TSTART & TSTOP keywords
For many of the Chandra data products, including the aspect solution files, the TSTART and TSTOP keywords give the time range of the files' content, ie the range of TIME column values in the file.
However, for Chandra event files, the TSTART and TSTOP keywords give the time range of the entire Observation Interval. The observation interval includes all time from the mid-way point between the previous target until the mid-way point to the next target. This is illustrated in the following figure.
Illustration of TSTART and TSTOP time boundaries
It is common to see a gap of several hundred seconds between TSTART and TSTOP and the first and last event in the Level 2 event file since time while the space craft is performing a slew is not included in the GTIs.
Note that the mid-slew to mid-slew definition is modified during entry and exit into the radiation belts or in the event of other anomalies.