brief intro about hubble space telescope

8/14/2019 Brief intro about hubble space telescope

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1. Introduction

Gazing through the first crudetelescopes, Galileo, Kepler, and their contemporaries of the 17th century

discovered the moons craters, thesatellites of Jupiter, and the rings of Saturn, leading the way to todays questfor in-depth knowledge andunderstanding of the cosmos. Since itslaunch in April 1990, NASAs Edwin P.Hubble Space Telescope (HST) hascontinued this historic quest, providingscientific data and photographs of unprecedented resolution from whichmany new and exciting discoveries have

been made.This unique observatory operates aroundthe clock above the Earths atmosphereto gather information for teams of scientists studying virtually all theconstituents of our universe, including planets, stars, starforming regions of theMilky Way galaxy, distant galaxies andquasars, and the tenuous hydrogen gaslying between the galaxies.

The Telescope can produce images of the outer planets in Earths solar systemthat rival the clarity of those achieved bythe Voyager flybys. Astronomers have been able to resolve previouslyunsuspected details of star-formingregions of the Orion Nebula in the MilkyWay. They have detected expanding gasshells blown off by exploding stars.Using the high resolution and light-

gathering power of the Telescope,scientists have calibrated the distances toremote galaxies and detected cool disksof matter trapped in the gravitationalfield of the cores of galaxies that portendthe presence of massive black holes.

Spectroscopic observations at ultravioletwavelengths inaccessible from theground have given astronomers their first opportunity to study the abundanceand spatial distribution of intergalactic

hydrogen in relatively nearby regions of the universe and have forced scientists torethink some of their earlier theoriesabout galactic evolution. TheTelescopes mission is to spend 15 years probing the farthest and faintest reachesof the cosmos. Crucial to fulfilling this promise is a series of on-orbit mannedservicing missions.

1.1 Mission Operations andObservations

HSTs mission objective was to place a 2m-class astronomical telescope and itsassociated instrumentation above theatmosphere in low Earth orbit, operatedand maintained for more than 15 yearsas an international observatory. AlthoughHST operates around the clock, not all of its time is spent observing. Each orbit

lasts about 95 minutes, with timeallocated for housekeeping functions andfor observations. "Housekeeping"functions includes turning the telescopeto acquire a new target, switchingcommunications antennas and datatransmission modes, receiving commandloads and downlinking data, calibratingthe instruments and similar activities. Onaverage, the telescope spends about 50%of the time observing astronomical

targets. About 50% of the time the viewto celestial targets is blocked by theEarth, and that time is used to carry outthese support functions.

Each year the STScI (Space TelescopeScience Institute) solicits ideas for scientific programs from the worldwide

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astronomical community. Allastronomers are free to submit proposalsfor observations. Typically, 700-1200 proposals are submitted each year. Aseries of panels, involving roughly 100

astronomers from around the world, areconvened to recommend which of the proposals to carry out over the next year.There is only sufficient time in a year toschedule about 1/5 of the proposals thatare submitted, so the competition for Hubble observing time is tight.

After proposals are chosen, the observerssubmit detailed observation plans. TheSTScI uses these to develop a yearlong

observing plan, spreading theobservations evenly throughout the period and taking into account scientificreasons that may require someobservations to be at a specific time.This long-range plan incorporatescalibrations 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 onboardcomputers. Computer loads are uplinkedseveral times a day to keep the telescopeoperating efficiently.

When possible, two scientificinstruments are used simultaneously toobserve adjacent target regions of thesky. For example, while a spectrographis focused on a chosen star or nebula, acamera can image a sky region offsetslightly from the main viewing target.During observations the Fine GuidanceSensors (FGS) track their respectiveguide stars to keep the telescope pointedsteadily at the right target.

Engineering and scientific data fromHST, as well as uplinked operationalcommands, are transmitted through theTracking Data Relay Satellite (TDRS)system and its companion ground station

at White Sands, New Mexico. Up to 24hours of commands can be stored in theonboard computers. Data can be broadcast from HST to the groundstations immediately or stored on asolid-state recorder and downlinkedlater.

The observer on the ground can examinethe "raw" images and other data within afew minutes for a quick-look analysis.

Within 24 hours, GSFC (Goddard SpaceFlight Center) formats the data for delivery to the STScI. STScI isresponsible for calibrating the data and providing them to the astronomer whorequested the observations. Theastronomer has a year to analyze the datafrom the proposed program, drawconclusions, and publish the results.After one year the data becomeaccessible to all astronomers. The STScI

maintains an archive of all data taken byHST. This archive has become animportant research tool in itself.Astronomers regularly check the archiveto determine whether data in it can beused for a new problem they are workingon. Frequently they find that there areHST data relevant for their research, andthey can then download these data freeof charge.

Hubble has proven to be an enormouslysuccessful program, providing newinsight into the mysteries of theUniverse.

