Brief Intro About Hubble Space Telescope

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1. Introduction Gazing through the first crude telescopes, Galileo, Kepler, and their contemporaries of the 17th century discovered the moon’s craters, the satellites of Jupiter, and the rings of Saturn, leading the way to today’s quest for in-depth knowledge and understanding of the cosmos. Since its launch in April 1990, NASA’s Edwin P. Hubble Space Telescope (HST) has continued this historic quest, providing scientific data and photographs of unprecedented resolution from which many new and exciting discoveries have been made. This unique observatory operates around the clock above the Earth’s atmosphere to gather information for teams of scientists studying virtually all the constituents of our universe, including planets, stars, starforming regions of the Milky Way galaxy, distant galaxies and quasars, and the tenuous hydrogen gas lying between the galaxies. The Telescope can produce images of the outer planets in Earth’s solar system that rival the clarity of those achieved by the Voyager flybys. Astronomers have been able to resolve previously unsuspected details of star-forming regions of the Orion Nebula in the Milky Way. They have detected expanding gas shells blown off by exploding stars. Using the high resolution and lightgathering power of the Telescope, scientists have calibrated the distances to remote galaxies and detected cool disks of matter trapped in the gravitational field of the cores of galaxies that portend the presence of massive black holes.

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HST Spectroscopic observations at ultraviolet wavelengths inaccessible from the ground have given astronomers their first opportunity to study the abundance and spatial distribution of intergalactic hydrogen in relatively nearby regions of the universe and have forced scientists to rethink some of their earlier theories about galactic evolution. The Telescope’s mission is to spend 15 years probing the farthest and faintest reaches of the cosmos. Crucial to fulfilling this promise is a series of on-orbit manned servicing missions.

1.1 Mission Observations

Operations

and

HST’s mission objective was to place a 2 m-class astronomical telescope and its associated instrumentation above the atmosphere in low Earth orbit, operated and maintained for more than 15 years as an international observatory. 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 (Space Telescope Science Institute) solicits ideas for scientific programs from the worldwide

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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.

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.

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.

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 (Goddard Space Flight Center) 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.

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.

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

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1.2 Its History Two disadvantages of ground based observing were well known to early−twentieth−century astronomers: Because of the turbulent motions of Earth's atmospheric gases, the paths of light rays passing through the atmosphere are constantly shifting, distorting our view of astronomical objects. To human eyes these distortions are rather subtle; for example, they are responsible for the twinkling of stars. However, this effect limits the sharpness of most images taken with ground based telescopes to a resolution of no better than 1 arc second The Earth' atmosphere is quite transparent to visible light but blocks much of the infrared and ultraviolet light from the cosmos. Both of these wavelengths are scientifically important. In the 1970s, NASA and ESA took up the idea of a space-based telescope. Funding began to flow in 1977. Later, it was decided to name the telescope after Edwin Hubble. Although the Hubble Space Telescope (HST) was downsized later to a 2.4 m primary mirror diameter from the initial 3 m, the project started to attract significant attention from astronomers. The precision-ground mirror was finished in 1981 and the assembly of the entire spacecraft was completed in 1985. The plan called for a launch on NASA’s Space Shuttle in 1986 but just months before the scheduled launch, the Challenger disaster caused a 2-year delay of the entire Shuttle programme. HST was finally launched on 24 April 1990. Soon after, the tension built up as astronomers examined the first images through the new telescope’s eyes. It was soon realized that its mirror

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HST had a serious flaw: the mirror edge was too flat by a mere fiftieth of the width of a human hair, enough of focusing defect to prevent it from taking sharp images. Fortunately, the HST was the first spacecraft ever to be conceived as serviceable. That made it possible for engineers and scientists at the Space Telescope Institute in Baltimore (USA) to come up with a cleverly designed corrective optics package that would restore the telescope’s eyesight completely. A crew of astronauts including Claude Nicollier carried out the repairs necessary to restore the telescope to its intended level of performance during the first Hubble Servicing Mission (SM1) in December 1993. This mission captured the attention of both astronomers and the public at large to a very high degree: meticulously planned and brilliantly executed, the mission succeeded on all counts. It will go down in history as one o f the great highlights of human space f light. Hubble was back in business!

