How Hubble Space Telescope Works

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Have you ever stared at the night sky and wondered what the universe looks like up close? Most of us are forced to stargaze with just our eyes, searching for pinpricks of light in the vast black night. Even if you're lucky enough to have access to a ground-based telescope, whose clarity depends on atmospheric factors such as clouds and weather, it still doesn't offer the kind of lucidity these stunning celestial objects deserve.
In 1946, an astrophysicist named Dr. Lyman Spitzer Jr. proposed that a telescope in space would reveal much clearer images of distant objects than any ground-based telescope. That sounds logical, right? But this was an outrageous idea, considering no one had even launched a rocket into outer space yet.
As the U.S. space program matured in the 1960s and 1970s, Spitzer lobbied NASA and Congress to develop a space telescope. In 1975, the European Space Agency (ESA) and NASA began drafting the initial plans for it, and in 1977, Congress approved the necessary funds. NASA named Lockheed Missiles (now Lockheed Martin) as the contractor that would build the telescope and its supporting systems, as well as assemble and test it.
The famous telescope was named after U.S. 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.
After a long delay due to the Challenger disaster in 1986, the Hubble Space Telescope shot into orbit on April 24, 1990, piggybacking aboard the Discovery space shuttle. Since its launch, Hubble has reshaped our view of space, with scientists writing thousands of papers based on the telescope's clear-eyed findings on important stuff like the age of the universe, gigantic black holes or what stars look like in the throes of death.
In this article, we'll talk about how Hubble has documented outer space and the instruments that have allowed it to do so. We'll also talk about a few of the problems the venerable telescope/spacecraft has encountered along the way.

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COSTAR Saves the Day.
Almost immediately after it was deployed in 1990, astronomers discovered a problem with their beloved $1.5 billion, 43.5-ft (13.3-m) telescope. Their new tractor-trailer-sized eye in the sky couldn't focus properly. They realized that the telescope's primary mirror had been ground to the wrong dimension. Although the defect in the mirror -- roughly equal to one-fiftieth the thickness of a human hair -- would seem ridiculously minute to most of us, it caused the Hubble Space Telescope to suffer spherical aberration and produce fuzzy images. Surely the astronomers didn't spend years working on the telescope only to be satisfied with unremarkable snapshots of outer space.
Scientists came up with a replacement "contact" lens called COSTAR (Corrective Optics Space Telescope Axial Replacement) to repair the defect in the HST. COSTAR consisted of several small mirrors that would intercept the beam from the flawed mirror, fix the defect and relay the corrected beam to the scientific instruments at the focus of the mirror.
NASA astronauts and staff spent 11 months preparing for what would be one of the most challenging space missions ever attempted. Finally, in December 1993, seven men aboard the space shuttle Endeavour rocketed into space for the HST's first servicing mission.
It took the crew one week to make all of the necessary repairs, and when the telescope was tested after the servicing mission, the images were vastly improved. Today, all of the instruments placed in the HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.
There's more to Hubble than COSTAR, though, and we'll talk about some of those critical parts next.

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Anatomy of the HST
Like any telescope, the HST has a long tube that is open at one end to let in light. It has mirrors to gather and bring the light to a focus where its "eyes" are located. The HST has several types of "eyes" in the form of various instruments. Just as insects can see ultraviolet light or we humans can see visible light, Hubble must also be able to see the various types of light raining down from the heavens.
Specifically, Hubble is a Cassegrain reflector telescope. That just means that light enters the device through the opening and bounces off the primary mirror to a secondary mirror. The secondary mirror in turn reflects the light through a hole in the center of the primary mirror to a focal point behind the primary mirror. If you drew the path of the incoming light, it would like the letter "W," except with three downward humps instead of two.
At the focal point, smaller, half-reflective, half-transparent mirrors distribute the incoming light to the various scientific instruments. (We'll talk more about those instruments in the next section.) As you might have guessed, these aren't just ordinary mirrors that you might gaze in to admire your reflection.
HST's mirrors are made of glass and coated with layers of pure aluminum (three-millionths of an inch thick) and magnesium fluoride (one-millionth of an inch thick) to make them reflect visible, infrared and ultraviolet light. The primary mirror is 7.9 feet (2.4 meters) in diameter, and the secondary mirror is 1.0 feet (0.3 meters) in diameter.
Next we'll talk about what Hubble does with all that light after it hits the telescope's mirrors.