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1.2 Its History

Two disadvantages of ground basedobserving were well known toearlytwentiethcentury astronomers:Because of the turbulent motions of Earth's atmospheric gases, the paths of light rays passing through theatmosphere are constantly shifting,distorting our view of astronomicalobjects. To human eyes these distortionsare rather subtle; for example, they areresponsible for the twinkling of stars.However, this effect limits the sharpnessof most images taken with ground basedtelescopes to a resolution of no better

than 1 arc second The Earth' atmosphereis quite transparent to visible light but blocks much of the infrared andultraviolet light from the cosmos. Bothof these wavelengths are scientificallyimportant.

In the 1970s, NASA and ESA took upthe idea of a space-based telescope.Funding began to flow in 1977. Later, itwas decided to name the telescope after Edwin Hubble. Although the HubbleSpace Telescope (HST) was downsizedlater to a 2.4 m primary mirror diameter from the initial 3 m, the project started toattract significant attention fromastronomers. The precision-groundmirror was finished in 1981 and theassembly of the entire spacecraft wascompleted in 1985. The plan called for alaunch on NASAs Space Shuttle in 1986 but just months before the scheduled

launch, the Challenger disaster caused a2-year delay of the entire Shuttle programme. HST was finally launchedon 24 April 1990. Soon after, the tension built up as astronomers examined thefirst images through the new telescopeseyes. It was soon realized that its mirror

had a serious flaw: the mirror edge wastoo flat by a mere fiftieth of the width of a human hair, enough of focusing defectto prevent it from taking sharp images.Fortunately, the HST was the first

spacecraft ever to be conceived asserviceable. That made it possible for engineers and scientists at the SpaceTelescope Institute in Baltimore (USA)to come up with a cleverly designedcorrective optics package that wouldrestore the telescopes eyesightcompletely. A crew of astronautsincluding Claude Nicollier carried outthe repairs necessary to restore thetelescope to its intended level of

performance during the first HubbleServicing Mission (SM1) in December 1993. This mission captured theattention of both astronomers and the public at large to a very high degree:meticulously planned and brilliantlyexecuted, the mission succeeded on allcounts. It will go down in history as oneo f the great highlights of human space f light. Hubble was back in business!

1.3 Telescope DetailsWeight : 24,500 lb (11,110

kg)Length : 43.5 ft (15.9 m)Diameter : 14 ft (4.2 m)Optical system : Ritchey-Chretien

Design cassegrainTelescope

Focal length : 189ft (56.7m) foldedto 21ft (6.3m)

Primary mirror : 94.5 in. (2.4m) indiameter 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 Hubblespace telescope.

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2. Current and Planned ScienceInstruments

Wide Field Planetary Camera 2. Space Telescope Imaging

Spectrograph. Near Infrared Camera and Multi-

Object Spectrometer. Advanced Camera for Surveys Fine Guidance Sensors. Cosmic Origins Spectrograph. Wide Field Camera 3.

2.1 Wide Field PlanetaryCamera 2

The original Wide Field/PlanetaryCamera (WF/PC1) was changed out anddisplaced by WFPC2 on the STS-61shuttle mission in December 1993.WFPC2 was a spare instrumentdeveloped by the Jet PropulsionLaboratory in Pasadena, California, atthe time of HST launch.

WFPC2 is actually four cameras. Therelay mirrors in WFPC2 are sphericallyaberrated in just the right way to correctfor the spherically aberrated primarymirror of the observatory. (HST's primary mirror is 2 microns too flat atthe edge, so the corrective optics withinWFPC2 are too high by that sameamount.)

The "heart'' of WFPC2 consists of an L-shaped trio of wide-field sensors and asmaller, high resolution ("planetary")camera tucked in the square's remainingcorner.

The WFPC-2 is Hubbles workhorsecamera. It records images throughselection of 48 colour filters covering a

spectral range from the far ultraviolet tovisible and near-infrared wavelengths. Ithas produced most of the pictures thathave been released as public outreach

images over the years. Its resolution andexcellent quality have made it the mostused instrument in the first ten years of Hubbles life.

WFPC2 will be removed in the 2009servicing mission and be replaced by theWide-Field Camera 3 (WFC3).

2.2 Space Telescope ImagingSpectrograph

A spectrograph spreads out the lightgathered by a telescope so that it can beanalyzed to determine such properties of celestial objects as chemical compositionand abundances, temperature, radialvelocity, rotational velocity, andmagnetic fields. The Space TelescopeImaging Spectrograph (STIS) can studythese objects across a spectral rangefrom the UV (115 nanometers) through

the visible red and the near-IR (1000nanometers).

STIS uses three detectors: a cesiumiodide photocathode Multi-AnodeMicrochannel Array (MAMA) for 115 to170 nm, a cesium telluride MAMA for 165 to 310 nm, and a Charge CoupledDevice (CCD) for 165 to 1000 nm. Allthree detectors have a 1024 X 1024 pixelformat. The field of view for each

MAMA is 25 X 25 arc-seconds, and thefield of view of the CCD is 52 X 52 arc-seconds.

