On April 24, 1990, something happened that forever altered mankind’s view of the universe. It was on that day that the Hubble Space Telescope (HST) was launched into space aboard the Space Shuttle Discovery.

The Hubble Space Telescope is named for Edwin Hubble, a pioneering American astronomer who used the giant 100-inch Hooker Telescope atop Southern California’s Mount Wilson in 1924 to observe numerous galaxies beyond our Milky Way, all of which appeared to be moving away from each other. Those observations led him to conclude that the universe is expanding.

The problem with Earth-based telescopes is that they must peer through Earth’s atmosphere, which introduces a tremendous amount of distortion and absorbs certain wavelengths of light, making it difficult to observe and analyze them. This led forward-thinkers like German rocket scientist Hermann Oberth and Princeton astrophysicist Lyman Spitzer to propose putting a telescope into space, well above Earth’s atmosphere.

The feasibility of this concept was tested by NASA with the launch of four instruments called Orbiting Astronomical Observatories (OAO) between 1966 and 1972.

Meanwhile, Lyman Spitzer continued his efforts to rally support among the world’s astronomers for the design and construction of a large, orbiting space telescope. His efforts received a major boost in 1969 when the National Academy of Sciences endorsed NASA’s plans for a Large Space Telescope (LST) project that would feature a mirror 3 meters (9.9 ft) in diameter.

The first servicing mission corrected the telescope’s flawed optics. (NASA)
When Congress cut all funding for it in 1974, public outcry and a massive lobbying effort from the scientific community prompted the Senate to restore half the funds. Less money meant the project had to be reduced in size and scope. The size of the mirror, for example, was changed from 3m to 2.4m (7.9 ft.) and an even smaller (1.5m) prototype telescope for system validation purposes was scrapped altogether. A partnership was formed in 1975 with the European Space Agency (ESA), which agreed to absorb fifteen 15 percent of the project’s cost by providing one of the instruments, known as a Faint Object Camera (FOC), the solar panels needed to power the telescope, and manpower to support the project. In return, the European science community was guaranteed 15 percent of the telescope’s operational time.

In 1977, Congress agreed to provide $36 million for the project in their 1978 budget and work finally got underway. The anticipated launch date was 1983.

With funding in place, NASA set about structuring the project. They designated Marshall Space Flight Center (MSFC) in Huntsville, AL to design and build the newly renamed Space Telescope (ST), and Goddard Space Flight Center in Greenbelt, MD to manage the scientific instruments and handle ground control duties once the telescope was launched. MSFC, in turn, hired the Perkin-Elmer Corporation to manufacture the optical telescope assembly, including the mirror, and the system’s fine guidance sensors. A contract to build the telescope’s airframe and assemble the instrument was awarded to the Lockheed Missiles and Space Company (now Lockheed Martin).

On the instrumentation front, Goddard solicited proposals from the scientific community, and from those proposals selected five instruments for inclusion: a Faint Object Camera (FOC); a Wide Field/Planetary Camera (WFPC); a Faint Object Spectrograph (FOS); a High Resolution Spectrograph (HRS); and a High Speed Photometer (HSP).

The Original Instruments

The iconic Pillars of Creation image captured by Hubble. (NASA)
Faint Object Camera (FOC): The FOC, built by the European Space Agency, was an optical and ultraviolet instrument capable of capturing images over a broad spectrum from ultraviolet to near infrared. The FOC was equipped with two separate detector systems. Incoming light was filtered to isolate specific wavelengths, which would then be passed on to a detector for processing and recording. This data would then be digitized for transmission to Earth where it could be processed and analyzed. It operated in the 122 nm to 550 nm wavelength range.

Wide Field and Planetary Camera (WFPC1): This instrument, which was proposed by California Institute of Technology professor James Westphal and built by NASA’s Jet Propulsion Laboratory, was made up of two separate cameras, each using four 800 × 800 pixel CCDs manufactured by Texas Instruments. The Wide Field Camera was designed to capture panoramic views of distant light sources, while the Planetary Camera was designed to capture higher resolution images. It operated in the 115 nm to 1000 nm wavelength range.

