Light and the invisible

LIGHT AND THE INVISIBLE: THE UNIVERSE IN DIFFERENT WAVELENGTHS

The atmosphere that envelops the Earth also represents an effective screen that protects the living organisms, and therefore us, from the intense ultraviolet radiations, X rays and other penetrating radiations that are emitted in large quantities by the Sun and other celestial bodies.

Only radio waves, visible light and a small fraction of the infrared radiation reach the ground, and this is the reason why, on the Earth, we only have telescopes that receive light or radio antennae. In order to register ultraviolet rays, X rays, Gamma rays, the far infrared we must launch, outside the terrestrial atmosphere, satellites which carry instruments capable of "seeing" such radiations. They receive a large quantity of new information and help us to better understand the Universe that surrounds us. (JPEG, 175 K)
(Drawing: Michelangelo Miani)

RADIOASTRONOMY

Radio waves are electromagnetic radiations, just like visible light, but compared to the latter their frequency is a millionfold lower, ranging between 100 and 100,000 MHz approximately, that is with wavelengths between 1 km and 1 mm.
Radioastronomical research started in the year 1933, when Karl Jansky, an American engineer who worked at the Bell Telephone laboratories announced that he had received radio signals of definite cosmic origin, coming from the centre of our Galaxy. To understand the novelty of his discovery you must think that Jansky DID NOT see the Sun or the Moon in such radio waves but the centre of the Milky Way!
Instead, the great development of Radioastronomy dates back to the years that followed the end of the second world war, when the technology derived from telecommunications and from the radar discovery allowed to build new precise and sensitive radiotelescopes. Thanks to radioastronomy man discovered an unexpected panorama: from the discovery of the emissions of Hydrogen to those of many molecules, including extremely complex organic molecules that lead to a completely new vision of interstellar gas, extremely important for the study of the structure and evolution of galaxies, and of all the problems related with the formation of the stars and maybe of life. Other highly energetic phenomena have also been discovered, in the so-called QUASARS (quasistellar radio sources) and in the nuclei of the active galaxies, that allow us to probe the Universe reaching long distances. The pulsing radiosources, called pulsars, the emission of which "switches on" and "off" every fraction of a second allowed to confirm the existence of exotic objects such as neutron stars.
Fundamental for Cosmology has been the discovery of the universal background radiation at 3K (-270 C) which took place in 1965, again at the Bell Labs, at millimetric wavelengths.
Among the huge developments of radioastronomical techniques we must mention the interferometric nets with very long base, European, American, Australian antennae, all synchronized by precise maser clocks. Such technique, known as Very Long Baseline Interferometry (VLBI), allows to carry out observations with a very high resolution (a thousandth of an arc second).
Italy strongly collaborates to this international net. So we can affirm today that the radioastronomical discoveries have completely changed our comprehension of the Universe and of the objects that compose it.

Cygnus A.
This radiogalaxy was one of the first radio waves' sources discovered in the sky. Although its distance is about one billion light years, it is one of the most brilliant radiosources. The little dark spot in the centre of the radio image is the nucleus, which coincides with a giant elliptic galaxy. The radio emission spreads outside the visible galaxy, up to about 500,000 light years. The radio nucleus produces the energy that supplies the whole radiosource, and the "external lobes" through canals called "jets". (JPEG, 218 K)
(NRAO)

Detail of the radio jet revealed in NGC6251. This jet is over 300,000 light years long, and represents the canal through which the energy produced in the nucleus (bottom left) is transported to the external zones of the radiosource. The spurt expands while moving away from the nucleus, and shows brilliant zones "knots" that represent shock waves formed during propagation. (JPEG, 146 K)
(NRAO)

NGC1265, prototype radiogalaxy for a class of radiosources called "tail radiosources". The galaxy, which coincides with the red nucleus visible in the bottom centre, moves downwards through the gas of the Perseus cluster of galaxies at the speed of approximately 2,000 km/sec. Therefore the spurts bend upwards because of the dynamic pressure, and the resulting radioemission resembles a wake. The galaxy is about 270 million light years far. (JPEG, 436 K)
(NRAO)

