ROENTGENSATELLIT (ROSAT)


MAPPING AND STUDYING A UNIVERSE INVISIBLE TO THE HUMAN EYE

Whether curiosity or fascination first prompted man to study the heavens,
unsatiated, he has continued through the centuries to probe cosmic puzzles
using instruments born of his imagination.  One of these instruments, the
"Roentgensatellit," known as ROSAT, rode into orbit atop a Delta-II rocket on
June 1, 1990.  Designed specifically to detect high-energy radiation, ROSAT's
telescopes are investigating X-ray and ultraviolet emissions, regions of the
electromagnetic spectrum that cannot be seen and that cannot penetrate the
Earth's atmosphere.

Named for German scientist Wilhelm Conrad Roentgen, who discovered X rays in
1895, ROSAT began as a national space program in the Federal Republic of
Germany. It grew into an international astronomical observatory project with
the involvement of the United Kingdom and the United States.

ROSAT's science mission is divided into two phases.  With its in- orbit
check-out period complete, ROSAT has begun phase one of its mission, an all-sky
survey to map the heavens.  When the 6-month mapping survey is complete, the
satellite will begin phase two and be pointed at selected objects, studying
individual targets, for the remainder of its mission.  All three participating
countries have invited potential Guest Observers to submit proposals for
investigations to be conducted during ROSAT's pointed phase.

In the United States, an extensive Guest Observer program is underway.  Through
the program, ROSAT's X-ray observing time will be shared by scientists from the
United States and throughout the world.  NASA supports the Guest Observers with
two staffed facilities and with special software to aid in the analysis of
data.  In addition, an on-line data base provides updates on ROSAT's stat's and
information needed to prepare proposals for additional pointed investigations.

With ROSAT, mankind continues its attempt to understand the energetics of
processes at work in the universe.  The discoveries of ROSAT are expected to
raise new questions to be investigated by observations of the next generation
of X-ray satellites.


WILHELM CONRAD ROENTGEN (1845-1923)

A scientist with active curiosity, Wilhelm Conrad Roentgen discovered X rays by
chance.  While trying to understand the cause of luminescence in a Crookes tube
(the forerunner of the cathode ray tube), Roentgen covered one end of the tube
to ensure that no light could escape.  When he turned on the tube, a nearby
screen that had been coated with fluorescent material began to glow.  He knew
that the glow was not caused by cathode-ray electrons.  Cathode rays couldn't
penetrate the black cardboard he had placed at the end of the tube.  His
curiosity piqued, Roentgen interrupted his study of cathode rays to learn about
the radiation he called "X."

Roentgen's discovery was serendipitous -- a matter of good luck -- but as Louis
Pasteur once said, "Chance favors only the mind that is prepared." Other
scientists had noticed the fluorescent glow while using Crookes tubes, but they
did not associate it with a new form of radiation.  Roentgen later explained,
"I didnUt observe, I investigated."

Like the scientist for whom it is named, ROSAT is prepared for serendipitous
discovery.  With its enhanced observing and all-sky survey capabilities, ROSAT
is well positioned to discover the unexpected.


A NEW AGE OF ASTRONOMY

Stargazing has entered a new age -- an age of space astronomy.  For centuries,
astronomical observations were limited to what could be seen with the naked eye
and the visible light captured in Earthbound telescopes.  Now, by placing
instruments above the obscuring atmosphere, astronomers can scan the heavens
across the entire electromagnetic spectrum to answer questions about the
cosmos.  Light visible to the human eye represents only a fraction of the
electromagnetic radiation emitted by objects throughout the universe.  The new
astronomy has scientists investigating old mysteries with new "eyes" --
instruments that view the universe in the electromagnetic wavelengths outside
the visible band of the spectrum, emissions that do not penetrate to the
Earth's surface.

Just as visible light passing through a prism is dispersed into a rainbow of
colors determined by their wavelengths, the invisible part of the
electromagnetic spectrum can also be separated into differing bands of
wavelengths.  These range from very long radio wavelengths to extremely short
gamma-ray wavelengths.  The emission of X rays from astrophysical objects
indicates the presence of high-energy phenomena in the universe.  The X rays
may originate in very hot gases, or plasmas, with temperatures of several
million degrees Kelvin (K).  Alternatively, they may be produced by the
interactions of streams of highly energetic particles with other particles or
magnetic fields.  Ultraviolet emissions are produced at somewhat cooler
temperatures ranging from 10,000 to 100,000 degrees K.

When instruments that sense these various emissions were turned to the heavens,
scientists discovered a previously invisible aspect of the universe.


A HISTORY OF X-RAY ASTRONOMY

The observation of high-energy radiation depends upon the ability to send
detectors above Earth's atmosphere because it blocks such radiation.  The study
of celestial objects that emit X-ray, gamma- ray, and ultraviolet radiation
only became possible with the advent of the space age.