The Space Telescope Imaging Spectrograph (STIS). (NASA)
Faint Object Spectrograph (FOS): The FOS, built by Martin Marietta, was designed to analyze the light gathered by the HST to determine a variety of properties such as chemical composition and quantities, magnetic fields, temperature, etc. The instrument, which used a pair of digicon red and blue detectors, could operate in either high-resolution or low-resolution mode and could make spectroscopic observations across a broad spectrum of light from near ultraviolet to near infrared. It typically operated in the 115 nm to 850 nm wavelength range.

High Resolution Spectrograph (HRS): More commonly known as the Goddard High Resolution Spectrograph (GHRS), this instrument was manufactured by Ball Aerospace & Technologies Corp. Like the FOS, it was designed to analyze incoming light to determine an object’s physical and chemical properties. The primary difference between the two was that the GHRS focused strictly on UV spectroscopy in the 1150 Angstrom (115 nm) to 3200 Angstrom (320 nm) wavelength range.

High Speed Photometer (HSP): The HSP was designed and built at the University of Wisconsin, Madison by a group of scientists, engineers and students from the Space Astronomy Laboratory and the Space Science and Engineering Center. Although it measured 3' × 3' × 6' and weighed 600 lbs., the instrument was unique in that it had no moving parts. The HSP was designed to make very high- speed photometric measurements from near ultraviolet to visible wavelengths. It was equipped with four image dissector tubes and a selection of 23 broad- and narrow-band filters ranging from 1200 Angstroms to 7500 Angstroms. Its apertures offered three different fields of view – 0.4 arcseconds, 1.0 arcseconds, and 10.8 arcseconds – and the instrument was capable of making up to 100,000 measurements per second. Because it had no moving parts, directing radiation from the target through the correct set of filters and apertures was accomplished by aiming the entire telescope, a process that could take 30 seconds or more.

Overruns and Delays

Technicians prepare the replacement SI C&DH for testing. (NASA)
Perkin-Elmer began work on the all-important mirror in 1979 using a special low-expansion glass manufactured by Corning Inc. After numerous de lays, work on the mirror finally concluded in late 1981, forcing NASA to postpone the planned 1983 launch date. Finally, in 1985, the recently renamed Hubble Space Telescope (HST) was ready for launch. NASA established a new launch date of October 1986, but on January 28, 1986, the Space Shuttle Challenger disintegrated on takeoff, forcing NASA to suspend all shuttle operations for almost three years while it investigated the cause of the accident. With no means to deploy it, the HST was put into climate-controlled storage where NASA’s engineers continued to make upgrades including improved solar arrays and more sophisticated computer and communications systems.

The space shuttle program resumed operations on September 29, 1988. On April 24, 1990, the Space Shuttle Discovery lifted off on mission STS-31, carrying the HST into space. When testing of the new telescope began shortly thereafter, it soon became obvious that there was a flaw in the mirror that prevented the optical system from focusing properly. The result was images that, although sharper than those produced by ground-based telescopes, were not as clear as they should be.

The problem was traced to a spherical aberration caused by an incorrectly ground edge on the mirror. This caused light reflecting off the edge of the mirror and light reflecting off its center to focus at two different points. It was later determined that one of the sophisticated instruments used to test the mirror, called a null corrector, was the culprit. Either it had been assembled incorrectly or misused, resulting in the mirror’s outer edge being ground 2.2 microns too flat. Fortunately, there was a way to fix it.

Servicing Missions

SM1 (STS-61) December 2 – 13, 1993

Replacing the Wide Field and Planetary Camera. (NASA)
On December 2, 1993, theSpace Shuttle Endeavour took off to conduct the first-ever servicing mission on the HST. The primary purpose of the mission was to correct the flawed optics caused by the spherical aberration in the main mirror.

