HST's Pointing Control System (PCS) has two guiding modes available. The default guide mode uses Fine Guidance Sensors (FGSs) to provide high precision pointing control by using guide stars to actively control the telescope pointing. However, the telescope pointing can also be controlled using the rate-sensing gyroscopes.
Gyro-only pointing and single guide star acquisitions will not be available for science observations in two-gyro mode. See the HST Two-Gyro Handbook for detailed information about pointing and guiding in two-gyro mode. In Cycle 15, all GO observations will be taken in two-gyro mode. On-orbit data shows that there is essentially no difference in guiding performance between 2 and 3 gyro operations.
The default operational practice is to schedule observations using Dual Guide Star mode. In a Dual Guide Star Acquisition, two FGSs lock onto separate guide stars. The combined pointing information is used to control the pitch, yaw, and roll axes of the telescope (by contrast to ground-based telescopes, which generally only use one guide star). Dual Guide Star Acquisition times are typically 6 minutes. Re-acquisitions following interruptions due to Earth occultations also take about 6 minutes. This pointing control method was designed to keep telescope jitter below 0.007" rms, which is now routinely achieved. A drift of up to 0.05" may occur over a timescale of 12 hours and is attributed to thermal effects as the spacecraft and FGSs are heated or cooled.
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