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The EXOSAT Data Archive at the HEASARC
L. Angelini, N. E. White
The European X-ray Observatory Satellite (EXOSAT) was operational from May 1983
to April 1986 and in that time made 1780 detailed observations of a wide
variety of astronomical objects. This article is an overview of the EXOSAT
archive available from the HEASARC. The first part gives a historical overview
of the EXOSAT Observatory, describing the EXOSAT instrumentation, operational
phase and post-operational data. The following sections provide a description
of the different levels of data currently present in the HEASARC archive and
their format. Future plans for reformatting the raw data to FITS format are
EXOSAT was launched on 1983 May 26 from the Vandenberg complex in the USA on a
Thor-Delta rocket and put in a highly eccentric orbit (e ~ 0.93) with a
90.6 hr period and an inclination of 73deg.. The mission lifetime was
ultimately limited by orbital decay. The spacecraft performed within
specifications for almost three years. On 1986 April 9 a failure in the
attitude control system caused the loss of the spacecraft. The natural decay of
the orbit caused EXOSAT to re-enter on 1986 May 6.
The scientific payload consisted of two low energy imaging telescopes (LEITs or
LE), sensitive over the 0.05-2 keV band, a medium energy (ME) proportional
counter and a gas scintillation proportional counter (GSPC) providing a
coverage over the 1-50 keV band. At the focus of each telescope was a moveable
instrument bench upon which a channel multiplier array (CMA) and a positional
sensitive detector (PSD) were located. A transmission grating spectrometer
(TGS) could be interposed into the light path of each telescope. The high
energy instruments, ME and GSPC, were collimated telescopes, with a full width
half maximum field of view , FOV, of 45 arcmin. The three instruments (LE, ME
and GSPC) were complimentary and designed to give a complete coverage over a
wide band pass of 0.05-50 keV.
The low energy imaging telescopes gave both medium quality spectral resolution
using the gratings, or broad band filter spectroscopy. The field of view was
about 2deg., with a peak effective area of 10 cm2 (which was reduced
by a factor of ten when the gratings were utilized). The ME detector gave
spectra with 20% resolution at 6 keV, with a total effective area of 1600
cm2. Each half of the ME detector array could be offset to monitor
the particle background. The gas scintillation proportional counter provided a
factor of 2 improved spectral resolution in the 2-30 keV band, but with a
factor of 10-20 less effective area than the ME. The optical axes of the four
telescopes were coaligned.
An on board computer (OBC), programmable from the ground, preprocessed and
compressed the data. The flexibility given by the OBC and the capability to
load new OBC programs, enhanced the quality of the science obtained, by writing
new data modes to address science discoveries not foreseen before launch. It
also proved invaluable to work around problems with the instrumentation and the
spacecraft. Figure 1 shows an exploded view of the EXOSAT satellite.
The orbit of EXOSAT was quite different from that of any previous X-ray
astronomy satellite. The initial apogee was 191,000 km and the perigee 350 km
(Parkes 1985). The science instruments were operated above 50,000 km, outside
the earth's radiation belts. This allowed scientific operations for up to 76 hr
per 90 hr orbit, without interruption. EXOSAT was visible from the ground
station at Villafranca in Spain for practically the entire time that the
science instruments were operated and there was no need for any onboard data
storage. EXOSAT operated in a 3-axis stabilized mode. A propane cold-gas
thruster system was used for both slew maneuvers and fine pointing. The
attitude was controlled using one of two star trackers, three gyros, and a sun
sensor and could be maintained to within ~1 arcsec. The first 8 weeks after
launch were dedicated to a performance verification phase. After this, the
regular guest observer program was undertaken. By the end of the operational
phase, four announcements of opportunity had been made with 98 %, 94 %, 64 %
and 18 % of each program completed. During these 3 years, scientists around the
world used EXOSAT to study most classes of X-ray sources and made many new
discoveries. Perhaps the most notable were the discovery of QPO from low mass
X-ray binaries, soft excesses from AGN, the red and blue shift iron K line from
SS433, many orbital periods from low mass X-ray binaries, and several new
transient sources. In ~ 3 years operation the total volume of data was about ~
160 Gbytes. The public availability of these data contributed to many notable
systematic studies of individual classes. The EXOSAT archive continues to be a
heavily used resource and is still yielding new results.
