The Hubble Space Telescope (HST) - Remote Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
The Hubble Space Telescope (HST)

This Section does not attempt to be a complete textbook on Astronomy. Some topics covered in an Astronomy course are not included here. Before we start the more detailed review of Astronomy and Cosmology in the next 11+ pages, we will examine what most scientists acclaim as the greatest astronomical instrument ever built by Man - the Hubble Space Telescope, or HST. If you may be sceptical of this claim, just feast your eyes on these two images - typical of HST's extraordinary output:

Part of the Carina nebula.
An exploding star.

This review of the HST may pique your curiosity about "telescopes" and how they work. The main function of a telescope is to gather photons from a source, concentrate them (focus) so as to improve detectability, and display them as discrete images or numerical data sets. If interest is aroused, try these two websites for a primer on optical telescopes: Wikipedia, and How Stuff Works.

Probably the most famous telescope of all time is at the Mt. Wilson Observatory, in the San Gabriel mountains north of Pasadena, California. The observatory was the brainchild of the astronomer George Hale, who reasoned that building it atop a high peak would avoid problems with the atmosphere and city lights. Work began in 1904 to build the observatory; its first telescope, the 60 inch Hale refractor, began observing in 1908. Here is the present-day view of the buildings including the main observatory at 5715 feet:

The Mount Wilson Observatory.

Mt. Wilson's most famous telescope - and commensurate with the Hubble Space Telescope in importance - is the 100 inch Hooker (named after its benefactor), begun in 1908 and operational by 1917. It uses a refracting mirror. This is the telescope that Edwin Hubble used to demonstrate the existence of galaxies beyond the Milky Way and to discover that the Universe is expanding. Here it the scope in its current setting:

The Hooker Telescope at Mt. Wilson.

Prior to 1990 all telescope observations of the heavens were confined to instruments on the ground. These have one obvious advantage: they can be visited by astronomers. But they have a major disadvantage: the atmosphere tends to interfere with the observations, causing distortions and diminution of the radiation signals; building telescopes on top of high mountains - the current practice - reduces that problem. But as populations grew and cities cast more light into the sky, this unwanted background effect caused further reduction in viewing efficiency. Still, ground telescopy remains a mainstay of astronomy. Click on these web sources for lists of Observatories grouped by country and largest ground telescopes. Among the top sites for observatories is Mauna Kea (more than 4200 meters [14000 ft] above sealevel) on the Big Island of Hawaii. Various nations have observatories there. Here is a view of the complex:

Some of the observatories on Mauna Kea.

Prior to the 1990s, surveying and studying stars and galaxies as visible entities required the use of optical telescopes at ground-based locations. This ground photo shows a typical cluster of observatories, the Kitt Peak complex in Arizona.

The Kitt Peak Observatories.

Telescope observatories are distributed in mountain ranges across the world. One example is the ESO (European Southern Observatory) Paranal facility in the Atacama desert of Chile. The cluster consists of 18 telescopes, operated by 14 nations, and includes the VLT - four conjoined telescopes each with 8.2 meters mirrors.

ESO's La Silla observatory complex.

In this Section we will see some stunning images acquired by space telesccopes. Images taken through ground telescopes are usually less spectacular but some can rival those acquired by space observatories. Examine below the pair of images (in the visible [left] and infrared [right]) of the Flame Nebula as seen through the newly operational VISTA telescope operated by the European Space Organization (ES0) on a high mountain in the Chilean Andes:

The Flame Nebula.

Before setting out to explore the Universe's development and history since the first moments after the Big Bang, we want to pay homage to what this writer (NMS; and many, many others) consider the greatest scientific instrument yet devised by mankind - the first of the Great Observatories: the Hubble Space Telescope (which we will often refer to as HST). No other instrument has advanced our knowledge of astronomy and the Universe as much as this splendid observatory in outer space. Perhaps no other astronomical observatory has captured the public's imagination, with its numerous sensational pictures, as has the Hubble. HST has provided many extraordinary views of stars, galaxies, dust clouds, exploding stars, and interstellar and intergalactic space, extending our view to the outermost reaches of the Universe. HST has brought about a revolution in our understanding of Astronomy and Cosmology. One good reason for placing this HST review on this second page is simply that many of the subsequent illustrations of the Cosmos used in this Section were made by this telescope.

