There are few scientific instruments made to-day which call for such precision in their assembly as modern telescopes. As these instruments may weigh 100 tons or more and have mirrors several tons in weight, their building calls for great engineering skill
THE WESTERN MEMBER OF THE POLAR AXIS of the 100-in Hooker telescope being swung into position. This enormous telescope was built for the Mount Wilson Observatory, California, at a cost of £125,000. The polar axis (parallel to an imaginary line joining the poles of the earth) is in the form of a yoke supported at north and south by enormous pedestals.
IN man’s effort to conquer space and to learn something of the marvels of the universe he has built many mighty telescopes. This work demands the skill and ingenuity of the engineer, as well as that of the optician and the electrician The largest of existing telescopes is the 100-in Hooker reflector housed in a circular building on a mountain top in Southern California. The telescope weighs 96 tons, and has a mirror of 8 ft 4-in diameter which alone weighs 4½ tons. It is to be outclassed by a still bigger instrument, with an object glass 16 ft 8-in diameter. This is the largest piece of glass ever cast and weighs 17 tons.
Half a century ago the most powerful of telescopes weighed no more than 15 or 20 tons, with eye-pieces of correspondingly small diameter. It was in 1888 that the Lick Observatory first opened its doors and astounded the astronomical world with a 36-in refractor telescope. It was built according to the provisions of the will of James Lick, an American millionaire, who left a sum of £140,000 to found and equip an observatory to contain a telescope “superior to and more powerful than any instrument ever yet made”. The telescope, which has a focal length of 56 feet, was built and set up in a steel house on Mount Hamilton, not far from San Jose, in California.
Lick’s example was quickly followed by other wealthy men. Telescopes grew in size and power, and were named after their donors. There developed a race among the great observatories of America as to which of them should hold the blue riband of the astronomical world. The Lick was soon followed by another mighty refractor with an object glass of 40-in diameter, known as the Yerkes telescope, the gift of Mr. Charles T. Yerkes to the University of Chicago.
A sum of £250,000 was spent upon the telescope and on the observatory that houses it at Williams Bay, not far from Chicago. The observatory’s circular, movable domed roof was the largest that had been built at that time and was rightly regarded as a clever piece of engineering work. So also was the floor, which was so designed that the astronomer sitting at the eye-end of the telescope could cause it to rise or fall by the mere pressure of a button. The telescope, which has a focal length of 64 feet, weighs, with its pedestal, 75 tons. Three years were spent in grinding and polishing the mirror, and when this work was finished and the lens ready for use it weighed half a ton.
Scarcely had the Yerkes telescope begun to astonish the astronomical world with its success, when work was started on a still larger instrument with an eye-piece of 60-in diameter. It took six years to build. Completed in 1916, it was set up in a steel house on Mount Wilson, not far from Pasadena, in California. The observatory here is under the control of the Carnegie Institution. Whereas the Lick and the Yerkes telescopes were of the refractor type, the 60-in instrument was a reflector. There are two distinct types of telescopes - refractors and reflectors. In the refractor the observer looks through a convex lens at a star; in the reflector he sees the reflection of the image in a concave mirror.
AN OBSERVER AT THE 100-INCH TELESCOPE in Mount Wilson Observatory. Observations can be made from four points of the instrument according to the focus used. The observer is using the Cassegrain focus, in which the image is reflected by a convex mirror to the top of the tube and back again, so that the image is focused at a point at the side of the tube near its base.
If these optical giants are to do good work, they must be housed in suitable quarters. Hence we find observatories on the tops of mountains where the atmosphere is clear and undisturbed by vibrations set up in busy crowded centres. The building of a modern observatory on a lonely mountain top, and the assembling within its walls of a telescope, turning the scale at 100 tons, are formidable tasks.
