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The Hubble Space Telescope
Bill Gutsch helped translate this information from more technical NASA
documents.
THE SCIENCE INSTRUMENTS
The Hubble Space Telescope has a variety of scientific instruments.
These include two cameras, two devices called spectrographs and additional
instruments called fine guidance sensors. Because the HST orbits in outer
space above the atmosphere, it can see more clearly than telescopes on
the surface of the earth. When astronomers speak about how clearly a telescope
sees, they talk about how fine a detail the telescope can see or how close
together two objects like stars can be and still be seen as two distinct
objects by the telescope.
The moon is about a half of a degree across. Since astronomers divide
degrees up into 60 minutes of arc, the moon may also be said to be about
30 minutes of arc across. A circle, as you know, has 360 degrees. Each
degree is sub-divided into 60 arc minutes, and each arc minute into 60
arc seconds. The best telescopes on the ground can rarely see detail that
is less than 1 second of arc wide (or split two stars that are less that
one second of arc apart in the sky) because turbulence in the earth's
atmosphere causes the image to ripple and shimmer. By comparison, the
HST can see detail down to less than 0.1 seconds of arc across or, in
other words, more than 10 times clearer. This is one of the main reasons
astronomers like to use the HST.
WIDE FIELD / PLANETARY CAMERA 2
(called "Wiff-Pick") This instrument was not on the HST originally but replaced
another camera when astronauts also went up to fix other things on the HST
during the first servicing mission in December 1993. The Wide Field / Planetary
Camera 2 has special optics that correct for the problem in the HST's main
mirror and is actually four imaging regions (rather like four pieces of
film) in one. However, these cameras do not use film like cameras you have
at home that you use to take snapshots but instead take pictures electronically
rather like a home camcorder. Three of the image taking regions are arranged
in an "L" shape and can take pictures of fairly large things down to a certain
level of detail. Together these three create what is called the Wide Field
Camera. The fourth region can only see a tiny piece of the sky but produces
images that show very fine detail. It is known as the Planetary Camera.
The Live from observations of Neptune will use WF/PC 2.
ADDITIONAL "EYE GLASSES" FOR THE HST
During the 1993 Servicing Mission, the other instruments on board the
HST also got special "eye glasses" to correct for the wrongly shaped main
mirror. Together, these special lenses are called COSTAR and they now
help HST see with wonderful clarity. The instruments COSTAR helps are:
THE FAINT OBJECT CAMERA
This camera was developed for NASA by the European Space Agency and
can see details better than any other device on board the HST. This camera
is also very, very sensitive to light and helps astronomers study some
of the most distant galaxies in the universe. Indeed the Faint Object
Camera (FOC) must be used with special filters if the HST is pointed at
anything less than 200 millions times fainter than the faintest star which
can be seen with the unaided eye! The FOC will be used to image pluto
during the Live from observations.
THE FAINT OBJECT SPECTROGRAPH
A spectrograph is an instrument which takes the light from planets,
stars and galaxies and spreads it out into a rainbow or spectrum of colors
(just like a prism). From the spectrum of an object, astronomers can determine
an amazing amount of information including the object's temperature, what
it's made of and whether it's moving toward or away from us and how fast.
As its name suggests, the Faint Object Spectrograph can examine very faint
objects such as distant or small galaxies. It can examine the visible
part of an object's spectrum as well as some of the radiation the object
gives off at both ends of the spectrum (infrared and ultraviolet) beyond
what the human eye can see.
GODDARD HIGH RESOLUTION SPECTROGRAPH
This instrument was named after the scientist who was the first to design
and launch liquid fueled rockets in the US. This spectrograph looks totally
within the ultraviolet part of the spectrum (that is, the part that is responsible
for giving us a sun burn). It can't study objects in space as faint as the
Faint Object Spectrograph but it can examine the spectrum of brighter objects
in as much as 100,000 times greater detail. Astronomers use this instrument
to get very detailed information on an object's chemical make up as well
as motions within an object such as the way a star is blasted apart in a
supernova or the way gas is swirling in a bright galaxy to help them find
evidence for black holes.
ATTITUDE AND ALTITUDE CONTROL
When parents or teachers talk about your "attitude", they may be referring
to how you feel about things or people around you. When scientists talk
about the attitude of a spacecraft, however, they are referring to which
way the spacecraft is pointing. Attitude control, in turn, refers to how
accurately the spacecraft can be kept pointing in a particular direction.
The pointing of some spacecraft is done using little rockets attached
to the spacecraft that gently turn it in different directions. But, that
is not the way the Hubble Space Telescope is pointed. The HST is so sensitive
and has to work for so many years in space, that scientists use a system
of gyroscopes (little devices that look like and spin like toy tops) to
measure the rate the HST is moving. Reaction wheels are used to turn the
HST. The system applies Newton's laws of action and reaction in a very
interesting way. This all happens very slowly. In the process of being
re-aimed from one object to the next, the HST turns about as fast as the
minute hand on a clock. Other gyroscopes keep the HST pointed steadily
at its target just as gyroscopes help keep planes and rockets on course.
SOLAR POWER
The HST runs on sunlight. Flanking the telescope's tube are two thin, blue
panels containing solar cells. (They look like curtains being stretched
out to dry). Each is about 7 and a half feet by 39 feet in size. They convert
sunlight directly into electricity to run the telescope's scientific instruments,
computers and radio transmitters. Some of the energy generated is also stored
in on board batteries so the telescope can continue to have power even when
it's in the earth's shadow (which is about 25 minutes out of each 96 minute
orbit).
The solar panels, along with the FOC, were built by the European Space
Agency. The Live From Broadcast on March14 will receive a videoconference
uplink from The European Space Agency's facility at Garching, near Munich,
Germany.
COMMUNICATIONS EQUIPMENT
Scientists remain in contact with the HST by means of radio signals. In
turn, the HST sends its images and other data to the ground by radio signals
as well. These signals do not go directly from the Space Telescope Science
Institute to HST and back. Instead, they follow a complicated "bounce around"
route using different earth orbiting communications satellites and other
relay stations on the ground.
Information for this description of the Hubble Telescope came from the
National Space Science Data Center and the Space Telescope Science Institute.
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