This, the most versatile optical telescope up to the present and perhaps the penultimate remote sensing system, receives its name to honor Edwin Hubble, the man who confirmed much about the existence, distribution, and movement of galaxies, leading to the realization of an expanding Universe which in turn brought about the Big Bang theory. Here he is at work in the 1920s on the 100-inch Mt. Wilson telescope:

Edwin Hubble - one of the greats in Astronomy and Cosmology - at work observing through the Mt Wilson 100-inch telescope.

As the space programs developed, astronomers dreamed of placing the telescopes in space orbit where viewing conditions are optimized. HST is the outgrowth of a concept first suggested in 1946 by Lyman Spitzer who argued that any telescope placed above Earth's atmosphere would produce significantly better imagery from outer space. (Spitzer has been honored for this idea by having the last of the Great Observatories named after him; see page 20-4.) The HST was launched from the Space Shuttle on April 24 of 1990 after 20 years of dedicated efforts by more than 10000 scientists and engineers to get this project funded and the spacecraft built. Here is the HTS in the Bay of the Shuttle:

Color photograph of the Hubble Space Telescope docked in the Shuttle cargo bay.

And this is a photo of the HST in orbit, as seen from the Shuttle:

The HST in orbit.

The HST is big - commonly described as the size of a school bus. An idea of the "bigness" - and why it just barely fitted into the Space Shuttle's cargo bay - is evident in this photo taken during the fourth repair mission, in which the astronauts provide a convenient scale.

Astronauts repairing Hubble during the fourth servicing mission.

A general description of the Hubble Space Telescope and its mission is given in this review by the Space Telescope Science Institute.

This cutaway diagram shows the major features and components of the HST:

Schematic diagram of HST showing location of its main components.

But, as scientists examined the first images they were dismayed to learn that these were both out of focus and lacked expected resolution. HST proved unable to deliver quite the sharp pictures expected because of a fundamental mistake in grinding the shape of its primary (2.4 m) mirror. The curvature was off by less than 100th of a millimeter but this error prevented focusing of light to yield sharp images. The distortion that resulted is evident in this early HST image of a star:

A distorted star image.

This dismaying result was a major blow to astronomers. NASA was urged by the scientific community to find a way to salvage HST. This took three and a half years to come up with and apply the solution. But during that time, HST did some limited but useful work. Consider this somewhat blurred image of NGC 4261. The small central bright spot was interpreted to be caused by the action of a black hole

Distorted view of NGC 4261.

Astronomers and engineers put their heads together to solve this egregious problem and designed optical hardware that could restore a sharp focus. In December of 1993 the Hubble telescope was revisited by the Space Shuttle. (This mission to salvage the HST is a definitive answer to the critics of manned space missions - only human intervention could remedy an otherwise lost cause.) At that time 5 astronaut spacewalks succeeded in installing corrective mirrors and servicing other sensors. The package was known as COSTAR (Corrective Optics Space Telescope Axial Replacement).

After the first servicing mission, the striking improvement in optical and electronic response is evident in the set of images below made by the telescope, which show the famed M100 (M denotes the Messier Catalog number) spiral galaxy viewed by the Wide Field Planetary Camera before (bottom left) and after (bottom right) the correction. For an indication of how much HST improves astronomers' views of distant astronomical bodies, one of the best earth-based telescope images, from Kitt Peak, is shown at the top:

Spiral Galaxy M100, shown in a Kitt Peak ground telescope image (top pair), then as it appeared through the HST before its optics were correct (left bottom) and after correction (right bottom).

Another way to judge the improvement that HST provides by being above the atmosphere is to compare absorption spectra for Hydrogen in the Visible and Ultraviolet coming from a quasar source as recorded by a ground based telescope and HST.

Hydrogen spectra in the Visible and UV recorded by a telescope looking through the atmosphere and by HST above the atmosphere.

The increased sensitivity of the HST instrumentation, unimpeded by atmospheric absorption, provides more detected Hydrogen lines in both the UV and Visible regions of the EM spectrum.

In some respects, the HST shares remote sensing features found on Landsat. HST has filters that narrow the wavelengths sensed. The filters range through part of the UV, the Visible, and the Near-IR. This permits individual chemical elements to be detected at their diagnostic wavelengths. The resulting narrow band images can then be combined through filters to produce the multicolored imagery that has made many Hubble scenes into almost an "abstract art" form - one of the reasons that the general public has taken so positively to this great instrument. As an example, here is an HST multifilter image of the Crab Nebula in which the blue is assigned to radiation from neutral hydrogen, the green relates to singly ionized oxygen, and the red doubly ionized oxygen.

HST image of the Crab nebula.