1.3 Telescope Details Weight

: 24,500 lb (11,110 kg) Length : 43.5 ft (15.9 m) Diameter : 14 ft (4.2 m) Optical system : Ritchey-Chretien Design cassegrain Telescope Focal length : 189ft (56.7m) folded to 21ft (6.3m) Primary mirror : 94.5 in. (2.4m) in diameter Secondary mirror : 12.2 in. (0.3m) in diameter Orbit : 320nmi (593km) Inclination : 28.5 degrees from equator

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FIGURE: 1 diagram of Hubble space telescope.

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2. Current and Planned Science Instruments • • • • • • •

Wide Field Planetary Camera 2. Space Telescope Imaging Spectrograph. Near Infrared Camera and MultiObject Spectrometer. Advanced Camera for Surveys Fine Guidance Sensors. Cosmic Origins Spectrograph. Wide Field Camera 3.

2.1 Wide Camera 2

Field

Planetary

The original Wide Field/Planetary Camera (WF/PC1) was changed out and displaced 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. WFPC2 is actually four cameras. The relay mirrors in WFPC2 are 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 are too high by that same amount.) The "heart'' of WFPC2 consists of an Lshaped trio of wide-field sensors and a smaller, high resolution ("planetary") camera tucked in the square's remaining corner. The WFPC-2 is Hubble’s workhorse camera. It records images through selection of 48 colour filters covering a

spectral range from the far ultraviolet to visible and near-infrared wavelengths. It has produced most of the pictures that have been released as public outreach images over the years. Its resolution and excellent quality have made it the most used instrument in the first ten years of Hubble’s life. WFPC2 will be removed in the 2009 servicing mission and be replaced by the Wide-Field Camera 3 (WFC3).

2.2 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 arcseconds. The main advance in STIS is its capability for two-dimensional rather than one-dimensional spectroscopy. For example, it is possible to record the

2008-2009 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 will attempt to repair the STIS by removing the circuit card containing the failed power supply and replacing it with a new card. STIS was not designed for in-orbit repair of internal electronics, so this task is a substantial challenge for the astronaut crew, and has required the development of clever tools and processes to accomplish.

2.3 Near Infrared Camera and Multi-Object Spectrometer The Near Infrared Camera and MultiObject 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,

HST 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 installation of the cooler in 2002 the NICMOS has been operating flawlessly.

2.4 Advanced Surveys

Camera

for

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 Å, and solar blind far-UV imaging. The primary design goal of the ACS WideField 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 mega pixel 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

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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 have developed an approach to remove and replace the failed electronics, to be carried out in the 2009 servicing mission. As with the STIS repair, the ACS repair is very challenging since the instrument was not designed originally with this repair in mind.

2.5 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).

2.6 Cosmic Spectrograph

Origins

The Cosmic Origins Spectrograph (COS) is a fourth-generation instrument to be 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. COS will significantly enhance the spectroscopic capabilities of HST at ultraviolet wavelengths, and will provide 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-byside 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.

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2.7 Wide Field Camera 3

•

The Wide Field Camera 3 (WFC3) is also a fourth generation instrument that has been built for installation during the 2009 servicing mission. Equipped with state-of-the-art detectors and optics, WFC3 will provide 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, which 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 will provide 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 mega pixel in the WFC3/IR vs. 0.06 mega pixels 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.

3. Previously Flown Instruments •

Faint Object Spectrograph.

Goddard High Resolution Spectrograph. Corrective Optics Space Telescope Axial Replacement. Faint Object Camera. High Speed Photometer.

3.1 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

2008-2009 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.

3.2 Goddard High Resolution Spectrograph

HST 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.

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.