Long before the 60-in telescope was ready to wrest new secrets from the heavens, Canadian astronomers, envious of the big instruments owned by the American observatories, induced their Government to enter the race for the world’s greatest telescope, and a reflector with a glass 72-in in diameter was planned. It represented the largest single contribution ever made at that time by any Government to the cause of astronomical research. The Canadian telescope, the largest in the British Empire, has an object glass with a diameter twice that of the largest telescope in Great Britain, the 36-in Yapp reflector at the Royal Observatory, Greenwich. This instrument was the gift of an American, Mr. W. J. Yapp.
The home of the 72-in Canadian telescope is a circular domed building, 66 feet in diameter and 75 feet in height. It has a movable roof, perched on the hump of Saanich Hill, 732 feet above sea level, on the outskirts of the city of Victoria, British Columbia. The telescope rests upon a massive monolithic pier, somewhat resembling in form a bent arm, the higher end corresponding to the shoulder and the lower to the hand. The gap between these two points is bridged by the long polar axis of the instrument, set in a direction parallel to the earth’s axis and supported at either end by heavy bearings. It is built up in three parts and, complete, is nearly 23 feet long and weighs about 10 tons. Attached to it is the declination axis, 15 feet in length and weighing 5¼ tons.
The telescope tube is built in three sections and is sufficiently large for an ordinary motor car to pass through it. To its upper end is attached a skeleton extension measuring over 23 feet long, 7½ feet in diameter and weighing, with all attachments, about 2 tons. Despite the heavy weights of the integral parts - the total weight of the moving parts is about 45 tons - a pull of only 5 lb upon the upper end of the tube is sufficient to set the instrument in motion.
The mirror, the most vital part of the whole instrument, was made from a special high-grade optical glass, in which Belgian manufacturers excel. Many months were spent in grinding and polishing the glass and boring the necessary hole in the centre. The surface was given a thin skin of silver, deposited by chemical means. When finally finished and ready for mounting, the mirror measured 73-in across its face and was 12-in thick at the edge, with a hole in its centre 10½-in diameter. The mirror weighed 4,340 lb, or nearly two tons.
Canada was not long to hold the record for owning the largest telescope. The 60-in reflector at Mount Wilson was doing splendid work, and the fact that the object glass of the Canadian instruments had been successfully increased another 12-in led American astronomers to contemplate building an instrument with a mirror of 100-in diameter.
SEEN THROUGH THE HOOKER TELESCOPE, the topography of the moon can be closely examined and photographed. This photograph shows the vicinity of the huge crater of Copernicus, which has a diameter of 56 miles.
Casting such large disks is expensive and largely experimental. There is no guarantee that they will be free from flaws. In making the 100-in mirror the manufacturers had eight failures before they felt confident of success. Even then there were internal irregularities in the glass, and it was not until it had been partly ground and polished that it was found to be suitable for astronomical work. A gift of £9,000 by Mr. John D. Hooker, a citizen of Los Angeles, had enabled the experiment of casting the 100-in mirror to be made, and the telescope, which was built from funds contributed by the Carnegie Institution, was named after him. All told, a sum of £125,000 was spent on the instrument. It is the astronomical wonder of the Mount Wilson Observatory.
The Mount Wilson Observatory is open to the public one evening a week. Fifteen thousand people visit it annually. It is a novel and interesting experience. This extraordinary workshop of the scientists is perched 5,704 feet above sea level on one of the higher peaks of the Sierra Madre range and is approached by a zigzag road from Pasadena. The summit of the mountain is littered with steel towers and strange buildings. Visitors are conducted over the buildings, shown the various instruments and given an opportunity of looking through one of the smaller telescopes at the moon, one of the planets, or some distant nebulae or star.
One structure of wood and canvas, standing out boldly on the mountain side, has been not inaptly nicknamed “The Ark”. Its work is to take photographs of the sun with a spectroheliograph. In connexion with this building there are two tower telescopes. 60 feet and 150 feet in height respectively. At the top of these skeleton steel towers are special mirrors which catch the sun’s light and send it vertically downward through the lens to spectrographs mounted in wells under the towers. The depth of the well is half the height of the tower. Here the image is caught and examined. These tower telescopes play an important part in the determination of the magnetic fields in sun-spots and in the taking of measurements of solar rotation.
