Chandra X-Ray Observatory CXO : Overview

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Chandra X-Ray Observatory (CXO): Overview

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M. C. Weisskopf ,H.D.Tananbaum ,L.P.Van Sp eybro eck , and S. L. O'Dell ,

NASA Marshall Space Flight Center

Huntsville, AL 35812 USA

Smithsoniam Astrophysical Observatory

Cambridge, MA 02138 USA

ABSTRACT has joined the Hubble Space Telescop e (HST) and the

Compton Gamma-Ray Observatory (CGRO) as one of

The Chandra X-Ray Observatory (CXO), the x-ray com-

NASA's "Great Observatories". CXO provides unprece-

p onent of NASA's Great Observatories, was launched

dented capabilities for sub-arcsecond imaging, sp ectro-

early in the morning of 1999, July 23 by the Space Shuttle

metric imaging, and for high-resolution disp ersive sp ec-

Columbia. The Shuttle launchwas only the rst step in

troscopyover the x-ray band 0.08-10 keV (15-0.12 nm).

placing the observatory in orbit. After release from the

With these capabilites a wide variety of high-energy phe-

cargo bay, the Inertial Upp er Stage p erformed two r-

nomena in a broad range of astronomical ob jects is b eing

ings, and separated from the observatory as planned. Fi-

observed.

nally, after ve rings of Chandra's own Integral Propul-

sion System | the last of which to ok place 15 days after

Chandra is a NASA facility which provides scienti c

launch | the observatory was placed in its highly ellipti-

data to the international astronomical community in re-

cal orbit of 140,000 km ap ogee and 10,000 km p erigee.

sp onse to scienti c prop osals for its use. The CXOis

After activation, the rst x-rays fo cussed by the telescop e

the pro duct of the e orts of many commercial, academic,

were observed on 1999, August 12. Beginning with these

and government organizations in the United States and

initial observations one could see that the telescop e had

Europ e. NASA Marshall Space Flight Center (MSFC,

survived the launchenvironment and was op erating as

Huntsville, Alabama) manages the Pro ject and provides

exp ected. The month following the op ening of the sun-

Pro ject Science; TRW Space and Electronics Group (Re-

shade do or was sp ent adjusting the fo cus for each set of

dondo Beach, California) served as prime contractor;

instrument con gurations, determining the optical axis,

the Smithsonian Astrophysical Observatory (SAO, Cam-

calibrating the star camera, establishing the relative re-

bridge, Massachusetts) provides the technical supp ort

sp onse functions, determining energy scales, and taking

and is resp onsible for ground op erations including the

a series of "publicity" images. Each observation proved

Chandra X-ray Center (CXC). There are also ve sci-

to b e far more revealing than was exp ected. Finally, and

enti c instruments ab oard the Observatory provided by

despite an initial surprise and setback due to the dis-

anumb er of di erent institutions. These instruments are

covery that the Chandra x-ray telescop e was far more

discussed in x2.3 and x2.4.

ecient for concentrating low-energy protons than had

b een anticipated, the observatory is p erforming well and

is returning sup erb scienti c data. Together with other

In 1977, NASA/MSFC and the Smithsonian Astro-

space observatories, most notably the recently activated

physical Observatory (SAO) b egan the phase-A study

XMM-Newton, it is clear that we are entering a new era

leading to the de nition of the then AXAF mission. This

of discovery in high-energy astrophysics.

study, in turn, had b een intiated as a result of an unso-

licited prop osal submitted to NASA in 1976 by Prof. R.

Keywords: Chandra, CXO, space missions, x rays,

Giacconi and Dr. H. Tananbaum. During the intervening

grazing-incidence optics, gratings, detectors, x-ray imag-

years, several signi cant milestones transpired, including

ing, x-ray sp ectroscopy, x-ray astronomy.

the highest recommendation by the National Academy

of Sciences, selection of the instruments, selection of the

1. INTRODUCTION

prime contractor, demonstration of the optics, restructur-

ing of the mission, and ultimately the launch (Figure 1)

The Chandra X-Ray Observatory (CXO), formerly known

as the Advanced X-Ray Astrophysics Facility (AXAF),

Invited Pap er, in X-Ray Optics, Instruments, and Missions,

J.Truemp er and B. Aschenbach, eds., Proc. SPIE 4012, 2000.

We b egin by describing the Chandra systems (x2) and

M.C.W.: [email protected].nasa.gov; 256-544-7740.

the ground calibration (x3). We then describ e Chan-

H.D.T.: [email protected]; 617-495-7248.

dra's on-orbit p erformance and its ability(x4) to serve

L.V.S.: [email protected]; 617-495-7233.

as NASA's premier facility for x-ray astrophysics. S.L.O.: o [email protected]; 256-544-7708. 1

orbital p erio d, as is the fraction of the time when the

detector backgrounds are high as the Observatory dips

into Earth's radiation b elts. Consequently, more than

70% of the time is useful and uninterrupted observations

lasting more than 2 days are p ossible.

The sp eci ed design life of the mission is 5 years; how-

ever, the only exp endable (gas for maneuvering) is sized

to allow op eration for more than 10 years. The orbit will

b e stable for decades.