In 1962, the science of X-ray astronomy was born with the flight of a small
Aerobee rocket launched from White Sands, New Mexico. A team of scientists sent
aloft a payload of three Geiger counters to investigate whether celestial
sources other than the Sun also emitted X rays.  The instruments recorded an
unexpected, brilliant source of X rays located in the constellation Scorpius,
later dubbed Sco X-1.

During the next 8 years, instruments launched on rockets and balloons detected
several dozen bright X-ray sources in the Milky Way Galaxy and a few sources in
other galaxies.  The excitement over X-ray astronomy was growing and, in 1970,
NASA launched the first satellite devoted to X-ray astronomy, the first Small
Astronomy Satellite (SAS-1).

Also known as "Uhuru" (Swahili for freedom), SAS-1's task was to perform the
first survey of the X-ray sky from which a catalog of X- ray sources could be
developed.  Uhuru discovered several hundred sources.  They included binary
star systems -- systems in which two stars travel in tandem, revolving around
one another; supernova remnants -- the remains of stars that have exploded
violently; the nearby Andromeda Galaxy -- a galaxy similar to the Milky Way;
and several galaxy clusters -- large gravitationally-bound groupings of
galaxies.

During the next 7 years, X-ray sources were studied by instruments on several
satellites: among them a small X-ray telescope aboard NASA's Copernic's, two of
NASA's Orbiting Solar Observatory satellites, the Defense Department's Vela
5-A, the Astronomical Netherlands Satellite, the British Ariel 5, and NASA's
SAS-3. In addition, a vigorous program of rocket and balloon experiments was
continued.

Numerous discoveries are credited to these early explorations: binary X-ray
pulsars -- a neutron star orbiting a normal companion and creating an X-ray
emission that appears to wink on and off; X- ray bursters -- compact objects
that suddenly increase in intensity and then fade; X-ray emission from active
stars; and active galaxies where the central regions (known as active galactic
nuclei) emit huge amounts of X rays.  Among the latter are "radio" galaxies,
known for producing strong radio waves; "Seyfert" galaxies, named for their
discoverer Carl Seyfert and known for intense levels of energy emanating from
small central regions; and quasars, the most luminous objects in the universe,
radiating up to a thousand times as much energy as the Milky Way Galaxy from an
area no larger than the solar system.

In addition to a wide variety of discrete sources, these early experiments
detected the presence of an isotropic X-ray background radiation arriving from
all directions, the origin of which was a subject of intense speculation.  A
fraction of the observed sources, due to their X-ray faintness, distance, or
the faintness of their optical counterparts, remained unidentified with any
known astronomical objects.

In 1977, NASA launched its first large orbiting X-ray observatory, HEAO-1, one
in a series of three High-Energy Astronomy Observatory satellites.  Weighing
3.5 tons, HEAO-1 carried into orbit four experiments that surveyed the sky and
pinpointed sources of X-ray and gamma-ray emission but had no capability of
producing images of emitting objects.  The observatory conducted a sky survey,
increasing the number of cataloged X-ray sources to approximately 1,500.

Accomplishments credited to HEAO-1 are many: the first precise measurement of
the energy spectrum of the diffuse X-ray background radiation, implying a
possible origin in a universal hot plasma; a very large bubble of hot gas in
the constellation Cygn's stretching across more than 1,000 light years of space
and containing the mass of several hundred thousand Suns; a new black hole
candidate; and the discovery that the class of objects known as active galactic
nuclei are powerful sources of X rays.  HEAO-1 remained in operation until
early 1979.


Until the launch of the second High Energy Astronomy Observatory in 1978,
scientists studied X-ray sources primarily by determining their positions,
measuring their X-ray spectra, and monitoring changes in their X-ray brightness
over time.  With HEAO-2 (known as the Einstein Observatory), it became possible
to routinely produce images of cosmic X-ray sources rather than to simply
locate their positions.  The Einstein Observatory was the first imaging X-ray
telescope to be deployed in Earth orbit.  With it, astronomers obtained X-ray
images of such extended optical objects as supernova remnants, normal galaxies,
clusters of galaxies, and active galactic nuclei.  Einstein observations
revealed that all classes of objects known to classical optical astronomy were
also sources of X-rays. Among the Einstein Observatory's most unexpected
discoveries was that all stars, from the coolest to the very hottest, emit
significant amounts of X-rays.

Thousands of cosmic X-ray sources became known after discoveries from NASA's
Einstein Observatory and the European Space Agency's EXOSAT Observatory
(launched in 1983) were added to the X-ray catalog.  Astronomers now recognize
that a significant fraction of the radiation emitted by virtually every type of
object in the cosmos emerges as X-rays. Each succeeding X-ray mission has made
discoveries at the limit of its capability and has tantalized astronomers to
push on to higher capabilities of resolution and sensitivity.



SCIENTIFIC POTENTIAL OF ROSAT

A myriad of unsolved questions awaits investigation by ROSAT. The observatory's
unique capabilities will provide high-resolution imaging of objects with a
precision and sensitivity that match or exceed those of previous observations.

