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Hubble Space Telescope
HST Overview

Introduction to Hubble

picture of HSTThe Hubble Space Telescope (HST) is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) to operate a space-based observatory for the benefit of the international astronomical community. HST is an observatory first envisioned in the 1940s, designed and built in the 1970s and 80s, and operational since the 1990. Since its preliminary inception, HST was designed to be a different type of mission for NASA -- a long-term, space-based observatory. To accomplish this goal and protect the spacecraft against instrument and equipment failures, NASA planned on regular servicing missions. Hubble has special grapple fixtures, 76 handholds, and is stabilized in all three axes. HST is a 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990.

Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).

HST's current complement of science instruments includes three cameras, two spectrographs, and fine guidance sensors (primarily used for accurate pointing, but also for astrometric observations). Because of HST's location above the Earth's atmosphere, these science instruments can produce high-resolution images of astronomical objects. Ground-based telescopes are limited in their resolution by the Earth’s atmosphere, which causes a variable distortion in the images. Hubble can observe ultraviolet radiation, which is blocked by the atmosphere and therefore unavailable to ground-based telescopes. In the infrared portion of the spectrum, the Earth’s atmosphere adds a great deal of background, which is absent in Hubble observations.

When originally planned in the early 1970s, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5-year interval between servicing missions. In the late 70s, contamination and structural loading concerns associated with return to Earth aboard the shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15-year design life. A three-year cycle of on-orbit servicing was adopted. HST servicing missions in December 1993, February 1997, December 1999, March 2002 and May 2009 were enormous successes and validated the concept of on-orbit servicing of Hubble.

The years since the launch of HST in 1990 have been momentous, with the discovery of spherical aberration in its main mirror and the search for a practical solution. The STS-61 (Endeavour) mission of December 1993 corrected the effects of spherical aberration and fully restored the functionality of HST. Since then, servicing missions have regularly provided opportunities to repair aging and failed equipment as well as incorporate new technologies in the telescope, especially in the Science Instruments that are the heart of its operations.

See OPO's Hubble Primer for more information about HST.

The Current Science Instruments

Space Telescope Imaging Spectrograph

A spectrograph spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. The Space Telescope Imaging Spectrograph (STIS) can study these objects across a spectral range from the UV (115 nanometers) through the visible red and the near-IR (1000 nanometers).

STIS uses three detectors: a cesium iodide photocathode Multi-Anode Microchannel Array (MAMA) for 115 to 170 nm, a cesium telluride MAMA for 165 to 310 nm, and a Charge Coupled Device (CCD) for 165 to 1000 nm. All three detectors have a 1024 X 1024 pixel format. The field of view for each MAMA is 25 X 25 arc-seconds, and the field of view of the CCD is 52 X 52 arc-seconds.

The main advance in STIS is its capability for two-dimensional rather than one-dimensional spectroscopy. For example, it is possible to record the spectrum of many locations in a galaxy simultaneously, rather than observing one location at a time. STIS can also record a broader span of wavelengths in the spectrum of a star at one time. As a result, STIS is much more efficient at obtaining scientific data than the earlier HST spectrographs.

A power supply in STIS failed in August 2004, rendering it inoperable. During the servicing mission in 2009, astronauts successfully repaired the STIS by removing the circuit card containing the failed power supply and replacing it with a new card. Since STIS was not designed for in-orbit repair of internal electronics, this task was a substantial challenge for the astronaut crew.

Near Infrared Camera and Multi-Object Spectrometer

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is an HST instrument providing the capability for infrared imaging and spectroscopic observations of astronomical targets. NICMOS detects light with wavelengths between 0.8 and 2.5 microns - longer than the human-eye limit.

The sensitive HgCdTe arrays that comprise the infrared detectors in NICMOS must operate at very cold temperatures. After its deployment, NICMOS kept its detectors cold inside a cryogenic dewar (a thermally insulated container much like a thermos bottle) containing frozen nitrogen ice. NICMOS is HST's first cryogenic instrument.

The frozen nitrogen ice cryogen in NICMOS was exhausted in early 1999, rendering the Instrument inoperable at that time. An alternate means of cooling the NICMOS was developed and installed in the March 2002 servicing mission. This device uses a mechanical cooler to cool the detectors to the low temperatures necessary for operations. The technology for this cooler was not available when the instrument was originally designed, but fortunately became available in time to support the reactivation of the instrument.