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Hubble's Scientific Instruments: WFPC2, NICMOS and STIS
By looking at the different wavelengths, or the spectrum of light, of a celestial object, you can discern many of its properties. To do this, HST is equipped with several scientific instruments. Each instrument uses charge-coupled devices (CCDs) rather than photographic film to capture the light. The light detected by the CCDs are turned into digital signals, which are stored in onboard computers and relayed to Earth. The digital data are then transformed into amazing photos. Let's look at how each instrument contributes to those images.
The Wide Field and Planetary Camera 2 (WFPC2) is Hubble's main "eye," or camera. It sees with the help of four CCD chips arranged in an "L" shape to catch the light -- three low-resolution, wide-field CCD chips, plus one high-resolution planetary camera CCD chip. All four chips are exposed simultaneously to the target, and the target image is centered on the desired CCD chip. This eye can see visible and ultraviolet light, and can take images through various filters to make natural color pictures, such as this well-known image of the Eagle nebula.
Often, interstellar gas and dust can block our vision of the visible light from various celestial objects. No problem: Hubble can see the infrared light, or heat, from the objects hidden in the dust and gas. To see this infrared light, HST has three sensitive cameras that make up the Near Infrared Camera and Multi-object Spectrometer (NICMOS).
Besides illuminating a celestial object, the light emanating from that object can also reveal what it's made of. The specific colors tell us what elements are present, and the intensity of each color tells us how much of that element is present. The Space Telescope Imaging Spectrograph (STIS) separates the incoming colors of light much as a prism makes a rainbow.
In addition to describing the chemical composition, the spectrum can convey the temperature, density and motion of a celestial object. If the object is moving, the chemical fingerprint may shift toward the blue end (moving toward us) or the red end (moving away from us) of the spectrum. Unfortunately, the STIS lost power in 2004 and has been inactive ever since.
Keep reading to find out what other scientific instruments Hubble has up its telescopic sleeve.

Hubble's Scientific Instruments: ACS and FGS
During a servicing mission in February 2002, astronauts added the Advanced Camera for Surveys (ACS), doubling the Hubble's field of view and replacing the Faint Object Camera, which served as the HST's telephoto lens.
The ACS, which sees visible light, was installed to help map the distribution of dark matter, detect the universe's most distant objects, search for massive planets and examine the evolution of clusters of galaxies. Scientists estimated it would last five years, and right on cue, an electrical shortage disabled two of its three cameras in January 2007.
Diagram of the Hubble Space Telescope. Mouse over the "Telescope Functions" to examine each function. Note: The Faint Object Camera was replaced by the Advanced Camera for Surveys in 2002.
The final instrument on board the HST are its Fine Guidance Sensors (FGSs), which point the telescope and precisely measure the positions and diameters of stars, as well as the separation of binary stars. The Hubble has three of these sensors overall; two to point the telescope and keep it fixed on its target, looking for "guide" stars in the HST field near the target. When each FGS finds a guide star, it locks on to it and feeds information back to the HST steering system to keep that guide star in its field. While two sensors are steering the telescope, one is free to make astrometric measurements (star positions). Astrometric measurements are important for detecting planets because orbiting planets cause the parent stars to wobble as they move across the sky.
Multiple repairs to these instruments, along with a few additions, are scheduled for the next servicing mission potentially in early 2009.
Now you know how Hubble takes all those pictures. We'll learn about Hubble's other life as a spacecraft next.

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Hubble's Spacecraft Systems: Generating Power and Talking to Ground Control
Hubble isn't only a telescope with highly specialized scientific instruments. It's also a spacecraft. As such, it must have power, communicate with the ground and be able to change its attitude (orientation).
All of the instruments and computers on board the HST require electrical power. Two large solar panels fulfill this responsibility. Each winglike panel can convert the sun's energy into 2,800 watts of electricity. When the HST is in the Earth's shadow, energy that has been stored in onboard batteries can sustain the telescope for 7.5 hours.
In addition to generating power, the HST must be able to talk with controllers on the ground to relay data and receive commands for its next targets. To communicate, the HST uses a series of relay satellites called the Tracking and Data Relay Satellite (TDRS) system. Currently, there are five TDRS satellites in various locations in the sky.
Hubble's communication process is also helped by the 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 and various housekeeping functions. Hubble also has backup computers in the event of an emergency.
But what's used to retrieve data? And what happens to that information after it has been collected? Four antennae positioned on the telescope transmit and receive information between Hubble and the Flight Operations Team at the Goddard Space Flight Center in Greenbelt, Md. After receiving the information, Goddard sends it to the Space Telescope Science Institute (STScI) in Maryland, where it's translated into scientific units such as wavelength or brightness.
Learn how Hubble navigates next.