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


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spectrum of many locations in a galaxysimultaneously, rather than observingone location at a time. STIS can alsorecord a broader span of wavelengths inthe spectrum of a star at one time. As a

result, STIS is much more efficient atobtaining scientific data than the earlier HST spectrographs.

A power supply in STIS failed in August2004, rendering it inoperable. During theservicing mission in 2009, astronautswill attempt to repair the STIS byremoving the circuit card containing thefailed power supply and replacing it witha new card. STIS was not designed for

in-orbit repair of internal electronics, sothis task is a substantial challenge for theastronaut crew, and has required thedevelopment of clever tools and processes to accomplish.

2.3 Near Infrared Camera andMulti-Object Spectrometer

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is an

HST instrument providing the capabilityfor infrared imaging and spectroscopicobservations 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 thatcomprise the infrared detectors in NICMOS must operate at very coldtemperatures. After its deployment,

NICMOS kept its detectors cold inside acryogenic dewar (a thermally insulatedcontainer much like a thermos bottle)containing frozen nitrogen ice. NICMOSis HST's first cryogenic instrument.

The frozen nitrogen ice cryogen in NICMOS was exhausted in early 1999,

rendering the Instrument inoperable atthat time. An alternate means of coolingthe NICMOS was developed andinstalled in the March 2002 servicingmission. This device uses a mechanical

cooler to cool the detectors to the lowtemperatures necessary for operations.The technology for this cooler was notavailable when the instrument wasoriginally designed, but fortunately became available in time to support thereactivation of the instrument. Sinceinstallation of the cooler in 2002 the NICMOS has been operating flawlessly.

2.4 Advanced Camera for

Surveys

The ACS is a camera designed to provide HST with a deep, wide-fieldsurvey capability from the visible tonear-IR, imaging from the near-UV tothe near-IR with the point-spreadfunction 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 discoveryefficiency is defined as the product of imaging area and instrument throughput.These gains are a direct result of improved technology since the HST waslaunched in 1990. The Charge CoupledDevices (CCDs) used as detectors in theACS, are more sensitive than those of the late 80s and early 90s, and also havemany more pixels, capturing more of the

sky in each exposure. The wide fieldcamera in the ACS is a 16 mega pixelcamera.

The ACS was installed during the March2002 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 surveysof varying breadths and depths, as wellas for detailed studies of specific objects.The ACS worked well until January2007, at which time a failure in the

electronics for the CCDs occurred andhas prevented use of those detectors.Engineers and astronauts have developedan approach to remove and replace thefailed electronics, to be carried out in the2009 servicing mission. As with theSTIS repair, the ACS repair is verychallenging since the instrument was notdesigned originally with this repair inmind.

2.5 Fine Guidance SensorsThe Fine Guidance Sensors (FGS), inaddition to being an integral part of theHST Pointing Control System (PCS), provide HST observers with thecapability of precision astrometry andmilliarcsecond resolution over a widerange of magnitudes (3 < V < 16.8). Itstwo observing modes - Position Modeand Transfer Mode - have been used to

determine the parallax and proper motion of astrometric targets to a precision of 0.2 mas, and to detectduplicity or structure around targets asclose as 8 mas (visual orbits can bedetermined for binaries as close as 12mas).

2.6 Cosmic OriginsSpectrograph

The Cosmic Origins Spectrograph(COS) is a fourth-generation instrumentto be installed on the Hubble SpaceTelescope (HST) during the 2009servicing mission. COS is designed to perform high sensitivity, moderate- andlow-resolution spectroscopy of astronomical objects in the 115-320 nm

wavelength range. COS willsignificantly enhance the spectroscopiccapabilities of HST at ultravioletwavelengths, and will provide observerswith unparalleled opportunities for

observing faint sources of ultravioletlight. The primary science objectives of the COS are the study of the origins of large scale structure in the Universe, theformation and evolution of galaxies, theorigin of stellar and planetary systems,and the cold interstellar medium.

The COS achieves its improvedsensitivity through advanced detectorsand optical fabrication techniques. At

UV wavelengths even the best mirrorsdo not reflect all light incident uponthem. Previous spectrographs haverequired multiple (5 or more) reflectionsin order to display the spectrum on thedetector. A substantial portion of theCOS improvement in sensitivity is dueto an optical design that requires only asingle reflection inside the instrument,reducing the losses due to imperfectreflectivity. This design is possible only

with advanced techniques for fabrication, which were not availablewhen earlier generations of HSTspectrographs were designed.

