Identifying Suddenly Saturated Objects in the Prompt Pipeline

Will there be a pipeline that can recognize object saturation “on the fly”, just after the first visit to a certain field?

Happy to clarify Data Management’s current plans for handling saturated sources.

First, a bit of background and context for this thread. The Prompt Pipeline performs Difference Imaging Analysis (DIA) by subtracting an older template image from each new image, and performing source detection on the difference image. All difference-image sources detected with a signal-to-noise ratio greater than 5 (in positive or negative flux) are considered “detected”. Each detection becomes an entry in the DIASource catalog, and an Alert is generated with 60 seconds. The DIAObject catalog is the collection of all DIASources, associated by coordinate. Each new DIASource is either associated with an existing DIAObject, or spawns a new DIAObject record. Both the DIASource and DIAObject catalogs, and the new and difference images, are available in the Data Access Center within 24 hours.

Second, there are two main scenarios for how saturated sources in a new image will be treated by the Prompt Pipeline’s Difference Imaging Analysis.

(1) An object which has never exhibited any variability before, and is thus not already in the DIAObjects catalog, suddenly saturates in a new image. This scenario does not lead to a |S/N|>5 detection in the difference image: saturated objects lead to artifacts in the difference image, which are rejected. No DIASource entry is made, no Alert is generated, and there is no DIAObject for this newly saturated transient/variable.

(2) An object which has exhibited variability before, and is already in the DIAObjects catalog, suddenly saturates in a new image. This scenario does not lead to a |S/N|>5 detection in the difference image, no DIASource entry is made, and no Alert is generated. However, as part of DIA, forced photometry is performed at the location of all existing DIAObjects. In this case, forced photometry would be performed for this object, and the fact that there is a saturated source at that location becomes a flag in the DIAObject catalog.

In practice, the second scenario will cover most cases of saturating objects, and LSST users will simply be able to query the DIAObjects catalog for their targets of interest, and know within 24 hours whether it recently saturated.

The first scenario is expected to be quite rare, but an LSST user would be able to create a user-generated pipeline with the tools in the LSST Science Platform in order to, for example, make image cutouts of their targets and run source characterization tasks to assess recent saturation.

Hopefully this answers your question, but please do reply if you have any more questions about this or any related topic! Thanks very much for posting to Community.