3.2 HST Guiding Performance

FGS - Dual Guide Star Acquisitions

FGS - Single Guide Star Acquisitions

HST’s Pointing Control System (PCS) has two guiding modes available. The default guide mode uses Fine Guidance Sensors (FGSs) that maintain 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.

FGS - Dual Guide Star Acquisitions

The default operational practice is to schedule observations using the Dual Guide Star mode. In a Dual Guide Star Acquisition, two FGSs are used, each locked on a different guide star. The combined pointing information is used to control the pitch, yaw, and roll axes of the telescope. Dual Guide Star Acquisition times are typically six minutes. Re-acquisitions following interruptions due to Earth occultations take about four 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 more–this 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 peak-up maneuver every four orbits (see Section 6.4.2).

FGS - Single Guide Star Acquisitions

In cases where two suitable guide stars are not available, a single guide star acquisition can be used. In this scenario, HST’s translational motion is controlled by a guide star in one of the FGSs, while roll motion is controlled by gyros. Therefore, a gyro drift around the guide star will be present that is approximately 1.5 milliarcseconds (mas) of roll angle per second. This introduces a translational drift across the target; the exact size of that drift depends on the roll drift rate, as well as the distance between the single guide star and the instrument aperture (see Figure 2.2). Note, however, that the gyro drift builds up through occultations, typically limiting a visit duration to a few orbits.

There are also occasions when a dual guide star acquisition is planned, but one of the guide stars cannot be acquired. In this case, the Pointing Control System (PCS) will usually carry out the observations using single FGS guiding. A description of this can be found at:

http://www.stsci.edu/hst/observatory/pointing/obslog/OL_7.html

under the header “Single FGS Pitch and Yaw plus Gyro Roll Control” in which a specific application to WFPC2 (a legacy instrument) is discussed. Additional information on single guide star guiding issues, particularly those that require very accurate knowledge of the PSF (including coronagraphic and astrometric programs), or accurate sub-pixel dithering can be found at:

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

The ACS Instrument Handbook also contains complementary examples.

Most science programs will not be adversely affected because the drift rates are generally small.