The left figure shows the overlapping of the visible image of NGC4261 elliptic galaxy (centre, white), and its radio emission (yellow and orange), characterized by two symmetric spurts and diffused lobes. The right figure shows an enlarged image of the nucleus of the galaxy, taken by Hubble Space Telescope. You can see a dark disc of dust and gas, that surround the small brilliant nucleus in the centre, that represents the active region where energy production takes place.
This image is regarded as evidence for the existence of a black hole in the centre of the radiogalaxy. (JPEG, 316 K)
(NASA-STScI)

3C273 was the first quasar to be optically identified in 1963 by Maarten Schmidt using the Mount Palomar 5m telescope. The radiosource coincides with a 12ma (magnitude) star situated at a huge distance compared to galaxies with the same luminosity. The high angular resolution radio image shows the radio nucleus (top left) and an asymmetric spurt, that propagates almost at light speed up to the distance of approximately 20,000 light years.(JPEG, 70 K)
(MERLIN)

Radio image of Jupiter:
this is how the planet appears when using radio waves with a wavelength of 21 cm. Jupiter has a strong magnetic field that captures the electrons in the surrounding space. These electrons emit radio waves, producing the two curved structures that can be seen in the picture. Jupiter's radio emissions are particularly strong along the equatorial zone, where the particles enter the magnetic field. (JPEG, 71 K)
(VLA Radiotelescope, New Mexico)

THE INFRARED

The stars have "colours" we cannot see. Just as we cannot see the light emitted in the form of X rays or Gamma rays, we do not receive the infrared radiation that is almost completely trapped by the atmosphere and do not reach the surface of the Earth. ISO, the ESA satellite, over 5 m high, with a diameter of 3m and a weight of almost 2.5 tons, has a special task: to reveal the invisible sky, that is the sky of infrared radiation, a light that is too cold to be "seen" by the human eye, that reveals extraordinary cosmic events. ISO is a space telescope that "sees" the infrared thanks to four detectors that work at very low temperatures (-271 degrees). Since its launch, ISO produced results of exceptional quality: some galaxies, for example, apparently "insignificant" in visible light, showed to be a place of high activity in the infrared, where a multitude of new and brilliant stars are born.

ISO

The ESA infrared satellite ISO (Infrared Space Observatory)l satellite infrarosso ISO (Infrared Space Observatory) was launched by an Ariane vector on November 15^th 1995. ISO orbits on a very elliptic trajectory, from 1,000 to 70,000 Km around the Earth.(JPEG, 134 K)
(ESA)

Artistic view of ISO in orbit. Its sensitivity is so high it could register the heat emitted by an ice cube at a distance of 100 Km. (JPEG, 157 K)
(ESA)

Mosaic of pictures of the infrared emissions in the Milky Way. The luminous horizontal band is produced by the emissions of the cold dust in the plane of our galaxy.(JPEG, 121 K)
(ESA)

The orbit of ISO is very elliptic so that, for approximately 16 hours a day, it works outside the bands of electric particles that surround the Earth, and the sensitivity of its instruments is maximum. (JPEG, 134 K)
(ESA)

The Rho Ophiuchi dark cloud.
The brilliant spots in this image are young stars, the smallest with the dimensions of the Sun and many bigger ones, in a nebula at a distance of 500 light years. These stars are invisible even to Hubble Space Telescope, because a cloud of dust obscures them completely. But the infrared radiation makes it through the dust and ISO reveals their presence. (JPEG, 136 K)
(ESA)

Regions of star formation in the M51 "whirlpool" spiral galaxy, 20 million light years far. The most luminous zones correspond to warm clouds of dust where new stars are condensing. (JPEG, 635 K)
(ESA)

The main control room of ESOC (European Space Agency Operation Centre) in Darmstadt, Germany, where the early phases of the launch of ISO were supervised.(JPEG, 409 K)
(ESA)

Region of the sky, in false colours, around the Orion constellation. These are zones of stellar formation. The ring toward the centre of the image is caused by the expanding gas around a newborn star. (JPEG, 404 K)
(NASA-JPL)

The telescope tha forms the heart of ISO satellite is tested at the temperature of liquid Helium at the Institute of Astrophysics of Liege, Belgium. (JPEG, 436 K)
(ESA)

ISO satellite during the tests carried out at ESTEC laboratories, the ESA technologic centre in Noordwijk, Holland. (JPEG, 204 K)
(ESA)