2.2. Flight system

Figure 2. Chandra attached to the IUS (to the right)

after deployment from Columbia but b efore IUS separa-

tion. Note that the apparent narrowing of the CXO opti-

Figure 1. Photograph of the Columbia launch with the

cal b ench is an illusion due to shadowing. NASA Photo.

CXOpayload (STS-93). NASA Photo.

With the IUS attached (Figure 2), the Chandra was the

largest and heaviest payload ever deployed from an STS

2. CHANDRA SYSTEMS

space shuttle. Once deployed and separated from the

2.1. Mission and Orbit

IUS, the Chandra ight system is 13.8-m (43.5-ft) long by

4.2-m (14-ft) diameter, with a 19.5-m (64-ft) solar-panel

The Space Shuttle Columbia carried and deployed the

wingspan. With extensive use of graphite-ep oxy struc-

Chandra intoalow earth orbit, as NASA's Space Trans-

tures, the mass of the Chandra ight system is 4,800 kg

p ortation System mission STS-93. Ab out 8 hours after

(10,600 p ounds). The Chandra ight system (Figure 3)

launch Chandra was deployed (Figure 2) at an altitude

itself has 3 ma jor mo dules or systems | the Spacecraft

of ab out 240 km (130 nautical miles). At this time, an

Mo dule (x2.2.1), the Telescop e System (x2.2.2), and the

Inertial Upp er Stage (IUS), a two-stage solid-fuel ro cket

Integrated Science Instrument Mo dule (x2.2.3).

b o oster develop ed by the Bo eing Company Defense and

Space Group (Seattle, Washington) for the US Air Force,

prop elled the Chandra ight system into a highly ellipti-

2.2.1. Spacecraft mo dule

cal transfer orbit. Subsequently,over a p erio d of days,

TRW Space and Electronics Group (Redondo Beach, Cal-

Chandra's Internal Propulsion System (IPS), built by

ifronia) built the Spacecraft Mo dule which is made up of:

TRW, placed the observatory into its initial op eration or-

bit - 140-Mm (87,000-nautical-mile) ap ogee and 10-Mm

(6,200-nautical-mile) p erigee, with a 28.5 initial inclina-

1. The Pointing Control and Asp ect Determination

tion.

(PCAD) subsystem which p erforms on-b oard at-

titude determination, solar-array control, slewing, Chandra's highly elliptical orbit, with a p erio d of 63.5

p ointing and dithering control, and momentum man- hours, yields a high observing eciency. The fraction of

agement. the sky o cculted by the earth is small over most of the 2

Telescop e System also provides mounts and mechanisms

for the Chandra Observatory's 2 ob jective transmission

gratings (x2.4.2). In addition, Ball Aerospace and Tech-

nologies Corp oration (Boulder, Colorado) fabricated the

Asp ect Camera Assembly, a visible-light telescop e and

CCD camera which attaches to, and is coupled with, the

Telescop e System through a ducial-lightTransfer Sys-

tem, which e ectively maps the x-ray fo cal plane onto

the sky.

2.2.3. Integrated Science Instrument Mo dule

Ball Aerospace and Technologies Corp oration (Boulder,

Figure 3. Expanded view of the Chandra ight system,

Colorado) built the Science Instrument Mo dule (SIM),

showing several subsystems of the 3 ma jor mo dules |

which includes mechanisms for fo cussing and translating

the Telescop e System, the Integrated Science Instrument

Chandra's fo cal-plane science instruments (x2.4.3). The

Mo dule, and the Spacecraft Mo dule. TRW drawing.

translation is necessary as the instruments cannot realis-

tically share the fo cal plane and must b e translated into

p osition at the HRMA fo cus. The Integrated Science In-

2. The Communication, Command, and Data Manage-

strument Mo dule (ISIM) simply denotes the SIM with

ment (CCDM) subsystem which p erforms commu-

the 2 fo cal-plane science instruments integrated.

nications, command storage and pro cessing, data

acquisition and storage, and computation supp ort,

2.3. Electron Proton Helium Intrument

timing reference, and switching of primary electrical

(EPHIN)

power for other Chandra systems or subsystems.

Mounted on the spacecraft and near the HRMA is a par-

3. The Electrical Power Subsystem (EPS) which gen-

ticle detector called the Electron, Proton, Helium INstru-

erates, regulates, stores, distributes, conditions, and

ment (EPHIN). The EPHIN instrumentwas built by the

controls the primary electrical p ower.

Institut fur Exp erimentelle und Angewandte Physik und

4. The Thermal Control Subsystem (TCS) which fur-

Extraterrestrishce Physik at the University of Kiel, Ger-

nishes passive thermal control (where p ossible),

many. The EPHIN detector is used to monitor the lo cal

heaters, and thermostats.

charged particle environment as part of the scheme to

protect the fo cal-plane instruments from particle radia-

5. The structures and mechanical subsystem which en-

tion damage. EPHIN consists of an array of 5 silicon

compasses the spacecraft structures, mechanical in-

detectors with anti-coincidence. The instrument is sensi-

terfaces among the spacecraft subsystems and with

tive to electrons in the energy range 150 keV - 5 MeV,

the telescop e system and external structures.

and protons/helium isotop es in the energy range 5 - 49

MeV/nucleon. The eld of view is 83 degrees. The fore-

6. The propulsion subsystem which comprises the Inte-

runner of the Chandra-EPHIN was own on the SOHO

gral Propulsion Subsystem (IPS) - delib erately dis-

satellite.

abled once nal orbit was obtained - and the Mo-

mentum Unloading Propulsion Subsystem (MUPS).

