Hubble's science instruments act as the astronomer's electronic eyes. Light hits the primary mirror, is reflected onto the secondary mirror and from there passes through a small opening in the primary mirror to the focal plane. The focal plane is the size and shape of a dinner plate and is shared by the scientific instruments that sample light and record the information. Once the telescope observes a celestial object, its onboard computers convert the image or data into long strings of numbers that are sent down to Earth via the Tracking and Data Relay Satellite System (TDRSS). From TDRSS the data are sent to the TDRSS ground station and on to the Goddard Space Flight Center. NASA's Goddard Space Flight Center then forwards the data over landlinks to the Space Telescope Science Institute (STScI) in Baltimore on the east coast of the United States where it is reconverted into data and images, analysed and stored. Observers can then study the data at STScI or at their home institutions.
The telescope operates 24 hours a day. However, scheduling is tricky. At certain times of the year a celestial object may not be visible because it appears too close to the Sun. Furthermore, Hubble is not in constant communication with the ground stations. Consequently, technicians must plan each observation down to the second, telling the telescope what to do and when, using detailed computer-coded instructions transmitted to and stored inside the telescope's onboard computer.
Although Hubble operates around the clock, not all of its time is spent observing. Each orbit lasts about 97 minutes, with time allocated for "housekeeping" functions and for observations. "Housekeeping" functions include turning the telescope to acquire a new target, or avoiding the Sun or Moon, switching communications antennae and data transmission modes, receiving command loads and downlinking data, calibrating and similar activities.
When STScI completes its master observing plan, the schedule is forwarded to Goddard's Space Telescope Operations Control Center (STOCC), where the science and housekeeping plans are merged into a detailed operations schedule. Each event is translated into a series of commands to be sent to the onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.
Whenever possible two scientific instruments are used simultaneously to observe adjacent target regions of the sky. For example, while a spectrograph is focused on a chosen star or nebula, WFPC2 can image a sky region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed steadily at the right target.
If an astronomer wishes to be present during the observation, there is a console at STScI and another at the STOCC, where monitors display images or other data as the observations occur. Some limited real-time commands for target acquisition or filter changing can be given from these stations, if the observation program has been set up to allow for it, but spontaneous control is not possible.
Before the Hubble Space Telescope can make any kind of observation, it must find a pair of guide stars located alongside the observational target. To find these direction beacons, mission planners refer to an immense catalogue, Hubble Guide Star Catalogue, containing the sky 'addresses' of 15 million stars.
Since its launch in 1990, Hubble has made 97 percent of its scheduled observations. The other three percent - usually missed because of minor equipment malfunctions - are rescheduled to later dates.