Figure 1: An exploded view of the EXOSAT Observatory
The Low Energy Imaging Telescopes
The two LEIT telescopes (de Korte et al (1981)) and detectors assemblies were
identical. The telescopes consisted of double nested gold coated Wolter type I
grazing incidence optics, with a focal length of 1.1 meter and an outer
diameter of 0.3 meters. The telescope parameters give a high energy cut-off of
~2 keV. The on-axis half energy width of the point spread function is 24 arc
seconds, which degrades to 4 arc minutes 1 degree off-axis. Vignetting in the
telescopes reduces the off-axis effective area to 45% its peak value 1 degree
In the focal plane of each telescope either a channel multiplier array, CMA, or
a position sensitive proportional counter detector, PSD, was interchanged.
Also, a transmission grating could be inserted behind each telescope and the
dispersed spectrum imaged by the CMAs. The gratings were 500 lines mm-1 in one
telescope (CMA2) and 1000 line mm-1 in the other (CMA1). The
spectral resolution is 2Å and 1Å respectively for energies >
0.25 keV, and 5Å at 304Å in both telescopes.
The field of view of each CMA covers a diameter of 2deg.. A CMA has no
intrinsic energy resolution in the X-ray band, but a number of different
filters gave coarse spectral information (analogous to UBV photometry). The
choice of filter dictates the energy response with the overall energy window
covered by the filter combinations ranging from 0.05 to 2.0 keV. The CMAs were
sensitive to ultraviolet photons and this caused contamination for observation
of bright O and B stars, pointed or serendipitous in the field. Filters were
used to determinate the degree of contamination. Observations obtained with the
boron filter were free of UV contamination and the Aluminum/Parylene is
relatively immune to it except for fields with the very brightest and earliest
stars. Also the sum signal distribution of events in the CMA provides a crude
method of differentiating between X-ray and UV sources. The most commonly used
filters during the mission were 3000 Lexan , Al/P and boron.
The CMA particle background counting rate was typically 8 x 10-6
count s-1 pixels-2 (a pixel is 4 arcsec) in the central
region. The background counting rate depends on the strength of the solar wind.
In 90% of the observations the average background counting rate is within a
factor of 2 of the quiescent value. The average source detection threshold for
a 104 second exposure (a typical minimum observation time) within
the central 12 arc minute radius region of the detector, using the 3000A lexan
filter, was 2 x 10-3 count s-1. For observations longer
than a few thousand seconds the sensitivity of the CMA is background limited.
Source position can be estimated to 6 arc seconds or 8 arcsec error radius at
67% or 90% confidence level respectively within the central 12 arc min.