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Hubble's Spacecraft Systems: Steering and Focusing the Eye in the Sky
Hubble zooms around the Earth every 97 minutes, so focusing on a target can be difficult. Three onboard systems allow the telescope to remain fixed on an object: gyroscopes, the Fine Guidance Sensors that we talked about in the previous section, and reaction wheels.
The gyroscopes keep track of Hubble's gross movements. Like compasses, they sense its motion, telling the flight computer that Hubble has moved away from the target. The flight computer then calculates how much and in what direction Hubble must move to remain on target. The flight computer then directs the reaction wheels to move the telescope.
Hubble's Fine Guidance Sensors help keep the telescope fixed on its target by sighting on guide stars. Two of the three sensors 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 sensors are more sensitive than the gyroscopes, but the combination of gyroscopes and the sensors can keep the HST fixed on a target for hours, despite the telescope's orbital motion.
The HST can't use rocket engines or gas thrusters to steer like most satellites do, because the exhaust gases would hover near 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 or pitch/roll/yaw). The reaction wheels are flywheels, like those found in a clutch. 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 force. 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.
Is there anything Hubble can't do?

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Hubble's Limitations
Although the HST is responsible for countless incredible images and discoveries, it does have a few limitations.
One of these limitations is that the HST can't observe the sun because the intense light and heat would fry its sensitive instruments. For this reason, the HST is always pointed away from the sun. That also means that Hubble can't observe Mercury, Venus and certain stars that are close to the sun either.
In addition to the brightness of objects, Hubble's orbit also restricts what can be seen. Sometimes, targets that astronomers would like Hubble to observe are obstructed by the Earth itself as Hubble orbits. This can limit the time spent observing a given object.
Lastly, the HST passes through a section of the Van Allen radiation belts, where charged particles from the solar winds are trapped by the Earth's magnetic field. These encounters cause high background radiation, which interferes with the instruments' detectors. It's impossible for the telescope to make observations during these periods.
Next learn what the future holds for the great observatory in the sky.

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Plans for Hubble: Final Servicing Mission and Replacement
At the moment, Hubble's future is a bit uncertain. The final servicing mission was scheduled for Oct. 10, 2008. However, NASA lost a week of preparation time due to a forced evacuation of Mission Control in Houston as Hurricane Ike swept through Texas.
Then, space shuttle Atlantis was set to blast off Oct. 14, 2008, carrying a crew of seven astronauts to complete the mission -- a journey that would have taken 11 days and extended the telescope's life until at least 2013.
However, on Sept. 29, 2008, NASA postponed the final mission until sometime in early 2009, due to a serious breakdown. A failure occurred in Hubble's command and data handling instruments, and it simply stopped capturing and sending the data needed to produce the deep space images that we know and love.
When Atlantis is finally launched, NASA may send a replacement part for the failed component. However, before that can happen, NASA has to test the replacement part and train the astronauts how to install it. In the meantime, the agency is also attempting to activate the backup channel for the command and data-handling system so that the telescope may resume transmitting data.
What's planned for life after Hubble?
Hubble's successor, the James Webb Space Telescope (JWST), named after former NASA administrator James Webb, will study every phase in the history of the universe. From its orbit approximately 1 million miles (1.6 million km) from Earth, the telescope will uncover information about the birth of stars, other solar systems and galaxies, and the evolution of our own solar system.
To make these fascinating discoveries, the JWST will rely primarily on four scientific instruments: a near-infrared (IR) camera, a near-IR multi-object spectrograph, a mid-IR instrument and a tunable filter imager.
Formerly known as the "Next Generation Space Telescope," the JWST is slated to launch in 2013 and has been an international collaboration among NASA, the European Space Agency and the Canadian Space Agency.
But before we move on to the JWST and forget about Hubble, perhaps the hardworking telescope deserves a moment. Thanks to Hubble's unparalleled discoveries, captivating images of what lies beyond the Earth's atmosphere have been made accessible for everyone to enjoy. From a rare alignment between two spiral galaxies to a powerful collision between galaxy clusters, Hubble has brought a little piece of the heavens closer to home.
Keep reading for more links on amazing telescopes and our adventures in space.

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