3.3 Corrective Optics Space Telescope Axial Replacement

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

3.4 Faint Object Camera

COSTAR is not a science instrument; it is a corrective optics package that displaced the High Speed Photometer during the first servicing mission to HST. COSTAR is designed to optically correct the effects of the primary mirror's aberration on the Faint Object Camera (FOC), the High Resolution Spectrograph (HRS), and the Faint Object Spectrograph (FOS). All the other instruments, installed since HST's initial deployment, were designed with their own corrective optics. When these firstgeneration instruments are replaced by other instruments, COSTAR will no longer be needed. COSTAR is no longer used in operations, and will be removed from Hubble during the servicing mission in 2008.

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

2008-2009 100,000 times brighter than the light received. This phosphor image was then scanned by a sensitive electronbombarded 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 arcseconds. The f/288 field of view was 3.6 X 3.6 arc-seconds square, with resolution down to 0.0072 arc-seconds.

3.5 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.

3.6 Solar Arrays The SAs provide power to the spacecraft. They are mounted like wings on opposite sides of the Telescope, on the forward shell of the SSM. Each array stands on a 4-ft mast that supports a retractable wing of solar panels 40 ft (12.2 m) long and 8.2 ft (2.5 m) wide.

HST The SAs are rotated so the solar cells face the sun. Each wing’s solar cells absorb the sun’s energy and convert that light energy into electrical energy to power the Telescope and charge the spacecraft’s batteries, which are part of the Electrical Power Subsystem (EPS). Batteries are used when the Telescope moves into Earth’s shadow during each orbit. Prior to the First Servicing Mission, as the Telescope orbited in and out of direct sunlight, the resulting thermal gradients caused oscillation of the SAs that induced jitter in the Telescope’s line of site. This in turn caused some loss of fine lock of the FGSs during science observations. New SAs installed during the First Servicing Mission with thermal shields over the array masts minimized the effect.

4. Computers The Hubble Telescope Data Management Subsystem (DMS) contains two computers: the DF-224 flight computer and the Science Instrument Control and Data Handling (SI C&DH) unit. The DF-224 performs onboard computations and also handles data and command transmissions between the Telescope systems and the ground system. The SI C&DH unit controls commands received by the science instruments, formats science data, and sends data to the communications system for transmission to Earth. During the First Servicing Mission, astronauts installed a coprocessor to augment the capacity of DF-224 flight computer. The new 386- coprocessor increased flight computer redundancy and significantly enhanced on-orbit computational capability.

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FIGURE: 2 Hubble Space Telescope – exploded view

HST

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5. Observatory status WF/PC is one of the primary instruments aboard HST and the one that creates the bulk of HST’s data. It is a photometric camera covering the spectrum over 1151100 nm. It is the instrument of choice for imaging that requires a wide field of view if the full diffraction-limited resolution of the telescope is not essential, and for imaging at red wavelengths of about 500 nm. Unlike the original WF/PC I, WF/PC II has only one imaging mode, with three CCDs serving the wide-field mode with a scale of 0.1 arcsec per pixel and only one CCD serving the planetary mode with a scale of 0.046 arcsec per pixel. The field of the wide-field mode has an unusual Lshape, with the longer legs spanning 2.5 arcmin, while the planetary mode has a field of 35 arcsec square. WF/PC II has a built-in correction for HST’s spherical aberration, which leads to near diffraction-limited Point Spread Function (PSF) over the whole field of view. Unfortunately, the WF pixel size does not provide a proper sampling of the PSF, which, however, can be achieved by the so-called ‘dithering’ technique: two or more images of the same field are taken after changing HST’s pointing by a fraction of a pixel size. These images are later recombined into a single image which has an improved spatial resolution with respect to the single frame. An additional advantage of this technique is the possibility for effectively removing the spurious point-like sources generated by cosmic rays. The core software package, based on the Lucy algorithm, for reconstructing the dithered images was contributed by the ST-ECF.