There were a number of notable problems with the instrumentation associated
with the LEITs. Both PSDs failed early on during the performance verification
phase. One of the CMA (CMA2) failed on 1983 October 28. The mechanism to insert
the grating behind the other telescope (CMA1) failed in 1983 September 15,
eliminating this spectroscopic capability. Only 24 sources were observed with
the gratings. These spectra provided some of the highest quality spectra
obtained at that time of nearby stars (e.g., Capella), white dwarfs (e.g.,
HZ43) and Sco X-1. The surviving CMA functioned well until the end of the
The Medium Energy Proportional Counter Array
The Medium Energy (ME) instrument consisted of an array of eight proportional
counter, with a total geometric area of 1600 cm2 and 45 arcmin FWHM
field of view, providing spectral and temporal data in the 1-50 keV energy
range (Turner, Smith and Zimmermann 1981). Each proportional counter had two
gas chambers separated by a 1.5 mm beryllium window with argon in the top layer
and xenon in the lower. Each proportional counter used a multi-wire design with
a [[Delta]]E/E of 21(E/6 keV)-0.5% FWHM for the argon chambers and
18(E/22 keV)-0.5% FWHM for the xenon chambers. The argon and xenon
spectra were pulse height analyzed into 128 channels each, sensitive to 1-20
keV and 5-50 keV energy ranges, respectively. The ME background was very stable
and dominated by particle events from the solar wind and radioactive lines
caused by the decay of residual plutonium in the Beryllium windows and detector
bodies. The background count rate per detector were in the argon chamber 2.4,
4.3 and 9.4 count/s in the energy range 1-6, 1-10 and 1-20 keV respectively and
40.6 and 59.1 count/s in the xenon chamber in the 10-30 and 10-50 keV energy
range, respectively. Occasional background flares occurred in both the aligned
and offset halves caused by enhancements in the solar wind. A major solar storm
typically happened every six months, causing the background to increase by
several orders of magnitude. The detectors were turned off during these
To optimize the background subtraction each half of the ME array detector could
be offset (also known as an `array swap') from the source, pointing at a
source-free region of the sky to monitor the particle background. The offset
half was alternated every few hours. Since the background obtained from the
offset half was slightly different for the half of the detector on source,
difference spectra were created to correct this effect. Background was also
obtained using the slew on and/or off the source. This technique was used when
the detector halves were coaligned and no array swaps made during the
observation. In 1985 August 20 one of the detectors in half-1 failed.
Occasionally because of small detector breakdown and/or reduction in the gain,
observations were carried out with one or more detectors off for a few hours.
An important component in the operation of the ME instrument was the usage of
the OBC. Depending on the objective of the observation, the OBC programs traded
time resolution against spectral information. Depending on the telemetry load
and the OBC programs running for the other two experiments, two or three ME
programs could be run simultaneously. A typical ME observation was carried out
with a primary spectral oriented program and a secondary high time resolution
program. One of the major problems with the ME+OBC setup was a dominant
deadtime effect mainly due to the OBC sampling.
The Gas Scintillation Proportional Counter
The gas scintillation proportional counter, GSPC, (Peacock et al (1981) on
EXOSAT has a [[Delta]]E/E of 4.5(E/6 keV)-0.5% FWHM, (a factor of 2
better than the ME experiment) with a peak effective area of ~100
cm2. The energy spectra were pulse height analyzed into 256
channels. Three different electronic gain modes were used: gain 1 = 2-32 keV,
gain 2 = 2-16 keV and gain 0.5 = 2-64 keV. The latter was only used on the very
bright source Sco X-1. Variations in the intrinsic detector gain caused by
temperature changes were removed using two background line features at 10.54
and 12.70 keV caused by fluorescence in the lead collimator and the radioactive
decay of residual plutonium in the beryllium window. In addition a Xenon L
feature at 4.78 keV could also be used for bright sources to calibrate the
The particle background rejection used burst length discrimination, which
rejects events which have exceptionally long or short duration. The background
shape spectrum remains constant during the short timescale variations, but
shows small changes on longer timescales. The standard OBC mode used in the
GSPC gives 256 channel spectra every 8 seconds. Higher time resolution was used
only for bright sources and if the telemetry needs of the other experiments was
low. The GSPC worked perfectly for the entire mission.
During the 4-year post-operational phase the EXOSAT observatory staff
concentrated on producing a database of EXOSAT results and data products
accessible via computer network by the whole community. During the 1987-1990
period two parallel efforts were undertaken. The first was creating basic
products, lightcurves, spectra and images, for all the three experiments. The
second was developing a database management system (later known as EXOSAT DBMS)
not only to make those result products available, but also giving the
possibility to manipulate, cross-correlate and derive results from the
database. The computer resources available were a HP1000 running RTE operating
system used to process the data and a VAX/VMS system dedicated to create the
DBMS and provide the on-line access. The EXOSAT results database first went put
on-line in April 1989. This was one of the first on-line astronomical archive
systems to allow the on-line retrieval and analysis of scientific data. When
the HEASARC was established in 1990, the EXOSAT system was adopted as its
on-line system, and is now also in use at several other high energy
astrophysics sites around the world.