Apart from these structures there are two circular domed buildings and the “monastery”, as the living quarters of the astronomers are termed. The smaller of the domed buildings houses the 60-in telescope, the newer and larger one being the home of the 100-in Hooker reflector.
From the time the work of polishing the mirror began until the great telescope stood complete in its steel house on the mountain top, six years had elapsed. Transporting the steel girders, massive pieces of machinery, the heavy framework and finally the finished mirror up the steep mountainside was no light task. Some years before a road had been cut to the site, but in places it was exceedingly steep; so the roadway was widened and a specially-designed motor truck was built for negotiating the road. The wheels of the truck were of small diameter, but wide, resembling rollers, and the truck was geared down to a speed of four miles an hour. Although the distance from the observatory workshops in Pasadena to the mountain site is only nine miles, frequently a whole day was occupied in hauling a single heavy piece of machinery. It took fourteen hours to drag up the mirror.
Over 600 tons of steel and 200 tons of cement were carried up in all, and with this material a suitable home for the new telescope was built. Working here on the bleak mountain top in mid-winter was trying and even dangerous, because of the sudden storms. Snowstorms were frequent and, in the cramped space available, care had to be taken not to miss a foothold, which meant being precipitated down the mountainside. One of the carpenters engaged thus had a narrow escape. The timber piling was buried in snow, and while the carpenter was searching for a piece of wood, he started a snow-slide. Before he realized what had happened, he found himself half-buried in snow 600 feet below the summit. He was stopped in his downward flight by striking a tree. He was uninjured. The mounting of the telescope has required the building of an enormous dome, 100 feet in diameter and 100 feet high, beyond which the shutter and fin project 4 feet Up to a height of 28 feet the dome is stationary; all above this level rotates. The dome is of metal, double-walled throughout to preserve a more uniform temperature through the day. Forty massive piers, arranged in two concentric rings round the main pier on which the telescope is mounted, provide substantial footings, heavy enough to stand severe winter winds.
THE 200-INCH MIRROR which was cast for a telescope on Mount Palomer, Southern California, has its back ribbed so that the weight of the mirror has been reduced from 40 to 17 tons. The thickness of the disk is about 27-in and the centre hole has a diameter of 40-in.
The main pier which carries the telescope is of hollow reinforced concrete and rises to a height of 33 feet, to remove the telescope from ground radiations at night. A solid wall extends to the south of the pier. On the side of this wall is hung the Coude spectrograph, with a focal length of 30 feet. The pier and wall are placed on a bed of sand 6½ feet deep, to prevent any vibrations of the dome from being felt at the telescope. No part of the dome is allowed to touch cither the pier or the wall.
Rotating 96-Tons Weight
At the top of the pier is a circular floor 52 feet in diameter. This carries on its outer rim a metal wall rising 9 feet to the upper floor of the dome, and designed to shut off air currents between the top and bottom of the dome. Within the pier there is a system of tanks and pumps arranged to maintain a constant temperature system for the mirror, and here are located the dark room, silvering room and the like.
The mounting of the telescope is of the English type, in which the polar axis is in the form of a yoke supported on pedestals at either end, with the tube swinging in the yoke. The pedestals are made of cast iron, that at the north consisting of five sections. The yoke is a massive structure built up of four sections, each weighing about 10 tons. The smaller parts of the telescope were made in the observatory’s own workshops at Pasadena, but the heavy castings and large sections of the tube were specially forged in the steel works of a shipbuilding company.
Before the yoke was transported to the observatory it was assembled, and the seats for the trunnions and bearings for the declination axis were carefully bored in line and at right angles to each other.
It was necessary to fit the various sections together with mathematical accuracy; the least miscalculation, even to the smallest fraction of an inch, would have spelt ruin. Work of this kind must not be hurried, and has to be checked constantly with delicate scientific instruments.
