3.2 HST Guiding Performance

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 only the rate-sensing gyroscopes.

3.2.1 FGS - Dual Guide Star Acquisitions

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. Reacquisitions following interruptions due to Earth occultations take about 5 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. As a result, observers planning extended observations in 0.1" or smaller STIS slits should execute a target peakup maneuver every 4 orbits (see Section 6.4.2).

3.2.2 FGS - Single Guide Star Acquisitions

In cases where two suitable guide stars are not available, a single guide star acquisition can be used. The translational motion of the HST is then controlled by a guide star in one of the FGSs, while the roll motion is controlled by the gyros. Therefore, a gyro drift will be present that is approximately 1.5 mas/sec around the guide star. This introduces a translational drift across the target, the exact size of which depends on the roll drift rate and distance from the single guide star to the instrument aperture (target) in the field-of-view (see Figure 2.2). Note however that the gyro drift can build up through occultations and typically limits a visit duration to a few orbits.

There are also occasions when a dual guide star acquisition was planned, but one of the planned pair of guide stars cannot be acquired. In this case, the Pointing Control System (PCS) will usually carry out the observations using single FGS guiding. More details on single guide-star guiding issues specific to ACS programs, particularly those that require very accurate knowledge of the PSF (including coronagraphic programs and astrometric programs) or accurate sub-pixel dithering can be found at:

       http://www.stsci.edu/hst/acs/faqs/guide_star.html
 

or in the ACS Instrument Handbook.

3.2.3 Gyro-only Pointing Control

It is possible, but not common, to take observations without any guide stars, using only gyro pointing control (e.g., for extremely short exposures, or snapshots). The absolute one-sigma pointing accuracy using gyros is about 14". The pointing drifts at a typical rate of 1.4 +/- 0.7 mas/sec, but it can be somewhat larger depending on the slew history of HST. Note again that gyro drift can build up through occultations and typically limits a visit duration to 1-2 orbits.