This document describes the format to store DL3 event data for gamma-ray instruments.
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.
GTI table gives the “good time intervals”. The
EVENTS table should
GTI table, i.e. contain the relevant events within these time
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.
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
LIVETIME from the event header, and combine that with
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.