The moving parts of the telescope weigh 96 tons. To permit this enormous mass to be rotated without undue friction the bulk of the weight is taken up by a flotation system. Forty tons of the weight is taken at the north pedestal and 56 tons at the south pedestal. The displacements are obtained by floating steel cylinders built up in cast-iron tanks with less than ⅛-in clearance between them. This is filled with mercury in which the cylinders float. The tanks had to be carefully machined to keep the volume of mercury small and to ensure freedom of contact with the float. As the different cages and attachments of the telescope have weights differing by several tons, a plunger device is installed to adjust the mercury to any desired level. Mounted at the lower end of the polar axis is a worm wheel which meshes with the driving train of the clock. This wheel has 1,440 teeth accurately cut to avoid periodic error. After assembly in final position on the telescope the graduations of an accurately divided circle were transferred to a brass strip in the rim and checked to one thousandth part of an inch. The teeth on this worm were then cut while the wheel was in position to ensure accuracy. The clock is driven by a massive weight which tips the scale at 1½ tons, the speed being regulated by a governor of the pendulum type. The weight is wound up automatically at intervals of twelve minutes. The rewinding is done without affecting the driving of the clock. The clock stands 6 feet high and occupies a floor space 5½ by 4 feet. It connects with the driving worm by a double universal joint.
The tube, which is mounted within the yoke, is 11 feet in diameter and 34 ft 11-in long. On this are added cages measuring from 5 ft 5-in to 10 ft 3-in. All sections of the tube were placed in the boring mill and machined true to size, so that upon assembly the tube as a whole would be strained uniformly.
INTERIOR OF THE DOME which contains the 100-in Hooker telescope. The moving parts of the telescope weigh 96 tons and the reflector, mounted at the bottom of the tube, weighs 4½ tons. The enormous dome which houses the telescope has a diameter of 100 feet. The telescope is mounted on a pier of hollow reinforced concrete 33 feet high.
The mounting of the mirror required the greatest accuracy. It lies in the bottom of the great tube in a specially designed steel cell. The diameter of the mirror is 101-in. It is 12¾-in thick at the edge and its weight is about 4½ tons. Four years were spent in grinding and polishing it and boring the hole in the centre. What was originally a flat disk of glass was ground until it became parabolic in contour. Any light reflected from any point of its surface comes to a focus at the same point, 42 feet away. Altogether a ton of glass was removed in the grinding and polishing operations. Then followed a coating of silver, deposited by chemical means, when the mirror was ready for insertion in its cell. Mirrors of this size are sensitive to changes of heat and cold, and therefore an elaborate 1-in cork-board chamber is built round the glass, which, with other apparatus, including pipes and coils and fans, keeps the temperature as desired. Mirrors also become tarnished and have to be re-silvered. Resilvering a 4½ tons giant glass is no light job, but it is done at Mount Wilson twice a year. The mirror is first unfastened in the tube and lowered through the floor in a plunger, or lift, which rises to receive it. Here the resilvering process begins.
The old coat of silver is first removed with acid and the glass given a thorough scrubbing. The silvering solution is then poured on. Three pounds of silver nitrate solution are necessary for the job. The surface of the glass is next dried. Then a huge pad worked by a power crank burnishes the film with circular strokes, the disk meanwhile slowly rotating. The silvering of the mirror occupies a dozen men a whole day, preparations beginning as early as 4 o’clock in the morning. The silver film deposited over the glass is about one hundred-thousandth of an inch in thickness.
250,000 Times as Bright
The instrument is of the reflector type, and it is the reflection of a star which the observer sees. The star’s light rays are caught by the large mirror at the bottom end of the telescope tube and reflected back upon a secondary mirror or prism, and there held until examined or photographed. It makes a star appear 250,000 times as bright as the eye sees it. This is because a mirror with a diameter of 100-in has an area 250,000 times as large as the pupil of the human eye. Only a negligible amount of light is lost in the process of reflection. The use of the mirror brings our nearest neighbour in space, the moon, which is about 240,000 miles away, within a few hundred miles of the earth. It has brought within photographic range 1,500,000,000 stars, whereas fewer than 5,000 can be seen by the naked eye.