VISIBLE LIGHT AND ULTRAVIOLET, FROM THE EARTH AND IN SPACE

The human eye can detect only part of the radiation emitted by the Sun or by other celestial bodies, in particular it can detect the radiations that make it through the barrier of the terrestrial atmosphere and that form the so-called visible light. Since the age of Galileo our capability of seeing far bodies using instruments that can detect a lot more light than the human eye which has an opening of a few mm, has increased.
First lens telescopes were used, then instruments with concave mirrors that could focus the flux of light coming from the celestial bodies. In our century the development of telescopes lead on one hand to mirrors with increasingly higher diameters, and on the other hand to sophisticated techniques which have the aim of improving the quality of astronomical images. Nevertheless, although transparent to light, our atmosphere disturbs the rectilinear route of light due to its turbulence and to the presence of materials in suspension. The atmosphere also completely blocks all radiations with a wavelength smaller than 3,000 Angstrom, that is the radiations beyond the violet.
So in 1990 Hubble Space Telescope (HST) was launched; this telescope, orbiting around the Earth, not subject to the problems caused by the atmosphere, gave us the most exciting images of the last few years. Besides, HST, as did the smaller but useful Ultraviolet Explorer, opened for us the ultraviolet region where other phenomena, at higher energy and temperature, take place: the warmest stars, the aurora borealis of Jupiter, the explosive phenomena.

HUBBLE SPACE TELESCOPE

Since 1990 a large telescope with a mirror of 2.4 m, situated in a 13 m long cylinder with a 4.5 m diameter and a weight of 11 tons orbits around the Earth at a height of approximately 600 Km: it's Hubble Space Telescope, built by NASA in collaboration with ESA. Hubbble is the first large telescope to investigate the sky directly from space. It was launched by the Shuttle on April 24^th 1990.
Outside the dense layers of the atmosphere that disturb the observation of the sky from the Earth (just think at the twinkling of the stars when you look at them at night), Hubble can see sources up to 30 times less luminous than those ever observed, thus revealing a new Universe, full of black holes, stars about to be born, far galaxies Nothing can escape Hubble's eye: to give you an idea of the sensitivity and the resolution of its observations, we can say that if the telescope were in Padua, it could see a firefly in Sidney, and it could distinguish two fireflies at a mutual distance of at least three metres.

Hubble telescope after the tests carried out in the United States. (JPEG, 203 K)
(ESA)

The mechanical arm of the Shuttle places Hubble Space Telescope in orbit. (JPEG, 196 K)
(NASA)

Cross-section of Hubble that shows the position of the 2.4 m mirror at three quarters of the instrument, and in the background the five instruments that analyze the received light. The large solar panels at the two sides of the telescope provide energy, while the two antennae send the data to the Earth. (JPEG, 372 K)
(ESA/NASA)

Maintenance operations on Hubble. On this occasion some instruments have also been replaced, thus increasing the sensitivity of the telescope. (JPEG, 210 K)
(NASA)

The improvement of the quality of the performances obtained thanks to the replacement of some parts of the equipment is evident if we compare these two images of the nucleus of the M100 galaxy. (JPEG, 284 K)
(NASA-STScI)

Images of Saturn taken by the space telescope. Top (August 1995): the rings were taken edgewise, and were almost invisible except for the shadow projected on the planet. In such conditions we could see many of the satellites of Saturn. The dark spot is the shadow of Titanus, the largest of them all. (JPEG, 552 K)
(NASA-STScI)

The 30 Doradus nebula with an enlargement of the R136 globular cluster. They are part of the Large Magellanic Cloud, a satellite galaxy of our Milky Way. (JPEG, 334 K)
(NASA-STScI)

UVSTAR

UVSTAR (JPEG, 207 K)
(NASA)

UVSTAR (UltraViolet Spectrograph Telescope for Astronomical Research) is an ultraviolet telescope/spectrograph.
The instrument has unique capacities of acquiring spectral images of celestial bodies as large as planets, stars towards the end of their existence and that form the "planetary nebulae", remains of supernovae, globular clusters and external galaxies such as the famous Magellanic Clouds.
The data acquired by UVSTAR refer to that part of the ultraviolet spectrum, the extreme ultraviolet, which is rich in diagnostic elements for studies of chemical abundance, of temperature and dynamics of the observed celestial bodies. UVSTAR flew onboard the Shuttle Endeavour in 1995.