Normal Stars -- Normal stars are excellent candidates for ROSAT observations.
While all classes of stars have been found to emit X rays at some level,
different types of stars apparently emit X rays via several different
mechanisms.

Cool stars, like the Sun, are known sources of X rays that originate in a layer
above the visible photosphere.  The Sun's outermost layer seethes with an
intensely hot, low density gas that creates a stellar corona, or crown, which
is visible when the brilliant photosphere is masked out, as in an eclipse.
X-rays are thought to be produced in a stellar corona by the dynamo action of a
star's magnetic field in which turbulent motion of the field heats gases to a
million degrees K or more.  The Sun will not be observed with ROSAT because it
would burn out the sensitive instruments designed to observe very faint
nonsolar X-ray sources.  However, ROSAT will add to astronomers' knowledge of
the stellar corona phenomenon by extending the study of coronae in other cool
stars to a very large sample.

In hot stars, those which are 5 or 10 times hotter than the Sun and 10 to 100
times more massive, scientists believe stellar winds carry shock-heated blobs
of gas that emit X rays.  For observations of this emission, the less massive
sources should prove the most informative because of the absence of dense
stellar winds that absorb X-rays. Many such objects are expected to be
detectable for the first time with ROSAT.

Very young stars also exhibit substantial X-ray emission, although the origin
of this radiation remains largely a mystery.  Stars are born in incubators of
collapsing gas and dust called molecular clouds, which often prevent the escape
of X rays from their cores.  As a molecular cloud collapses, temperatures climb
and nuclear reactions begin; from this protostar, a star bursts to life.
ROSAT's sensitivity may allow observation of these heavily obscured objects.

Supernova Remnants -- A pool of expanding supernova remnants has disrupted and
enriched the interstellar medium since shortly after the birth of the Milky Way
Galaxy. In the process of a massive star collapsing into a neutron star or
black hole, much of its mass is expelled in a violent explosion known as a
supernova.  X-ray studies of the expanding stellar remnant produced by the
explosion tell 's much about the progenitor star, its evolution, and the nature
of the surrounding interstellar medium.

Scientists hope the high resolution of ROSAT will be able to reveal structural
details of supernova remnants, adding to their understanding of remnant
evolution.

Compact Objects -- Reacting to the exhaustion of its nuclear fuel supply and
the inexorable forces of gravity, a star of mass greater than that of the Sun
will eventually collapse.  Depending upon the star's exact mass, it will become
either a white dwarf (approximately the size of the Earth), a neutron star (no
larger than 10 kilometers in radius), or a black hole, a massive object so
compact and with gravity so great that not even light can escape it.

The X-ray emission from the hot gas surrounding and falling onto such compact
objects is a key to their detection and study.  Accurate positions can be
obtained by ROSAT for several known compact X-ray sources for which positional
data have been poorly defined.  Identification of these sources with optical
objects will provide a critical tool in determining the nature of their basic
physical parameters.

X-Ray Binaries -- A majority of stars travel in pairs, revolving around one
another in binary systems.  In X-ray binaries, where the compact star is
attracting a flow of X-ray emitting gas from its companion, analyses of X-ray
flux variations help define the emitting regions.  Such analyses are
particularly useful in identifying an eclipsing binary system, where a
nonemitting companion acts as a shutter being drawn across the emitting region.
Observing the eclipse helps to establish the shape and size of the region and
reveal the physical processes at work.

Beyond the Milky Way Galaxy, a seemingly infinite number of other galaxies,
either isolated in space or members of clusters, are available for study by
ROSAT.

Galaxies -- Normal galaxies are known to be sources of X-rays, but because they
tend to be less X-ray active than other extragalactic objects, they have been
difficult to study.  Normal galaxies are generally divided into two classes:
spiral galaxies, which are flattened disks of gas, dust, and stars, often with
bar or spiral-arm patterns; and elliptical galaxies, which are spheroidal
systems of stars that are usually more massive than spirals.

The predominant X-ray emission mechanisms differ in spiral and elliptical
galaxies.  In spirals, the X rays that are detected represent the combined
emission from many individual sources, such as X-ray binaries and supernova
remnants.  ROSAT, with its improved sensitivity and resolution, will allow
detection of these individual sources in many galaxies.

Dark Matter -- In contrast to the emission from spiral galaxies, X rays from
elliptical galaxies appear to originate in a diffuse gas that is heated to
several million degrees K and is gravitationally bound to the galaxy.  This gas
is of particular interest because it provides information on nonluminous
material, the so-called "dark matter," that may be present in a galaxy.
Because the gas is bound by gravity, a knowledge of the gas's density and
temperature will enable scientists to estimate the total mass of the galaxy.
The difference between this total mass and that fraction observed in the form
of luminous stars and X-ray emitting gas represents the amount of dark matter
associated with the galaxy.