HST

In its original design, ESA’s Faint Object Camera (FOC) is capable of operating in four basic modes: direct imaging at f/48, f/96 and f/288, and a 20 ´0.1 arcsec (R=1000) long-slit spectrographic mode. It uses two sophisticated photon-counting intensified cameras as detectors and is equipped with a large bank of filters, polarisers and objective prisms. Following COSTAR’s installation, the f/96 camera became f/150 and the f/48 camera f/75, with fields of view of 7 and 14 arcsec, respectively. The FOC optics are performing flawlessly, showing textbook diffraction-limited images of stars. Indeed, due to the change in focal length, the increased spatial resolution is providing a very good sampling of the PSF, which can be used to further improve the image and photometric quality by deconvolution techniques. Unfortunately, the f/48 detector has developed an intermittent problem in its high voltage section that prevents regular use of this channel. Currently, the f/48 capability is reserved for observations with the long-slit spectrographic mode, a unique HST capability provided by the FOC. STIS is designed not only to replace the capabilities of the previous spectrographs (GHRS and FOS), but to provide an expanded spectroscopic facility for HST. By using large-format (1024´1024 pixels) Multi-Anode Microchannel Array (MAMA) and CCD, STIS can record many echelle spectral orders in a single observation, while in the lowresolution modes it provides spatiallyresolved long-slit spectroscopy for

2008-2009 extended objects. The main observing modes are: • Objective-prism UV spectroscopy (115-310 nm, R~26-1000); • Long-slit spectroscopy (1151000 nm, R~415-13 900); • Echelle spectroscopy (115310 nm, R~23 500-100 000); • UV and visible imaging (1151000 nm). STIS is operating nominally and producing excellent results. Thanks to the expanded onboard memory allowing parallel mode operations, it is producing a large number of serendipity observations that are offered immediately to the community through the HST Archive. NICMOS has three cameras designed for simultaneous operation in the near-IR at 0.8-2.5 mm. The optics present the detectors with three adjacent, but not spatially contiguous, fields-of-view of different image scales. NICMOS employs low-noise, high-quantum efficiency, 256´256 HgCdTe arrays in a passive dewar using solid nitrogen as a coolant. Thermoelectric cooling was designed to prolong the nominal mission lifetime to about 5 years. A variety of filters, grisms and polarisers can be used for IR imaging and spectroscopy, with each camera having its own set of 19 different filter elements. Unfortunately, during the months before its Shuttle delivery HST, it was realized that too much solid nitrogen had been loaded into the dewar. This stressed the dewar and, while not endangering its integrity, caused it to expand, shifting the detectors from their nominal position. In an emergency move before the launch, a new focusing mechanism offering a

HST greater range was installed. Unfortunately, after installation on HST, the expansion of the dewar continued to move the detector of Camera 3 (the wide-field camera also serving the grism mode) beyond the capacity of the focusing mechanism. In addition, an internal baffle came into physical contact with the external structure, creating a thermal bridge that considerably increased the nitrogen’s consumption rate and consequently decreased the expected operational lifetime of NICMOS. As compensation for these unexpected problems, the initial observations indicated that HST’s induced IR background was much lower than projected, making NICMOS more efficient. Indeed, in spite of the problems, NICMOS is producing excellent scientific results, reviewed by the scientific community in a dedicated Workshop organized by ESA’s ST-ECF (Pula, 25- 26 May 1998). The focusing problem with Camera 3 is being tackled by observing campaigns using HST’s secondary mirror to bring the camera into focus (but obviously defocusing all other instruments). The problem of the shorter operational lifetime has been tackled by giving NICMOS preference over the other instruments and by announcing a supplemental Call for Proposals exclusively dedicated to NICMOS. NASA Goddard Space Flight Center engineers are studying the possibility of installing, during the 2000 M&R Mission, a mechanical cryocooler to extend the instrument life, albeit with inferior performance.

