THE HUBBLE SPACE TELESCOPE LOOKING FORWARD TO LOOKING BACK "If the doors of perception were cleansed everything would appear to man as it is, infinite". The English poet William Blake shows remarkable insight, not to say foresight, when he reflects in "The Marriage of Heaven and Hell" on what lies beyond man's immediate ability to perceive. To cleanse the doors of perception one must go "above and beyond". Even from the highest mountain peaks, ground based astronomical telescope suffer from major disadvantages caused by the Earth's atmosphere. Ever since became a reality, astronomers have dreamed about escaping this frustrating restriction. The joint American-European Hubble Space Telescope (HST) will sharpen our eyes when it is launched from Cape Canaveral in March 1990. Soaring above our planet to "see" much deeper into the Universe, observing stars and galaxies fifty times fainter than have even been detected before, the HST will also look back some fifteen billion years in time. Because of the time light takes to travel, looking further out into Space also means looking further back into the past. Acting like a time machine, the HST will reveal the Universe in different stages of it evolution, back to its very origins. By studying far distant quasars, these mysterious sources of energy thought to be the oldest visible cosmic object, the HST may also hold the key to our knowledge of the distant future and help predict the fate of our Universe. As T.S.Eliot wrote, "Time present and time past are both perhaps present in time future, and time future contained in time past". Above and beyond Launched by the Space Shuttle in a low Earth orbit, The Space Telescope will be well above our planet's atmosphere, which limits ground-based telescopes. From the ground, shifting air currents constantly make the image of a celestial object flicker, jumping from its true position tens of times per second, so that a point- like source is blurred. Freed from the distortions of the atmosphere and from stray light, the Space Telescope will have a resolving power improved by a factor of ten compared to conventional telescope and will detect galaxies seven times further away than can be seen from the ground. Electronic eyes in Space The new telescope is a large structure about the size of a city bus, with a main mirror 2.4 m in diameter. Five instruments to detect and analyse the light are housed at the focus behind the main mirror. Four are American, a wide field/planetary camera, two spectrographs and a photometer, the fifth instrument, the Faint Object Camera, is European. The two cameras will electronically scan regions of the sky. The wide field/planetary camera will "see" relatively large fields of view and photograph many objects at once. The first ever high resolution images in the ultraviolet ESA's Faint Object Camera will produce images which will exploit the maximum resolving and light-gathering power of the telescope's optics. It will image stars in ultraviolet light at high resolution and this has never been done before. The camera uses a television tube arrangement to count individual photons ( the smallest quantum of light energy) and reject background noise. Some exposures may take as long as ten hours to produce an image of a very faint object. The instruments will register individual photons of starlight, revealing faint sources that have never been detected before, and showing an unprecedented level of detail in familiar astronomical objects. A wide choice of filters, prisms and polarizers will make the Faint Object Camera even more versatile. Capturing faint starlight The Faint Object Camera is made up of two separate and entirely independent cameras each with a vidicon television tube and an image intensifier. One camera magnifies the HST image by a factor of two and also includes a spectrograph, the second camera works at a magnification of four but can also work at the higher magnification of 12. The image intensifier system operates in the 115 to 650 mm wavelength range. Weak light from a celestial source is focused on a light sensitive cathode. Each photon is converted into electrons which are accelerated and focused to fall on a screen where they create a small circle of light. These minute flashed are recorded by a television camera at the back of an electronic tube, and they are fed to a microprocessor which will record and memorise the location of each spot. The full image of the faint light captured in front of the instrument will be gradually built up by the electronic memory from 250,000 picture elements. A Long Lived Observatory The eleven tonne orbiting observatory has a planned lifetime of 15 years. Almost all the major system have been designed for on-orbit repair or replacement by astronauts. The Space Shuttle is expected to visit the HST approximately every five years. Indeed the solar panels are expected to have a lifetime of about five years and several other spacecraft components, such as the batteries, reaction wheels and gyroscopes may fail during the satellite's long life. The Space Shuttle will also make it possible to change or update the instruments on board as they become obsolete. "The HST is the first of a new breed of spacecraft designed for in- orbit maintenance to provide astronomers with a long lived and updatable observatory" says Robin Laurance, ESA Hubble Space Telescope project manager. The Hubble Space Telescope is a joint ESA/NASA project with the European Space Agency contributing 15 per cent of the cost, which accounts for the Faint Object Camera, the solar panels, and manpower. For the user, a magnetic tape The dramatic improvement over exiting observatories means that the services of the HST will be in great demand among astronomers throughout the world. Already observation time on the HST is heavily oversubscribed. During the first 12 months, 11,000 hours of observing time were requested, with only 1,200 hours available. The average length of time allocated for observing is ten hours. A panel of international scientist selected 162 proposals out 556 submitted by astronomers from 30 countries, and 20% of these proposals came from ESA Member States. The Space Telescope Science Institute in Baltimore, United States, will allocate time on the Telescope and schedule the various observations. While astronomers from all over world will be to go and spend several weeks at the Space Telescope Science Institute, they can also stay at home and send in their proposals. Providing these are accepted, when the observations are completed, they will be sent a magnetic tape containing their data. Whether the tape is mailed or handed over to him in Baltimore, the astronomers must analyse the data and get his study published within one year, since after that time the information falls into the public domain. Since