- Timing information: All times are given in UTC.
- Predicted data: All predicted information is extracted or derived from the Mission Schedule which is a time ordered list of commands and activities sent to the spacecraft. During normal operations, the duration of a schedule is 7 days and it is prepared within a few days of the execution date. The Mission Schedule uses the most accurate orbit data available at that time.
- Coordinate reference frames: The guide star pointing and line-of-sight angles are given with respect to the orthogonal vehicle reference frame as projected on the sky: +V1 points outward and is the axis around which the vehicle is rolled, (near but not exactly the optical axis or the mechanical axis), +V3 is perpendicular to the solar arrays and points in the direction of the sun, and +V2 forms a right-handed frame. Motion around V2 is called pitch, motion around V3 is yaw, and motion around V1 is roll. This coordinate system is described in the instrument handbooks and proposal instructions. Note that the "U2, U3" axes, also used in the handbooks, are simply rotated 180 degrees from the V2,V3 axes, i.e., they share the same parity.
- HST line of sight: The +V1 axis and the term, HST line of sight, are used interchangeably in this documentation.
- X,Y,Z: Geocentric Inertial Coordinate Frame (GCI). Equivalent to the celestial coordinate system but centered on the Earth.
- Velocity Aberration: The apparent shift in the position of a star due to the motion of HST is corrected onboard the spacecraft. When using guide stars or gyros to reconstruct the pointing, the observation log includes appropriate velocity aberration and differential aberration corrections (small differences between the guide star and target positions). The shifts due to velocity aberration between the FGSs and the apertures in the focal plane are about +/- 50 milliarcsec.
- Baseline acquisition and re-acquisition: A baseline guide star acquisition occurs when initially acquiring guide stars at the start of an observing program, i.e., after a large slew. A re-acquisition of guide stars is executed at the start of each orbit thereafter for the same pointing.