2.4. X-Ray Subsystems

7. The ight software which implements algorithms for

Chandra's x-ray subsytems are the High-Resolution Mir-

attitude determination and control, command and

ror Assembly (HRMA, x2.4.1), the ob jective transmission

telemetry pro cessing and storage, and thermal and

gratings (x2.4.2), and the fo cal-plane science instruments

electrical p ower monitoring and control.

(x2.4.3).

2.4.1. High-Resolution Mirror Assembly

2.2.2. Telescop e system

(HRMA)

The Eastman Ko dak Company (Ko dak, Ro chester, New

Hughes Danbury Optical Systems (HDOS, Danbury, Con- York) integrated the Telescop e System. Its princi-

necticut) - nowRaytheon Optical Systems Incorp orated pal subsytems are the High-Resolution Mirror Assembly

(ROSI) - precision gured and sup erp olished the 4- (HRMA, x2.4.1) and the Optical Bench Assembly (OBA).

mirror-pair grazing-incidence x-ray optics out of Zero dur Comp osite Optics Incorp orated (COI, San Diego, Califor-

blanks from Schott Glaswerke (Mainz, Germany). Opti- nia) develop ed the critical light-weight comp osite materi-

cal Coating Lab oratory Incorp orated (OCLI, Santa Rosa, als for the OBA (and for other Chandra structures). The 3

ab out 991-nm p erio d, the LETG provides high-resolution

sp ectroscopy from 0.08 to 2 keV (15 to 0.6 nm).

Figure 4. Photograph of the High-Resolution Mirror

Assembly (HRMA) during alignment and assembly in the

Figure 5. Photograph of the LETG and HETG mounted

HRMA AlignmentTower at Ko dak. In the picture, 7 of

to the spacecraft structure. Photograph is from TRW.

the 8 mirrors are already attached to the center ap erture

plate. Photograph is from Ko dak.

High-Energy Transmission Grating (HETG) The

Massachusetts Institute of Technology (MIT, Cambridge,

California) coated the optics with iridium, chosen for high

Massachusetts) designed and fabricated the High-Energy

re ectivity and stability. The Eastman Ko dak Company

Transmission Grating (HETG, Figure 6). The HETG

(Ro chester, New York) aligned and assembled the mir-

employs 2 typ es of grating facets | the Medium-Energy

rors into the 10-m fo cal length High-Resolution Mirror

Gratings (MEG), mounted b ehind the HRMA's 2 out-

Assembly (HRMA, Figure 4), which also includes thermal

ermost shells, and the High-Energy Gratings (HEG),

pre- and p ost-collimators and forward and aft contami-

mounted b ehind the HRMA's 2 innermost shells | ori-

nation covers. The forward contamination cover houses

ented at slightly di erent disp ersion directions. With

16 radioactive sources, develop ed by MSFC, for verifying

3{5

p olyimide-supp orted gold bars of 400-nm and 200-nm p e-

transfer of the ux scale from the ground to orbit.

rio ds, resp ectively, the HETG provides high-resolution

sp ectroscopy from 0.4 to 4 keV (MEG, 3 to 0.3 nm) and

2.4.2. Ob jective transmission gratings

from 0.8 to 8 keV (HEG, 1.5 to 0.15 nm).

Aft of the HRMA are 2 ob jective transmission gratings

2.4.3. Fo cal-plane science instruments

(OTGs) - the Low-Energy Transmission Grating (LETG)

and the High-Energy Tranmission Grating (HETG). Posi-

The ISIM (x2.2.3) houses Chandra's 2 fo cal-plane science

tioning mechanisms may insert either OTG into the con-

instruments | the (micro channel-plate) High-Resolution

verging b eam to disp erse the x-radiation onto the fo cal

Camera (HRC) and the Advanced CCD Imaging Sp ec-

plane pro ducing high-resolution sp ectra read-out by one

trometer (ACIS). Each instrument provides b oth a so-

of the fo cal-plane detectors (x2.4.3).

called (as all the detectors are imagers) imaging detector

(I) and a sp ectroscopy detector (S), the latter designed

for reading out the high-resolution sp ectra disp ersed by

Low-Energy Transmission Grating (LETG) The

the Chandra's Observatory's insertable OTGs | HRC-S

Space Research Institute of the Netherlands (SRON,

with the LETG and ACIS-S with the HETG.