COS has a far-UV and near-UV channelthat use different detectors: two side-by-side 16384 x 1024 pixel Cross-DelayLine Microchannel Plates (MCPs) for the far-UV, 115 to 205 nm, and a1024x1024 pixel cesium tellurideMAMA for the near-UV,170 to 320 nm.The far-UV detector is similar todetectors flown on the FUSE spacecraft,and takes advantage of improvedtechnology over the past decade. Thenear-UV detector is a spare STISdetector.


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2.7 Wide Field Camera 3

The Wide Field Camera 3 (WFC3) isalso a fourth generation instrument thathas been built for installation during the2009 servicing mission. Equipped withstate-of-the-art detectors and optics,WFC3 will provide wide-field imagingwith continuous spectral coverage fromthe ultraviolet into the infrared,dramatically increasing both the survey power and the panchromatic sciencecapabilities of HST.

The WFC3 has two camera channels, theUVIS channel that operates in theultraviolet and visible bands (from about200 to 1000 nm) and the IR channel,which operates in the infrared (from 900to 1700 nm). The performance of thetwo channels was designed tocomplement the performance of theACS. The UVIS channel will providethe largest field of view and bestsensitivity of any ultraviolet camera

HST has had. This is feasible as a resultof continued improvement in the performance of Charge Coupled Devicesdesigned for astronomical use. The IR channel on WFC3 represents a major improvement on the capabilities of the NICMOS, primarily as a result of theavailability of much larger detectors, 1mega pixel in the WFC3/IR vs. 0.06mega pixels for the NICMOS. Inaddition, modern IR detectors like that in

the WFC3 have benefited fromimprovements over the last decade indesign and fabrication.

3. Previously Flown Instruments

Faint Object Spectrograph.

Goddard High ResolutionSpectrograph.

Corrective Optics SpaceTelescope Axial Replacement.

Faint Object Camera. High Speed Photometer.

3.1 Faint Object Spectrograph

A spectrograph spreads out the lightgathered by a telescope so that it can beanalyzed to determine such properties of celestial objects as chemical compositionand abundances, temperature, radialvelocity, rotational velocity, andmagnetic fields. The Faint ObjectSpectrograph (FOS) was one of theoriginal instruments on Hubble; it wasreplaced by NICMOS during the secondservicing mission in 1997. The FOSexamined fainter objects than the HighResolution Spectrograph (HRS), andcould study these objects across a muchwider spectral range -- from the UV(1150 Angstroms) through the visible redand the near-IR (8000 Angstroms).

The FOS used two 512-element Digiconsensors (light intensifiers). The "blue"tube was sensitive from 1150 to 5500Angstroms (UV to yellow). The "red"tube was sensitive from 1800 to 8000Angstroms (longer UV through red).Light entered the FOS through any of 11different apertures from 0.1 to about 1.0arc-seconds in diameter. There were alsotwo occulting devices to block out lightfrom the center of an object whileallowing the light from just outside thecenter to pass on through. This couldallow analysis of the shells of gas aroundred giant stars of the faint galaxiesaround a quasar.

The FOS had two modes of operation:low resolution and high resolution. At


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low resolution, it could reach 26thmagnitude in one hour with a resolving power of 250. At high resolution, theFOS could reach only 22nd magnitude inan hour (before noise becomes a

problem), but the resolving power wasincreased to 1300.

3.2 Goddard High ResolutionSpectrograph

The Goddard High ResolutionSpectrograph (GHRS) was one of theoriginal instruments on Hubble; it failedin 1997, shortly before being replaced bySTIS during the second servicing

mission. As a spectrograph, HRS alsoseparated incoming light into its spectralcomponents so that the composition,temperature, motion, and other chemicaland physical properties of the objectscould be analyzed. The HRS contrastedwith the FOS in that it concentratedentirely on UV spectroscopy and tradedthe extremely faint objects for the abilityto analyze very fine spectral detail. Likethe FOS, the HRS used two 521-channel

Digicon electronic light detectors, butthe detectors of the HRS weredeliberately blind to visible light. Onetube was sensitive from 1050 to 1700Angstroms; while the other was sensitivefrom 1150 to 3200 Angstroms.

The HRS also had three resolutionmodes: low, medium, and high. "Lowresolution" for the HRS was 2000 --higher than the best resolution available

on the FOS. Examining a feature at 1200Angstroms, the HRS could resolve detailof 0.6 Angstroms and could examineobjects down to 19th magnitude. Atmedium resolution of 20,000; that samespectral feature at 1200 Angstroms could be seen in detail down to 0.06Angstroms, but the object would have to

be brighter than 16th magnitude to bestudied. High resolution for the HRSwas 100,000, allowing a spectral line at1200 Angstroms to be resolved down to0.012 Angstroms. However, "high

resolution" could be applied only toobjects of 14th magnitude or brighter.The HRS could also discriminate between variations in light from objectsas rapid as 100 milliseconds apart.