UVSTAR on the Shuttle.
NASA approved 5 flights of the Shuttle for UVSTAR; the first took place in the period 7^th -18^th September 1995. The next flight for UVSTAR is scheduled for the month of July, 1997. (JPEG, 397 K)
(ASI)

Logo of the UVSTAR mission of September 1997.
UVSTAR is a scientific and technological programme of the Italian Space Agency carried out, for the scientific part, by the University of Trieste (with the collaborations of the University of Arizona, Tucson) and for the technological part by CARSO (Center for Advanced Research in Space Optics). CARSO is a consortium between the University of Trieste and the Officine Galileo of Florence. (JPEG, 204 K)
(ASI)

The instrument during the vibration tests carried out by NASA to verify its resistance to the stress of a launch in space and subsequent re-entry. The technological programme has the opportunity to retrieve the instrument after every flight to update it and improve it from the technical point of view. (JPEG, 372 K)
(ASI)

VERY LARGE TELESCOPE

VLT (Very Large Telescope) of ESO represents the most advanced element of a new generation of extremely powerful telescopes, in which completely new technological concepts and materials are employed. It will be the best optical telescope of the 21^st century; infact it is composed of 4 telescopes (with a diameter of 8.2 m) that will be able to work separately or in an interferometric battery.
Thanks to its huge sensitivity and exceptional resolution, VLT will be able to see beyond today's observative horizons in space and time. By using VLT scientists will be able to solve most of the fundamental problems that cannot be dealt with today.

A unique concept: within the VLT programme is the construction of an observatory that comprises all the infrastructures necessary to run a high technology, modern research centre in the desert.

VLT is made of a group of four telescopes (with a diameter of 8.2 m), besides various auxiliary mobile telescopes (with a diameter of 1.8 m). Together they reach the power of a telescope with a diameter of 16 m. The most important configuration of the VLT project consists in the use of a common focus where the optic sheafs arrive together from the various telescopes. With this configuration, the sheafs of light coming from all the telescopes interfere in phase. VLT becomes a giant interferometer that allows the astronomers to study the celestial objects in detail.
As VLT uses the most advanced methods, designs, materials, so the scientific research will follow new ideas and principles. This giant telescope will give a determinant contribution to future science. (JPEG, 325 K)
(ESO)

Drawing of the dome and the telescope of one of the large 8.2 m instruments that form the VLT system. (JPEG, 245 K)
(ESO)

Works at the VLT site in May 1996.
The Paranal Cerro is absolutely the best site for VLT in the Southern Hemisphere. Paranal is situated 130 Km south of the city of Antofagasta, 12 Km from the Pacific coast, in an area among the driest in the world.

The investigations demonstrate that the atmospherical turbulence at the Cerro Paranal is extraordinarily low: the stellar light is diffused on an angular disc with a diameter of less than 0.45 second of arc for 15% of the time, and a diameter of less than 0.65 seconds of arc for 50% of the time. There are almost 350 clear nights a year. At the moment there are approximately 400 people working at Cerro Paranal, at the height of 2640 metres. They first removed 350,000 cube metres of stones and debris and then they created a platform on which the telescopes stand.
Besides the construction of the various telescopes, at the Paranal Observatory there will be laboratories and technical offices to maintain and run the whole Observatory, that is generators, water tanks, warehouses, a building for the maintenance of the main mirrors, workrooms, laboratories, lodgings for the staff. The various VLT components will operate between 1997 and 2001. When it will be finished, it will be the world's largest optical telescope. (JPEG, 198 K).
(ESO)

The eye of the giant
The final check on the firs 8.2 m mirror at REOSC. (JPEG, 91 K)
(ESO)

The transport of the VLT gigantic mirror. (JPEG, 405 K)
(ESO)

ESO and the European Industry: a positive collaboration.
Astronomy has a long tradition in the development of new techniques, many of which are actually used later in time. Industry plays a role of vital importance in the realization of the VLT project. ESO collaborates with a large number of industrial companies in the various member States and in Chile.
The huge 8m mirrors were produced by Schott Glasswerke, Germany, then optically cleaned by REOSC, a French company. The Italian Consortia AES and SEBIS were responsable for the construction of the supporting structures of the telescope. The highly precise mechanical techniques and the vastness of the structures represented a fascinating goal from the engineering point of view.