The products, created for each experiment, were obtained through automatic
processing. If the automatic analysis failed in creating reliable results, an
effort was made to correct the nature of the failure. The images, lightcurves
and spectra produced were in binary format designed to optimize disk space,
which was a major constraint at that time. The binary table extensions to the
FITS format were not well established when the EXOSAT database effort was begun
(1987). Therefore, the resulting files were not portable between different
operating systems, and intermediate ASCII files were used. The HEASARC has
taken all the EXOSAT data products and converted them to FITS. The FITS data
format was designed as general as possible to accommodate not only the EXOSAT
data but also data from other missions. Guidelines for the FITS format for
spectra, response matrices and lightcurves are described in the following
Legacy articles: Arnaud et al. 1992, George et al. 1992
and Angelini et al. 1993, respectively. To analyze the data the XANADU
software packages XSPEC (spectral fitting), XIMAGE (image analysis) and XRONOS
(timing analysis) have been modified to read the data in the new format. The
volume of all the products in the original binary format was about 1.7 Gbyte
for a total number of files of about 40100, which expanded by a factor of 2
after transforming in FITS. The data available from HEASARC archive have been
compressed using the standard unix `Z' compression to minimize network traffic.
All the header information present in the original binary file has been kept in
the current FITS header and stored in keywords. In some cases the original
value was changed because it was not compatible with the keyword definition. To
maintain the original information in all the EXOSAT FITS files, the original
header information has been preserved in a COMMENT card.
The LE Products
The LE automatic analysis consisted of:
1) generating an image for each filter used during an observation;
2) searching the image for point-like sources;
3) for each detection, generating a background-subtracted lightcurve;
4) for each image, generating a background lightcurve.
After each run, the image and the detections were visually inspected. The
source detection algorithm used is based on a sliding cell method. The size of
the search cell was such to maximize the sensitivity across the field of view.
When a source was detected its sky position (RA and Dec in 1950 coordinates),
count rate and error, significance of the detection, the X and Y image pixel
position, dead time correction and several other parameters were calculated.
The information recorded for each detected source was written as a unique entry
in a database together with the associated products. The integration time of
the background-subtracted light curve varies from source to source and was
chosen such that the average count per second per bin was about 0.3. The
automatic analysis did not include a test for UV contamination, therefore a
fraction of all the sources detected are due to such contamination.
The ME Products
The ME automatic analysis was designed to cope with the complex observing
configurations as well as the myriad of OBC mode used during the mission. For
each observation the automatic analysis creates the following products:
1) a source lightcurve background-subtracted in the energy range between 1-8
keV with a time resolution of 30 seconds;
2) a background lightcurve in the energy range 1-8 keV with a time resolution
of 30 seconds;
3) a multiband on-source background-subtracted lightcurve in the energy range
1-3.8 keV, 3.8-8 keV and 1-8 keV. The time resolution for those lightcurves
varied from observation to observation and corresponds to the maximum available
time resolution for the pulse height analyzed data. Typically this ranges
between 1-10 seconds.
4) a background-subtracted on-source spectrum integrated over the entire
observing interval. Spectra of burst sources were obtained excluding the burst
The number of files expected for a particular pointing observation depends on
how the data have been divided (either because different ME array
configurations were used throughout the observation, or because there was a
change in the OBC mode). Typically, different lightcurve files were created for
different array configurations as well as OBC configurations within one
observation. Spectral files obtained from different array configurations were
corrected for the difference spectra between the halves and then averaged
together. The spectra and lightcurves were obtained only using the argon
detectors sampled by the primary OBC mode (energy mode). Because of the limited
resources in available disk space, the automatic analysis did not create
lightcurves with high time resolution data.