Since late 2008, the NICMOS Cooling System (NCS) has experienced difficulties maintaining the instrument’s nominal scientific operating state, in which the detectors are maintained at ~ 77K. Repeated restart attempts have demonstrated that it is not possible to restart the NCS in a cold state immediately following safing events. The main culprit for the problems is believed to be water ice in the primary (circulator) loop of the NCS. An inefficient approach to this problem would be to put the NCS through a several-month warm-up/cooldown cycle and hope that there is an opportunity for science prior to the next payload safing event.

The only feasible path towards satisfactory operation of NICMOS is to remove the putative water by venting the existing contaminated Ne coolant and replacing it with a fresh charge, which is available onboard but has never actually been used on-orbit. Based on the Cycle 18 proposal review results, STScI and Goddard HST Project, with the concurrence of NASA Headquarters, have decided that NICMOS will not be available for science in Cycle 18. A decision on the availability of NICMOS beyond Cycle 18 has not yet been made and awaits further discussion.

Advanced Camera for Surveys

The ACS is a camera designed to provide HST with a deep, wide-field survey capability from the visible to near-IR, imaging from the near-UV to the near-IR with the point-spread function critically sampled at 6300 , and solar blind far-UV imaging. The primary design goal of the ACS Wide-Field Channel is to achieve a factor of 10 improvement in discovery efficiency, compared to WFPC2, where discovery efficiency is defined as the product of imaging area and instrument throughput. These gains are a direct result of improved technology since the HST was launched in 1990. The Charge Coupled Devices (CCDs) used as detectors in the ACS, are more sensitive than those of the late 80s and early 90s, and also have many more pixels, capturing more of the sky in each exposure. The wide field camera in the ACS is a 16 megapixel camera.

The ACS was installed during the March 2002 servicing mission. As a result of the improved sensitivity it instantly became the most heavily used Hubble instrument. It has been used for surveys of varying breadths and depths, as well as for detailed studies of specific objects. The ACS worked well until January 2007, at which time a failure in the electronics for the CCDs occurred and has prevented use of those detectors. Engineers and astronauts then developed an approach to remove and replace the failed electronics, which was carried out during the 2009 servicing mission. As with the STIS repair, the ACS repair was challenging, since the instrument was not designed originally with this type of repair in mind.

Fine Guidance Sensors

The Fine Guidance Sensors (FGS), in addition to being an integral part of the HST Pointing Control System (PCS), provide HST observers with the capability of precision astrometry and milliarcsecond resolution over a wide range of magnitudes (3 < V < 16.8). Its two observing modes - Position Mode and Transfer Mode - have been used to determine the parallax and proper motion of astrometric targets to a precision of 0.2 mas, and to detect duplicity or structure around targets as close as 8 mas (visual orbits can be determined for binaries as close as 12 mas).

Cosmic Origins Spectrograph

The Cosmic Origins Spectrograph (COS) is a fourth-generation instrument that was installed on the Hubble Space Telescope (HST) during the 2009 servicing mission. COS is designed to perform high sensitivity, moderate- and low-resolution spectroscopy of astronomical objects in the 115-320 nm wavelength range. It significantly enhances the spectroscopic capabilities of HST at ultraviolet wavelengths, and provides observers with unparalleled opportunities for observing faint sources of ultraviolet light. The primary science objectives of the COS are the study of the origins of large scale structure in the Universe, the formation and evolution of galaxies, the origin of stellar and planetary systems, and the cold interstellar medium.

The COS achieves its improved sensitivity through advanced detectors and optical fabrication techniques. At UV wavelengths even the best mirrors do not reflect all light incident upon them. Previous spectrographs have required multiple (5 or more) reflections in order to display the spectrum on the detector. A substantial portion of the COS improvement in sensitivity is due to an optical design that requires only a single reflection inside the instrument, reducing the losses due to imperfect reflectivity. This design is possible only with advanced techniques for fabrication, which were not available when earlier generations of HST spectrographs were designed.

COS has a far-UV and near-UV channel that use different detectors: two side-by-side 16384 x 1024 pixel Cross-Delay Line Microchannel Plates (MCPs) for the far-UV, 115 to 205 nm, and a 1024x1024 pixel cesium telluride MAMA for the near-UV,170 to 320 nm. The far-UV detector is similar to detectors flown on the FUSE spacecraft, and takes advantage of improved technology over the past decade. The near-UV detector is a spare STIS detector.

Wide Field Camera 3

The Wide Field Camera 3 (WFC3) is also a fourth generation instrument that was installed during the 2009 servicing mission. Equipped with state-of-the-art detectors and optics, WFC3 provides wide-field imaging with continuous spectral coverage from the ultraviolet into the infrared, dramatically increasing both the survey power and the panchromatic science capabilities of HST.