The "Norte Grande" as it appears in the photographs taken from the NASA Space Shuttle.(JPEG, 215 K)

Optical Interferometry: forefront technology.
One of the most extraordinary aspects of VLT is the possibility of using it as a gigantic optical interferometer. By using stellar interferometry scientists try to produce a coherent interference, or "in phase", of the light coming from the various telescopes. Although this is a very complicated technology, the scientific results are huge.

Here are some simulated images that show how better is the resolution, that is the brightness of the VLT interferometer called VLTI, compared to the images obtained with the ESO 3.5 Telescope or with Hubble Space Telescope (HST).
Apart from the telescopes, the whole optical structure of VLTI is underground, to assure the maximum thermal stability and the minimum atmospherical turbulence. VLTI will have an extraordinary angular resolution and sensitivity; it will be the best optical interferometer and will open endless possibilities to the astronomers of the 21^st century. (JPEG, 231 K)
(ESO)

THE GALILEO NATIONAL TELESCOPE (TNG)

The TNG is the optical instrument that will serve the entire Italian astronomical community. It is under construction (almost finished) at La Palma, the Canary Islands. It is a new technology telescope, with a main mirror (3.5 m the diameter) that applies the principles of Active Optics and Adaptive Optics.
It has an alt-azimuth monture, and a three mirror optical configuration called Ritchey-Chretien with two lateral Nasmyth f/11 focuses.
It was designed following the ESO New Technology Telescope model, although with many innovations that regard the control of the movements of the secondary and the tertiary mirrors, the possibility to have other focal stations in the future, the size of the building and its rotation system, the fact that that the control room is situated far from the dome, and much more.
The telescope is moved by 8 engines, 4 for each axis, while the position of the telescope is determined each moment by "encoders" with the precision of 0.06 seconds of arc. The instrument will be ready in the course of 1997.

The site of TNG.
The GALILEO National Telescope is situated at a height of 2370 m on the northern side of the Caldera de Taburiente, near the peak of the Roque de los Muchachos, an extinct volcano on the La Palma island (The Canary Islands, Spain).

Such site was chosen because of the ideal quality of its sky, protected for most of the year by an anticyclon which bars the way to bad weather. The dominant winds, although they can be strong, are of oceanic origin and laminar. A quite frequent thermal inversion forms a layer of clouds at the lower heights, leaving the air on top clear and dry, and hiding the lights of the towns on the coast.
The TNG is part of the complex of the Roque de los Muchachos Observatory, together with other prestigious plants such as for example the English telescopes (left), the Swedish solar tower and the 2.5 m telescope of the northern countries (centre). The TNG is at the extreme right. The coordination of the Observatory is taken care of by the local Institute of Astrophysics of the Canary islands (IAC, Tenerife) in a spirit of full European collaboration. (JPEG, 364 K)
(TNG)

The TNG in the dome.
The mechanical structure of the TNG, over 120 tons, is visible here in its stateliness. The instrument is now tied in the dome and rotates together with it. The floor is at the height of the superior surface of the fork, while the whole large azimuthal box with the relative engines and electronic racks are in the floor below.
The inner space is separated by two walls situated at the two sides of the telescope, so that the space has the shape of a parallelepipedon where air flows freely without creating vortices and turbulence that would damage the quality of the astronomical images. On the upper part, rotating along with the telescope, are the two large sliding doors the opening of which allows the vision of the sky.(JPEG, 127 K)
(Photo: Franco e Matteo Danesin)

The light is directed by three mirrors towards the foci called "Nasmyth" at the sides of the fork. Here, the scientific equipment will be placed in two darkrooms; such equipment includes high and low resolution spectrographs, cameras for visible and infrared images, and the unit of "adaptive optics".
Such unit counteracts the unrequired effects of atmospheric turbulence by using sophisticated devices, among which is a small flexible mirror, that restore the original characteristics of the luminous beam at the best extent.

ACTIVE OPTICS

Active optics.
The most constraing request of the Italian astronomical community to TNG is to produce very high resolution images. Such requirement pushed the designers towards an Active Optics Instrument. The traditional mirrors, which are thick in order to maintain their rigidity, undergo deformations under the action of their own weight when the telescope is inclined to point the stars, thus worsening the quality of the image.

