IACT events

This document describes the format to store DL3 event data for IACTs.

The main table is EVENTS, which at the moment contains not only event parameters, but also key information about the observation needed to analyse the data such as the pointing position or the livetime.

The GTI table gives the “good time intervals”. The EVENTS table should match the GTI table, i.e. contain the relevant events within these time intervals.

No requirement is stated yet whether event times must be sorted chronologically; this is under discussion and might be added in a future version of the spec; sorting events by time now when producing EVENTS tables is highly recommended.

The POINTING table defines the pointing position at given times, allowing to interpolate the pointing position at any time. It currently isn’t in use yet, science tools access the pointing position from the EVENTS header and only support fixed-pointing observations.

We note that it is likely that the EVENTS, GTI, and POINTING will change significantly in the future. Discusssion on observing modes is ongoing, a new HDU might be introduced that stores the “observation” (sometimes called “tech”) information, like e.g. the observation mode and pointing information. Other major decision like whether there will be on set of IRFs per OBS_ID (like we have now), or per GTI, is being discussed in CTA. Another discussion point is how to handle trigger dead times. Currently science tools have to access DEADC or LIVETIME from the event header, and combine that with GTI if they want to analyse parts of observations. One option could be to absorb the dead-time correction into the effective areas, another option could be to add dead-time correction factors to GTI tables.