceRank = 1 would be set. If it is the second nearest, it would be 2, etc. In this framework, multiple DiaSources may include the ssSource record for the same known solar system object, but each would have a different value assigned for the diaDistanceRank according to their proximity to the predicted position of that SSO. To find the best association match between a given diaSourceId to the predicted position of a known solar system object, filter on diaDistanceRank == 1 in the SSSource table. In addition, because SSSources (detections) are attributed to SSObjects, a given ssObjectId may have SSSources associated to that ssObjectId that include different diaDistanceRank values. Similarly in this case, to find the best association match between a given ssObjectId to the predicted position of a known SSO, filter on diaDistanceRank == 1 in the SSSource table.

Also included in the ssSource catalog is the ephemeris offset (ephOffset) that measures the total observed versus predicted angular separation on the sky between the observed DiaSource and the predicted position of a known solar system object. This field is also useful for quality control, association confidence, and filtering uncertain matches. The along-track and cross-track ephemeris offsets (ephOffsetAlongTrack and ephOffsetCrossTrack, respectively) as well as the RA and Dec ephemeris offsets (ephOffsetRa and ephOffsetDec, respectively) between the observed DiaSource and the predicted position of a known solar system object are also included in the ssSource catalog.

To decipher ambiguous associations, the diaDistanceRank, ephemeris offsets, and multiple associated detections can be used. Orbit-fitting, performed by the IAU Minor Planet Center (MPC), after DiaSource detections associated with known solar system objects in the above ranked system are submitted to the MPC and subsequently ingested by the Solar System Processing pipelines in the mpc_orbits catalog, will provide the most concrete solar system object association.

Variability characterization

A small set of light curve characterization parameters (timeseries features) are calculated for all DiaObjects.

Alert Production Database

The live results of association are held in a Cassandra database known as the Alert Production Database (APDB).

Daily exports of the APDB contents will be make available to Data Rights holders for query in the Prompt Products Database (PPDB).

Duplication

Because is run in real time, if two consecutive images of the same sky area are processed simultaneously, it is possible for two new DiaObjects to be created at the same position. These duplicates will be cleaned up in morning processing. Similarly, alerts may not include the DiaSources from images taken immediately prior to the current image. Such DiaSources will appear in future alerts to that DiaObject, however. This situation is most common for deep drilling fields but can also occur in field overlaps.