Although astronomers who work with
ground-based telescopes have to deal with
bad weather and atmospheric filtering, they
do have one advantage over astronomers
working with instruments in space. The
ground-based astronomers can work directly
with their instruments. That means that they
can constantly check and adjust their
instruments first-hand. Astronomers
working with satellite-based instruments
must do everything remotely. With the
exception of telescopes mounted in the
Space Shuttle's payload bay and the Hubble
Space Telescope, which was serviced by
Shuttle astronauts in 1993, astronomers can
only interact with their instruments via radio
transmissions. That means that the
instruments have to be mounted on a
satellite that provides radio receivers and
transmitters, electric power, pointing control,
data storage, and a variety of computer-run
subsystems. Data collection, transmission, and analysis is of primary importance to astronomers. The development of photomultiplier tubes and CCDs or charged coupled devices (See introduction in Unit 3.) provides astronomers with an efficient means of collecting data in a digital form, transmitting it via radio, and analyzing it by computer processing. CCDs, for example, convert photons falling on their light sensitive elements into electric signals which are assigned numeric values |
representing their strength. Spacecraft
subsystems convert numeric values into a
data stream of binary numbers that are
transmitted to Earth. Once received,
computers reconvert the data stream to the
original numbers that can be processed into
images or spectra. If the satellite is in a geostationary orbit, which permits it to remain above one location on Earth, these data may be continuously transmitted to ground receiving stations consisting of one or more radio antennas and support equipment. Geostationary satellites orbit in an easterly direction over Earth's equator at an elevation of approximately 40,000 kilometers. They orbit Earth in one day, the same time it takes Earth to rotate, so the satellite remains over the same part of Earth at all times. Satellites at other altitudes and orbital paths do not stay above one point on Earth. As a result, they remain visible to a particular ground station for a short time and then move out of range. This requires many widely-spaced ground stations to collect the satellite's data. In spite of this, the satellite still spends much of its time over parts of Earth where no stations exist (oceans, polar regions, etc.). For this reason, one of the subsystems on astronomical satellites are tape recorders that store data until they can transmit it to ground stations. |