The idea to switch to thin mirrors with the shape of a meniscus, supported by variable actuators which control their shape, was developed by ESO. In the TNG 78 supports (or actuators) are inserted in the cell on which the primary mirror rests, and correct the distortions due to gravitational force, to the thermal variations, or to other disturbances. Moreover, the TNG system actively controls even the secondary mirror to correct aberrations such as coma and defocusing, while the tertiary mirror, with an elliptic shape and a major diameter of 87 cm, can be made oscillate with frequencies up to 10 Hz to contrast the image degradation due to tracking errors and atmospheric turbulence.
The information on optical quality is obtained by the analysis of the luminous beam coming from the celestial body, which, through a computer system, transmits to the actuators of the three mirrors the impulses necessary to maintain the image quality. (JPEG, 80 K)
(TNG)

78 actuators, inserted in the cell, control the shape of the TNG main mirror by exerting the push necessary to maintain the optimal curvature. The mirror rests the hemispheric head on the extreme upper portion, through small flat plates glued to its lower side. Each actuator can register up to 200 Kg with the precision of 10g.

The rotating system of the TNG building is made of a rail that can be directly fixed to the supporting surface, on which a carriage travels.
The diameter of the system used for TNG is 9.2 m, the largest ever built with this mechanism, with the distribution of the weight of the dome (300 tons) on 100 trolleys.

The TNG assembled at the Ansaldo workshops, Milan, for the motion and control tests. The alt-azimuthal monture, typical of all modern large telescopes, allows to move the instrument only in two directions: downwards and upwards (height movement) and along arcs parallel to the horizon (azimuth movement).

Due to the rotation of the Earth though the celestial bodies appear to move from east to west along arcs parallel to the equator. In order to point and follow the stars is therefore necessary to control the movements of the telescope both in height and in azimuth at the same time, by using a complex computerized system and dedicated programmes. (JPEG, 242 K)
(Ansaldo Energia)

The three mirrors of TNG, made of Zerodur glass ceramics by Schott di Mainz (Germany), were worked at ZEISS of Oberkochen (Germany). The results were exceptional: the mirror behaviour is that imposed by the tre diffraction limit! 80% of the light falls within a disc with a diameter of 0.10 arc seconds. Here you can see the first step of the workout of the main mirror with a special rotating instrument. (JPEG, 189 K)
(Zeiss)

The primary mirror is hoisted from the cell after the tests are completed. The hemispherical heads of the actuators emerge from the upper side of the cell, while you can appreciate the reduced thickness of the mirror, only 24 cm, while the diameter is 358 cm. You can note along the edge the points of support for the 24 lateral supports that complete the positioning system of the primary mirror. By the use of Active Optics the mirror is maintained in its position with the precision of less than the millionth part of the diameter of a hair: this should allow to see a coin with a diameter of 2 cm at a distance of 200 Km! (JPEG, 357 K)
(TNG)

The lower side of the primary mirror of TNG is not flat but has the shape of a convex meniscus and it is possible to note the small supporting plates for the heads of the actuators, made of INVAR metal.

In order to render the glass surface of the "eye of TNG" reflective, so that it becomes a real mirror, a thin alluminium coating is placed on its surface, inside a vacuum chamber.
Such operation must be periodically repeated because the dust, the water vapour etc, slowly steam the reflecting surface. The mirror was alluminized by the technicians of the Willian Herschel English Telescope, who obtained a surface that reflects over 90% of the stellar radiation from the ultraviolet to the near infrared.

78 openings for active control.
One of the most important components of the active optics is the support system for the primary mirror, that is its active cell. It is equipped by 78 openings where the actuators controlling the push to be applied to the mirror to maintain a constant curvature, are placed.

The cabling of the actuators and their connection to a transputer network form the most complex part of the system. The actuators are inserted on the lower side of the cell and emerge from its upper side to provide the point of support and control of the mirror. (JPEG, 233 K)
(TNG)

Assembly of the azimuthal box.
The base of the fork (or azimuthal box), was placed on the hydrostatic layer with a very delicate operation considering its large weight and size, as well as the very strong wind of that day. The assembly has been done before the construction of the dome around the pillar, since the large size of the piece would not have allowed to put it in place later, as for other parts of the telescope has been done. (JPEG, 441 K)
(TNG)

The assembly of the TNG building. The whole structure is made by a system of metallic latticework mounted in place. The "skeleton" was then filled with alluminium panels. (JPEG, 403 K)
(TNG)

A new shape for the traditional dome. The buildin housing the TNG is a structure 24 m heigh, made by an octagonal rotating dome, by a lower round part wich encloses the central pillar, and by a service building and a flat surface connected to it. A strong bridge connects the upper road to the observing floor, in order to allow maintenance and to carry out the mirror for the periodical alluminization operations.

