An observation can begin under FGS control and then switch over to gyros when the guide stars have moved out of the FGS field of view. Targets moving too fast for FGS control, but slower than 7.8 arcsec per second, can be observed under gyro control , with a loss in precision that depends on the length of the observation. The track for a moving target is derived from its orbital elements. The size of the offset is limited by the requirement that both guide stars remain within the respective FOVs of their FGSs. Patterns are used to place the telescope at multiple positions to allow for dithering or mosaic construction.
Patterns can define a linear, spiral, or parallelogram series of observation points.
Patterns can also be combined to produce a more complex series of observation points. If guide stars are used, the possible pattern area is limited by the requirement that the same guide stars be used throughout the pattern. This implies about 120 arcsec of maximum linear motion. The continuum background for HST observations is a function of when and how a given target is observed. Therefore, using the LOW restriction must have adequate scientific justification included in a Phase I proposal.