Utrecht, Netherlands) and the Max-Planck-Institut fur

extraterrestrische Physik (MPE, Garching, Germany) de-

signed and fabricated the Low-Energy Transmission Grat- High-Resolution Camera (HRC) The Smithso-

ing (LETG, Figure 5). The 540 grating facets, mounted nian Astrophysical Observatory (SAO, Cambridge, Mas-

3 p er mo dule, lie tangent to the Rowland toroid which sachusetts) designed and fabricated the High-Resolution

includes the fo cal plane. With free-standing gold bars of Camera (HRC, Figure 7). Made of a single large-format 4

Figure 7. Photograph of the fo cal plane of the Chandra

ight High-Resolution Camera (HRC). The HRC-I (im-

ager) is at the b ottom; the HRC-S (sp ectroscopic read-

out), at the top. Photograph is from the HRC team.

lters.

Figure 6. Photograph of the High-Energy Transmission

Grating (HETG). Photograph is from the HETG team.

(10-cm-square) micro channel plate, the HRC-I provides

high-resolution imaging over a large (31-arcmin-square)

eld of view. Comprising 3 rectangular segments (3-cm-

by-10-cm each) mounted end-to-end along the OTG dis-

p ersion direction, the HRC-S serves as the primary read-

out detector for the LETG. Both detectors are coated

with a cesium{io dide photo catho de and covered with

aluminized-p olyimide UV/ion shields.

Advanced CCD Imaging Sp ectrometer (ACIS)

The Pennsylvania State University (PSU, University

Park, Pennsylvania) and the Massachusetts Institute of

Technology (MIT, Cambridge, Massachusetts) designed

and fabricated the ACIS (Figure 8), with charge-coupled

devices (CCDs) pro duced by MIT Lincoln Lab oratory

Figure 8. Photograph of the fo cal plane of the Chan-

(Lexington, Massachusetts) and some subsystems and

dra ight Advanced CCD Imaging Sp ectrometer (ACIS),

systems integration provided byLockheed{Martin Astro-

prior to installation of the optical blo cking lters. The

nautics (Littleton, Colorado). Made of a 2-by-2 arrayof

ACIS-I (imager) is at the b ottom; the ACIS-S (sp ectro-

large-format (2.5-cm-square) CCDs, the ACIS-I provides

scopic read-out), at the top. Photograph is from the ACIS

high-resolution sp ectrometric imaging over a 17-arcmin-

team.

square eld of view. The ACIS-S, a 6-by-1 arrayofthe

large-format CCDs mounted along the OTG disp ersion

direction, serves b oth as the primary read-out detector

2.5. Ground system

for the HETG, and, using BI CCD lo cated at the center

of the array, also provides high-resolution sp ectrometric The Chandra ground system comprises the Deep-Space

imaging extending to lower energies but over a smaller (8- Network (DSN, x2.5.1), the Chandra Op erations Control

arcmin-square) eld than ACIS-I. Both ACIS detectors Center (OCC, x2.5.2), and the Chandra X-Ray Center

are covered with aluminized-p olyimide optical blo cking (CXC, x2.5.3). 5

3. GROUND CALIBRATION 2.5.1. Deep-Space Network (DSN)

The Jet Propulsion Lab oratory (JPL, managed for NASA

The calibration of Chandra included an intensive and ex-

by the California Institute of Technology,Pasadena, Cal-

tensive ground calibration program for calibrating the full

ifornia) op erates NASA's Deep-Space Network (DSN).

Observatory (x3.1) and its individual subsystems. The

Through its 3 antenna stations (Goldstone, California;

on-ground, and now on-orbit, calibration results (x3.2)

Madrid, Spain; and Canb erra, Australia), the DSN com-

clearly demonstrate that the Chandra Observatory pro-

municates directly with the Chandra spacecraft, up-

vides the science capabilities (x 4) | high-resolution (sub-

linking commands and down-linking telemetered science

arcsec) imaging and sp ectrometric imaging and high-

and engineering data. During normal op erations, DSN

resolution disp ersive sp ectroscopy | to address its sci-

contacts, at 8-hour intervals, the Chandra spacecraft,

ence ob jectives. With a goal of a high accurate calibra-

which stores 16.8 hours (1.8 Gb) of data in its solid-state

tion, the calibration program is an on-going e ort which

recorder(s).

requires continued analysis and interpretation. Our previ-

7,8

ous overviews provide additional details and references.

2.5.2. Op erations Control Center (OCC)

3.1. Observatory ground-calibration

The Chandra Op erations Control Center (OCC) is lo-

cated in Cambridge, Massachusetts, and is part of the

From 1996 December until 1997 May, the Chandra teams

Chandra X-Ray Center (CXC, x 2.5.3). The Chandra

calibrated the Chandra Observatory at the MSFC X-Ray

prime contractor, TRW, sta s the OCC's Flight Op era-

9,10

Calibration Facility (XRCF, Figure 9). Calibration

tions Team (FOT), which is resp onsible for the control,

of the the Observatory used, of course, the ight High-

health, and safety of the spacecraft. The OCC receives

Resolution Mirror Assembly (HRMA, x 2.4.1) and ight

Observation Requests, which it uses to build the Detailed

ob jective transmission gratings (OTGs, x 2.4.2), the Low

Op erations Timeline, and command loads to b e sentto

Energy Transmission Grating (LETG) and the High En-

the spacecraft through the DSN. From the spacecraft,

ergy Transmission Grating (HETG), and the ight fo cal-

through the DSN, the OCC receives the telemetered data,

plane detectors ACIS (x 2.4.3) and HRC(x2.4.3).

converts its format, extracts the engineering stream, and

analyses the engineering data. The OCC utilizes soft-

ware develop ed by MSFC and by the Computer Sciences

Corp oration (CSC, El Segundo, California).