3.3 Corrective Optics SpaceTelescope Axial Replacement

COSTAR is not a science instrument; itis a corrective optics package that

displaced the High Speed Photometer during the first servicing mission toHST. COSTAR is designed to opticallycorrect the effects of the primary mirror'saberration on the Faint Object Camera(FOC), the High ResolutionSpectrograph (HRS), and the FaintObject Spectrograph (FOS). All the other instruments, installed since HST's initialdeployment, were designed with their own corrective optics. When these first-

generation instruments are replaced byother instruments, COSTAR will nolonger be needed. COSTAR is no longer used in operations, and will be removedfrom Hubble during the servicingmission in 2008.

3.4 Faint Object Camera

The Faint Object Camera (FOC) was built by the European Space Agency asone of the original science instrumentson Hubble. It was replaced by ACSduring the servicing mission in 2002.

There were two complete detector systems for the FOC. Each used animage intensifier tube to produced animage on a phosphor screen that is


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100,000 times brighter than the lightreceived. This phosphor image was thenscanned by a sensitive electron- bombarded silicon (EBS) televisioncamera. This system was so sensitive

that objects brighter than 21st magnitudehad 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 accuratelymeasured was 20th magnitude.

The FOC offered three different focalratios: f/48, f/96, and f/288 on a standardtelevision picture format. The f/48 imagemeasured 22 X 22 arc-seconds and

yielded a resolution (pixel size) of 0.043arc-seconds. The f/96 mode provided animage of 11 X 11 arc-seconds on eachside and a resolution of 0.022 arc-seconds. The f/288 field of view was 3.6X 3.6 arc-seconds square, withresolution down to 0.0072 arc-seconds.

3.5 High Speed Photometer

The High Speed Photometer (HSP) was

one of the four original axial instrumentson the Hubble Space Telescope (HST).The HSP was designed to make veryrapid photometric observations of astrophysical sources in a variety of filters and passbands from the near ultraviolet to the visible. The HSP wasremoved from HST during the firstservicing mission in December, 1993.

3.6 Solar Arrays

The SAs provide power to thespacecraft. They are mounted like wingson opposite sides of the Telescope, onthe forward shell of the SSM. Each arraystands on a 4-ft mast that supports aretractable wing of solar panels 40 ft(12.2 m) long and 8.2 ft (2.5 m) wide.

The SAs are rotated so the solar cellsface the sun. Each wings solar cellsabsorb the suns energy and convert thatlight energy into electrical energy to power the Telescope and charge the

spacecrafts batteries, which are part of the Electrical Power Subsystem (EPS).Batteries are used when the Telescopemoves into Earths shadow during eachorbit. Prior to the First ServicingMission, as the Telescope orbited in andout of direct sunlight, the resultingthermal gradients caused oscillation of the SAs that induced jitter in theTelescopes line of site. This in turncaused some loss of fine lock of the

FGSs during science observations. NewSAs installed during the First ServicingMission with thermal shields over thearray masts minimized the effect.

4. Computers

The Hubble Telescope DataManagement Subsystem (DMS) containstwo computers: the DF-224 flightcomputer and the Science Instrument

Control and Data Handling (SI C&DH)unit. The DF-224 performs onboardcomputations and also handles data andcommand transmissions between theTelescope systems and the groundsystem. The SI C&DH unit controlscommands received by the scienceinstruments, formats science data, andsends data to the communicationssystem for transmission to Earth. Duringthe First Servicing Mission, astronauts

installed a coprocessor to augment thecapacity of DF-224 flight computer. Thenew 386- coprocessor increased flightcomputer redundancy and significantlyenhanced on-orbit computationalcapability.


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FIGURE: 2 Hubble Space Telescope exploded view


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extended objects. The main observingmodes are:

Objective-prism UVspectroscopy (115-310 nm,R~26-1000);

Long-slit spectroscopy (115-1000 nm, R~415-13 900); Echelle spectroscopy (115-

310 nm, R~23 500-100 000); UV and visible imaging (115-

1000 nm).

STIS is operating nominally and producing excellent results. Thanks tothe expanded onboard memory allowing parallel mode operations, it is producinga large number of serendipityobservations that are offeredimmediately to the community throughthe HST Archive.