There are four ways in which observations can be made through the telescope. The four methods are known as the prime, the Newtonian, the Cassegrain, and the Coude focuses.
When photographing celestial objects or when obtaining spectra of faint objects, the photographic plate or small spectrograph may be placed at the upper end of the tube, 36 feet above the mirror. This is called the prime focus. In the Newtonian focus a plane mirror is set about 6 feet down the tube and inclined at an angle of 45 degrees. This catches the ray of light reflected from the mirror, bends it at right angles and throws it upon the plate or spectrograph set at the side of the tube.
AT THE ROYAL OBSERVATORY, GREENWICH, LONDON. Here is housed Great Britain’s largest telescope, the gift of an American, Mr. W. J. Yapp. The reflector has a diameter of 36-in. This photograph shows the Astronomer Royal at work.
The Cassegrain form involves inserting a convex mirror a few feet below the prime focus. This throws or reflects the light received from the great, mirror to the top of the tube and back again, but in such a way as to pass through the central hole in the big mirror to a focus at the side of the tube near its base where the light can be observed, photographed or analysed by the spectrograph.
By making the rays travel up and down the tube in this way the focal length is considerably increased, from 42 feet with the prime and Newtonian methods, to 134 feet with the Cassegrain. With the latter method images are secured of exactly the same size as would be obtainable with a refracting instrument having a tube as much as 134 feet long.
In the Coude method the focal length is 250 feet, due to the fact that the beam has to pass down the hollow polar axis of the telescope to a room in the pier where the spectrograph is situated.
The telescope and dome are controlled and operated from a deck on the pier at the 42-feet level just south-west of the telescope. To this deck all circuits and circle readings are carried, as well as to the siderial clock and setting circle. Auxiliary controls are also placed at the various foci or observation points of the instrument. All motions are controlled by buttons in the hands of the observer, no matter from what points he may be making observations. Altogether there are used in the dome and on the telescope forty motors of one-twentieth to 7½ horse-power in size, with an aggregate of 50 horse-power and more than thirteen miles of wiring.
Disk of 200-in Diameter
When this telescope was built it was thought, the limit had been reached, at least for many years to come. Glass manufacturers declared it was impossible to cast mirrors of larger size. Then someone thought of the possibility of using a special material manufactured by melting boro-silicate glass at an extremely high temperature. Experiments showed that. mirrors of this material were suitable for telescopic work, and a disk 200-in in diameter has been cast.
Casting such a huge slab of glass presented many problems. After the molten glass had been poured into its mould it was allowed ten months to cool to avoid strains and other blemishes. The mirror was not cast in a solid piece as it would have weighed more than 40 tons. This weight was reduced by more than half without loss of structural strength by casting the disk with a ribbed back. The mirror is about 27-in thick, with a hole of 40-in diameter in the centre, and weighs about 17 tons. A special machine, weighing over 50 tons, had to be built in which the grinding and polishing operations could be carried out.
It is not expected that the great mirror will be ready for work before 1940. It will then be placed in the bottom of a telescope to be built and set up on Mount Palomer, in Southern California. The giant telescope with its tube, mirror and mountings will weigh 450 tons.
Mount Palomer was selected as a site for the new telescope after five years’ study of the heavens in this region. The sky between the stars is darker here than at any other point, in south-west America. The telescope will undertake no work available to the 100-in. instrument at Mount Wilson, but will be confined to studies of selected celestial bodies.
Astronomers declare that it will be powerful enough to penetrate more than three times as far into space as any existing telescope, and to open up an unexplored sphere of the heaven thirty times the volume of that known.
MOUNT WILSON OBSERVATORY stands at a height of 5,704 feet above sea level, on one of the peaks of the Sierra Madre, a few miles from Pasadena, California. The dome which houses the 100-in. telescope rotates, except for the lower 28 feet of its walls.