THE EXPANDING UNIVERSE

New Zealand Listener, Volume 18, Issue 448, 23 January 1948, Page 6

THE EXPANDING UNIVERSE

Mit. Palomar’s Giant Telescope Opens New Era in Human Knowledge

| Written for "The Listener’ |

|} Sy Dr.

C. M.

Focken

Senior | |

Lecturer in Physics, Otago University.

physicists throughout the world are awaiting an event of the first magnitude in the long history of astronomy. Very soon now the giant 200-inch telescope situated on top of Mt. Palomar in Southern California will have undergone its final tests and be ready for service, and a crowded programme for extending man’s knowledge of the universe has already been’ mapped out for it. The great eye of the telescope-the glass disc with an accurately ground concave paraboloidal surface-was transported up the mountain last November. There were sighs of relief when this precious piece of glass, insured at Lloyd’s for £150,000, was safely lowered on to its massive supports at the base of the telescope tube. The completion of this telescope, probably the world’s most costly single mechanism, crowns nearly 20 years of expert labour. Although the Project was well advanced in 1939, all work on it was suspended during the war years. The last event of a comparable kind was the commissioning of the 100inch telescope at the Mt. Wilson Observatory in 1917. th and ss astroThe year 1948 will most probably begin a new era in knowledge of the cosmos, a subject which has always had a peculiar fascination for mankind. It is no exaggeration to say that our information about celestial bodies has come

to us on rays of light. We cannot experiment with these remote sources of radiation in the ordinary way. We can do nothing but observe, from an incredible distance, and reason on the results of our _ observations. The astronomer’s instruments are therefore all concerned with light, principally with its collection and with its analysis into component colours (or wavelengths). Until aided by scientific weapons such as the telescope, the spectroscope, the photographic plate and the photoelectric cell, man’s concepts of the universe were childlike.. Physics and mathematics have played such a prominent part in advances of modern astronomy that it can more exactly be referred to as astrophysics. Millions of Universes Only four centuries ago men began peering through telescopes to find out what existed beyond the limits of unaided vision. During the present century accumulation of data has been rapid and extensive, and has led to a complete revision of all older concepts of celestial structure. It was not

long ago that astronomers spoke of the "universe" as if it were one. entity, namely, the galaxy which includes our solar system. Now they speak glibly of universes or galaxies; Our own Milky Way is one; the Andromeda Nebula another, It is known that there are millions more out in space and each one may contain several thousand million ‘stars. How are thése universes related? What changes have occurred and will occur in the future in their relationships? Surely there is some fundamental plan. If so it has not yet been discovered. Nor is it known whether there is life, in any way similar to ours, o any other celestial body. True, this question may not be of fundamental importance, but it does interest us. How may the structure of the universe be described? Is it expanding or static, finite or infinite? Profoundly different hypotheses have been Proposed by eminent astronomers. The answer to these problems will be sought beyond the present limits of exploration, inconceivably vast though these are. The Mt. Palomar Observatory has been designed and built to help solve the mystery of the universe. + Six Thousand Feet Up This observatory has been most carefully planned and will possess when completed many valuable auxiliary instruments in addition to the giant tele. ‘scope. It embodies the ingenuity, brains, ‘and experience of the Mt. Wilson

Uoservatory staff and other experts who were consulted. Liberal financial backing has been provided since its inception'in 1928 by the Rockefeller Foundation of New York. The construction was undertaken by the California Institute of TechnBlogy. A plan of co-opera-tive research with the staff of Mt. Wilson Observatory has been developed, which should be highly advantageous to both institutions-the equipment at the two observatories is largely supplementary. And these unrivalled facilities will be available for approved work to astronomers the world over. Mt. Palomar was selected as the best site available within working distance of Pasadena, the common focus of the joint project. It is only 125 miles away, is unaffected by the lights of Los Angeles, which have been detrimental to certain work at Mt. Wilson, and has excellent atmospheric conditions. The mountain is about 6000 feet high with a large, relatively flat top. The construction of a mirror to satisfy the exacting requirements was physically the most interesting of the construction problems. Years of experimentation with fused silica or quartz which had some admirable properties including non-expansibility, revealed that it was too difficult to cast in a large disc. It was also too expensive. Consequently a special pyrex (borosilicate) glass was decided on. The Corning Glass Works finally. completed the unprecedented task after gaining experience by pouring several smaller discs and one unsuccessful 200-inch one. The back of the