HST images can be combined with those made from other space observatories that sense at wavelengths outside the visible. This provides information on chemical composition as well as temperatures and the types of radiation involved. Consider this example:

Multiwavelength image of a supernova, using inputs from different satellites.

These multiwavelength images give rise to one technique for picking out galaxies that are located at various great distances from Earth - the so-called Deep Field galaxies (page 20-3a) that formed early in the Universe's history. These galaxies are moving away at ever greater velocities. The redshift method (see page 20-10) of determining distance relies on the Doppler effect in which motion relative to the observer reduces the frequency (lengthens the wavelength towards/to/past red) of light radiation as the galaxies move away from Earth as a result of expansion. Those ever farther away, moving at progressively greater velocities, experience increasing redshifts. A galaxy emitting light at some maximum frequency can be imaged through, say, a narrow bandpass blue filter. This frequency translates to a specific redshift and hence a particular distance. A galaxy farther away has its redshift toward/to the green and will appear brightest through a green filter. Filters passing longer wavelengths will favor detection of greater redshifts - thus galaxies still more distant from Earth. Younger/closer galaxies may not even shine bright enough at shorter wavelengths to be detectable in filters whose bandpass cutoffs exclude those wavelengths.

Information on both original Hubble instruments and those added later appears in this site prepared by the Space Telescope Science Institute. The history of HST in terms of instrument placements and servicing missions, from the early days to the present and a look to the future is given in this chart prepared by the Space Telescope Institute:

The history of HST and its instrument maintenance and installation; note - the fourth servicing mission in 2004 did not take place but will in 2009.

The original 5 instruments onboard HST were: the FOC (Faint Object Camera); FOS (Faint Object Spectrograph) GHRS (Goddard High Resolution Spectrograph); HSP (High Speed Photometer) and WFPC1 (Wide Field Planetary Camera); added since (by subsequent visits using the Space Shuttle) are NICMOS (Near Infrared Camera and MultiObject Spectrometer); STIS (Space Telescope Imaging Spectrograph); ACS (Advanced Camera Surveyor); FGS (Fine Guidance Sensor); and WFPC2; future additions (by Shuttle flights) may be the COS (Cosmic Origins Spectrograph) and WFPC3.

Thus, HST has been further improved even beyond its initial ten year life expectancy - now extended well into the second decade of the 21st century. A third Shuttle servicing mission was successfully completed in two stages: December 1999 and March of 2002. In addition to replacing or "repairing" existing systems on the satellite bus, a new instrument, the ACS (Advanced Camera for Surveys) was added; it represents a tenfold improvement in resolution and clarity. During this repair mission the NICMOS (Near Infrared Camera and Multi-Object Spectroscope) sensor, out of working order for nearly three years, was repaired and upgraded. This pair of images, ACS on the left and NICMOS on the right, shows the improved quality of imaging of part of the Cone Nebula, bringing out more details of the dust that dominates this feature:
Gas and dust within the Cone Nebula, imaged by the upgraded ACD and NICMOS sensors on the HST.

Many of the most informative HST images can be viewed on the Space Telescope Science Institute's (Baltimore, MD) Home Page . HST has imaged numerous galaxies at different distances - almost to the edge of space - from Earth that are therefore also at different time stages in the general evolution of the Universe. The following illustration shows both spiral and elliptical galaxies (but not the same individuals) at 2, 5, 9, and 14 billion years after the Big Bang in a sequence that represents different stages in this development.

 Looking back in time at elliptical and spiral galaxies at different stages of their history (age).

The Hubble Space Telescope has had a remarkable impact on the study of the Universe. In its honor, the Astronomy Picture of the Day (APOD) web site, in celebration of its 10th anniversary, has compiled a collage of a variety of the more spectacular images acquired by HST, supplemented with a few images made by other instruments. This is reproduced here; be on the lookout for many of the individual embedded images in this montage elsewhere in this Section.

Collage of APOD images published on the Web in the last 10 years.

However, technology and design are allowing ground-based telescopes to "catch up" with the HST, at least for those galaxies that are relatively close to Earth. The resolution and clarity of some recently activated ground telescopes are on a par with their Hubble counterparts, at least within the depth range (lookback time) of a few billion light years. This results from better detectors, improved optics, the ability of a ground telescope to dwell on the target for much longer time spans (allowing buildup of the incoming radiation to generate a bright image), and, for some location on high mountain tops, above most of the atmosphere. This is illustrated with this pair of images which show a Highton Compact Group galaxy (HCG87) imaged by ESO's southern hemisphere telescope (left) and by the Hubble ST (right):

Galaxy HCG87 seen through the ESO ground telescope (left) and HST (right)

This diagram summarizes the current and anticipated status of space telescopes' ability to see back in time towards the earliest events following the Big Bang:

Schematic showing the limits to which space telescope can now or will soon be able to look back in time to the beginning events in Universe history.