NICMOS has three cameras designed for simultaneous operation in the near-IR at0.8-2.5 mm. The optics present thedetectors with three adjacent, but notspatially contiguous, fields-of-view of different image scales. NICMOSemploys low-noise, high-quantumefficiency, 256256 HgCdTe arrays in a passive dewar using solid nitrogen as acoolant. Thermoelectric cooling wasdesigned to prolong the nominal missionlifetime to about 5 years. A variety of filters, grisms and polarisers can be usedfor IR imaging and spectroscopy, witheach camera having its own set of 19different filter elements. Unfortunately,during the months before its Shuttledelivery HST, it was realized that toomuch solid nitrogen had been loadedinto the dewar. This stressed the dewar and, while not endangering its integrity,caused it to expand, shifting thedetectors from their nominal position. Inan emergency move before the launch, anew focusing mechanism offering a

greater range was installed.Unfortunately, after installation on HST,the expansion of the dewar continued tomove the detector of Camera 3 (thewide-field camera also serving the grism

mode) beyond the capacity of thefocusing mechanism. In addition, aninternal baffle came into physicalcontact with the external structure,creating a thermal bridge thatconsiderably increased the nitrogensconsumption rate and consequentlydecreased the expected operationallifetime of NICMOS. As compensationfor these unexpected problems, theinitial observations indicated that HSTs

induced IR background was much lower than projected, making NICMOS moreefficient. Indeed, in spite of the problems, NICMOS is producingexcellent scientific results, reviewed bythe scientific community in a dedicatedWorkshop organized by ESAs ST-ECF(Pula, 25- 26 May 1998). The focusing problem with Camera 3 is being tackled by observing campaigns using HSTssecondary mirror to bring the camerainto focus (but obviously defocusing allother instruments). The problem of theshorter operational lifetime has beentackled by giving NICMOS preferenceover the other instruments and byannouncing a supplemental Call for Proposals exclusively dedicated to NICMOS. NASA Goddard Space FlightCenter engineers are studying the possibility of installing, during the 2000M&R Mission, a mechanical cryocooler to extend the instrument life, albeit withinferior performance.


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6. Scientific results

Since its launch on 24 April 1990, HSThas provided a stunning view of our Universe by making unique discoveriesand capturing spectacular images of infant galaxies, distant quasars,exploding stars, mysterious black holesand colliding galaxies. In its 8 years of space exploration, the 12.5 t orbitingobservatory has allowed astronomers to publish 1700 scientific papers. The totalamount of Hubble data placed inarchives is 4.44 Tbytes, which fills 71030-cm optical disks (6.66 GB/disk). Thetelescope has taken about 120 000

exposures, observing about 10 000astronomical 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 infantgalaxies, some of which date to almostthe beginning of the Universe. Whatthey found was astonishing: a bewildering assortment of about 2000

galaxies at various stages of evolution.This deepest, most detailed optical viewof the Universe is known as the HubbleDeep Field. For 10 consecutive days in1995, the telescope was pointed at akeyhole-sized piece of sky. Most of thegalaxies are so faint (about 4 billiontimes fainter than can be seen by thehuman eye) that they have never beenseen by even the largest telescopes onEarth. This observation has revealed the

shapes of galaxies in the distant past.Astronomers have continued to analyzethe Hubble Deep Field images to tracethe evolution of stars and galaxies. Thishas led to intriguing evidence that theBig Bang may have been followed by astellar baby boom. The early Universemay have had an active, dynamic youth

when stars formed out of dust and gas ata ferocious rate. Consequently, most of the stars the Universe will ever makemay have already been formed, and itnow contains largely mid-life stars.

For decades, astronomers have debatedthe question: how old is our Universe?Hubble is helping them to determine theanswer by studying distant supernovaeand pulsating stars. Peering halfwayacross the Universe to analyze light fromexploded stars that died long before theSun was born, Hubbles crisp vision hasallowed astronomers to determine thatthe Universe and all its objects may have

not slowed down since their creation andmay continue to balloon outward. Basedon preliminary observations of severaldistant supernovae one of whicherupted 7.7 billion years ago thecosmos is not packed with enoughmaterial to halt its infinite expansion. If these early conclusions are true, then theUniverse could be 15 billion years old.

Other teams of astronomers are usingdifferent techniques to calculate theUniverses age. They are using Hubbleto measure accurately the distances togalaxies, an important prerequisite for calculating age. Hubble is measuringdistances to neighboring galaxies byfinding reliable distance markers, theCepheid variable stars. These, in turn,are being used to calibrate more remotemarkers. By calculating these distances,astronomers can determine the rate atwhich the Universe is expanding (theHubble constant) and, ultimately, itsage.Astronomers have long pondered theorigins of one of the Universes greatestenigmas: periodic bursts of gamma raysin deep space. Hubble has made animportant contribution towards


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identifying the source and nature of these fireballs, natures most powerfulexplosions. The telescope has allowedastronomers to follow the fading visible-light counterpart of an invisible gamma-

ray burst, whose position in the sky wasdetected by the Italian-Dutch BeppoSAX satellite. After monitoring thevisible afterglow of the gamma rayexplosion, Hubbles sensitiveinstruments have given astronomersimportant information by pinpointingsome gamma-ray bursts within distantgalaxies.