EEE disc is cellular or honeycombed to save weight without sacrificing much strength. To remove all trace of internal strain the disc was slowly annealed during a period of 10 months. Rough grinding began in Pasadena in 1936 and was followed by years of polishing to bring the surface of the glass to that of a perfect paraboloid. This slimming and figuring reduced the disc by ‘about five tons. Many people fail to realise that the light is reflected from the front surface of a telescope mirror, not from the back surface as in the case of the usual plate glass mirror. The glass, however, must maintain its shape to a high degree of precision under all conditions to which it is subjected.. It supports the reflecting layer, a thin coating of aluminium which is sprayed on the surface of the disc. Aluminium itself, like silver, tarnishes in the air. But the film of oxide which forms has a composition like emery or sapphire and provides splendid protective film. When dirty with dust, as will happen in time even in the clear mountain air of California, the precious film can be washed with plain soap and water, An electronic mechanism was devised to counteract automatically the earth’s daily rotation. It enables a star to be kept exactly in focus as it crosses the sky. This is an essential requirement for the lengthy exposures extending over successive nights, which are necessary when photographing the faint messages from distant stars. The support for the disc, which must not introduce any distortion; the yoke type of equatorial mounting; the 60-foot telescope tube, and the 137-foot hemispherical dome mounted on concrete piers running deep’

into granite were all skilled engineering feats. Altogether the moving parts total 450 tons. The yoke holding the telescope tube is not.on bearings, it, is floating in oil. Popular Fallacies ‘Popular misconceptions in regard to large telescopes are not uncommon. Since these telescopes are designed for photography, visual observations are incidental and generally brief. Gone are the days wher an astronomer glued his eye to the .elescope and swept the skies hoping for some undiscovered body to swim into his ken "Surély* the largest telescope must have an immense magnifying power? Broadly speaking this, too, is a fallacy. The primary purpose is not high magnification, it is to collect more light than was ever collected before, and direct it all to a small, sharp, undistorted image of unequalled brilliance. Magnification is relatively easy-it is the main function of a ‘small System of lenses or mirrors trained on the primary image. And the insuperable limitation to magnification lies in our atmosphere which is almost always turbulent. "Twinkle, twinkle little star" is the manifest evidence of atmospheric irregularities about which astronomers can do little else but cuss. Neither rapid nor spectacular results are to be expected when work. begins with the Mt. Palomar telescope. Investigations will be mainly directed to distant frontiers of space, not to neighbouring planets. So do not expect to

find headlines in your newspaper such s ‘Martians Construct Strange Edifice," or "Lunar Craters Grow Bigger," or "Six New Comets Last Night." There is one other important consideration worth mentioning about the design of the _ telescope, Amateur photographers know that a "fast" lens has a short focal length. -The large concentration of light on a relatively small image means a_ reduction of exposure time. This principle has been applied to make the 200-inch mirror more effective. The ratio of its focal length to diameter (or aperture) is only 3.3, so the focal length (about 55 feet) is comparatively short. This property of extreme light concentration will make it incomparable in the photography of very faint object:. By other supplementary mirrors, higher focal ratios of 16 and 30 can be used in work on any source which gives enough light, such as the sun, moon and nearer planets. These longer focal lengths give a higher magnification. The observations will be mainly of two kinds@#-direct photographs of selected areas or faint objects, and spectograms. A spectrogram is a photographic record of the analysis of the radiation. received.. It. shows the intensities and wavelengths of. the component radiations. Two Kinds of Telescope Telescopes fall naturally into two classes; the refractors which use a lens (like the eYe), and the reflectors which use a mirror. It has not proved practicable to construct successfully a lens greater than 40 inches in diameter, which is the size of the one in the Yerkes Observatory telescope. The true function of a large telescope may be made clear ‘by simple considerations. Since the only light which enters the instrument is that which falls on the lens or mirror,.as the case may be, this should be as large as possible. When fully dilated the pupil of your eye has a diameter of about a third of an inch. In that condition it can record the light of a few thousand bright stars. Dimmer stars are swallowed up on the black