To do this, NASA reviewed a number of proposals before settling on one submitted by Dr. Mark Bottema, an optics engineer at Ball Aerospace & Technologies. Bottema proposed installing a device that would place small mirrors no bigger than the size of a quarter in front of the Faint Object Spectrograph, the Faint Object Camera, and the Goddard High Resolution Spectrograph. Called COSTAR (Corrective Optics Space Telescope Axial Replacement), it basically did for the HST’s three instruments what eyeglasses do for the human eye. The hard part was finding room for it aboard the HST. Something had to be removed, and since COSTAR was roughly the same size as the HSP, the least used instrument aboard Hubble, officials decided to make the switch.

Instead of installing corrective vision mirrors on the WFPC, officials decided to replace it with a new unit that incorporated corrective optics, as well as other enhancements including better UV performance and more sophisticated detectors. The astronauts also replaced the HST’s solar arrays and related electronics, replaced gyroscopes and magnetometers, and upgraded the onboard computers.

SM2 (STS-82) February 11 – 21, 1997

The second Hubble servicing mission involved replacing the FOS with an instrument called the Space Telescope Imaging Spectrograph (STIS), and replacing the GHRS with an instrument called the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The STIS was a combination spectrograph and camera designed to cover a broad spectrum of light from near-infrared to ultraviolet wavelengths. It was equipped with special two-dimensional detectors that were capable of collecting “30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble,” according to NASA.

The second new instrument, NICMOS, was a cryogenically cooled instrument consisting of three cameras, all designed to operate simultaneously and focus their images in the same plane. Unfortunately, stress and deformation problems with the cryogenic storage dewar prevented one of the cameras – NIC3 – from focusing properly and created a heat sink that depleted the nitrogen coolant quicker than expected. Despite these problems, NICMOS still provided valuable images of the universe in the near-infrared range.

SM3A (STS-103) December 19 – 27, 1999

The third servicing mission was an unscheduled emergency repair mission hastily assembled and executed following the failure of three of HST’s six gyroscopes. When a fourth gyro failed on November 13, 1999, putting Hubble to sleep, the wisdom of splitting SM3 into two parts became evident. For Christmas that year, Hubble got six new gyros, a more powerful main computer, another solid-state data recorder, a more sophisticated FGS, battery system improvements, and better thermal insulation.

SM3B (STS-109) March 1 – 12, 2002

The second half of SM3 took place in March 2002. Astronauts aboard the Space Shuttle Columbia replaced the original instrument aboard Hubble, with a new device called the Advanced Camera for Surveys (ACS). The ACS featured a large detector area and three cameras capable of recording wavelengths from ultraviolet to near infrared. The Wide Field Camera, which is equipped with a 16- megapixel detector that operates in the 350 – 1100 nm spectral range, is designed to search for galaxies that date back to some of the earliest days of the universe. The High-Resolution Camera (HRC) is designed to take finely detailed high-resolution images of things like galaxies, star clusters and gaseous nebulae. The Solar Blind Camera increases its sensitivity to ultraviolet light, particularly in the 1150 to 1700 Angstrom range, by blocking visible light using a Multi-Anode Microchannel Array (MAMA) similar to the one used in the STIS.

The ACS functioned well for almost five years, but beginning in mid-2006, a series of electrical problems forced NASA to come up with a variety of creative workarounds to keep the instrument operational. A short circuit in the unit’s backup power supply in January 2007, however, temporarily killed the HRC.

In addition to installing the ACS, new solar arrays were installed, Hubble’s power control unit (PCU) was replaced, and NICMOS was retrofitted with a new experimental cryogenic cooling system, returning it to service.

SM4 (STS-125) May 11 – 24, 2009

The final Hubble servicing mission, originally scheduled for February 2005, might not have happened at all due to safety concerns following the Space Shuttle Columbia disaster. There was serious debate over the risk vs. value of one last mission to Hubble, but in the end it was approved by NASA administrator Michael D. Griffin and scheduled for October 2008.