The WFC3 has two camera channels: the UVIS channel that operates in the ultraviolet and visible bands (from about 200 to 1000 nm), and the IR channel that operates in the infrared (from 900 to 1700 nm). The performance of the two channels was designed to complement the performance of the ACS. The UVIS channel provides the largest field of view and best sensitivity of any ultraviolet camera HST has had. This is feasible as a result of continued improvement in the performance of Charge Coupled Devices designed for astronomical use. The IR channel on WFC3 represents a major improvement on the capabilities of the NICMOS, primarily as a result of the availability of much larger detectors, 1 megapixel in the WFC3/IR vs. 0.06 megapixels for the NICMOS. In addition, modern IR detectors like that in the WFC3 have benefited from improvements over the last decade in design and fabrication.

Mission Operations and Observations:

Although HST operates around the clock, not all of its time is spent observing. Each orbit lasts about 95 minutes, with time allocated for housekeeping functions and for observations. "Housekeeping" functions includes turning the telescope to acquire a new target, switching communications antennas and data transmission modes, receiving command loads and downlinking data, calibrating the instruments and similar activities. On average, the telescope spends about 50% of the time observing astronomical targets. About 50% of the time the view to celestial targets is blocked by the Earth, and that time is used to carry out these support functions.

Each year the STScI solicits ideas for scientific programs from the worldwide astronomical community. All astronomers are free to submit proposals for observations. Typically, 700-1200 proposals are submitted each year. A series of panels, involving roughly 100 astronomers from around the world, are convened to recommend which of the proposals to carry out over the next year. There is only sufficient time in a year to schedule about 1/5 of the proposals that are submitted, so the competition for Hubble observing time is tight.

After proposals are chosen, the observers submit detailed observation plans. The STScI uses these to develop a yearlong observing plan, spreading the observations evenly throughout the period and taking into account scientific reasons that may require some observations to be at a specific time. This long-range plan incorporates calibrations and engineering activities, as well as the scientific observations. This plan is then used as the basis for detailed scheduling of the telescope, which is done one week at a time. Each event is translated into a series of commands to be sent to the onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.

When possible, two scientific instruments are used simultaneously to observe adjacent target regions of the sky. For example, while a spectrograph is focused on a chosen star or nebula, a camera can image a sky region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed steadily at the right target.

Engineering and scientific data from HST, as well as uplinked operational commands, are transmitted through the Tracking Data Relay Satellite (TDRS) system and its companion ground station at White Sands, New Mexico. Up to 24 hours of commands can be stored in the onboard computers. Data can be broadcast from HST to the ground stations immediately or stored on a solid-state recorder and downlinked later.

The observer on the ground can examine the "raw" images and other data within a few minutes for a quick-look analysis. Within 24 hours, GSFC formats the data for delivery to the STScI. STScI is responsible for calibrating the data and providing them to the astronomer who requested the observations. The astronomer has a year to analyze the data from the proposed program, draw conclusions, and publish the results. After one year the data become accessible to all astronomers. The STScI maintains an archive of all data taken by HST. This archive has become an important research tool in itself. Astronomers regularly check the archive to determine whether data in it can be used for a new problem they are working on. Frequently they find that there are HST data relevant for their research, and they can then download these data free of charge.

Hubble has proven to be an enormously successful program, providing new insight into the mysteries of the Universe.

Previously Flown Instruments:

Wide Field/Planetary Camera

The Wide Field/Planetary Camera (WF/PC1) was used from April 1990 to November 1993, to obtain high resolution images of astronomical objects over a relatively wide field of view and a broad range of wavelengths (1150 to 11,000 Angstroms).

Wide Field Planetary Camera 2

The original Wide Field/Planetary Camera (WF/PC1) was replaced by WFPC2 on the STS-61 shuttle mission in December 1993. WFPC2 was a spare instrument developed by the Jet Propulsion Laboratory in Pasadena, California, at the time of HST launch. It consisted of four cameras. The relay mirrors in WFPC2 were spherically aberrated in just the right way to correct for the spherically aberrated primary mirror of the observatory. (HST's primary mirror is 2 microns too flat at the edge, so the corrective optics within WFPC2 were too high by that same amount.). The "heart'' of WFPC2 consisted of an L-shaped trio of wide-field sensors and a smaller, high resolution ("planetary") camera tucked in the square's remaining corner.

WFPC2 was removed in the May 2009 servicing mission and replaced by the Wide-Field Camera 3 (WFC3).

