The Telescope | HubbleSite

Orbiting high above the Earth, the Hubble Space Telescope has a clear view of the universe free from the blurring and absorbing effects of the atmosphere. In addition to observing visible and near-infrared light, Hubble detects ultraviolet light, which is absorbed by the atmosphere and visible only from space. The telescope has beamed hundreds of thousands of celestial images back to Earth during its time in space.

Hubble is a Cassegrain reflector telescope. Light from celestial objects travels down a tube, is collected by a bowl-like, inwardly curved primary mirror and reflected toward a smaller, dome-shaped, outwardly curved secondary mirror. The secondary mirror bounces the light back to the primary mirror and through a hole in its center. The light is focused on a small area called the focal plane, where it is picked up by its various science instruments.

Hubble’s science instruments, the astronomer’s eyes to the universe, work together or individually to provide the observations. Each instrument is designed to examine the universe in a different way. Hubble holds two main varieties of instruments: cameras, which capture Hubble's famed images, and spectrographs, which break light into colors for analysis.

Hubble's current suite of instruments includes the Wide Field Camera 3 (WFC3), Cosmic Origins Spectrograph (COS), Advanced Camera for Surveys (ACS), Space Telescope Imaging Spectrograph (STIS) and Fine Guidance Sensors (FGS).

Learn more about Hubble’s instruments

These are not the only instruments that have flown aboard Hubble. The telescope was designed to be visited periodically by astronauts, who brought new instruments and technology, and made repairs from December 1993 to May 2009.

After launch in April 1990, NASA discovered that the primary mirror was flawed. The flaw was tiny, only about 1/50th the width of a human hair, but significant enough to distort Hubble’s vision. During Servicing Mission 1 in December of 1993, astronauts added corrective optics to compensate for the flaw. The optics acted like eyeglasses to correct Hubble’s vision.

Hubble is operated by commands from the ground. Several spacecraft systems are in place to keep Hubble functioning smoothly.

Hubble performs in response to detailed instructions from people on the ground. The antennas allow technicians to communicate with the telescope, telling it what to do and when to do it. Four antennas receive and send information to a set of satellites, which in turn communicate with Earth.

Hubble is powered by sunlight. Each wing-like array has solar cells that convert the Sun’s energy into electricity. Some of that electricity runs the telescope, some is stored in onboard batteries for the periods when Hubble is in Earth’s shadow.

Several computers and microprocessors reside in Hubble’s body and in each science instrument. There are two main computers. One talks to the instruments, sends commands and other information, and transmits data; the other handles pointing control, gyroscopes and other system-wide functions.

Hubble has blanket of multilayered insulation, which protects the telescope from temperature extremes.

Hubble uses a combination of gyroscopes, reaction wheels and Fine Guidance Sensors to orient itself.

Hubble is one of NASA’s most successful and long-lasting science missions. It has beamed hundreds of thousands of images back to Earth, shedding light on many of the great mysteries of astronomy.

Among its many discoveries, Hubble has revealed the age of the universe to be about 13.8 billion years, much more accurate than the old range of anywhere from 10 to 20 billion years. Hubble played a key role in the discovery of dark energy, a mysterious force that causes the expansion of the universe to accelerate.

Hubble has shown scientists galaxies in all stages of evolution, including galaxies that were around when the universe was still young, helping them understand how galaxies form. It found protoplanetary disks, clumps of gas and dust around young stars that likely function as birthing grounds for new planets. It discovered that gamma-ray bursts — strange, incredibly powerful explosions of energy — can occur in far-distant galaxies when massive stars collapse. And these are only a handful of its many continuing contributions to astronomy.

Read news stories about Hubble discoveries

The telescope is an instrument for the entire astronomical community. Any astronomer in the world can submit a proposal and request time on the telescope — alone or in coordination with other observatories in space and on the ground — as well as for support to make use of Hubble’s extensive data archives. Astronomers compete for time to use Hubble.

More scientists want to use the telescope than time allows, so a review committee of astronomy experts has to pick out the best proposals from the bunch. To avoid bias, the competition process is double-blind. This means that not only are proposers unaware of the identity of the reviewers, but the reviewers are also not aware of the identities of proposers.

The winning proposals are the ones that make the best use of the telescope’s capabilities while addressing pressing astronomical questions. Each year around 1,000 proposals are reviewed and approximately 200 are selected, for a total of about 20,000 individual observations.