|HST Primer for Cycle 25|
Objects within the Solar System move with respect to the fixed stars. HST has the capability to point at, and track moving targets, including planets, their satellites and surface features on them, with sub-arcsecond accuracy. However, there are a variety of practical limitations on the use of these capabilities that must be considered before addressing the feasibility of any particular investigation. Proposals to observe the Moon, for example, will be permitted in Cycle 25. Please consult the Lunar Observations User Information Report (UIR-2007-01) for more information.HST is capable of tracking moving targets with the same precision achieved for fixed targets. This is accomplished by maintaining FGS Fine Lock on guide stars, and driving the FGS star sensors in the appropriate path, thus moving the telescope to track the target. Tracking under FGS control is technically possible for apparent target motions up to 5 arcsec/s. In practice, however, FGS tracking of a fast moving target may be able to accommodate a full HST orbit’s worth of observing even if the target motion forces the guide stars out of the FGS apertures before completion of the orbit; in such cases, it is possible to continue observing under (less accurate) gyro guiding.The track for a moving target is derived from its orbital elements. Orbital elements for all planets and their satellites are available at STScI. For other objects, the Principal Investigator (PI) must provide orbital elements for the target in the Phase II proposal. The “Requires Ephemeris Correction” special requirement, described in the Phase II Proposal Instructions can be used to insert an offset shortly before the observation is executed to eliminate “zeropoint” errors due to an inaccurate ephemeris. See Section 4.1.4 of the Call for Proposals for restrictions on observations of Solar System targets.