2.5.3. Chandra X-Ray Center (CXC)

The Smithsonian Astrophysical Observatory (SAO), with

the Massachusetts Institute of Technology (MIT), op er-

ates the Chandra X-Ray Center (CXC). As Chandra's

day-to-dayinterface with the scienti c community, the

CXC supp orts NASA for soliciting observing prop os-

als, provides prop osal-preparation information and to ols,

organizes p eer reviews on b ehalf of NASA, makes the

long-term schedule from the approved observing prop os-

als, generates Observation Requests (OR) from the long-

term schedule and calibration needs, and submits the

OR to the OCC for scheduling. The CXC receives the

re-formatted telemetry from the OCC, extracts the sci-

Figure 9. Aerial photograph of the X-Ray Calibration

ence stream, pro cesses the science data, constructs time-

Facility (XRCF) at NASA Marshall Space Flight Center

tagged event lists for each observation, p erforms higher

(MSFC). The small building to the far left houses the X-

level data pro cessing (e.g., generates images and sp ectra),

ray Source System (XSS); the large building to the near

op erates the data archive, maintains (with NASA Pro ject

right houses the instrumentchamb er, and control ro om,

Science) the calibration, and provides data, analysis to ols,

and provides space for data pro cessing. MSFC photo-

and other supp ort to users. The CXC is also resp onsible

graph.

for developing and maintaining the software to supp ort

its functions. 6

3.2. Summary of ground results

The Chandra science teams provided detail results of the

ground calibration in various Calibration Rep orts, acces-

sible, for example, through the Pro ject Science calibra-

tion web pages (see A). In addition to the Calibration

11{20,5

Rep orts, some of the results app ear in these and

21{35,9,10

previous pro ceedings.

Perhaps the most outstanding anomaly from the

ground calibration had b een a discrepancy (at ab out the

10-15% level) b etween the measured HRMA e ective area

and the mo delled (predicted) e ective area at mo derate

(few keV) to high (several keV) energies. The discrep-

ancy was sp eculated, and now con rmed, to have arisen

from di erent and inconsistent approaches used in mo d-

elling the e ects of surface roughness. The synchrotron

program to determine the optical constants of iridium-

coated ats had used one technique, while the analysis of

the Chandra HRMA calibration had used another. The

measured e ective area (Figure 10) is now in reasonable

agreement with mo del predictions. Details are presented

bySchwartz et al. elsewhere in these pro ceedings.

4. ON-ORBIT PERFORMANCE

Chandra's mission is to provide high-quality x-ray data.

In this section, we summarize certain key p erformance

Figure 10. Comparison of the mo del of the HRMA ef-

capabilities and address the degree to which they have

fective area with various data obtained during the Obser-

b een accomplished.

vatory calibration at the XRCF. Plots are from Telescop e

Science and Mission Supp ort Teams at SAO.

4.1. Capabilities

Chandra is a unique x-ray astronomy facility for high-

resolution imaging (x 4.1.1) and for high-resolution sp ec-

troscopy(x 4.1.2). Indeed, Chandra's p erformance advan-

tage over other x-ray observatories is analagous to that

of the Hubble Space Telescop e (HST) over ground-bases

observatories.

4.1.1. Imaging p erformance

The angular resolution of Chandra is signi cantly b et-

ter than any previous, current, or even currently-planned

x-ray observatory. Figure 11 qualitatively,yet dramati-

Figure 11. Chandra (left) and ROSAT (right) images

cally, illustrates this p ointby comparing the early Chan-

of CAS-A.

dra image of the sup ernova remnant Cassiop eia-A, based

on ab out 2700 s of data, with a 200,000 s ROSAT im-

age. (Prior to the development necessary to pro duce the

Quantitatively, Chandra's p oint spread function (PSF),

Chandra optics, the ROSAT observatory represented the

as measured during ground calibration, had a full width

state of the art in high-resolution x-ray imaging.) The

at half-maximum (FWHM) less than 0.5 arcsec and a

improvement broughtby Chandra's advance in angular

half-p ower diameter (HPD) less than 1 arcsec. The pre-

resolution is dramatic, and the p oint source at the cen-

diction for the on-orbit encircled-energy fraction was that

ter | undetected in the ROSAT image | simply leaps

a 1-arcsec-diameter circle would enclose at least half the

out of the Chandra image.

ux from a p oint source. The relatively mild dep endence 7

on energy (resulting from di ractive scattering by surface

microroughness) attested to the excellent sup erp olished

nish of the Chandra optics. The ground measurements

were, of course, taken under environmental conditions

quite di erent than those encountered on-orbit. Most

notably the e ects of gravity on the optics and the nite

distance and size of the various x-ray sources used were

unique to the ground calibration. On the other hand, on

the ground there was no Observatory motion to deal with.

On-orbit the p erformance folds in the spatial resolution

of the ight detectors and any uncertainties in the asp ect

solution which determines, p ost-facto, the direction the

observatory was p ointing relative to the instruments and

to celestial co ordinates.