Invisible to optical telescopes, dark matter therefore contributes a
gravitational force that cannot be accounted for by luminous matter.  The
presence of dark matter in several galaxies, as implied by X-ray observations,
was initially established by the Einstein Observatory. The greater sensitivity,
spatial resolution, and spectral resolution of ROSAT will increase the sample
of galaxies studied and provide a more precise determination of the total mass
and distribution of dark matter in elliptical galaxies.

Active Galactic Nuclei (AGNs) -- In addition to the more common spiral and
elliptical galaxies, a small fraction of galaxies release very large amounts of
energy from highly compact regions inside their nuclei.  These so-called active
galactic nuclei (AGNs) release more energy than can be accounted for by the
stars contained within the galaxies.  A well-known class of AGN is the
quasi-stellar object, or "quasar." The most luminous objects in the universe,
quasars are also the most distant objects ever observed.  How such objects
radiate more power than the entire Milky Way Galaxy from an area smaller than
the solar system is one of the most challenging questions of present-day
astrophysics.

The high luminosities of AGNs suggest that they may be powered by the release
of gravitational energy as matter is accreted, or accumulated, onto a compact
massive central object, such as a black hole.  Current ideas favor the
formation of a disk of matter, heated by friction as material is pulled inward
by gravity, accreting onto the central object.  A large fraction of the energy
emitted by AGNs is in the soft X-ray band, -- the X-ray band closest to the
ultraviolet region of the spectrum.  ROSAT, with its unprecedented soft X-ray
sensitivity, is well equipped to help scientists understand these energetic
objects.

Galaxy clusters -- clusters of galaxies, in which many galaxies are
gravitationally bound together, represent another area of study for ROSAT.
Early X-ray astronomy experiments discovered these clusters to be copious
sources of X-ray emission, now known to originate in hot (multimillion degree
K) gas permeating each cluster.

The mass of this gas is usually comparable to or greater than that of the
galaxies that can be seen in visible light.  The total mass of a cluster --
including member galaxies, the X-ray emitting gas, and any Rdark matterS -- can
be estimated by using X-ray observations in the same way as for elliptical
galaxies.  ROSAT will be especially effective for observing the
lower-temperature clusters that radiate predominantly in the soft X-ray region.

Diffuse X-Ray Background -- In addition to discrete sources of X rays, the
existence of an apparently uniform and isotropic X-ray glow, called the diffuse
X-ray background, has been known since the earliest rocket experiments.
Although this radiation has been extensively studied, its source remains a
subject of debate.  Two possible origins for the X-ray background have been
proposed: an intergalactic hot gas, more or less smoothly distributed
throughout the universe; or the combined emission from a large number of
discrete sources too numerous and weak to be individually detected by past
instruments.  A strong constraint on a possible diffuse source origin for the
background was recently provided by results from experiments on NASA's Cosmic
Background Explorer (COBE), which indicate that any such hot gas would have to
be highly clumped and not uniformly distributed.  A number of candidates for
the underlying source population in the discrete-source theory of the X-ray
background have been proposed, including such possibilities as starburst
galaxies, active galactic nuclei, quasars, or a class of objects not yet known.

ROSAT's enhanced sensitivity and spatial resolution can be used to help
determine the origin of the diffuse X-ray background, by making deep exposures
of selected sky regions otherwise devoid of known sources.  ROSAT will attempt
to detect and resolve the individual objects that may be contributing to the
diffuse background.

By virtue of its enhanced capabilities for observing the X-ray characteristics
of a wide range of astrophysical objects and processes, ROSAT offers
astronomers a new window on the universe.  Each new observation holds the
potential for discovery.  Each new discovery holds the promise of solving a
cosmic mystery and providing a clearer picture of the universe.


ROSAT'S MISSION IN A NEW DECADE OF DISCOVERY

Mankind's understanding of the origin and fate of the universe and the birth,
nature, and evolution of the objects within it is expected to increase greatly
during the decade of the 1990s.  Scientists will be studying celestial objects
across the entire electromagnetic spectrum with several major space
observatories scheduled for launch during this decade: the Hubble Space
Telescope (HST), already in orbit, for visible, infrared, and ultraviolet
wavelengths; the Gamma Ray Observatory (GRO) for gamma rays; the Advanced X-Ray
Astrophysics Facility (AXAF) for X-rays; and the Space Infrared Telescope
Facility (SIRTF) for infrared radiation.

In the investigation of X-ray sources, ROSAT will follow the path set by HEAO-2
(the Einstein Observatory) and will be a key link in preparing for AXAF
observations.  ROSAT contributes to this evolution in instrument capability
with its enhanced sensitivity, resolution, and completeness of sky coverage.
ROSAT has a sensitivity five times greater and angular resolution (capability
to separate adjacent sources) three times greater than HEAO-2, which was the
most sensitive X-ray observatory previously flown.

