How wonderful it would have been if we could look out at distant stars, galaxies, nebulas and other heavenly bodies with unmatched clarity. With such a telescope, you could see billions of light years away and see the things that happened billions of light years ago. Astronomers are doing just that with the Hubble Space Telescope.
Let us understand the world's most powerful telescope that is not on ground but out there in the heavens looking not the ground but everything else. We will understand that how good is Hubble Space Telescope and why it so special for astronomers.
The major problem with observing the light from distant stars using ground-based telescopes is that the light must pass through the Earth's atmosphere. Besides clouds and weather, the Earth's atmosphere is a boiling place – there is dust, currents of warm air rising, currents of cold air falling and water vapor. All of these factors can produce fuzzy images of the stars and limit the usefulness of ground-based telescopes.
History
Dr. Lyman Spitzer (1914-1997), an eminent astrophysicist proposed in 1946, that a telescope in space would reveal much sharper images, of even farther-off objects, than any ground-based telescopes. This was a magnificent idea considering no one had yet launched a rocket into outer space. As the U.S. space program excelled in 1960s and 1970s, Spitzer pushed the matter of developing such a telescope. In 1975, NASA and European Space Agency (ESA) started developing the telescope that would stay out of Earth's environment. The space telescope was named after American astronomer Edwin Hubble, whose observations of variable stars in distant galaxies confirmed that the Universe was expanding and gave support to the 'Big Bang' theory. The Hubble Space Telescope took 8 years to build, has five scientific instruments, had more than 4,00,000 parts and had 41,600 km of electrical wiring. HST was reported to be 50 times more sensitive than the ground-based telescopes, with 10 times more resolution. After a long delay, HST went into orbit in 1990. Today HST is considered as an icon of modern astronomy.
Immediate Problems
Almost immediately after HST was deployed, astronomers found that they could not focus the telescope. They discovered that the primary mirror had a very minute flaw, as minute as one-fiftieth the size of human hair, due to this the images became fuzzy. But, very soon scientist came up with a replacement contact lens to correct the defect in HST. This repair was carried out in 1993 while the HST was in orbit around the Earth. After the service mission when the HST was tested, the images were vastly improved.
Inside Hubble
HST is not just a simple telescope but also a smart spacecraft that maneuvers HST in space in perfect harmony with the instructions submitted by its mission people from here on the Earth.
As the ground-based telescopes, HST has a long tube that is open at one end. It has mirrors to gather light and bring the light to a focus where its "eyes" are located. HST is capable of looking at not just visible light but also at ultraviolet and infrared spectrum of the electro-magnetic radiation. To see at these different types of spectrums HST is packed with different instruments.
HST Mirrors
HST is a compound telescope design. Light enters through the tube and bounces of the primary mirror situated at the other end of the tube. Light than hits the secondary mirror, which again reflects the light through a small hole in the primary mirror to a point behind the primary mirror. At this point, which is called focal point, smaller half reflective, half transparent mirrors distribute the light to the various scientific instruments.
HST's mirrors are made of glass and coated with layers of pure aluminum (three-millionths of an inch) and magnesium fluoride (one-millionth of an inch) to make them reflect visible light, infrared and ultraviolet light. The primary mirror weighs 828 kg and the secondary mirror weighs 12.3 kg. Aperture (diameter) of the primary mirror is 94.5 in and of secondary mirror it is 12 in.
Some Facts
The total length of HST is 43.5 ft and width is 14 ft. Weight of HST is 12 tons. HST is at a distance of 612 km, far far away from any atmospheric disturbances and pollutions. Orbit is HST is inclined at 28.5 degrees relative to the Earth's equator. To complete one orbit around the Earth HST takes 97 minutes, a little above 1-½ hour. In a day, HST makes almost 15 revolutions around the Earth at the speed of 28,000 km/h. The estimated life span of HST is of 20 years out of which 14 years are already completed successfully.
Images from HST
What ever HST sees is stored through CCD (Charged-Coupled Devices) rather than photographic films. The light detected by CCDs is converted into digital signals, which are then stored in on-board computers and relayed to Earth. The digital data are then transformed into amazing pictures that we see in the news and Internet.
Power
All of the instruments and computers on board the HST need electrical power. This electrical power is supplied by two large solar panels, each panel measuring 40 feet. The solar panels provide 2400 watts of electricity, which is enough to power sixty 40 watts light bulbs. When the HST is in the Earth's shadow, electrical power is provided by six nickel-hydrogen batteries, which provide the same power as 20 car batteries. The batteries are recharged by the solar panels when the HST comes around to the sunlight again.
Communications
The HST much be able to talk with controllers on the ground to relay data from its observations and receive command for its next mission. For communications, HST uses a series of relay satellites.
Incoming light from an object gets received by the HST and converted to digital data. The data from HST is then sent to the satellites in orbit, which then transmits it to the Ground Receiving Station. From here the data is sent to NASA's Goddard Spaceflight Control Center, where the HST operations are centered. Various teams of scientists then analyze the data. Most of the time, commands are relayed to the HST in advance of a planned observing run; however, real-time commands are possible when necessary.
Steering
The HST must remain fix on a target while observing it for taking images, which could take several hours depending upon how much detail is required and which type of spectrum is being examined. Bear in mind that HST is orbiting Earth at a speed of 28,000 km/h, so focusing on a target is like keeping sight of a small object on the shore from the deck of a boat that is rapidly moving over the coast, bobbing up and down in the waves. To remain on the target, HST has three on-board systems:
- Gyroscopes – sense small to large motions
- Reaction Wheels – move the telescope
- FGS – sense fine motion
The gyroscopes keep the track of the gross movements of the HST. Like a compass, they sense the motion of the HST, telling the flight computer that the HST has moved away from the target. The flight computer calculates how much and in what direction the HST must move to remain on target. The flight computer then directs the reaction wheels to move the telescope.
The HST cannot have rocket engines or gas thrusters to steer like most satellites do, because the exhaust gases would hover neat the telescope and cloud the surrounding field of view. Instead, the HST has reaction wheels oriented in the three directions of motion (x/y/z). The reaction wheels are flywheels. When the HST needs to move, the flight computer tells one or more flywheels which direction to spin in and how fast, which provides the action. In accordance with Newton's third law of motion (for every action there is an equal and opposite reaction), the HST spins in the opposite direction of the flywheels until it reaches its target.
The FGS help keep the telescope fixed on its target by sighting on guide stars. Two of the three FGS find guide stars around the target within their respective fields of view. Once found, they lock onto the guide stars and send information to the flight computer to keep the guide stars within their field of view. The FGS are more sensitive than the gyroscopes; but the combination of gyroscopes and FGS can keep the HST fixed on a target for hours despite the telescope's orbital motion.
Computing
The HST has two main computers that fit around the telescope's tube above the scientific instrument bays. One computer talks to the ground to transmit data and receive commands. The other computer is responsible for steering the HST, as well as various housekeeping functions. There are also backup computers in the event of an emergency.
After Hubble Space Telescope
HST is performing well, yielding much scientific data and beautiful images. However, the HST will not last forever. Plans are underway for a new space telescope, which will be even more sensitive than HST and provide better images of even more distant objects.
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