The High Resolution Camera (HRC) has the b est spa-

tial resolution ( 20m, 0.4 arcsec) of the two imaging

instruments ab oard Chandra and thus is b est matched to

the telescop e. Figure 12 illustrates the extrap olation of

the ground calibration to on-orbit p erformance and com-

pares the predictions at two energies with an observed

PSF. Figure 13 shows a similar comparison using the

HRC-S. The angular resolution of the Chandra X-Ray

Figure 12. The predicted and observed encircled en-

Observatory has b een as exp ected. Further details con-

ergy as a function of radius for an on-axis p oint source.

cerning the Chandra p oint-spread function are presented

The detector is the HRC-I. The calculations, p erformed

by Jerius and colleagues in these pro ceedings. The

at two energies | 0.277 keV and 6.40 keV, include a real-

on-orbit p erformance of the HRC is discussed in more

istic (0.22") estimate of the contribution from the asp ect

15 18

detail by Murray et al., Kenter et al., and Kraft et

solution. Flight data from the calibration observation of

al.. Similarly, these pro ceedings contain more detailed

AR Lac are also shown. Figure pro duced byTelescop e

discussion of the p erformance of the asp ect camera and

Science.

the attitude control in the pap ers by Aldcroft et al. and

Cameron et al..

4.2.1. Proton damage to the front-illuminated

4.1.2. Sp ectroscopic p erformance

CCDs

The unprecedented angular resolution of the Chandra

The ACIS front-illuminated CCDs originally approached

optics, combined with Chandra's micro-ruled ob jective

the theoretical limit for the energy resolution at almost

transmission gratings (OTGs), provides the capability

all energies, while the back-illuminated devices exhibited

for high-resolution disp ersive sp ectroscopy. Chandra has

p o orer resolution. Subsequent to launch and orbital acti-

two sets of OTGs | the Low-Energy Tranmission Grat-

vation, the energy resolution of the front-illuminated (FI)

ing (LETG) is optimized for longer x-raywavelengths,

CCDs has b ecome a function of the rownumb er, b eing

and the High-Energy Tranmission Grating (HETG) for

nearer pre-launchvalues close to the frame store region

shorter wavelengths. Hence, with an appropriate com-

and progressively degraded towards the farthest row. An

bination of Chandra's gratings, Chandra allows mea-

illustration of the current dep endence on rowisshown in

surements with sp ectral resolving p ower (Figure 14) of

Figure 15.

(= =(E= E) > 500 for wavelengths >0:4nm

(energies < 3keV).

Foranumb er of reasons, we b elieve that the damage

was caused bylow energy protons, encountered during

4.2. Performance anomalies

radiation b elt passages and re ecting o the x-ray tele-

The p erformance of the Chandra Observatory, the instru- scop e onto the fo cal plane. Subsequent to the discovery

ments and subsystems have b een remarkably free from of the degradation, op erational pro cedures were changed

problems and anomalies. Here we discuss the two di- and the ACIS is not left at the fo cal p osition during ra-

culties that have b een encountered that have had some diation b elt passages. (The HRC is left at the fo cal p o-

impact on the scienti c p erformance. We note, however, sition, but with its do or partially closed for protection.)

that neither are preventing the mission from accomplish- Since this pro cedure was initiated, no further degrada-

ing its scienti c ob jectives. tion in p erformance has b een encountered. The BI CCDs 8 Wavelength ( Angstrom )

200 100 50 20 10 5 2 1

2,000 HRMA--OTG

1,000 LETG HEG

500 MEG

200 Resolving Power 100

10 .05 .1 .2 .5 1 2 5 10

Energy ( keV )

Figure 14. Sp eci ed sp ectral resolving p ower of Chan-

dra OTGs. Preliminary results indicate slightly b etter

Figure 13. The predicted and observed encircled en-

p erformance. Plot is from the Chandra Pro ject Science

ergy as a function of radius for an on-axis p oint source.

team.

The detector is the HRC-S. The calculations, p erformed

at two energies | 0.277 keV and 6.40 keV, include a real-

Further details are provided elsewhere in these pro ceed-

istic (0.22") estimate of the contribution from the asp ect

14,20

ings.

solution. Flight data from the calibration observation of

SMC X-1 are also shown. Figure pro duced byTelescop e

4.2.2. HRC-S Anticoincidence Electronics

Science.