The major problem in running an automatic process on the ME data was the
selection of the method for the background subtraction. This mainly depended on
the configuration used throughout an observation. Typically, background taken
simultaneously to the source spectra gave better results, but this was not
For about 30% of the observations, a number of corrections were required,
mostly to fix unsatisfactory background subtraction. The main reasons for this
were: a) flaring in the background; b) detectors which appear significantly
noisier than others during a particular observation; c) for those observation
with no simultaneous background, the standard or the slew background was not
adequate. Most of the problems were recovered by running the analysis
interactively on each problematic dataset. Quality flags from 1 to 5 were
assigned to give an indication of the of the overall quality of the ME
The GSPC Products
Although the GSPC was on during most of the mission life time, data products
were obtained only for those observations where the source ME count rate was in
excess of 5 count/s/half. Sources with lower count rates typically do not have
sufficient signal-to-noise to justify a GSPC analysis. The products created
with the automatic processing for the GSPC instrument included:
1) background subtracted lightcurves for the source in the energy range between
2-8 keV and 8-15 keV with the original time resolution (8 seconds in most
2) an average background-subtracted spectra for the entire observation of the
source (excluding any bursts that occurred).
The background subtraction was done using both the slew spectra (to determinate
the gain) and standard spectra (adjusted to the proper gain). Again, the data
were quality checked and a flag from 1 to 5 assigned to indicate the overall
reliability of the products.
The TGS Products
A total of 24 sources were observed with one or both transmission grating
spectrometers, from which 19 gave a useful spectrum. For each observation the
following products were created:
1) a background subtracted lightcurve, per filter, centered on the zero order
in a box 16 by 16;
2) multiple spectra (see later);
3) background spectra.
The spectra were extracted from the CMA images, typically using a mask of 40 to
50 pixels in width (on the Y-axis) and integrating perpendicular to the
dispersion direction (along the X-axis). The spectra generated in this way
consisted of a one-dimensional array of 2048 channels. A second set of spectra
consisted of the positive and negative orders added together, resulting in a
1024-channel spectra. The background spectra was estimated over the same mask
size in the image of a long blank-field exposure and renormalized to the source
image. The source spectra obtained with the TGS are not background subtracted.
Within an observation, the 2048- and 1024-channel spectra were accumulated for
each filter or change in position. In addition, when it was possible, a
simultaneous GSPC and/or ME spectra covering the same interval of time was
The automatic processing also performed spectral and timing analysis of the
products. The analysis results were included as parameters in the databases,
although parameters associated with timing and/or spectral fitting should be
regarded with more attention, because in same cases, the automatic processing
All the products and analysis results were made available from the EXOSAT X-ray
Observatory as databases using the EXOSAT DBMS. Below is the list of the
* EXO_LOG contains the complete list of all EXOSAT observations, configuration
and observing modes, and principal investigators.
* EXO_PUBS contains information about EXOSAT publications in refereed
* CMAIMAGE contains references about all the images created for each
observation using both low energy telescopes (CMA1 and CMA2). Associated
* LE contains the detections found within 6 arcmin from the pointing position
obtained with both low energy telescopes (CMA1 and CMA2). Associated products:
* CMA contains the complete list of detections over the full field of view
obtained with both low energy telescopes (CMA1 and CMA2). Associated products:
* GS contains the results obtained from the GSPC only for sources with an ME
count rate of at least 5 count/s/half. Associated products: spectra, multiband
* ME contains the results obtained from the ME argon detectors. Associated
products: spectra and multiband lightcurves
* TGS contains the results obtained from the TGS instruments. Associated
products: multi spectra (L and R or L+R order), lightcurves
Each database contains several parameters specific to the instrument and
associated products, and others obtained by cross-correlating the different
databases with stellar catalogs.
