The Chandra X-ray observatory, one of NASA's "Great Observatories," provides high angular and spectral resolution X-ray data which is freely available to all. In this review I describe the instruments on chandra along with their current calibration, as well as the chandra proposal system, the freely-available Chandra analysis software package CIAO, and the Chandra archive. As Chandra is in its 6th year of operation, the archive already contains calibrated observations of a large range of X-ray sources. The Chandra X-ray Center is committed to assisting astronomers from any country who wish to use data from the archive or propose for observation

Smith, Randall K.

NASA Technical Reports Server

Astrophys Space Sci 
DO1 10.1007/~10509-006-9193-~ 
The Chandra X-ray Observatory: An Astronomical Facility 
Available to the world 
3 Randall K. Smith 
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Received 2 May 2006 / Accepted: 2 June 2006 
@ Springer Science+Business Media B.V. 2006 
Abstract The Chundru X-ray observatory, one of NASA’s 
“Great Observatories,” provides high angular and spectral 
resolution X-ray data which is freely available to all. In this 
review I describe the instruments on chandra along with 
tem, the freely-available chundra analysis software package 
CIAO, and the chandm archive. As chandra is in its 6th year 
of operation, the archive already contains calibrated obser- 
vations of a large range of X-ray sources. The chundra X- 
ray Center is committed to assisting astronomers from any 
country who wish to use data from the archive or propose for 
observations. 
Keywords 
To maximize the scientific return from 
sions, NASA created a science center fo 
would be responsible for c 
all instruments, organizing 
the mission archi 
located in Gamb 
entists hand 
2. 
the Hubble Space Telescope, was launched 
after being deployed Chandru used a secondary rocket 
st it into a highly elliptical orbit. Currently, Chundra’s 
the Moon), well outside the Earth’s radiation belts, and 
1. Introduction 
The Chalzdra X-ray Observatory 
launched on the Space Shuttle 
planning and development. ed to be a 
scopy. Chandru’s mir- 
a level comparable to 
antly better than any previous 
cannot be done within or near the radiation belts). As a result, 
Chandru regularly achieves an observing fraction of nearly 
70%, extremely high for satellite observations. 
2.1. Mirrors 
The Chundru High Resolution Mirror Assembly (HRMA) 
is responsible for its sub-arcsecond imaging. The HRMA 
consists of four pairs of nested mirrors in a Wolter Type-I 
NASA’s “Great Observatory” mis- 
ubble Space Telescope, the Compton 
Gamma Ray Observatory, and the Spitzer Space Telescope. 
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design. The mirrors are made of iridium-coated glass, and 57 - - 
R. K. Smith 
NASA’s Goddard Space Flight Centcr, Grccnbclt, MD 20771 and 
Johns Hopkins University, 3400 North Charles Street, Baltimore, 
MD 21218 
the entire system has a focal length of 10.07 meters. The 
unobstructed geometric effective area is 1145 cm2; in flight 
the total effective area is 800 cm2 at 0.25 keV, 400 cm2 at 
1s 
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e-mail: rsmith@milkyway.gsfc.nasa.gov 5 keV, and 100 cm2 at 8 key The ghost-free field of view has 61 
9 Springer 
https://ntrs.nasa.gov/search.jsp?R=20070020002 2019-08-30T01:03:56+00:00Z
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Astrophys Space Sci 
a 30’ diameter, which encompasses the entire ACIS detector 
array and most of the HRC (see below for details about the 
Chandra detectors). 
The primary differences between Chandra and XMM- 
Newton (Gabriel, Guainazzi and Metcalfe, 2005) are found 
in their mirrors. XMM-Newton has three X-ray telescopes, 
each with 58 nested gold-coated nickel shells, while Chan- 
&a’s single HRMA has only 4 mirror pairs made of precisely 
formed iridium-coated glass. As a result, XMM-Newton has 
a larger effective area than Chandra, but worse angular 
resolution. 
The HRMA was calibrated at the NASA/Marshall Space 
Flight Center X-ray Calibration Facility pre-flight, and these 
measurements have been used to cross-check the in-flight 
calibration with excellent results. More information about 
the HFWA and the latest calibration can be found at 
http://cxc.harvard.edu/cal/Hrma. 
2.2. ACIS 
The Advanced CCD Imaging Spectrometer (ACIS) (Garmire 
et al., 2003) is the most-used detector on Chaizdra. Unlike 
XMM-Newton, Chandra has only a single HRMA and the 
observer must chose the detector to put into the focal plane. 
The ACIS is a set of 10 CCDs arranged in two patterns: a 2 
x 2 array with a 17’ square field of view used primarily for 
imaging, and a 1 x 6 array with a 8.5’ x 51’ field of view, used 
primarily for grating observations. The user can select up to 
6 CCDs to be used for any observation (the 6 CCD limit is 
set by the power available from the solar panels). The CCDs 
are operated in single-photon counting mode, which provides 
tion is handled by the CXC as part of the standard pipeline 
analysis (see Section 3.2). For more information about the 
ACIS, please see http: //cxc . harvard. edu/proposer/ 
POG/html/ACIS.html. 