For the United States, ROSAT's specific mission is to advance the science of
astrophysics through the study of X-ray emission from nonsolar celestial
objects.  This will be realized primarily through the pointed phase studies of
selected sources and, to a lesser extent, through limited participation in the
X-ray all-sky survey.

ROSAT also carries a Wide-Field Camera, which will extend the satellite's
coverage of celestial phenomena to extreme ultraviolet wavelengths, 300 to 60
angstroms (0.042 to 0.21 kilo electron Volts, or keV).  This camera, developed
and supplied by the United Kingdom, will provide the first survey of the sky in
this little- studied region of the electromagnetic spectrum.

Objects to be studied during ROSAT's pointed phase are being selected by the
international astrophysics community through proposals to a Guest Observer
program.  Proposals for the first 6 months of pointed observations were invited
in 1989.  Additional calls for proposals will take place during the lifetime of
ROSAT.

ROSAT was launched into orbit aboard a two-stage Delta-II launch vehicle from
the Cape Canaveral Air Force Station in Florida by the US Air Force for NASA.
The Delta-II was augmented with a specially designed fairing to accommodate the
ROSAT spacecraft.  NASA assisted ROSAT operations by providing prelaunch
testing support, Deep Space Network (DSN) support in the first weeks after
spacecraft separation from the launch vehicle, and backup DSN support of the
German ground tracking and data system, if needed, throughout the mission.

ROSAT's orbit is nearly circular, at an altitude of approximately 580 km and at
an inclination to the Earth's equator of 53 degrees, with an orbital period of
96.2 minutes.  Designed to observe X-rays in the range from 0.1 keV to 2 keV,
commonly called the low-energy or soft X-ray band, the ROSAT telescope is so
sensitive that it can detect and record X rays from all known classes of
celestial sources.

During the all-sky survey, the X-ray telescope scans a band 2 degrees wide
during each revolution around the Earth, thus completing the survey in 6
months.  Scientists expect to locate more than 100,000 X-ray sources with a
positional accuracy of approximately 30 arc seconds during the ROSAT sky
survey.

In its second phase, ROSAT will be pointed at selected individual X- ray
sources.  Many X-ray sources are faint, and a typical ROSAT observation will
require approximately 10,000 seconds (about 3 hours) to record an X-ray signal
of adequate strength.

The German Space Operations Center (GSOC), located in Oberpfaffenhofen near
Munich, operates the spacecraft using the 15-meter antenna at the Deep Space
Station near Weilheim, Germany. The spacecraft contacts the ground station on
six consecutive orbits daily, for 6 to 8 minutes per contact.  During periods
when no communications are possible, commands are stored on the spacecraft and
data are stored on one of two tape recorders.  The tape recorders can hold 21
hours of data.

After telemetry capture at Weilheim, data are sent to the GSOC for a quality
check and initial processing.  Data are reformatted as necessary and
transmitted for evaluation to the German ROSAT Science Data Center at the Max
Planck Institute for Extraterrestrial Physics (MPE) in Garching. The GSOC
distributes Guest Observer data tapes to the ROSAT Science Data Centers in the
three participating nations.

MPE processes and analyzes X-ray data acquired during the survey mode and is
responsible for compiling an X-ray source catalog.  The processing,
distribution, analysis, and archiving of the data from the ultraviolet camera
are the joint responsibility of the United Kingdom and the Federal Republic of
Germany.


ROSAT's HIGHLY SENSITIVE INSTRUMENTS THE SATELLITE

The design of the ROSAT spacecraft was driven by the structure of its X-ray
telescope and Wide-Field Camera. Spacecraft support systems were built around
the telescope assembly, which is nearly 4 meters (13 feet) long.  The
spacecraft, which weighs 2429 kilograms (5,354 pounds), has a square body with
an adapter for the Delta-II rocket.  An array of three solar panels provides
1,000 Watts of power to the spacecraft and science payload.  The large,
unobstructed rear surface of the array dissipates excess heat into space.

During launch, two of the solar panels were folded over the central body of the
spacecraft.  These were deployed on orbit, along with antenna masts.
Protective telescope "doors" were then opened to permit the first observations.

Orbiting the Earth at 17,000 miles per hour, the satellite locates and locks
onto targets using gyroscopes, Sun sensors, and magnetometers for coarse
orientation information, and two star trackers for a highly accurate sky
reference based on known star positions.  A system of gyroscopes, reaction
wheels (angular momentum flywheels), and magnetic torquing devices are used to
maintain stable pointing at a selected target and to re-orient the spacecraft
to point at a new target.


THE X-RAY TELESCOPE

In X-ray astronomy, each new project and advance in technology has led to new
discoveries.  ROSAT carries the finest high-resolution X- ray mirrors ever
made.  The ROSAT X-ray telescope's principal subsystems are its mirror assembly
and its focal-plane detectors.  Two Position-Sensitive Proportional Counters
and a High-Resolution Imager are mounted on the turret in the focal-plane
instrument section, where they can be used one at a time.

