Associate gravity events with ATLAS footprints and PS skycells in database

[thespacedoctor]

Associate ATLAS footprints and PS skycells with gravity events (for each map produced) within the 90% contour area and -0.5 days to 7 days of the event.

These associations are needed for event plots and coverage stats.

[thespacedoctor]

To implement this match I decided to convert each alert map to a flat nside=128 healpix map and then import all pixels within the 90% contour into a database table called alert_pixels_128.

For each ATLAS exposure and PS skycell taken within -0.5 to 7 days of the alert event time, I calculate the overlapping healpix nside=128 pixels. The overlapping pixels are matched against the event map pixels and, if a match is found, the pixels are associated with the specific exposure/skycell (see below).

This implementation has the advantage of requiring the compute intensive matching to be done only once and then the results are cached. After this, finding the exposures/skycells overlapping the map regions and calculating the coverage statistics for GCN reports is trivial and fast.