2.3. HRC 
The High Resolution Camera (HRC) (Murray et al., 2000) 
instrument consists of two microchannel plate detectors. The 
HRC-I is designed for imaging and has a 30’ square field of 
view, while the HRC-S is designed to be used with LETG 
grating (see Section 2.5) and has a 6‘ x 99’ field of view. As 
these are microchannel plate detectors, they have no useful 
energy resolution but do have better timing and a smaller 
pixel size than the ACIS detectors. In addition, they do not 
suffer from photon pileup as happens in CCD detect0 
the event rate becomes large. 
toTs is 16ps, in flight the actual event 
approximately 4 ms due to a wiring e 
sible in special cases to regain 
cial instrument mode. For mo 
please see http : //cx 
html/HRC . html. 
Although the intrinsi 
ctors, Chaizdra has a two different 
an be inserted, although only one can 
e High Energy Transmis- 
G) (Canizares et al., 2000) was designed 
modest @/A E - 50 spectral resolution) and limited timi 
information. 
illuminated’ (BI) CCDs. The FI CCDs have th 
tures mounted facing the mirrors, whil 
inverted. As a result, the BI CCDs have 
at low energies since no X 
However, they also have 
lower spectral resolution. 
the H CCDs with 
sedp with the ACIS-S detector, although it could be 
ACIS-I or the HRC (in practice this is not useful 
fic observations). The HETG itself contains two 
ifferent gratings, the High Energy Grating (Ah = 0.012& 
The ACIS contains 8 ‘front-illuminated’ (FI) 
n the Chandra 
resolution now 
er. The CXC has de- 
t this effect as part of 
The pixel si ACIS CCDs is 0.492” on a side; as a 
result the ACIS barely samples the full HRNlA resolution. To 
reduce the effect of pixel-to-pixel calibration uncertainties, 
and to avoid damaging the on-axis pixels, Chandra dithers 
in a 16” lissajous pattern during all ACIS observations. A 
separate instrument using an optical CCD called the Aspect 
Camera is used to point Chandi-a and to remove the effect 
of dither from the final images and event files. The reduc- 
and the Medium Energy Grating (Ah = 0.023A). These two 
gratings are arranged so that their dispersion axes are an- 
gled -10” from each other, so that both dispersed spectra 
can be independently extracted. The HEG spectral range is 
1.2-15&0.8-10 keV), while the MEG spectral range covers 
2.5-31&0.4-5 keV). The HEG has twice the resolution of 
the MEG, balanced by a smaller effective area. 
Unlike theXMM-Newton RGS, the Chandra gratings lose 
resolution quickly for sources larger than a few arcseconds. 
For point sources, however, the HETG combines high spec- 
tral resolution with sufficient effective area to measure spec- 
tra from both Galactic and extragalactic sources. 
The HETG has significantly overlapping orders, but these 
can be easily separated using the energy resolution of the 
ACIS-S CCDs. By design, the HETG suppresses the even 
order peaks in order to maximize the first order intensity; 
for bright sources, the 3rd order spectrum can be used and 
provides even higher resolution. For more information about 
B Springer 
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Astrophys Space Sci 
162 the HETG, please see http : //cxc . harvard. edu/ pro- 
163 poser/POG/html/HETG.html. 
1.54 2.5. LETG 
16; The Low Energy Transmission Grating (LETG) (Predehl 
166 et al., 2000) complements the HETG, providing high resolu- 
167 tion spectra of point sources in the range 1.2-175&0.07-10 
168 keV). Unlike the HETG, the LETG has only a single grating, 
169 with resolution Ah = 0.05A. The LETG can be used with 
170 either the HRC or the ACE. The HRC-S is needed in order 3.2. Calibration 
171 to get the wide spectral coverage out to 175& but the data 
~n are then limited by the lack of energy resolution and the rela- 
173 tively high background in the HRC-S. As a result, a number 
174 of observations have been done using the LETG in combina-' 
175 tion with the ACIS-S detector. When used with the ACIS-S, 
176 the spectral range decreases to 1.2-65A. However, by design, 
observation, and then it is passed to the Operations Control 
Center (OCC), also located in Cambridge, MA at MIT to be 
uploaded to the sateilite. Uploads are done approximately 
once per day via NASA's Deep Space Network; unscheduled 
or continuous contacts are available as needed. This, along 
with the need to have 3 days worth of observations uploaded 
at all times, limits Chandra's ability to respond to a Target 
of Opportunity request; in most cases a 24-48 hour response 
is the best possible. 
The CXC calibration team both collects calibration data from 
the individual instrument teams (ACIS, HRC, HRMA, LETG 
& HETG) and reduces calibration data itself. Calibration 
data is regularly obtained from set-aside tim 
Chandra's performance and to address new 
1x0 background of the HRC-S. Complete information ab 
181 lution, the orders cannot be resolved directly. As a result, 
1 ~ 4  they have to be determined iteratively (if possible) or sim- 
1s ply modeled. Unlike the HETG, orders up to nz = 10 can 
1% contribute to the final result, so in many situations this is- 
ative positions are kn 
different detect 
resolution is 
ve areas for the 
f the calibration group is compiled in the cal- 
190 3. The Chandra X-ray center eline and the analysis software. 
