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Please note that the RA_SCX, DEC_SCX, RA_SCZ, DEC_SCZ keywords refer to the spacecraft (i.e. star tracker), but the boresight of the LAT should be calculated by applying a rototranslation from the star tracker position. The latter can be always improved, so that it is always better to have the starting point, that is the position from the star tracker.
(McEnery) I disagree that the data will be treated differently in inertially pointed and survey mode. The LAT is not pointing at a particular direction (in celestial coordinates) during survey mode, the direction of the LAT boresight is constantly changing. In pointed mode, there is no guarantee that the LAT will stay perfectly inertially pointed, it may move by a few degrees to satisfy contraints, and will move by a lot during an Earth avoidance maneuver. So even in a pointed observation RA_SCX, DEC_SCX, RA_SCZ, DEC_SCZ, OBJECT are not automatically defined, and a science analysis cannot assume that the pointing direction is constant.
5. MC_TRUTH keyword
(Digel) I think that the original intent was that this keyword indicate whether the data are entirely Monte Carlo truth values (i.e., actual directions, energies, etc.). We do not actually have files like this, and I propose removing this keyword. gtobsssim does add a column MC_SRC_ID to the files it generates, but this is easy enough to check for without the MC_TRUTH keyword.
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