The anti-coincidence shield of the HRC-S is not working

b ecause of a timing error in the electronics. The error is

were not impacted and this result is consistent with the

not correctable. As a result the rawevent rate is very high

proton-damage scenario as it is far more dicult for low

and exceeds the total telemetry rate limit. To cop e with

energy protons from the direction of the HRMA to de-

this the HRCTeam has de ned a "sp ectroscopy region"

p osit their energy in the buried channels (where damage

which is ab out 1/2 of the width and extends along the

is most detrimental to p erformance) of the BI devices,

full length of the HRC-S detector. With this change, the

since these channels are near the gates and the gates face

quiescent on-orbit background rate is ab out 85 cts s .

in the direction opp osite to the HRMA. Thus the en-

This background can b e further reduced in ground data

ergy resolution for the two BI devices remains at their

pro cessing by using pulse height ltering that preferen-

prelaunchvalues.

tially selects x-rays over the cosmic rayevents. A further

reduction in background of a factor of ab out three is p os-

The p osition dep endent energy resolution of the FI

sible. More details on the p erformance of the HRCmay

chips dep ends on the ACIS op erating temp erature. Since

15,18,16

b e found elsewhere in these pro ceedings.

activation, the ACIS op erating temp erature has b een

slightly lowered, based on considerations of molecular

4.3. Scienti c Performance

contamination, and is now set at the lowest temp erature

X-rays result from highly energetic pro cesses - thermal

now thought safely achievable (120 C).

pro cesses in plasmas with temp eratures of millions of de-

More recently, the ACIS team has b een able to repro-

grees or nonthermal pro cesses, such as synchrotron emis-

duce the damage characteristics after b ombarding test de-

sion or scattering from very hot or relativistic electrons.

vices with low-energy protons (< few hundred keV). Fur-

Consequently, x-ray sources are frequently exotic:

thermore, bysweeping charge through the system they

have b eeen able to ll the charge traps and further dra- Sup ernova explosions and remnants, where the ex-

matically reduce the impact. On-orbit testing of this plosion sho cks the ambientinterstellar medium or a

technique will take place in the near future (April 2000). pulsar (rotating neutron star) p owers the emission. 9

Figure 16. X-ray image of the source PKS0637 with

radio contours overlaid. The distance from the central

ob ject to the x-ray bright knot is 10". Image courtesy

CXC.

Figure 15. The energy resolution of S3 and I3 as a func-

tion of rownumb er. These data were taken at -120 C.

Note that these curves are representative of the varia-

tion | but they do not account for the row-dep endent

gain variation which also increases the energy resolution

by an additional 15-20% for the larger rownumb ers.

Accretion disks or jets around stellar-mass neutron

stars or black holes.

Accretion disks or jets around massive black holes in

galactic nuclei.

Hot gas in clusters of galaxies and in galaxies, which

traces the gravitational eld for determining the

mass.

Hot gas in stellar coronae, esp ecially during ares

(coronal mass ejection).

Here we give a few examples of observations with

Figure 17. X-ray image of the center of the Hydra clus-

Chandra which indicate the p otential for investigating

ter of galaxies. Image courtesy CXC.

these pro cesses and astronomical ob jects through high-

resolution imaging (x 4.3.1) and high-resolution sp ec-

troscopy(x4.3.2).

In addition to mapping the structure of extended

sources, the high angular resolution p ermits studies of en-

4.3.1. Imaging

sembles of discrete sources, whichwould otherwise b e im-

p ossible owing to source confusion. A b eautiful example Chandra's capability for high-resolution imaging (x 4.1.1)

comes from the recent observations of the center of M31 enables detailed high-resolution studies of the structure

(Figure 18) p erformed by M. Garcia and colleagues. of extended x-ray sources, including sup ernova remnants

The image shows what used to b e considered as emission (Figure 11), astrophysical jets (Figure 16, and hot gas

asso ciated with the black hole at the center of the galaxy in galaxies and clusters of galaxies (Figure 17). The

now resolved into several distinct ob jects. A most inter- supplementary capability for sp ectrometric imaging al-

esting consequence is that the emission from the region lows studies of structure, not only in x-rayintensity, but

surrounding the central black hole is now known to b e in temp erature and in chemical comp osition. Through

dramatically reduced and unexp ectedly and surprisingly these observations, astronomers will address several of

faint! Thus, Chandra observations will isolate individ- the most exciting topics in contemp orary astrophysics |

ual stars in clusters and star-forming regions and x-ray e.g., galaxy mergers, dark matter, and the cosmological

binaries in nearby normal galaxies. Furthermore, high- distance scale. 10

angular-resolution observations with Chandra's low-noise 4.3.2. Sp ectroscopy

fo cal-plane detectors will obtain photon-limited, deep-

Owing to their unprecedented clarity, Chandra images

eld exp osures which are likely to resolve most of the

will b e visually striking and provide new insights into

extragalactic cosmic x-ray background into faint, discrete

the nature of x-ray sources. Equally imp ortant to the

sources (Figure 19).

imaging science (x 4.3.1) will b e Chandra's unique con-

tributions to high-resolution disp ersive sp ectroscopy. In-

deed, as the capability for visible-light sp ectroscopybe-

gat the eld of astrophysics ab out a century ago, high-

resolution x-ray sp ectroscopy will contribute profoundly

to the understanding of the physical pro cesses in cosmic

x-ray sources.

High-resolution x-ray sp ectroscopy is the essential to ol

for diagnosing conditions in hot plasmas. It provides in-

formation for determining the temp erature, density, ele-

mental abundance, and ionization stage of x-ray emitting

plasma. The high sp ectral resolution of Chandra isolates

individual lines from the myriad of sp ectra lines which

would overlap at lower resolution. Furthermore, it en-

ables the determination of ow and turbulentvelo cities,

through measurement of Doppler shifts and widths.