The Raw Data
Data obtained from the four instruments were originally distributed to
observers as Final Observation Tapes, FOT, in a form of one or more 1600 BPI
tapes per instrument. A FOT contains all the scientific, housekeeping and
calibration data for a single observation and can be used to make a more
detailed analysis. There were a total of 8340 1600 BPI tapes , for a volume of
about 160 Gbyte of data.
The complete archive of tapes was kept at the ESA European Space Operations
Center, ESOC, in Germany. Requests for copies by the community were sent to
ESOC which sent back a copy to the observer. This process typically took 4-6
weeks. In 1992 ESIS/ESRIN/ESA (Italy) took the responsibility to copy all the
FOTs into a more permanent optical jukebox archive and distribute the tapes to
the community. An agreement made between ESA and HEASARC allowed HEASARC to
obtain a copy of all the FOT data, in return ESA obtained a copy of the data
reformatted to FITS format by the HEASARC. From ESRIN the data arrived at
HEASARC via EXABYTE magnetic tape, each of them containing about 1.8 Gbyte of
data (usually compressed using the `zoo' compression scheme). The incoming data
was loaded into the HEASARC rewriteable jukebox.
The HEASARC archive currently contains two different sets of the data files:
1) the data product files in FITS format and
2) the original raw data from the Final Observation Tape, FOT.
The Data Product File Contents
The FITS EXOSAT lightcurves in the HEASARC archive have two different layouts
which reproduce exactly the content of the original products. The ME and GS
experiment data are in a bintable containing two columns only: RATE and ERROR.
The RATE is the background subtracted count rate within the integration time,
corrected for collimator efficiency and dead time. Gaps are padded correctly.
The time associated with each bin can be calculated as TIMEZERO + ((n-1)
* TIMEDEL), where TIMEZERO and TIMEDEL are header keywords which represent the
zero time in mission elapsed time and the integration time, and n is the
bin number. The LE and TGS lightcurve data are in a bintable containing four
columns TIME, RATE, ERROR, and FRACEXP. The TIME column contains the residual
time from an offset stored in the TIMEZERO and the FRACEXP is the fractional
exposure for each bin. The RATE is the background subtracted count rate within
the integration time, corrected for vignetting and dead time.
The FITS EXOSAT spectral files have similar formats for the GS, ME and TGS
experiments. The ME file, in addition to the spectral data stored in the first
extension, also has a second extension containing information on the specific
detector used during the observation. During the re-formatting stage for each
ME and GS spectrum, the corresponding response matrices were generated and
formatted into FITS format.
The FITS EXOSAT images are stored using the FITS primary array. The images are
2048 x 2048, and are made using the linearized detector coordinates. The
projection is `tangent plane' and the images do not have the y-axis direction
aligned with celestial north. The FITS header contains two coordinates systems.
The first is the so called sky-coordinates system which maps each pixel in the
array into a RA and Dec in the sky. This system is a FITS standard and uses
the keywords CRPIXn, CDELTn, CROTAn, CRVALn, and CTYPEn where values of n are 1
and 2. The secondary system, which is not a FITS standard convention, describes
the detector characteristics. This system uses the keywords DLMINn, DLMAXn (the
minimum and maximum values for the X and Y axes), DLABELn (the type of
coordinates), DDELTn (the increment), DUNITn (unit for DDELTn), DRPIXn (value
of CRPIXn in this secondary frame defined by DLMINn, DLMAXn), and DOPTICn (X
and Y coordinates of the optical axis in the secondary frame).
The File Naming Convention for the Data Products
The product file names are composites of 6-character strings, a letter followed
by a 5-digit number, generated automatically by the computer, based on the
start time of the observation. The first character, which identifies the
product type has a different convention for the 3 experiments, but within an
experiment different products associated with the same observation share the 5
digit number. The LE images have `a' as the first character, `b' is used for
the background lightcurve, and from `c' on, alphabetically, are the lightcurves
for the different sources detected in the field of view. The ME 30-second
lightcurves have `d' as first character, and the multi-band use `a' (lower
energy),'b' (higher energy), and 'c' (total band). The background lightcurves
have `r' as a suffix and the spectra use `s.' GS lightcurves have the first
character `d' or `c' depending on the energy range and `s' for the spectra.