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6. Scientific results Since its launch on 24 April 1990, HST has provided a stunning view of our Universe by making unique discoveries and capturing spectacular images of infant galaxies, distant quasars, exploding stars, mysterious black holes and colliding galaxies. In its 8 years of space exploration, the 12.5 t orbiting observatory has allowed astronomers to publish 1700 scientific papers. The total amount of Hubble data placed in archives is 4.44 Tbytes, which fills 710 30-cm optical disks (6.66 GB/disk). The telescope has taken about 120 000 exposures, observing about 10 000 astronomical targets. In this report we can give only a brief sample of recent HST results. Astronomers have used Hubble to look back more than 10 billion years at infant galaxies, some of which date to almost the beginning of the Universe. What they found was astonishing: a bewildering assortment of about 2000 galaxies at various stages of evolution. This deepest, most detailed optical view of the Universe is known as the Hubble Deep Field. For 10 consecutive days in 1995, the telescope was pointed at a keyhole-sized piece of sky. Most of the galaxies are so faint (about 4 billion times fainter than can be seen by the human eye) that they have never been seen by even the largest telescopes on Earth. This observation has revealed the shapes of galaxies in the distant past. Astronomers have continued to analyze the Hubble Deep Field images to trace the evolution of stars and galaxies. This has led to intriguing evidence that the Big Bang may have been followed by a stellar ‘baby boom.’ The early Universe may have had an active, dynamic youth

HST when stars formed out of dust and gas at a ferocious rate. Consequently, most of the stars the Universe will ever make may have already been formed, and it now contains largely ‘mid-life’ stars. For decades, astronomers have debated the question: how old is our Universe? Hubble is helping them to determine the answer by studying distant supernovae and pulsating stars. Peering halfway across the Universe to analyze light from exploded stars that died long before the Sun was born, Hubble’s crisp vision has allowed astronomers to determine that the Universe and all its objects may have not slowed down since their creation and may continue to balloon outward. Based on preliminary observations of several distant supernovae – one of which erupted 7.7 billion years ago – the cosmos is not packed with enough material to halt its infinite expansion. If these early conclusions are true, then the Universe could be 15 billion years old. Other teams of astronomers are using different techniques to calculate the Universe’s age. They are using Hubble to measure accurately the distances to galaxies, an important prerequisite for calculating age. Hubble is measuring distances to neighboring galaxies by finding reliable distance markers, the Cepheid variable stars. These, in turn, are being used to calibrate more remote markers. By calculating these distances, astronomers can determine the rate at which the Universe is expanding (the ‘Hubble constant’) and, ultimately, its age. Astronomers have long pondered the origins of one of the Universe’s greatest enigmas: periodic bursts of gamma rays in deep space. Hubble has made an important contribution towards

2008-2009 identifying the source and nature of these fireballs, nature’s most powerful explosions. The telescope has allowed astronomers to follow the fading visiblelight counterpart of an invisible gammaray burst, whose position in the sky was detected by the Italian-Dutch Beppo SAX satellite. After monitoring the visible afterglow of the gamma ray explosion, Hubble’s sensitive instruments have given astronomers important information by pinpointing some gamma-ray bursts within distant galaxies. Massive black holes cannot be seen because they are so dense and compact that nothing, not even light, escapes from their gravitational clutches. But black holes do leave a swirling trail of clues: a whirlpool-like orbiting stew of gas, dust and stars. Hubble has provided convincing evidence of the existence of these powerhouses by measuring the speed of gas and stars in the cores of galaxies, where black holes reside. STIS has measured the increasing speed of a gas discs orbiting a black hole in M84, located 50 million light-years away in the Virgo cluster of galaxies. This is swirling around the unseen black hole at 1.42 million km/h. Astronomers have calculated that the black hole contains at least 300 million solar masses. HST has helped to prove that massive black holes are so common that almost every large galaxy has one. Hubble has followed the expanding wave of material from the explosion of supernova 1987A. The massive star’s self-destruction was first seen almost 11 years ago via ground-based telescopes. Hubble’s WF/PC II and STIS have shown that debris from the blast is slamming into a ring of material around

HST the dying star the collision has illuminated part of the ring, which was formed before the star exploded. The crash has allowed scientists to probe the structure around the supernova and uncover new clues about the final years of the progenitor star.

6.1 Shining a Light on Dark Matter Astronomers used Hubble to make the first three-dimensional map of dark matter, which is considered the construction scaffolding of the universe.