On September 27, 2008, just weeks before the mission’s scheduled launch, Hubble’s primary data handling unit, called the Science Instrument Command and Data Handling (SI C&DH) module, failed. This module, together with HST’s data management unit, controlled the processing, storage and communication of all science and engineering data collected by Hubble. NASA engineers successfully transitioned Hubble to the backup unit and postponed SM4 until a new SI C&DH module could be prepared.

On May 11, 2009, Space Shuttle Atlantis took off for the final trip to HST. In addition to replacing the faulty data handling unit, the crew installed two new instruments: the Wide Field Camera 3 (WFC3), which replaced WFPC2, and the Cosmic Origins Spectrograph (COS), which they installed in the space formerly occupied by COSTAR. In addition, they repaired the STIS and ACS. These repairs were crucial to Hubble’s mission because the ACS and WFC3 were designed to complement each other, as were the STIS and COS. WFC3 added ultraviolet and infrared capability to the visible light spectrum normally covered by the ACS. COS added the ability to observe pinpoint sources of light, such as those typically emitted by stars and quasars, to STIS’ ability to observe the broad spectrum of light typically emitted by larger bodies such as galaxies and nebulae.

In addition, the crew also replaced all six of the HST’s nickel-hydrogen (NiH2) batteries, installed a refurbished fine guidance sensor and new outer blanket layer insulation panels, and installed a device called the Soft Capture and Rendezvous System (SCRS), which will allow a future manned or robotic mission to recover the HST when it reaches the end of its life.

The Future

Nobody knows how much longer Hubble will continue to operate. NASA’s original projection for its life expectancy was 15 years, so it has already exceeded all expectations. But nothing lasts forever. Knowing this, in 1996, NASA began planning Hubble’s successor. Originally called the Next Generation Space Telescope (NGST), it was officially renamed the James Webb Space Telescope (JWST) in 2002 in honor of NASA’s second administrator.

The JWST is designed to pick up where Hubble left off, looking farther back in time through the universe. Where Hubble was designed to operate primarily in the visible and ultraviolet spectrum, JWST will operate primarily at infrared wavelengths of 0.6 to 28 microns. The best that Hubble’s instruments can do in the infrared spectrum is 0.8 to 2.5 microns.

The JWST will also have a much larger primary mirror than Hubble – 6.5 meters vs. 2.4 meters – and whereas Hubble orbits the Earth at an altitude of approximately 350 miles, the JWST will orbit the sun at the Earth-Sun L2 Lagrange point. This should give the JWST the ability to see the earliest stars and galaxies formed in the universe.

Hubble’s Legacy

Like the mythical Phoenix, Hubble rose from the ashes of what could have been an embarrassing failure to become one of NASA’s greatest accomplishments. What it has contributed to our knowledge and understanding of the universe is invaluable.

For example, data collected by Hubble on the expansion of the universe helped astronomers calculate its age to be about 13.7 billion years. Not only did Hubble discover that the universe is expanding, it also determined that the rate of expansion has recently begun to accelerate, leading astronomers to wonder why. It was Hubble that also provided compelling evidence of the existence of supermassive black holes and gave astronomers vital clues as to how planets are formed.

And let us not forget the thousands of breathtaking, historic images captured by Hubble such as the Hubble Deep Field and the iconic Pillars of Creation.

According to data provided by NASA, since 1990, Hubble has made well over 1 million observations of 38,000 celestial bodies. It typically collects data at a rate of 844 gigabytes per month, meaning that over its 25-year career it has contributed more than 100 terabytes of knowledge to our understanding of the universe. By any measure, that has to be considered an unqualified success.

For more information on the Hubble Space Telescope, visit www.nasa.gov/hubble .

Photonics Tech Briefs Magazine

This article first appeared in the March, 2015 issue of Photonics Tech Briefs Magazine.

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