How Does the TGS Mode Work?

HST requires information on all three axes (roll, pitch, and yaw) in order to perform slews to targets and maintain pointing stability (i.e., track targets). This is normally accomplished using three of the six onboard gyros. With three working gyros, HST slews to a target (pointing accuracy ~200") and uses the Fixed Head Star Trackers (FHSTs) to perform an initial correction to the pointing (pointing accuracy within 10"). The Fine Guidance Sensors (FGSs) acquire the guide stars needed to center the target in the field-of-view of an instrument, and then track the target during science exposures. For some targets, degraded tracking using only one FGS can be used. For multi-orbit programs, the telescope may have to return to gyro pointing while the FGSs are occulted, with a guide star re-acquisition required before science operations can resume. In some cases, the telescope will actually switch from one set of guide stars to a different set (e.g., moving targets).

Two Gyro Science Mode

Two Gyro Science (TGS) mode refers to the process HST will use to continue science operations in the event that there are only two working gyros available. When there are only two available, an additional sensor is needed to provide information on the third axis. Though the FHSTs and FGSs can provide this information, they are not always available (e.g., during Earth occultations). However, the magnetometers can provide information at all times, although their accuracy is dependent upon sensor noise and the magnetic field alignment. In TGS mode, the magnetometers will be used to support large HST maneuvers (i.e., slewing to targets) and for pointing control during periods when the FHSTs or FGSs are not available. The FHSTs will provide information when they are not occulted, as they provide more accurate information than the magnetometers. Note that while TGS mode will allow continuing HST observations with only two gyros, the pointing stability (i.e., telescope jitter) will be degraded from three gyro operation, and the scheduling of observations will be less efficient and flexible.

How TGS Mode Works

During and after slewing to a target, the telescope uses the magnetometers and two gyros to control pointing (M2G mode); this mode is also used when all the FHSTs are occulted and FGS guiding is not used. The pointing error in M2G mode can be as large as 10 degrees, although the nominal error is expected to be ~5 degrees.

Once an FHST becomes available, the tracker will replace the magnetometer and the telescope will enter T2G mode, which will be used for pointing determination and correction. While one FHST can be used to maintain the pointing and prevent further drifting, a second FHST is required to determine where the telescope is pointed. Star maps from the FHSTs will be analyzed on-board, and a coarse correction to the desired pointing will be made. A second set of maps will then be obtained to perform a fine pointing correction sufficient for the FGS to acquire guide stars. The pointing accuracy for T2G will be less than 50".

Once the guide star acquisition has completed and fine lock is obtained, information from the FGSs replaces the FHST and the telescope will enter F2G mode. The pointing stability in this mode is expected to be very similar to three gyro operations.

The figure below shows schematically how three gyro and two gyro modes differ.