A Diagram of the Hubble
As shown in this diagram, HST's Scientific Instruments (SIs) are mounted in bays behind the primary mirror. The Wide Field Planetary Camera 2 occupies one of the radial bays, with an attached 45 degree pickoff mirror that allows it to receive the on-axis beam. Three SIs (Faint Object Camera, Near Infrared Camera and Multi-Object Spectrometer, and Space Telescope Imaging Spectrograph) are mounted in the axial bays and receive images several arcminutes off-axis.
HST's Scientific Instruments (SIs) are mounted in bays behind the primary mirror. The Wide Field Planetary Camera 2 occupies one of the radial bays, with an attached 45 degree pickoff mirror that allows it to receive the on-axis beam. Three SIs (Faint Object Camera, Near Infrared Camera and Multi-Object Spectrometer, and Space Telescope Imaging Spectrograph) are mounted in the axial bays and receive images several arcminutes off-axis.
Major components are labelled, and definitions of V1,V2,V3 spacecraft axes are indicated.
During the servicing mission in December 1993, the astronauts installed the Corrective Optics Space Telescope Axial Replacement (COSTAR) in the fourth axial bay (in place of the High Speed Photometer). COSTAR deployed corrective reflecting optics in the optical paths in front of the Faint Object Camera, thus removing the effects of the primary mirror's spherical aberration. In addition the Wide Field and Planetary Camera (WF/PC) was replaced by the WFPC2, which contains internal optics to correct the spherical aberration.
The Fine Guidance Sensors (FGSs) occupy the other three radial bays and receive light 10-14 arcminutes off-axis. Since at most two FGSs are required to guide the telescope, it is possible to conduct astrometric observations with a third FGS. Their performance is unaffected by the installation of COSTAR.
HST receives electrical power from two solar arrays (see Fig. 1), which are turned (and the spacecraft rolled about its optical axis) so that the panels face the incident sunlight. During the 1993 servicing mission the astronauts installed new solar arrays, which have significantly reduced the thermally induced vibrations that the old arrays had been producing. Nickel-hydrogen batteries provide power during orbital night. The two high-gain antennas shown in Fig. 1 provide communications with the ground (via the Tracking and Data Relay Satellite System). Power, control, and communications functions are carried out by the Support Systems Module (SSM), which encircles the primary mirror.
In addition to STIS and NICMOS, the second servicing mission will replace several additional pieces of equipment. One of the FGSs (most likely FGS 2) will be replaced. The new FGS will have an adjustable fold flat to recover some of the performance capability lost by spherical aberration. A spare magnetic tape recorder will replace the failed ESTR-2. A solid state recorder (SSR) will replace ESTR-1, and will provide for a factor of 10 greater on-board data storage volume. This extra storage will be necessary to support parallel operations of the WFPC2, STIS, and NICMOS. It will also provide increased flexibility in scheduling HST observations, reducing the tight coupling with the TDRSS system.
HST Maneuvering and Pointing
HST is, in principle, free to roll about its optical axis. However, this freedom is limited by the need to keep sunlight shining on the solar arrays, and by a thermal design that assumes that the Sun always heats the same side of the telescope.
To discuss HST pointing, it is useful to define a coordinate system that is fixed to the telescope. This system consists of three orthogonal axes: V1, V2, and V3. V1 lies along the optical axis, V2 is parallel to the solar-array rotation axis, and V3 is perpendicular to the solar-array axis (see Fig. 1). Power and thermal constraints are satisfied when the telescope is oriented such that the Sun is in the half-plane defined by the +V1 axis and the positive V3 axis. The orientation that optimizes the solar-array positioning with respect to the Sun is called the "nominal orientation."
It should be noted that the nominal orientation angle required for a particular observation depends on the location of the target and the date of the observation. Observations of the same target made at different times will, in general, be made at different orientations.
Some departures from nominal orientation are permitted during HST observing (e.g., if a specific orientation is required at a specific date, or if the same orientation is required for observations made at different times). Roll is defined as the angle about the V1 axis between a given orientation and nominal orientation. Off-nominal rolls are restricted to approximately 5 degrees when the sun angle is between 50 degrees and 90 degrees, < 30 degrees when the sun angle is between 90 degrees and 178 degrees and is unlimited at anti-sun pointings of 178 degrees to 180 degrees.
HST utilizes electrically driven reaction wheels to perform all maneuvering required for guide-star acquisition and pointing control. A separate set of rate gyroscopes is used to provide attitude information to the pointing control system. The servicing mission restored or replaced three gyros that had failed since the original launch, so that the spacecraft currently has a total of six operational gyros. Any three of these are the minimum required for telescope pointing control.
The slew rate is limited to approximately 6 degrees per minute of time. Thus, about one hour is needed to go full circle in pitch, yaw, or roll. Upon arrival at a new target, up to 9 additional minutes must be allowed for the FGSs to acquire a new pair of guide stars. As a result, large maneuvers are costly in time and are generally scheduled for periods of Earth occultation or crossing of the South Atlantic Anomaly.
The telescope does not generally observe targets within 50 degrees of the Sun, 15.5 degrees of any illuminated portion of the Earth, 7.6 degrees of the dark limb of the Earth, nor 9 degrees of the Moon. Following the first servicing mission, the telescope is again allowed to point directly away from the Sun.
There are exceptions to these rules for HST pointing in certain cases. For instance, the bright Earth is a useful flat-field calibration source. However, there are onboard safety features that cannot be overridden. The most important of these is that the aperture door shown in Fig. 1 will close automatically whenever HST is pointed within 20 degrees of the Sun, in order to prevent direct sunlight from reaching the optics and focal plane.
Objects in the inner solar system, such as Venus or comets near perihelion, are unfortunately difficult or impossible to observe with HST, because of the 50 degree solar limit. When the scientific justification is compelling, observations of Venus and time-critical observations of other solar-system objects lying between 45 degrees and 50 degrees of the Sun may be carried out.
Data Storage and Transmission
The HST observing schedule is constructed at STScI and forwarded to the Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The HST is controlled from the Space Telescope Operations Control Center (STOCC), located at GSFC. Communication with the spacecraft is via the Tracking and Data Relay Satellite System (TDRSS), which consists of a set of satellites in geosynchronous orbit.
Commands to HST originate at the STOCC and are sent via land-line or communications satellites to the TDRSS ground station at White Sands. From there the commands are sent via the appropriate TDRSS to HST. Scientific data are sent from HST to the STOCC via the reverse path, and then from the STOCC to the STScI via dedicated high-speed links.
The TDRSS network supports many spacecraft in addition to HST. Therefore, use of the network, either to send commands or return data, must be scheduled. Because of limited TDRSS availability, command sequences for HST observations are normally uplinked periodically and stored in the onboard computers. HST then executes the observations automatically.
It is possible for observers at STScI to interact in real-time with HST for specific purposes, such as certain target acquisitions. In practice, real-time interactions are difficult to schedule. Historically, during normal operations, fewer than 50 real-time interactions have been required per year.
HST currently uses two onboard tape recorders to store scientific data. After the second servicing mission a much larger capacity Solid State Recorder will be available. Except when real-time access is required, most HST observations are stored to a recorder and read back to the ground several hours later. There are, however, limits to the amount of data that can be handled by the ground system supporting HST. Some scientific programs requiring very high data-acquisition rates cannot be accommodated, because the SIs would generate more data than either the links or ground system could handle.