Faint Object Spectrograph

A spectrograph spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. The Faint Object Spectrograph (FOS) was one of the original instruments on Hubble; it was replaced by NICMOS during the second servicing mission in 1997. The FOS examined fainter objects than the High Resolution Spectrograph (HRS), and could study these objects across a much wider spectral range -- from the UV (1150 Angstroms) through the visible red and the near-IR (8000 Angstroms).

The FOS used two 512-element Digicon sensors (light intensifiers). The "blue" tube was sensitive from 1150 to 5500 Angstroms (UV to yellow). The "red" tube was sensitive from 1800 to 8000 Angstroms (longer UV through red). Light entered the FOS through any of 11 different apertures from 0.1 to about 1.0 arc-seconds in diameter. There were also two occulting devices to block out light from the center of an object while allowing the light from just outside the center to pass on through. This could allow analysis of the shells of gas around red giant stars of the faint galaxies around a quasar.

The FOS had two modes of operation: low resolution and high resolution. At low resolution, it could reach 26th magnitude in one hour with a resolving power of 250. At high resolution, the FOS could reach only 22nd magnitude in an hour (before noise becomes a problem), but the resolving power was increased to 1300.

Goddard High Resolution Spectrograph

The Goddard High Resolution Spectrograph (GHRS) was one of the original instruments on Hubble; it failed in 1997, shortly before being replaced by STIS during the second servicing mission. As a spectrograph, HRS also separated incoming light into its spectral components so that the composition, temperature, motion, and other chemical and physical properties of the objects could be analyzed. The HRS contrasted with the FOS in that it concentrated entirely on UV spectroscopy and traded the extremely faint objects for the ability to analyze very fine spectral detail. Like the FOS, the HRS used two 521-channel Digicon electronic light detectors, but the detectors of the HRS were deliberately blind to visible light. One tube was sensitive from 1050 to 1700 Angstroms; while the other was sensitive from 1150 to 3200 Angstroms.

The HRS also had three resolution modes: low, medium, and high. "Low resolution" for the HRS was 2000 -- higher than the best resolution available on the FOS. Examining a feature at 1200 Angstroms, the HRS could resolve detail of 0.6 Angstroms and could examine objects down to 19th magnitude. At medium resolution of 20,000; that same spectral feature at 1200 Angstroms could be seen in detail down to 0.06 Angstroms, but the object would have to be brighter than 16th magnitude to be studied. High resolution for the HRS was 100,000, allowing a spectral line at 1200 Angstroms to be resolved down to 0.012 Angstroms. However, "high resolution" could be applied only to objects of 14th magnitude or brighter. The HRS could also discriminate between variations in light from objects as rapid as 100 milliseconds apart.

Corrective Optics Space Telescope Axial Replacement

COSTAR was not a science instrument; it was a corrective optics package that displaced the High Speed Photometer during the first servicing mission to HST. COSTAR was designed to optically correct the effects of the primary mirror's aberration for the Faint Object Camera (FOC), the High Resolution Spectrograph (HRS), and the Faint Object Spectrograph (FOS). All the other instruments that have been installed since HST's initial deployment, have been designed with their own corrective optics. When all of the first-generation instruments were replaced by other instruments, COSTAR was no longer be needed and was removed from Hubble during the 2009 servicing mission.

Faint Object Camera

The Faint Object Camera (FOC) was built by the European Space Agency as one of the original science instruments on Hubble. It was replaced by ACS during the servicing mission in 2002.

There were two complete detector systems for the FOC. Each used an image intensifier tube to produced an image on a phosphor screen that is 100,000 times brighter than the light received. This phosphor image was then scanned by a sensitive electron-bombarded silicon (EBS) television camera. This system was so sensitive that objects brighter than 21st magnitude had to be dimmed by the camera's filter systems to avoid saturating the detectors. Even with a broad-band filter, the brightest object that could be accurately measured was 20th magnitude.

The FOC offered three different focal ratios: f/48, f/96, and f/288 on a standard television picture format. The f/48 image measured 22 X 22 arc-seconds and yielded a resolution (pixel size) of 0.043 arc-seconds. The f/96 mode provided an image of 11 X 11 arc-seconds on each side and a resolution of 0.022 arc-seconds. The f/288 field of view was 3.6 X 3.6 arc-seconds square, with resolution down to 0.0072 arc-seconds.

High Speed Photometer

The High Speed Photometer (HSP) was one of the four original axial instruments on the Hubble Space Telescope (HST). The HSP was designed to make very rapid photometric observations of astrophysical sources in a variety of filters and passbands from the near ultraviolet to the visible. The HSP was removed from HST during the first servicing mission in December, 1993.

Technical Overview

For more complete technical information about HST and its instruments, see the HST Primer.

Updated: 06/21/2010