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Hubble space telescope as seen from the space.
What is Hubble Telescope?

It is a large space telescope launched by NASA in 1990. The telescope is named after astronomer Edwin P. Hubble who was instrumental in deciphering the expansion of the universe. Called 'Hubble' in short, it is 13.3 meters (43.5 feet) long-the length of a large school bus and it weighs about 24,000 pounds. Hubble travels around Earth at about 5 miles per second, in low Earth orbit at an altitude of approximately 540 kilometers and an inclination of 28.5°. Hubble has no thrusters. To change pointing angles, it uses Newton's third law by spinning its wheels in the opposite direction. It turns at about the speed of a minute hand on a clock, taking 15 minutes to turn 90 degrees. While orbiting around earth, Hubble takes pictures of planets, stars and galaxies. It can see galaxies that are trillions of miles away and take pictures of them. Astronomers and astrophysicists have learned a lot about space from these pictures. Hubble can see astronomical objects with an angular size of 0.05 arc seconds, which is like seeing a pair of fireflies in Tokyo from New York.

Chronology of various space-telescopes launched to observe the vast Universe.
Need for a space telescope

Ever since Galileo turned his telescope toward the heavens in 1610, the telescopes grew in size and complexity. They were placed far from city lights and as far above the haze of the atmosphere. Ground-based telescopes were limited to angular resolution (the smallest separation at which objects can be clearly distinguished) of 0.5 to 1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m (8.2 ft) in diameter.

A ground-based telescope can not observe infrared and ultraviolet light as these wavelengths are strongly absorbed by the atmosphere. On the other hand, a space telescope is limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle.

Hubble is the first major optical telescope to be placed in space above the distortion and light pollution of the atmosphere. Stability and clarity of Hubble's images combined with the ability to view the universe at wavelengths of light that are unavailable to observatories on Earth make Hubble by far the most powerful telescope available. Hubble's launch marked the most significant advance in astronomy since Galileo's telescope.

Image showing various components of hubble telescope
Optical system of hubble telescope

The telescope's optical system is a Ritchey-Chretien design, a reflector with a hyperbolic primary mirror and a hyperbolic secondary mirror, designed to minimize optical aberrations. Hubble's primary mirror is 2.4 meters (7 feet, 10.5 inches) across. The system has a focal length of 57.6 meters and is designed with instruments that capture data in visible, infrared, near-infrared, and near-ultraviolet wavelengths. Optically, the HST is a Cassegrain reflector of Ritchey–Chretien design, as are most large professional telescopes. This design has two hyperbolic mirrors which renders good imaging performance over a wide field of view.

When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS).

WF/PC was a high-resolution imaging device primarily intended for optical observations. The instrument contained eight charge-coupled device (CCD) chips divided equally between two cameras. Each CCD has a resolution of 0.64 megapixels. The "wide field camera" (WFC) covered a large angular field at the expense of resolution, while the "planetary camera" (PC) took images at a longer effective focal length than the WF chips, giving it a greater magnification.

Hubble accommodates five science instruments at a given time, plus the Fine Guidance Sensors, which are mainly used for aiming the telescope. Early instruments were replaced with more advanced ones during various Shuttle servicing missions.

Core of the spiral galaxy μ100 imaged with hubble
a. before corrective optics
b. after corrective optics
Launch and initial glitch

Hubble's launch marked the most significant advance in astronomy since Galileo's telescope. However, within weeks of the launch of the telescope, the images returned by HTS indicated a serious problem with the optical system. Although the images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus intended. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function (PSF) concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria.

Analysis of the flawed images showed that the primary mirror had been polished to the wrong shape. Although it was the most precisely figured optical mirror ever made, smooth to about 10 nm (0.4 μin) at the perimeter, it was too flat by about 2,200 nanometers (2.2 micrometers) which was enough to introduce severe spherical aberration.

Working backwards from images of point sources, astronomers determined that the conic constant of the mirror was –1.01390 ±0.0002, instead of the intended –1.00230. The disaster was based on a simple error: null corrector, an optical testing device, had been assembled incorrectly, leaving a lens within it 1.3 millimeters out of position.

After the problem with Hubble's mirror was discovered, the first servicing mission was ensued. Seven trained astronauts flew aboard Endeavour in December 1993. This first servicing mission put up an herculean effort to fix the telescope's flawed optics. Engineers, opticians, and allied scientists began an effort to create a set of corrective optics that would be added to Hubble to make its "vision" clear, dubbed COSTAR (Corrective Optics Space Telescope Axial Replacement).

In 1994, comet shoemaker-Levy a collided with Jupiter. The image shows impact sites of the comet on Jupiter's southern hemisphere.
Discoveries using hubble telescope

Hubble space telescope enabled astronomers to survey large number of galaxies to show that every galaxy with a bright central stellar bulge contains a supermassive black hole in its center. Many dwarf galaxies lack these central black holes. Most galaxies contain one black hole with a mass of millions to billions of times that of the Sun.

Astronomers also used Hubble to record a historic series of so-called deep fields to study distant galaxies and the star formation rate in the universe in unprecedented detail. Astronomers can see, the universe is the same smooth, familiar place everywhere and in every direction, involving more than 100 billion galaxies total.

Scientists using Hubble have shone light onto the earliest days of the cosmos in their quest to understand how the Big Bang transformed into the universe we now know. Soon after the Big Bang, the cosmos was an opaque sea of plasma, and only some 380,000 years later did it cool sufficiently to undergo a so-called phase transition – protons and electrons combined to form neutral hydrogen atoms.

Hubble has played a key role in enabling planetary scientists to study the atmospheres of exoplanets – planets orbiting stars other than the Sun. Scientists hope to find biosignatures on other worlds, marking the first discoveries of the evidence of life elsewhere in the cosmos. For now, Hubble has detected elements and molecules in the atmospheres of "hot Jupiters" orbiting other stars and has unveiled evidence of absorption lines in the spectra of several exoplanets indicating water vapor in their atmospheres. Hubble and its sister telescope Spitzer, the infrared instrument, have detected clouds in the atmospheres of several exoplanets.

The image of stellar recycling Most famous image captured by Hubble showing stars formation in the Eagle Nebula.
Pillars of Creation

Pillars of Creation is one of the astonishing photographs taken by the Hubble Space Telescope of elephant trunks of interstellar gas and dust in the the Serpens constellation of the Eagle Nebula, some 6,500-7,000 light years from Earth. They are so named because the gas and dust are in the process of creating new stars, while also being eroded by the light from nearby stars that have recently formed.

The pillars are composed of cool molecular hydrogen. They are being eroded by photoevaporation from the ultraviolet light of close and hot stars. The leftmost pillar is about four light years in length. The finger-like protrusions at the top of the clouds are larger than our solar system, and are made visible by the shadows of evaporating gaseous globules (EGGs). EGGs shield the gas behind them from intense UV flux from relatively near and hot stars. EGGs are the incubators of new stars. These EGGs get evaporated after giving birth to new stars.

Legacy of Hubble telescope:

NASA had established a schedule of servicing missions for Hubble, which would be needed to maintain the telescope and to upgrade instruments over time. Subsequent servicing missions were carried out in 1997, 1999, 2002, and 2009. Each year, about 10% of the most cited papers published are based on Hubble data. Public image processing of Hubble data is encouraged as most of the data in the archives has not been processed into color imagery. Hubble aroused the curiosity of astrophysicists and public alike throughout the last quarter-century.