Massive black holes cannot be seen

because they are so dense and compactthat nothing, not even light, escapesfrom 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 providedconvincing evidence of the existence of these powerhouses by measuring thespeed of gas and stars in the cores of galaxies, where black holes reside. STIShas measured the increasing speed of agas discs orbiting a black hole in M84,located 50 million light-years away inthe Virgo cluster of galaxies. This isswirling around the unseen black hole at1.42 million km/h. Astronomers havecalculated that the black hole contains atleast 300 million solar masses. HST hashelped to prove that massive black holesare so common that almost every largegalaxy has one.

Hubble has followed the expandingwave of material from the explosion of supernova 1987A. The massive starsself-destruction was first seen almost 11years ago via ground-based telescopes.Hubbles WF/PC II and STIS haveshown that debris from the blast isslamming into a ring of material around

the dying star the collision hasilluminated part of the ring, which wasformed before the star exploded. Thecrash has allowed scientists to probe thestructure around the supernova and

uncover new clues about the final yearsof the progenitor star.

6.1 Shining a Light on Dark Matter

Astronomers used Hubble to make thefirst three-dimensional map of dark matter, which is considered theconstruction scaffolding of the universe.

FIGURE:3 Three-Dimensional Distribution of Dark Matter in the Universe:a. 3.5 billion years ago; b. 5billion years ago; c. 6.5

billion years ago.

Dark matters invisible gravity allowsnormal matter in the form of gas anddust to collect and build up into stars andgalaxies. The Hubble telescope played astarring role in helping to shed light on


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dark matter, which is much moreabundant than normal matter.Although astronomers cannot see dark matter, they can detect it in galaxyclusters by observing how its gravity

bends the light of more distant background galaxies, a phenomenoncalled gravitational lensing. Astronomersconstructed the map by using Hubble tomeasure the shapes of half a millionfaraway galaxies.

The new map provides the best evidenceto date that normal matter, largely in theform of galaxies, accumulates along thedensest concentrations of dark matter.

The map, which stretches halfway back to the beginning of the universe, revealsa loose network of filaments that grewover time and intersect in massivestructures at the locations of galaxyclusters.

Astronomers also used gravitationallensing in a previous study to make thefirst direct detection for the existence of dark matter. Hubble teamed up with theChandra X-ray Observatory, theEuropean Southern Observatorys VeryLarge Telescope, and the Magellanoptical telescopes to make the discovery.Astronomers found that dark matter andnormal matter were pulled apart by thetremendous collision of two largeclusters of galaxies, called the BulletCluster.

6.2 A Speedy Universe

By witnessing bursts of light fromfaraway exploding stars, Hubble helpedastronomers discover dark energy. Thismysterious, invisible energy exerts arepulsive force that pervades our uni-verse.

Several years later, Hubble providedevidence that dark energy has beenengaged in a tug of war with gravity for billions of years. Dark energy, whichworks in opposition to gravity, shoves

galaxies away from each other at ever-increasing speeds, making the universeexpand at an ever-faster pace.

But dark energy wasnt always in thedrivers seat. By studying distantsupernovae, Hubble traced dark energyall 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 thegravitational pull of the universesmatter even before it began to win thecosmic tug of war. Dark energy finallywon the struggle with gravity about 5 billion years ago.

FIGURE:4 History of the universe:A cosmic tug of war.

By knowing more about how dark energy behaves over time, astronomershope to gain a better understanding of what it is. Astronomers still understand


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almost nothing about dark energy, eventhough it appears to comprise about 70 percent of the universes energy.

6.3 Planets, Planets Everywhere

Peering into the crowded bulge of our Milky Way galaxy, Hubble lookedfarther than ever before to nab a group of planet candidates outside our solar system.Astronomers used Hubble to conduct acensus of Jupiter-sized extrasolar planetsresiding in the bulge of our Milky Waygalaxy. 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 haveJupiter-sized, star-hugging planets. Sothis discovery means there are probably billions of such planets in our MilkyWay.

Five of the newly found planetcandidates represent a new extreme typeof planet. Dubbed Ultra-Short-Period

Planets, these worlds whirl around their stars in less than an Earth day.Astronomers made the discoveries bymeasuring the slight dimming of a star as a planet passed in front of it, an eventcalled a transit.

The telescope also made the first directmeasurements of the chemicalcomposition of an extrasolar planetsatmosphere, detecting sodium, oxygen,

and carbon in the atmosphere of theJupiter-sized planet HD209458b. Hubblealso found that the planets outer hydrogen-rich atmosphere is heated somuch by its star that it is evaporatinginto space. The planet circles its star in atight 3.5-day orbit.

These unique observations demonstratethat Hubble and other telescopes cansample the chemical makeup of theatmospheres of alien worlds.Astronomers could use the same

technique someday to determine whether life exists on extrasolar planets.Besides testing the atmosphere of anextrasolar planet, Hubble also made precise measurements of the masses of two distant worlds.

6.4 Monster Black Holes AreEverywhere

Hubble probed the dense, central regionsof galaxies and provided decisiveevidence that supermassive black holesreside in many of them. Giant black holes are compact monsters weighingmillions to billions the mass of our Sun.They have so much gravity that theygobble up any material that venturesnear them.