However, the Hubble Space Telescope remains the premier astronomical instrument - in many opinions, the finest instrument of any kind yet made - in the stable of space observatories. But, being complicated in its electronics and optics, like any fine instrument it has a finite lifetime. Being out in space, it is not easy to repair the HST whenever a major failure occurs.

Hubble has now been visited five times (1993; 1997; 1999; 2002; 2009) already for repair and upgrade. However, its components are now well beyond their planned lifetime and will likely fail in the next few years. Following the Columbia disaster, the perils of space travel for humans caused NASA to decide against another servicing mission that could be too dangerous at the higher altitude in which HST orbits. This raised a storm of protest and expressions of dismay from both the scientific community and an involved public. Sensitive to this outcry, the then NASA Administrator, Michael Griffin, ordered a "rethink" of that decision and on October 31, 2006 he announced that, with the resumption of the Shuttle program, the HST has been scheduled to be visited by astronaut-repairmen in the Spring of 2009 to rescue it from eventual failure.

The principal tasks (see below) were carried out in 5 (dangerous and challenging) EVAs. The ACS, which has had some periodic problems, apparently failed totally in January, 2007, owing to an electronic short-circuit; it is too large to be replaced. Other instruments and components have failed, or will soon, so that if not fixed or replaced, the HST would cease to function in the foreseeable future. This diagram indicates the major modifications and replacements that were executed during the 2009 mission:

Changes to HST made during the 2009 servicing mission.

This paragraph provides more details about the changes, all of which were successfully made:

* Installation of three new rate sensing units, or RSUs, containing two gyroscopes each to restore full redundancy in the telescope's pointing control system

* Installation of six new nickel-hydrogen batteries to replace the power packs launched with Hubble in 1990

* Installation of the Wide Field Camera 3 (in place of the current Wide Field Planetary Camera 2), providing high-resolution optical coverage from the near-infrared region of the spectrum to the ultraviolet

* Installation of the Cosmic Origins Spectrograph, sensitive to ultraviolet wavelengths. COS will take the place of a no-longer-used instrument known as COSTAR that once was used to correct for the spherical aberration of Hubble's primary mirror. All current Hubble instruments are equipped with their own corrective optics

* Repair of the Advanced Camera for Surveys

* Repair of the Space Telescope Imaging Spectrograph

* Installation of a refurbished fine guidance sensor, one of three used to lock onto and track astronomical targets (two of Hubble's three sensors suffer degraded performance). The refurbished FGS, removed from Hubble during a 1999 servicing mission, will replace FGS-2R, which has a problem with an LED sensor in a star selector subsystem

* Installation of the replacement science instrument command and data handling system computer

* Attachment of new outer blanket layer - NOBL - insulation to replace degrading panels

* Attachment of the soft capture mechanism to permit future attachment to a deorbit rocket motor or NASA's planned Orion capsule

The launch of the Space Shuttle Atlantis to begin the repair mission took place on May 12, 2009. The first space walk on May 14, shown below, successfully installed the new Wide Field Camera:

Astronauts repairing the Hubble Space Telescope.

This photo, taken by one of the astronauts during the third spacewalk, shows astronaut Grunsfeld next to the Hubble.

On May 13th an amateur photographer captured the picture shown below, which made all the evening news broadcasts. It shows the Shuttle and the Hubble during the brief moment they passed in front of the Sun.

The Space Shuttle and the Hubble Telescope silhouetted against the Sun's surface.

Baring the unforeseen, the HST should continue to operate in its improved state for years (with luck, into the 2020s).