FIGURE:3 ThreeDimensional Distribution of Dark Matter in the Universe: a. 3.5 billion years ago; b. 5 billion years ago; c. 6.5 billion years ago. Dark matter’s invisible gravity allows normal matter in the form of gas and dust to collect and build up into stars and galaxies. The Hubble telescope played a starring role in helping to shed light on

2008-2009 dark matter, which is much more abundant than normal matter. Although astronomers cannot see dark matter, they can detect it in galaxy clusters by observing how its gravity bends the light of more distant background galaxies, a phenomenon called gravitational lensing. Astronomers constructed the map by using Hubble to measure the shapes of half a million faraway galaxies. The new map provides the best evidence to date that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The map, which stretches halfway back to the beginning of the universe, reveals a loose network of filaments that grew over time and intersect in massive structures at the locations of galaxy clusters.

HST Several years later, Hubble provided evidence that dark energy has been engaged in a tug of war with gravity for billions of years. Dark energy, which works in opposition to gravity, shoves galaxies away from each other at everincreasing speeds, making the universe expand at an ever-faster pace. But dark energy wasn’t always in the driver’s seat. By studying distant supernovae, Hubble traced dark energy all the way back to 9 billion years ago, when the universe was less than half its present size. During that epoch, dark energy was struggling with gravity for control of the cosmos, obstructing the gravitational pull of the universe’s matter even before it began to win the cosmic tug of war. Dark energy finally won the struggle with gravity about 5 billion years ago.

Astronomers also used gravitational lensing in a previous study to make the first direct detection for the existence of dark matter. Hubble teamed up with the Chandra X-ray Observatory, the European Southern Observatory’s Very Large Telescope, and the Magellan optical telescopes to make the discovery. Astronomers found that dark matter and normal matter were pulled apart by the tremendous collision of two large clusters of galaxies, called the Bullet Cluster.

6.2 A Speedy Universe By witnessing bursts of light from faraway exploding stars, Hubble helped astronomers discover dark energy. This mysterious, invisible energy exerts a repulsive force that pervades our universe.

FIGURE:4 History of the universe: A cosmic tug of war. By knowing more about how dark energy behaves over time, astronomers hope to gain a better understanding of what it is. Astronomers still understand

2008-2009 almost nothing about dark energy, even though it appears to comprise about 70 percent of the universe’s energy.

6.3 Planets, Planets Everywhere Peering into the crowded bulge of our Milky Way galaxy, Hubble looked farther than ever before to nab a group of planet candidates outside our solar system. Astronomers used Hubble to conduct a census of Jupiter-sized extrasolar planets residing in the bulge of our Milky Way galaxy. Looking at a narrow slice of sky, the telescope nabbed 16 potential alien worlds orbiting a variety of stars. Astronomers have estimated that about 5 percent of stars in the galaxy may have Jupiter-sized, star-hugging planets. So this discovery means there are probably billions of such planets in our Milky Way. Five of the newly found planet candidates represent a new extreme type of planet. Dubbed Ultra-Short-Period Planets, these worlds whirl around their stars in less than an Earth day. Astronomers made the discoveries by measuring the slight dimming of a star as a planet passed in front of it, an event called a transit. The telescope also made the first direct measurements of the chemical composition of an extrasolar planet’s atmosphere, detecting sodium, oxygen, and carbon in the atmosphere of the Jupiter-sized planet HD209458b. Hubble also found that the planet’s outer hydrogen-rich atmosphere is heated so much by its star that it is evaporating into space. The planet circles its star in a tight 3.5-day orbit.

HST These unique observations demonstrate that Hubble and other telescopes can sample the chemical makeup of the atmospheres of alien worlds. Astronomers could use the same technique someday to determine whether life exists on extrasolar planets. Besides testing the atmosphere of an extrasolar planet, Hubble also made precise measurements of the masses of two distant worlds.