The Raw Data
The EXOSAT raw data were made available from the HEASARC in May 1994. Data can
be immediately retrieved from the anonymous FTP account at
heasarc.gsfc.nasa.gov. All the data have been compressed using the
`zoo' compression program, as originally provided by ESA (for these data, this
algorithm typically gives a factor 2 smaller files than the standard unix
compress). The zoo software is available from the legacy anonymous FTP
under the directory /software/zoo.
The database EXOFOT contains 6614 entries, each of them corresponding to a FOT.
It does not contain the coordinate information. To search for data within this
database, it is recommended to first use the EXOSAT observation log database
(EXOLOG) to retrieve the experiment, target name, start and stop time. With
this information, search in the EXOFOT database using the parameters TIME
and/or TIME END with a time window which includes the time value found in the
EXOLOG database for the parameters FOT TIME and FOT END. Currently we are
merging the EXOLOG and EXOFOT database such that the filename of FOT will be a
new parameter associated to an entry in the EXOLOG database. The matching is
quite tedious because it is based only on the start and stop time information
stored in the log which can be different from the FOT start time and stop
The data format is the original binary format, the description of which can be
found in the rather large FOT-handbook. This document was distributed by ESA,
with the data to the original PI. It does not exist in electronic form. On
request, a copy can be obtained by contacting us via our Feedback form.
Rather than rewrite software to access
these data it is recommended to use the EXOSAT Interactive Analysis, IA,
System. This can be obtained from the anonymous ftp site at ESTEC/SSD
(astro.estec.esa.nl). The IA system is fully described in the
companion article by Parmar et al. in this Legacy issue.
The HEASARC plans to reformat the EXOSAT raw data to FITS format over the
coming 2 years. The effort will make use of the IA system to write out the data
in FITS format for each of the modes used, at the highest time resolution
available. These FITS data will then be moved to the anonymous FTP area. This
effort began in July 1995 with the LE data, writing FITS files in the same
event format as is used by the imaging instruments on ROSAT, ASCA and Einstein.
The ME standard mode data will be reformatted next, followed by the GSPC. The
most commonly used modes will be done first, followed by the more complex,
rarely used modes. As each stage is completed, regular updates will appear in
future issues of Legacy.
Every member of the EXOSAT Observatory team gave a substantial contribution to
the realization and success of the EXOSAT database system. Particular thanks
are due to P. Barr, G. Giommi, P. Giommi, M. Gottwald, F. Haberl, L. Osborne,
J. Osborne, A. Parmar, L. Stella, G. Tagliaferri, G. Thorner, and H. van der
Woerd. The HEASARC staff are also thanked for their support. In particular, I.
George and B. O'Neel for helping in the reformatting effort of the products,
and Pat Tyler and Song Yom for creating and maintaining the databases and the
database management system.
Arnaud K.A., George I.M, and Tennant A, 1992 Legacy 2, 65.
Angelini L., Pence W. and Tennant A. 1993 Legacy 3, 32.
George I.M, Arnaud K.A., Pence W. and Ruamsuwan L. 1992 Legacy 2,
De Korte, P.A.J. et al, 1981, Sp. Sci. Rev., 30, 495.
Parmar, A, E, et al 1995, in this Legacy issue.
Peacock, A., et al. 1981, Sp. Sci. Rev., 30, 525.
Turner M.J.L., Smith, A., and Zimmermann, H.U., 1981, Sp. Sci. Rev.,
White N. E. and Peacock A., Memorie della Società Astronomica Italiana,
1988 Vol. 59,7.
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