Because X rays interact more strongly with metallic surfaces than does visible
light, a critical angle exists for the reflection of X-ray photons (particles
of electromagnetic radiation).  If the X ray strikes a mirror at an angle
greater than the critical angle, it is absorbed and lost.  To be reflected, it
must strike the mirror surface at a grazing angle, hence the name "grazing
incidence mirror."

ROSAT uses four pairs of nested grazing incidence mirrors to provide the total
reflecting area required for the specified energy range.  ROSAT's mirrors,
known as a Wolter Type 1 configuration, consist of tubelike shells nested
inside one another.  Each shell contains a pair of hyperbolic and parabolic
grazing incidence mirrors supported at one end by a central flange.  All of the
mirror shells are made of Zerodur, a glass ceramic, and coated with a thin
layer of gold to increase X-ray reflectivity.

The ROSAT mirrors yield higher angular resolution and produce less scattering
than any previous X-ray mirrors, thereby permitting greater image contrast.
The X-ray mirror assembly is the product of a joint endeavor between Germany's
Max Planck Institute for Extraterrestrial Physics (MPE) and the Carl Zeiss
Corporation.


HIGH-RESOLUTION IMAGER

The ROSAT High-Resolution Imager (HRI), which was developed for NASA by the
Harvard-Smithsonian Center for Astrophysics, is based upon a design flown
successfully on the Einstein Observatory. Several modifications have been made
to enhance the HRI's performance, including an increase in quantum efficiency
and a reduction in the level of internal background.

While the HRI has spatial resolution superior to that of the Position-
Sensitive Proportional Counters, it has very limited energy resolution and
covers a smaller field of view.  Consequently, the HRI is better suited for
precisely locating X-ray sources, for separating sources in regions where they
are too close together for study by the proportional counters, and for
resolving small-scale features of extended objects.

The detector consists of two microchannel plates in a cascade configuration,
with a grid of crossed wires for electronic readout.  Microchannel plates
absorb incident X rays and amplify the signal for position determination via
the crossed-wire grid below the plate.  Each microchannel plate is an array of
small hollow tubes or channels.  An X-ray photon striking the surface of a
channel frees an electron.  The electric field produced by a high voltage
applied across the microchannel plate accelerates this electron, which then
collides with the wall of the tube to produce more electrons.  A series of
electrons thus cascades down the tube, multiplying in number until a sufficient
signal is produced to be recorded electronically, revealing the location of the
incident X-ray photon.  The array of such events is used to produce the X-ray
image of a given field.


POSITION-SENSITIVE PROPORTIONAL COUNTERS

The two Position-Sensitive Proportional Counters (PSPCs) on ROSAT are improved
versions of those flown on sounding rockets by MPE. The PSPCs are a type of gas
counter in which X rays are photoelectrically absorbed.

X rays enter the detector through its entrance window and interact with the gas
inside.  The photoelectrons produced by the interaction are accelerated; as
they move through the gas, they produce more electrons.  Planes of wires locate
the electrical signals, recording the position and amplitude of each incoming
X-ray photon event.  The strength of the electronic signal is proportional to
the energy of the incident X ray.  The collection of all of the events from a
given source provides its position and energy spectrum.

While these detectors do not resolve sources in space as accurately as the HRI,
they cover a wider field of view and provide photon energy measurements not
possible with the HRI.


WIDE-FIELD CAMERA

The Wide-Field Camera (WFC) was developed and supplied by a consortium of
institutions in the United Kingdom led by the University of Leicester.
Complementing the X-ray telescope, the WFC extends ROSAT's spectral coverage
into the extreme ultraviolet region, 0.042 to 0.21 keV.

The WFC functions as an autonomous instrument, with its own star tracker (for
position information), thermal control system, and command and data handling
system.  Power, on-board data storage, command reception, and telemetry are
provided by the spacecraft.  Coaligned with the X-ray telescope, the WFC has a
wider field of view (5 degree circular diameter).  The optics consist of a
nested set of three grazing - incidence mirrors, known as Wolter-Schwarzschild
Type 1, fabricated from nickel-plated aluminum and coated with gold for optimum
reflectance.

Two identical detector assemblies are mounted on a focal-plane turntable so
that either one can be selected for use.  A filter-wheel assembly containing
eight spectral filters is located in front of the detectors.  Any one of the
filters may be chosen to select a specific energy band, depending on the target
to be studied.



PROGRAM FOR GUEST OBSERVERS

Scientists around the world were invited to submit proposals for the objects to
be studied by ROSAT during the first 6 months of the pointed phase.  Half of
the pointed observation time with the X-ray telescope will be devoted to
observations conducted under the US Guest Observer Program, with the remaining
50-percent allotted to the corresponding programs of the Federal Republic of
Germany and the United Kingdom.

Proposals for participation in the US Guest Observer Program are submitted to
NASA Headquarters, where a two-stage process is followed:

* A scientific and technical evaluation directed by NASA is conducted by
peer-review panels and by the US ROSAT Science Data Center (RSDC) staff to
assess feasibility.  All feasible proposals are prioritized according to
scientific merit.  Final selection of US proposals is made by the Director of
the NASA Astrophysics Division.