6.4 Monster Black Holes Are Everywhere Hubble probed the dense, central regions of galaxies and provided decisive evidence that supermassive black holes reside in many of them. Giant black holes are compact “monsters” weighing millions to billions the mass of our Sun. They have so much gravity that they gobble up any material that ventures near them. These elusive “eating machines” cannot be observed directly, because nothing, not even light, escapes their grasp. But the telescope provided indirect, yet compelling, evidence of their existence. Hubble helped astronomers determine the masses of several black holes by measuring the velocities of material whirling around them. The telescope’s census of many galaxies showed an intimate relationship between galaxies and their resident black holes. The survey revealed that a black hole’s mass is dependent on the weight of its host galaxy’s bulge, a spherical region consisting of stars in a galaxy’s central region. Large galaxies, for example, have massive black holes; less massive galaxies have smaller black holes. This close relationship may be evidence that

2008-2009 black holes co-evolved with their galaxies, feasting on a measured diet of gas and stars residing in the hearts of those galaxies.

6.5 The Most Powerful Blasts since the Big Bang Imagine a powerful burst of light and other radiation that can burn away the ozone in Earth’s atmosphere. Luckily,

HST occur in galaxies with fewer heavy elements, such as carbon and oxygen. The Milky Way galaxy, which is rich in heavy elements released by many generations of stars, is therefore an unlikely place for them to pop off.

6.6 How Old Is the Universe? Hubble observations allowed astronomers to calculate a precise age for the universe using two independent methods. The findings reduced the uncertainty to 10 percent. The first method relied on determining the expansion rate of the universe, a value called the Hubble constant. In May 1999 a team of astronomers obtained a value for the Hubble constant by measuring the distances to nearly two dozen galaxies, some as far as 65 million lightyears from Earth. By obtaining a value for the Hubble constant, the team then determined that the universe is about 13 billion years old.

FIGURE: 5 Four gamma-ray burst host galaxies. flashes of such strong radiation occur so far away they will not scorch our planet. These brilliant flashbulbs are called gamma-ray bursts. They may represent the most powerful explosions in the universe since the Big Bang. Hubble images showed that these brief flashes of radiation arise from far-flung galaxies, which are forming stars at enormously high rates. Hubble’s observations confirmed that the bursts of light originated from the collapse of massive stars. Astronomers using Hubble also found that a certain type of extremely energetic gamma-ray bursts are more likely to

FIGURE:6 Close-up of ancient, white-dwarf stars in the Milky Way galaxy. In the second method astronomers calculated a lower limit for the universe’s age by measuring the light from old, dim, burned-out stars, called white dwarfs. The ancient white dwarf stars, as seen by Hubble, are at least 12 to 13 billion years old.

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6.7 Quasars, the Light Fantastic

7. Conclusion

Quasars have been so elusive and mysterious that the hunt to define them would have taxed even the superior analytical skills of detective Sherlock Holmes. Since their discovery in 1963, astronomers have been trying to crack the mystery of how these compact dynamos of light and other radiation, which lie at the outer reaches of the universe, produce so much energy. Quasars are no larger than our solar system but outshine galaxies of hundreds of billions of stars.

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. Its gaze has helped determine the age of the universe, the identity of quasars, and the existence of dark energy. Eventually, Hubble's time will end. In the years after servicing mission, Hubble's components will slowly degrade to the point at which the telescope stops working. When that happens, Hubble will continue to orbit the Earth until its orbit decays, allowing it to spiral toward Earth. Astronauts or a robotic mission could either bring Hubble back to Earth or crash it safely into the ocean.

FIGURE:7 Looking “underneath” Quasar HE0450-2958. These light beacons have left trails of evidence and plenty of clues, but scientists have only just begun to understand their behavior. Astronomers using Hubble tracked down the “homes” of quasars to the centers of faraway galaxies. Hubble’s observations bolstered the idea that quasars are powered by a gush of radiation unleashed by black holes in the cores of these galaxies.

But Hubble's legacy — its discoveries, its trailblazing design, its success in showing us the universe in unparalleled detail — will live on. Scientists will rely on Hubble's revelations for years as they continue in their quest to understand the cosmos — a quest that has attained clarity, focus, and triumph through Hubble's rich existence.

8. Bibliography http://hubblesite.org/ http://hubblesite.org/newscenter/ http://www.spacetelescope.org/about/i ndex.html http://www.stsci.edu/hst/HST_overvie w/

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