In the service building, the auxiliary generatores are housed, as well as the workshops and laboratories, but, above all, the control room for the observing astronomer. The choice to mve the control room away from the dome is forced by the need to avid any thermal interference on the observing floor. The thermal stability is in fact a fundamental requirement, in order to obtain good images. To this aim, an effectivi air conditioning system has been devised. The air flux is controlled by ventilation panels on the back and by a screen on the front, which aid in lowering the air turbulence phenomena. (JPEG, 229 K)
(Foto Franco e Matteo Danesin)

The Universe in X rays

When at night we watch the sky, we expect to see a fascinating, but otherwise familiar show. If we could observe the same sky with eyes different from ours, sensitive to radiation much more penetrating than light, we would see a completely different sky. In particular, with X ray eyes, such like those offered by the SAX instruments, we could see constellations of stars different from the usual ones, since our brightes stars do no necessarily emit so much in the X range, and vice versa, apparently insignificant or even invisible bodies would acquire an extraordinary visibility.

An extreme example: important sources of X rays are the black holes, which, by nature, do not emit light, but where such energetic phenomena happen, that an impressive amount of X rays is generated.

SAX

SAX is a scientific satellite for the X ray Astronomy, developed by ASI in collaboration with the Dutch Space Agency NIVR. It has been launched on April 30, 1996. The satellite has been nicknamed "Beppo Sax" to honour Giuseppe 'Beppo' Occhialini, one of the founders of X ray Astronomy.

The satellite has been built by Alenia Spazio guiding a group of industries, among wich we list Laben, Officine Galileo, Fiar, BPD, TopRel and Fokker Space, while Nuova Telespazio is responsible for the groundbased part. The satellite has a diameter of 9m when the solar panels are unfolded, and weighs a total of 1,440kg.

On the satellite, there are two wide field X ray cameras, 4 low and medium energy telescopes and two hard X detectors, one of which can also be used as gamma ray detector. SAX will carry out an observational mission, open to scientists from all over the world, dedicated to the observation of Galactic and extragalactic objects.

During its two operating lifetime, the satellite will carry out between 2,000 and 3,000 different pointings, giving a significant contribution to the different areas of X Astronomy: active galactic nuclei, compact galactic sources, clusters of galaxies, supernova remnants, normal galaxies, neutron stars, black holes.

Artist's view of the orbiting SAX. (JPEG, 158 K)
(ASI)

Vibration tests on the SAX satellite made at ESTEC, the Dutch technological center of ESA. (JPEG, 437 K)
(ASI)

A likely black hole in our Galaxy.
Cygnus X-1 is in the Cygnus constellation and is located about 8,000 light years from the Earth.

In this type of system, the matter composing the companion star is pulled into the black hole and an enormous amount of energy is released during this process, particularly in the form of X rays. Here this energy is actually seen. Cygnus X-1 is the image almost at the center, represented in false colors, in order to highlight the intensity of the emission. The other two objects are the calibration sources present inside the instrument and used as comparison sources to mark the characteristics of the observed objects. (JPEG, 463 K)
(ASI)

The first image of a Gamma-Ray Burst.
Among the most relevant scientific aims of the SAX mission, there is the study of one of the most fascinating and less known phenomena of the Universe: the Gamma-Ray Bursts (GRB), that is sudden high energy flashes coming from remote regions of the sky.
This is what happened on July 20, 1996 at 11:36:53 UT, when SAX detected a GRB simultaneously making its light curve on several X and gamma bands, and its X image in the sky, with a resolution of 10'. The pre-burst and post-burst image has a longer exposure time (more than 1000 times) than the duration of the event (about 30 sec), in order to reveal even the faintest sources. (JPEG, 293 K)
(ASI)

Where did the gamma flash come from?
High resolution image obtained by the low energy telescope on board of SAX, about one month after the detection of the GRB. It is still not clear which of the numerous sources present in the field is responsible for the flash. The SAX Team scientists predict that observations like the one of July 20 will be obtained at a pace of one per month. One of these could lead to the definite solution of this enygma. (JPEG, 291 K)
(ASI)