the Space Telescope will nearly always operate in an autonomous preplanned fashion, astronomers will not necessary have to travel to Baltimore. In fact most may choose to stay at home. Europe's centre for coordinating observations from the Space Telescope will be at the Headquarters of the European Southern Observatory at Garching, FRG. At Garching, Space Telescope staff will be responsible for coordinating some of the software system that will be developed to analyse data. Space Telescope European Coordinators will also maintain an up-to- date archive of all HST data. As they fall into the public domain, these data will be made available to astronomers. More than meets the eye The Hubble Space Telescope will do far more than take spectacular pictures: several of the instruments on board are spectrographs that separate light from celestial objects into its constituent wavelengths, in the same way that a prism can be used to resolve white sunlight into a rainbow spectrum. The exact mixture and balance of wavelengths in the light from a distant object, say a quasar, will reveal a wealth of information about the chemical composition, temperature, distance and even motion and velocity of the object in question. The ultraviolet, the FM band of astronomy The various spectrographs on board HST will further push back the limits of the unknown. They all respond to wavelengths down to 115 nanometers, well into the ultraviolet. Although the results from the spectrographs may not look as impressive to the layman as pictures from the Cameras, the information they provide will have enormous scientific significance. Indeed ultraviolet observations reveal what is hidden from the visible region of the spectrum. "The Ultraviolet is like the FM band of astronomy because it provides very detailed information of very high quality" says Peter Jakobsen, the ESA Hubble Space Telescope project scientist. Cosmic fingerprints The strongest spectral lines of the commonest atom and ions can be seen in the ultraviolet. Each chemical element is only capable of absorbing and emitting light at specific wavelength and each has its own characteristic signature, as revealing as a fingerprint. By identifying and studying these features in the spectra of various celestial objects, astronomers are able to infer the physical make-up these distant object. As ultraviolet waves are strongly absorbed by the Earth's atmo- sphere, ultraviolet astronomy can only take place in Space. The ESA/NASA/SERC ( Science Engineering Research Council, UK) Interna- tional Ultraviolet Explorer, IUE satellite has already provide a wealth of data about many cosmic objects over the past ten years and it is still being used daily to study a broad range of astrophysical topics. However, the ultraviolet spectrographs on board the Space Telescope will be considerably more sensitive than IUE, primarily because the HST have a much larger mirror. The study of intergalactic clouds using quasars as background "light bulbs" will be among the most intriguing observations that are possible with the Space Telescope. Seeing in the ultraviolet, the faint object spectrograph should be able to study the mixture of gases they contain. Both their velocity and composition are crucial to the understanding of the dynamics of the Universe. The Big Bang theory predicts that only hydrogen and helium would have to be present in the early Universe. Helium along with hydrogen is not primary a product of the nuclear "burning" process in stars by which all heavier elements are formed, but is, according to that theory, a product of the Big Bang itself. Because of their enormous distance from Earth, the lines of sight to quasars can be used as probes to detect the presence of intervening material in the distant Universe. Scientists are particularly interested in the clouds which intervene along the sight line to quasars. They reveal themselves as absorption features in the spectrum of the distant quasars. Otherwise these clouds are invisible. Some of these clouds show traces of heavy elements, such as carbon, nitrogen or oxygen and therefore must have participated in star formation. By determining the heavy element content of these clouds at various distance from Earth and hence at various stage in time, astronomers hope to map the history of nuclear synthesis. Of special interest to astrophysicists is a second class of clouds that show no traces of heavy elements beyond hydrogen. These clouds are believed to consist of primeval material which has not yet participated in star formation. Astronomers hope that the HST in the ultraviolet will enable the helium content of these primeval clouds to be revealed. Detection of this helium will be a key consistency check on the Big Bang theory, but studying helium absorption in great detail will also yield unique information on the physical conditions in these clouds and hence in the early Universe. HST will also closer to home Paradoxically, astronomers know more about these intergalactic clouds in the very distant Universe than they do about what's happening closer to us in time and Space. The expansion of the Universe shifts the spectra of objects receding from us towards the red because of the Doppler effect. In the most distant quasars this red shift is so enormous that astronomers can view parts of the ultraviolet spectrum from the ground. However, in order to carry out matching observations closer to home ( and hence in more recent times) where the red shift is much smaller, astronomers need direct access to the ultraviolet and this can only be achieved from Space. "One important long-term goal of the Space Telescope is to determine the fate of these clouds of primeval gas that we know existed in the very distant past. We do not know whether they are still around today, or whether they have all evaporated or coalesced into galaxies" says Peter Jakobsen. Edwin P. Hubble, who gave his name to the Space Telescope, lived in the first part of our century, he would have celebrated his 100th birthday on November 11th, 1989. He analysed the speed of recession of a number of galaxies and showed that speed at which a galaxy recedes from us is proportional to its distance ( Hubble's law). By studying the fingerprints of galaxy formation in the form of chemical elements being gradually built up by stars a billion years only after the Big Bang, the Space Telescope holds the key to many unsolved problems in astronomy today. The Hubble Space Telescope will, to a great extent, cleanse the doors of perception, allowing us to see up to the very edge of the Universe. "We can anticipate some of the results that the new capability will bring, but we can also expect the unexpected: indeed some of the most exciting discoveries we will make cannot be predicted" says Pr. Roger Bonnet, ESA's Director of Science.