It took more than 3 months for the new equipment to "gas out" and otherwise be checked for functionality before routine observations could begin. On September 9, 2009 NASA released the first new images and spectral data from the refurbished HST. Here are some representative samples, some with commentary only in their captions:

The first pair of images show Stephan's Quintet, a group of colliding galaxies. The top image was taken with the old Wide Field Camera, the bottom with the new WFC3:

Stephan's Quintet.
Stephan's Quintet

The nearby (3200 light years) planetary nebula NGC6302, called the Butterfly Nebula (there is another by that name, M2-9) is shown here first with part of it from the new WFC3 imagery on the right and a previous image made by the WFC2 on the left. Then, the full WFC3 image of this beautiful nebula is shown; compare that with the best ground image made by the European Southern Observatory telescope, shown beneath it:

The nebula NGC6302, WPC3 image; the red is Hydrogen, the blue Oxygen gas.
ESO image of NGC6302

HST is not just an imaging system. It also has instruments that can make other kinds of measurements. Here are two such products:

Spectra from NGC6302 taken by the Cosmic Origins Spectrograph (COS).
Comparison of spectra obtained by the new COS and the older GHRS.

As we finish up this introduction to the HST, the writer would like to place an enlarged image of his favorite among all Hubble images seen to date: It is of the Carinae nebula, and shows one of its hydrogen gas and dust pillars from which stars are born. A few stars have already formed. More stars will eventually develop, destroying their pillars of creation over the next 100,000 years, and resulting in a new open cluster of stars. The pink dots around the image are newly formed stars. The technical name for the stellar jets are Herbig-Haro objects. How a star creates Herbig-Haro jets is an ongoing topic of research, but it likely involves an accretion disk swirling around a central star. A second impressive Herbig-Haro jet occurs diagonally near the image center.

Part of the Carina nebula.

The inquisitive reader of this Section may well ask: Is this the real appearance of the nebula; does it have the blues that give it the visual flare one sees? The answer is that the assigned colors are somewhat arbitrary or selective, done to enhance the appearance. The ways in which colors are chosen is the subject of this Hubble website: The Meaning of Color

This image inspires this statement: May the best happen to this most supreme of scientific instruments!

A significant number of other space telescopes have been placed in orbit. Most have instruments that cover other parts of the EM spectrum beyond the visible. Among these we mention here: SWIFT (gamma-ray bursts) the Chandra Telescope (X-ray region), XMM-Newton (X-ray region), FUSE and Galex (UV), and the Spitzer Space Telescope (Infrared). Most of those telescopes operate in various parts of the spectrum as described on page 20-4. For a comprehensive listing of nearly all space telescopes launched or planned consult this Space Observatories website.

As scientists learn ever more about the Cosmos from existing ground and space telescopes, they are constantly advocating the need to have a more powerful and sophisticated telescope in space as the eventual HST replacement. NASA and the astronomical community always seem to have new telescopes on the drawing boards. The big follow-up being planned by The Space Telescope Institute and Goddard Space Flight Center is NGST which stands for the Next Generation Space Telescope. In 2002, this telescope was formally renamed the James Webb Space Telescope (JWST), to honor the second NASA Administrator for his many accomplishments in galvanizing the space program, including his role in the Moon program. A launch date has been set for no sooner than 2014. It will move far from Earth to "park" at a Lagrangian point (about 1000000 miles away, where the Earth's and the Sun's gravitational forces balance out). A separate launch will place a big heat shield to block out the Sun's rays to keep the sensors at about 20° K above absolute zero. The JWST is an unusually shaped spacecraft, with a very large primary mirror, as evident from these three pictures:

Artist's sketch of the JWST.
Size comparison between the HST and JWST primary mirrors.
A full scale mockup of the JWST near the front entrance of Goddard Space Flight Center.

The HST has the detection capability and resolving power to look back to about the half billion years whereas the JWST will be able to detect and image events taking place about 300,000 years after the BB. Earlier than that will be difficult to examine by visual means because of the opacity of the Universe prior to that time. The principal scientific goal of JWST is to obtain improved information about the Universe's history between about 1 million and 2 billion years. The telescope main sensor will concentrate on the infrared region of the spectrum, with a range between 0.6 and 28 µm. Because of the spectral wavelength redshift that results from the expansion of space (see page 20-9), the visible light from these early moments in the Universe's history will have now, as received, extended into the infrared. (For further information, check out Goddard's JWST site.)

To reiterate, space telescopes have the advantage over most ground-based telescopes because they are above the distorting atmosphere. But those on the ground are constantly being improved, and more are being built on high mountains above most of the atmosphere. Several recent ones rival the HST. The Large Binocular Telescope in Arizona, which began operating in 2010, has captured images that are slightly sharper than HST can produce. This image pair shows the same region of outer space as imaged by HST on the left and LBT on the right:

Hubble image on left; LBT on right.

These images of both ground and space-based views into the Cosmos are solid proof that Astronomy has entered a "Golden Age" in the last quarter century.