Disp ersive sp ectroscopyachieves its highest resolution

for spatially unresolved (p oint) sources. Thus, OTG ob-

servations will concentrate on stellar coronae, x-ray bina-

Figure 18. X-ray image of the center of the galaxy M31.

ries, and active galactic nuclei. Figure 20 illustrates with

Image courtesy S. Murray.

an ACIS-S image of the sp ectra disp ersed by the HETG

during observations of HR1099. Figure 21 shows the line

rich extracted sp ectrum. Figure 22 shows a similarly ex-

tracted sp ectrum from LETG observations of Cap ella.

HEG

MEG

Figure 20. Image of the sp ectra disp ersed by the HETG

during observations of HR1099. Image courtesy Dan

Dewey and HETGS team.

It is interesting that the use of the zeroth order image

for the observations of extremely bright sources, which

would otherwise saturate the detectors and/or the teleme-

try, has proven quite useful. The utility for such obser-

vations is illustrated in Figure 23 where b oth the jet and

the central source of 3C273 are clearly resolved.

Besides observing the emission sp ectra of cosmic

Figure 19. A mo derately deep (100 ksec) Chandra im-

sources, Chandra high-resolution sp ectroscopy will also

age showing dozens of faint x-ray sources. Image courtesy

prob e the interstellar medium through its absorption of

R. Mushotzky.

x-rays from bright sources. Such observations of absorp-

tion sp ectra provide information on b oth interstellar gas 11

LETG+HRC-S 3C273 and Jet

Figure 21. The MEG sp ectrum of HR1099. Image cour-

tesy Dan Dewey and HETGS team.

Figure 23. Image of the disp ersed sp ectrum, including

zeroth order, of 3C273. The jet is clearly resolved in the

lower right hand p ortion of the gure. The six spikes

emanating from the central image are due to disp ersion

by the facet holders. Image courtesy Jeremey Drake and

LETGS team.

APPENDIX A. CHANDRA WEB SITES

The following lists several Chandra-related sites on the

World-Wide Web (WWW). Most sites are cross-linked to

one another.

Chandra X-Ray Center ASCA EUVE http://chandra.harvard.edu/

J.J. Drake

(CXC), op erated for NASA by the Smithsonian As-

trophysical Observatory (SAO).

Figure 22. Cap ella sp ectrum from LETGS. Image cour-

tesy Jeremey Drake and LETGS team.

http://wwwastro.msfc.nasa.gov/xray/axafps.html

Chandra Pro ject Science, at the NASA Marshall

Space Flight Center (MSFC).

and dust | the latter through the analysis of extended

40,41

x-ray absorption ne sturcture (EXAFS) and x-ray

http://hea-www.harvard.edu/HRC/

absoprtion near-edge structure (XANES).

Chandra High-Resolution Camera (HRC) team, at

the Smithsonian Astrophysical Observatory (SAO).

5. CONCLUSION

http://www.astro.psu.edu/xray/axaf/axaf.html

The Chandra X-Ray Observatory is p erforming as well, if

Advanced CCD Imaging Sp ectrometer (ACIS) team

not b etter, than anticipated | and the results are serving

at the Pennsylvania State University (PSU).

to usher in a new age of astronomical and astrophysical

discoveries.

http://acis.mit.edu/ Advanced CCD Imaging Sp ec-

trometer (ACIS) team at the Massachusetts Institute

of Technology (MIT).

6. ACKNOWLEDGEMENTS

http://www.ROSAT.mpe-garching.mpg.de/axaf/ We recognize the e orts of the various Chandra teams

Chandra Low-Energy Transmission Grating (LETG) whichhave contributed to the success of the observatory.

team at the Max-Planck Institut fur extrater- In preparing this overview, wehave used gures drawn

restrische Physik (MPE). from their work. 12

M. A. Gummin, eds., Proc. SPIE 3114, pp. 11{25, http://space.mit.edu/HETG/ Chandra High-

1997. Energy Transmission Grating (HETG) team, at the

Massachusetts Institute of Technology (MIT).

7. M. C. Weisskopf and S. L. O'Dell, \Calibration of

the AXAF observatory: Overview," in Grazing Inci-

http://hea-www.harvard.edu/MST/ Chandra Mission

dence and Multilayer X-Ray Optical Systems,R.B.

Supp ort Team (MST), at the Smithsonian Astro-

Ho over and A. B. Walker, eds., Proc. SPIE 3113,

physical Observatory (SAO).

pp. 2{17, 1997.

http://ipa.harvard.edu/ Chandra Op erations Con-

8. S. L. O'Dell and M. C. Weisskopf, \Advanced

trol Center, op erated for NASA by the Smithsonian

X-ray Astrophysics Facility (AXAF): Calibration

Astrophysical Observatory (SAO).

overview," in X-Ray Optics, Instruments, and Mis-

sions,R.B.Hoover and A. B. Walker, eds., Proc.

http://ifkki.kernphysik.uni-kiel.de/soho EPHIN

SPIE 3444, pp. 2{18, 1998.

particle detector.

9. J. J. Kolo dziejczak, R. A. Austin, R. F. Elsner,

M. K. Joy, M. E. Sulkanen, E. M. Kellogg, and

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