* The International ROSAT users' Committee, made up of representatives from the
three participating countries and chaired by the FRG's Project Scientist, meets
to resolve duplication among recommended proposals and to assign each proposal
an observational priority rank.

Observations selected from the first call for proposals in 1989 are scheduled
for execution during ROSAT's first pointed phase.  A second call for proposals
will be announced at a later date.  While the nominal ROSAT mission lifetime is
2 years, the satellite is expected to remain operational for a much longer
period.  New observing proposals will be sought periodically.

ROSAT will be pointed at selected individual X-Ray sources for varying lengths
of time, depending upon the intensity of each source.  An hour or more of
observing time may be required to obtain sufficient data for analysis of a
particular X-ray source.  Observations of the faintest sources will require
sustained pointing of the spacecraft at a given target over several orbits.

Data are processed initially at the German Space Operations Center, in
Oberpfaffenhofen, Federal Republic of Germany. Magnetic tapes containing master
data records are shipped to the US ROSAT Science Data Center (RSDC) at NASA's
Goddard Space Flight Center in Greenbelt, Maryland. The ROSAT Standard Analysis
Software System (SASS), developed by MPE and the Harvard-Smithsonian Center for
Astrophysics (CfA), is used to yield a standard data product for each
observation.  Following verification and SASS processing at CfA, the data are
released to original investigators and archived in the Goddard RSDC. Data are
treated as proprietary for the original investigators for 1 year from the date
of receipt, after which they become generally accessible.

The United States is providing extensive assistance to its ROSAT Guest
Observers. Two Guest Observer facilities have been developed: at the NASA
Goddard Laboratory for High-Energy Astrophysics in Greenbelt, Maryland, and at
the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

As part of the RSDC activities, the CfA has developed a standard set of
software packages for scientific analysis.  They are transportable and run
under the Image Reduction and Analysis Facility (IRAF).

IRAF is a product of the National Optical Astronomy Observatories and is
already a familiar tool to many astronomers.  Using this new package, called
"PROS" (for Post Reduction Off-Line Software), observers can extract and
display photon counts, smooth their data, perform analysis of X-ray spectra and
light curves (graphs showing a source's changes in brightness over time), as
well as perform other modeling.  Because PROS is compatible with the widely
used IRAF, it facilitates spectral studies and comparisons of X-ray data with
data collected at other wavelengths for the same object.

An on-line computer service for information retrieval is also being offered by
the US ROSAT Science Data Center. The Mission Information and Planning System
(MIPS) will provide readily accessible data to help potential ROSAT users plan
their observing proposals.  With it, a prospective observer can calculate
observing time and viewing windows, and can access a technical data base
providing performance specifications of the ROSAT X-ray instruments and
existing information on the source.  The system also contains a bulletin board
and mail facility where present observers and prospective proposers will find
information on the Guest Observer program, the status of observations and data
processing, and items of general interest.


ROSAT MILESTONES

1975: Max Planck Institute for Extraterrestrial Physics (MPE) proposed ROSAT as
a German national space program to the Federal Ministry for Research and
Technology (Bundesministerium Forschung und Technologie, BMFT)

1982: NASA and BMFT signed memorandum of understanding establishing cooperation
between United States and Federal Republic of Germany (FRG) on ROSAT, including
launch from Space Shuttle in 1987

1983: BMFT and British Science and Engineering Research Council (SERC) signed
memorandum of understanding establishing FRG- United Kingdom (UK) cooperation
on ROSAT

1987: NASA/BMFT decision to launch ROSAT on a Delta-II launch vehicle, rather
than the Space Shuttle

Feb 1989: First Research Announcement released soliciting proposals for pointed
observations

May 1989: US, FRG, and the UK received a total of 717 proposals

Oct 1989: Telescope and flight instruments calibrated

Oct 1989: ROSAT pre-ship review held in FRG

Oct 1989: Meeting of International users Committee to resolve conflicts in the
recommended national proposal selections

Feb 1990: ROSAT shipped to Cape Canaveral Air Force Station, Florida

June 1, 1990: ROSAT launched

July 29, 1990: Observatory checkout completed; all-sky survey begun Launch + 8
months: Begin pointed phase of mission.  Pointed observations will continue
throughout the mission, which is expected to last at least until January 1992