IYZ ing Chandra and maximizing its scien 
195 3.1. Proposals sional programmers at the CXC, working under the direction 
of X-ray astronomers to develop software that would (a) pro- 
cess data from Chandra into calibrated event lists, images, 
and spectra and (b) allow users to analyze these data easily. 
code and precompiled for a range of computer platforms, in- 
cluding Linux, OS X, and Solaris. System requirements are 
relatively modest, and useful work can be done with a 300 
MHz machine with 256 MI3 of RAM and 5 GB of disk space. 
If needed, the CXC will mail a CD containing CIAO (and 
the CALDB) anywhere in the world. 
CIAO contains over one hundred individual programs to 
perform various operations. However, to guarantee unifor- 
mity of interface and application, almost all of these tools 
s once per year, with 
in March. Proposals are 
198 welcomed 
199 are evaluat ndent international group of as- 
2w tronomers. ription of the information needed 
201 for a proposal can be found at http : //cxc . harvard . edu/ 
202 proposer/POG/index . html and is also available in hard- 
203 copy. 
201 Once a year's list of targets has been selected, the CXC 
2 s  mission operations team sorts the targets into a long- and 
20(1 short-term schedule. The principal investigator for each ob- 
Z~ servation is contacted to confirm the final settings for the 
MU 
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257 use the same set of code libraries. One such library is the 
2% “Data Model” (DM), which CIAO tools use to access the 
2 s  raw data files. In practice, tools that use the DM allow users 
2 ~ )  to filter event data “on-the-fly,” selecting only particular re- 
261 gions or times of interest for any individual process, such as 
262 creating a lightcurve or extracting a spectrum. Many CIAO 
263 tools also use a standard parameter interface library, similar 
264 to that used by the IRAF package. Recently, the CIAO devel- 
265 opers have incorporated the S-lang scripting language (see 
266 http : //www . s-lang . org for more information about S- 
267 lang) into some CIAO tools and developed S-lang interfaces 
268 to many CIAO libraries. This allows users to easily develop 
269 their own CIAO tools which access the same libraries as of- 
270 ficial CIAO tools. These can be submitted to the CXC for 
271 general use (and in some cases official incorporation into 
272 CIAO) at the website http: //cxc . harvard. edu/cont- 
273 soft/soft-exchange .html. 
274 CIAO is extensively documented both on the CXC web- 
275 site ( http : //cxc. harvard. edu/ciao) and within CIAO 
276 itself via the ahelp command, which returns a descrip- 
277 tion for every CIAO command as well as a large number 
278 of CIAO terms (e.g. ahelp dmf iltering for information 
279 about using DM filters). In addition, the website also con- 
ZRO tains “threads” with step-by-step descriptions of common 
2R1 tasks as well as “why threads” describing why particular 
282 tasks are needed. An active helpdesk is also supported where 
ZQ CIAO experts will answer questions submitted via email or a 
2% web interface. Approximately twice per year, 2-3 day CIAO 
2s workshops are held where CXC scientists describe how to 
2% reduce X-ray data using CIAO and users can get hands-on 
experience with immediate assistance from these experts. 
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2s 3.4. Archive 
uw All Chandra observations are stored in the Chandra archive, 
290 in a number of formats ranging from very nearly the raw 
291 telemetry to calibrated event lists, images, and spectra. Ac- 
292 cess to this data is determined by its purpose. Data taken 
293 for calibration is immediately available, while guest ob- 
2% server observations are generally restricted to the prin- 
295 cipal investigator for one year. Chandi-a’s Director has 
a pool of timediscretionary available for unexpected op- 
Astrophys Space Sci 
portunities; these data are also immediately available to 
all. 
Access to the archive is normaIIy done via the 
Internet, using either the standalone “chaser” java 
applet or the web interface “webchaser” available 
at http://cda.harvard.edu/chaser/main Entry.do. 
The main archive is stored at the CXC in Cambridge, but a 
mirror site is also available in the UK at the University of Le- 
icester. Chandra datasets vary in size from - 50 MI3 to over 
2GB, depending on the detector used, the source flux, and 
the length of the observation. If network speed is a limiting 
factor, a CD-ROM can be made at the CXC and sent to the 
user. 
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4. Conclusions 309 
Chandra’s launch opened the new era of X-ray astronomy, 
adding capabilities in the X-ray that could finally match op- 
tical telescopes in angular and spectral resolution. The open 
proposal process allows any astronomer to use Charzdra’s 
capabilities, and the relatively modest computing require- 
ments combined with the open archive allow relatively easy 
access to Chandra datasets. The CXC has actively supported 
the COSPAR X-ray astronomy schools in the past, and re- 
mains committed to supporting the use of Chandra around 
the world. 
Acknowledgements The author worked at the CXC from la 
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on any issue. 
References 
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es, C., Garmire, G., et al.: 332 
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