These elusive eating machines cannot be observed directly, because nothing,not even light, escapes their grasp. Butthe telescope provided indirect, yetcompelling, evidence of their existence.Hubble helped astronomers determinethe masses of several black holes bymeasuring the velocities of materialwhirling around them.

The telescopes census of many galaxiesshowed an intimate relationship betweengalaxies and their resident black holes.

The survey revealed that a black holesmass is dependent on the weight of itshost galaxys bulge, a spherical regionconsisting of stars in a galaxys centralregion. Large galaxies, for example,have massive black holes; less massivegalaxies have smaller black holes. Thisclose relationship may be evidence that


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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 Blastssince the Big Bang

Imagine a powerful burst of light andother radiation that can burn away theozone in Earths atmosphere. Luckily,

FIGURE: 5 F our gamma-ray burst

host galaxies.flashes of such strong radiation occur sofar away they will not scorch our planet.These brilliant flashbulbs are calledgamma-ray bursts. They may representthe most powerful explosions in theuniverse since the Big Bang.

Hubble images showed that these brief flashes of radiation arise from far-flunggalaxies, which are forming stars atenormously high rates. Hubblesobservations confirmed that the bursts of light originated from the collapse of massive stars.Astronomers using Hubble also foundthat a certain type of extremely energeticgamma-ray bursts are more likely to

occur in galaxies with fewer heavyelements, such as carbon and oxygen.The Milky Way galaxy, which is rich inheavy elements released by manygenerations of stars, is therefore an

unlikely place for them to pop off.6.6 How Old Is the Universe?

Hubble observations allowedastronomers to calculate a precise agefor the universe using two independentmethods. The findings reduced theuncertainty to 10 percent. The firstmethod relied on determining theexpansion rate of the universe, a value

called the Hubble constant. In May 1999a team of astronomers obtained a valuefor the Hubble constant by measuringthe distances to nearly two dozengalaxies, some as far as 65 million light-years from Earth. By obtaining a valuefor the Hubble constant, the team thendetermined that the universe is about 13 billion years old.

FIGURE: 6 Close-up of ancient,white-dwarf stars in the Milky Waygalaxy .

In the second method astronomerscalculated a lower limit for theuniverses age by measuring the lightfrom old, dim, burned-out stars, calledwhite dwarfs. The ancient white dwarf stars, as seen by Hubble, are at least 12to 13 billion years old.


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6.7 Quasars, the Light Fantastic

Quasars have been so elusive andmysterious that the hunt to define themwould 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 compactdynamos of light and other radiation,which lie at the outer reaches of theuniverse, produce so much energy.Quasars are no larger than our solar system but outshine galaxies of hundredsof billions of stars.

FIGURE:7 Looking underneathQuasar HE0450-2958.

These light beacons have left trails of evidence and plenty of clues, butscientists have only just begun tounderstand their behavior. Astronomersusing Hubble tracked down the homesof quasars to the centers of farawaygalaxies. Hubbles observations bolstered the idea that quasars are powered by a gush of radiationunleashed by black holes in the cores of these galaxies.

7. Conclusion

Hubble is one of NASA's mostsuccessful and long-lasting science

missions. It has beamed hundreds of thousands of images back to Earth,shedding light on many of the greatmysteries of astronomy. Its gaze hashelped determine the age of the universe,the identity of quasars, and the existenceof dark energy.

Eventually, Hubble's time will end. Inthe years after servicing mission,Hubble's components will slowly

degrade to the point at which thetelescope stops working.

When that happens, Hubble willcontinue to orbit the Earth until its orbitdecays, allowing it to spiral towardEarth. Astronauts or a robotic missioncould either bring Hubble back to Earthor crash it safely into the ocean.

But Hubble's legacy its discoveries,

its trailblazing design, its success inshowing us the universe in unparalleleddetail will live on. Scientists will relyon Hubble's revelations for years as theycontinue in their quest to understand thecosmos a quest that has attainedclarity, focus, and triumph throughHubble's rich existence.

8. Bibliography

http://hubblesite.org/http://hubblesite.org/newscenter/http://www.spacetelescope.org/about/index.htmlhttp://www.stsci.edu/hst/HST_overview/
http://hubblesite.org/http://hubblesite.org/newscenter/http://www.spacetelescope.org/about/index.htmlhttp://www.spacetelescope.org/about/index.htmlhttp://www.stsci.edu/hst/HST_overview/http://www.stsci.edu/hst/HST_overview/http://hubblesite.org/http://hubblesite.org/newscenter/http://www.spacetelescope.org/about/index.htmlhttp://www.spacetelescope.org/about/index.htmlhttp://www.stsci.edu/hst/HST_overview/http://www.stsci.edu/hst/HST_overview/


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brief intro about hubble space telescope

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