Forced source-fitting stage

(See also the relevant configuration parameters.)

When the source association stage is complete, the pipeline proceeds to handle forced-fits. These may be required either for measuring ‘null detections’ or sources added to the monitoringlist.

Null detection handling

A “null detection” is the term used to describe a source which was expected to be measured in a particular image (because it has been observed in previous images covering the same field of view) but was in fact not detected by the blind extraction step.

After the blindly-extracted source measurements have been stored to the database and have been associated with the known sources in the running catalogue, the null detection stage starts. We retrieve from the database a list of sources that serve as the null detections. Sources on this list come from the runningcatalog and

  • Do not have a counterpart in the extractedsources of the current image after source association has run.
  • Have been seen (in any band) at a timestamp earlier than that of the current image.

We determine null detections only as those sources which have been seen at earlier times which don’t appear in the current image. Sources which have not been seen earlier, and which appear in different bands at the current timestep, are not null detections, but are considered as “new” sources.

For all sources identified as null detections, we measure fluxes by performing a forced elliptical Gaussian fit to the expected source position on the image. The procedure followed is similar to that used for blind extraction, but rather than allowing the pixel position of the barycentre to vary freely, it is held to the known source position. No deblending is performed.

The results of these “forced” source measurements are marked as such and appended to the database.

After being added to the database, the forced fits are matched back to their running catalog counterparts in order to append it as a datapoint in the light curve. This matching is does not include the De Ruiter radius, since the source position came from the running catalog. It is sufficient to use the weighted positional error as a cone search, since the positions are identical. Therefore the forced fit position is not included as an extra datapoint in the position of the running catalog. The fluxes, however, are included into the statistical properties and the values are updated.

It is worth emphasizing that the above procedure guarantees that every known source will have either a blind detection or a forced-fit measurement in every image from the moment it was detected for the first time.

Null-detection depends upon the same job configuration file parameters as the “blind” source-extraction stage.


If monitoringlist positions have been specified when running a job, then these are always force-fitted whenever an image’s source-extraction region contains the designated co-ordinates. These forced fits are used to build up a special runningcatalog entry which is excluded from association with regular extractions.