background. If your eye had a diameter of just over two inches-the size of Galileo’s lenses-the light gathering area would be increased 60 times, and half-a-million stars could be viewed. The area of the Mt. Wilson mirror brings into photographie view over 1000 million stars. By doubling the diameter of the mirror the light-gathering power is increased four-fold, so that the Mt. Palomar telescope is expected to add

hundreds of millions of stars to the number previously observable. About a century ago Lord Rosse excited the wonder of the world by constructing his © great six-foot reflector in Ireland. He secured a twofold advance: he could see fainter stars, and he saw them more clearly defined. Nebulae which in smaller telescopes appeared as hazy masses of *gas were "resolved" into myriads of tiny faint . stars. There were, however, nebulae which his telescope could not resolve, and presumably others. which were too faint to be located at all. The situation was completely changed as the result of three important discoveries. The spectroscope showed that some of the nebulae, at any rate, were masses of luminous gas, not collections of stars. Later, photography provided an alternative to increasing the size of telescopes by prolongation of exposure time. The third discovery was a method of measuring the distances of stars and nebulae containing stars even when they are exceedingly remote,

The effect of these discoveries has profoundly influenced the deductions drawn from the records gathered at many observatories. One riddle in particular may remain unsolved © unless further evidence is available from the outermost reaches of the universe. Over a long period of years Dr. Edwin Hubble has made observations of the spectra of distant galaxies. The most straightforward explanation of these observations is that the universe is expanding at an ascertainable rate. This hypothesis, which is based on the well-established physical principle known as the Doppler effect, has received support from leading cosmologists. Dr. Hubble and some others, however, state that the consequences of this hypothesis appear to be unacceptable, so they prefer to await an alternative explanation, which they think may be found in a new principle of physics. Primeval Light The feature of the known universe is its vastness. This is inconceivable to the layman and hardly conceivable to the scientist, who, in general, has been trained to think in terms of atomic dimensions rather than in terms of stellar distances. Astronomers prefer a unit of length called the parsec, which is convenient for their measurements, but they sometimes use the light year, which is of suitable size and easier to explain. It is the distance light travels in one year at its colossal speed of 186,000 miles every second. A light year is approximately six million million miles.

I know of no adequate way)of forming a clear concept of this unit, whichis so totally different in order of magnitude from distances on the earth, The nearest star is more than three light years away from us. The farthest galaxy yet observed is about 500 million light years away, so the only information we can receive from it to-day is a message it sent out long, long ago. The new telescope is expected to more than double this limit of telescope recording. Thus it will increase the volume of space exposed to astronomical investigation at least eight times, probably considerably more, No time will be wasted with the world’s largest telescope. It is so valuable scientifically, and so costly an investment, that every moment of available time will be wisely used in accordance with a plan of co-operative research. Probably its most important applications will be the study of very remote nebulae, the # analysis of the nearer large spiral nebulae, and the complete spectrographic investigation of many stars in our own galaxy. There are many other problems both physical and astronomical, which await illumination from it. Judging from the case of its predecessor at Mt. Wilson, its most striking usefulness may eventually be in some direction at present unforeseen or just dimly discernible. This will add to the excitment in the astronomical world. It may even provide an excuse for the incompleteness of this outline of what is expected from Mt. Palomar Observatory. ;