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
![[A line plot showing time intervals for 3 observations. Vertical lines identify the mid-slew time boundaries. Pink shading is used to identify good time of the individual observations.]](tstart_tstop.png)
Illustration of TSTART and TSTOP time boundaries
In this illustration we show the time boundaries for three consecutive observations: 11447, 10992, 11557. The X-axis is time relative to the start of ObsID 11447. The Y-axis shows the pointing Declination. The nearly constant (flat) sections are the 3 separate observations (zooming in would show the typical dither pattern).
The sloped line segments show the time when Chandra is slewing from target to target. The gray vertical dashed lines show the TSTART and TSTOP values for each observation. These are the times at the middle of the slew between targets. Note that TSTOP11447 equals TSTART10992 and TSTOP10992 equals TSTART11557.
The pink shaded area shows the good time interval (GTI) for each of the observations.
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.