Technical Glossary

arc second
60 arc seconds = 1 arc minute, 60 arc minutes = 1 degree on the circumference
of a circle

cascade configuration
an arrangement of devices connected in a series so that they multiply the
effect of each device

electron Volt (eV)
a general unit of energy for fundamental particles and electromagnetic
radiation

extragalactic
beyond the Milky Way Galaxy

flux
quantity flowing across a given area

isotropic
quality of having the same intensity in all directions

Kelvin
the standard international unit of absolute temperature 

luminosity
the intrinsic energy output of a star

magnetometer
instrument for measuring intensity of a magnetic field

microchannel plates
plates that consist of extremely small cylinder-shaped electron multipliers
mounted side by side to provide image intensification

plasma 
a high-temperature ionized gas

progenitor star
the star responsible for an outburst or supernova

protostar
a star in the process of forming

spatial resolution
capability to distinguish separate radiation sources that appear close together

spectral resolution
the capability to resolve detailed features in the spectrum of a source

starburst galaxy
galaxy with a high rate of new star formation

telemetry
transmission of instrument readings to a remote location

torquing device
on ROSAT, a device that uses the Earthus magnetic field to maintain stability 

Small Magellanic Cloud
one of two small irregular galaxies close to the Milky Way Galaxy, known as the
Large and Small Magellanic Clouds; visible in the Southern skies.

X-ray burster
object in space repeatedly producing sudden, intense bursts of X-rays,
typically lasting only a few seconds. 


The US ROSAT Team 
Dr. Lennard A. Fisk, Associate Administrator, Office of Space Science and
Applications, NASA Headquarters 

Alphonso V. Diaz, Deputy Associate Administrator, Office of Space Science and
Applications, NASA Headquarters 

Dr. Charles J. Pellerin, Jr., Director, Astrophysics Division, Office of Space
Science and Applications, NASA Headquarters 

John A. Lintott, ROSAT Program Development Manager, Astrophysics Division, NASA
Headquarters 

Dr. Guenter Riegler, ROSAT Program Operations Manager, Astrophysics Division,
NASA Headquarters 

Dr. Alan N. Bunner, ROSAT Program Scientist, Astrophysics Division, NASA
Headquarters 

Dr. Louis Kaluzienski, Deputy Program Scientist, Astrophysics Division, NASA
Headquarters 

Dr. John W. Townsend, Jr., formerly Director, NASA Goddard Space Flight Center 

Gilbert W. Ousley, Sr., ROSAT Project Manager, NASA Goddard Space Flight Center

Dr. Stephen S. Holt, ROSAT Project Scientist, NASA Goddard Space Flight Center 

Dr. Robert Petre, Deputy ROSAT Project Scientist, NASA Goddard Space Flight
Center 

Dr. Robert Price, Director,  ROSAT Science Data Center, NASA Goddard Space
Flight Center 

John Gerdes, HRI Project Manager, Harvard- Smithsonian Center for Astrophysics

Dr. Martin Zombeck, HRI Project Scientist, Harvard- Smithsonian Center for
Astrophysics

Dr. Stephen S. Murray, Data Analysis Center Manager, Harvard-Smithsonian Center
for Astrophysics 


Acknowledgments 

Within NASA, ROSAT is managed by the Astrophysics Division of the Office of
Space Science and Applications. The detailed implementation of the US ROSAT
Program is under the management of the Goddard Space Flight Center. The Goddard
Flight Projects Directorate, ROSAT Project Office, provides overall United
States project management and the Space and Earth Sciences Directorate is
providing the Project Scientist and ROSAT Science Data Centers. The Flight
Projects Directorate, Orbital Launch Services Office, also provided NASA field
center management of the Delta-II launch vehicle.

The ROSAT telescope was designed and developed by Germany's Max Planck
Institute for Extraterrestrial Physics (MPE) and the Carl Zeiss Corporation,
under the direction of Professor Joachim Trmper. The spacecraft was built at
Dornier GmbH, also of Germany. The Position-Sensitive Proportional Counters
were provided by MPE. From the United States, the High-Resolution Imager was
provided by the Harvard-Smithsonian Center for Astrophysics, under the
direction of Drs. Harvey Tananbaum, Stephen Murray, and Martin Zombeck. From
the United Kingdom, the Wide-Field Camera was provided by a consortium of the
University of Leicester, Rutherford-Appleton Laboratories, and Mullard Space
Science Laboratory under the direction of Professor Kenneth Pounds.

Brochure Preparation 

Manager, Patricia Pengra, BDM International, Inc.; Editor, Marilyn Finley, BDM
International, Inc.; Graphic Artist, Jeff Lilly, BDM International, Inc.

Contributors:

Dr. Keith Arnaud, Goddard Space Flight Center/University of Maryland; Dr. Alan
Bunner, NASA ROSAT Program Scientist; Dr. Cynthia Cheung, BDM International,
Inc.; Dr. Jim Heppner, BDM International, Inc.; Dr. Jack Hughes,
Harvard-Smithsonian Center for Astrophysics; Dr. Louis Kaluzienski, NASA ROSAT
Deputy Program Scientist; Mr. John Lintott, NASA ROSAT Program Manager; Dr.
Albert Opp, BDM, International, Inc.; Dr. Eric Schlegel, Goddard Space Flight
Center/Universities Space Research Association; Dr. T. Jane Turner, Goddard
Space Flight Center/Universities Space Research Association.