Our National Observatory at Greenwich has still nothing larger than a 28in. refractor, erected some four years ago. In Europe, the most powerful telescope yet constructed is the 30in. refractor of the Russian Imperial Observatory at Pulkowa. Since 1888 the Lick telescope has held the world's record easily. It was a great task to beat, but another American millionaire, Mr. Charles T. Yerkes, has beaten it.
In the early autumn will be formally opened the Yerkes Observatory, at Lake Geneva, Wisconsin, which contains a refracting telescope with lenses 40in. in diameter—4in. larger than those of the Lick telescope—and a focal length of 61ft.
Work upon it was begun in 1862, and it has been completed in remarkably short time. Thirteen years were required for the construction and equipment of the Lick Observatory. Much of this delay was due to the difficulties encountered in casting the thick blocks of glass. At the centre of the Yerkes objective, as the combination of crown and flint glass lenses is termed, no less than 3 ¾in. of solid glass has to be pierced, and it will be readily understood that the light of a faint star would be entirely lost in passing through this thick medium unless the glass were of perfect purity.
Mr. Yerkes is a great Chicago street-railway magnate. Out of his immense fortune he set apart a million dollars for the construction of the telescope and the observatory, with liberty to draw upon him for any more money that might be required—and it is understood that liberal drafts have been made. He was fortunate in finding the glass casts for his telescope ready to hand.
Optical lenses 40in. in diameter, and of great purity and excellence, had been cast by M. Mantois, in Paris, for the University of Southern California, when the erection of a great observatory on Wilson's Peak was under consideration. A cheque for 40,000 dollars secured them in the rough. They were carefully packed, and shipped to Cambridgeport, Massachusetts, where Mr. Alvin Clark, the last surviving member of the famous firm of Alvin Clark and Sons, opticians, the constructors of the Lick and other giant objectives, spent four years in reducing them to the required figure.
This is work which requires an amount of mathematical exactness of almost inconceivable delicacy. Every ray of light falling on the great circular disc of glass 40in. in diameter, has to be bent to focus 61ft. distant, and to meet at a point not larger, if the result is to be perfectly satisfactory, than the dot over the letter "i" in this type.
As may be imagined, the process is a long and laborious one. First the glass is ground down with sand and water so as to get rid of the roughest projections on the surface. Then it is treated with emery, coarse at first, and grading down to the very finest, and the last touches are given with rouge. Generally there remain a number of corrections which keep the optician employed for many months. So exceedingly fine is the work, that a little rubbing with the finger on the hard glass will sufficiently deflect the ray of light out of its true course on its long journey to the focus as to distort the image. And this accuracy of curvature has to be obtained, be it remembered, not on one, but four surfaces.
When once the lenses are completed, what difficulties remain in the construction of a great telescope are for the engineers; but, as may be imagined, they are not light. Expressed in figures, the Yerkes telescope, with its attachments, has an extreme length of 74ft.; but figures convey a poor idea of what has been accomplished. Take a comparison with some well-known object. Cleopatra's Needle, the great granite obelisk on the Thames Embankment, is 68ft. in height. Add 6ft. to Cleopatra's Needle, poise it at a sufficient height to clear the ground, on bearings so strong and yet so delicately constructed that it can be moved in any direction by the hand, and over all place a movable, circular dome, and then you have an idea of the proportions of the Yerk es telescope.
The tube is of wrought steel, varying in thickness, the strain having been nicely calculated so that the weight could be cut down as much as possible. It is about ¼in. thick at the middle, and tapers to 1/8in. at the extremities. The tube is 4ft. 2in. in diameter at the middle. Its weight is six tons.
In the illustration on the next page, drawn in the Observatory recently, a very good impression is given of the telescope mounted on its pedestal, and of the arrangements for swinging the big tube. To the great kindness of Professor Hale, the director of the Yerkes Observatory, I am indebted for the Yerkes photographs which illustrate these pages.
At the farther extremity the tube carries a steel cell, in which the glasses which form the lens are securely fixed. The crown-glass is double-convex, 2½in. thick at the centre, and ¾in. at the edge, and weighs two hundred pounds. The flint-glass is 2in. thick at the edge, and 1¼in. at the centre, and weighs over three hundred pounds. Between the glasses a space of 83/8in. is left clear.
This steel cell with its burden weighs nearly half a ton; and no matter at what angle the tube may be placed, it is essential that it should remain perfectly rigid. A slight flexure would throw the image out of the range of the magnifying eye-piece.
Seen through the great eye of the telescope, faint stars come out with startling brilliancy and clearness, the divisions of Saturn's rings are plainly visible, while the satellites, always difficult objects to graspin a smaller telescope, shine forth like little moons in the sky.
Electricity has been largely brought into use in the manipulation of the telescope, so that by the mere pressing of a button the great mass is moved in any direction. A powerful driving clock, corrected to fractions of a second, enables the telescope automatically to follow the stars in their apparent motion across the sky.
One difficulty may confront the reader. When a star or planet is to be observed low on the horizon, how is the observer to get within reach of the eye-piece of the telescope? It may be 30ft, above his head. This has been met by making the observatory floor rise or fall by electrical power through a range of 23ft., a device first suggested by Sir Howard Grubb for the Lick Observatory. This elevating floor is a great circular iron platform 75ft. across.
There are seventy-five tons of metal in the Yerkes telescope and its pedestal. Over all, capping a solid masonry tower, is a huge revolving steel dome, 90ft. in diameter, and weighing 150 tons. This dome is, like the telescope, the largest of its kind in the world.
Now, as to the performance of the Yerkes telescope. Its magnifying eye-pieces will a give powers ranging from 200 to 4000. With the highest power the moon will be brought within sixty miles of the earth, that is to say, it it will be seen through the telescope just as it would be seen with the naked eye if it were suspended sixty miles over our heads, and under the most favourable conditions with more distinctness.
But with each increase in magnifying power there is a contraction of the field of view, so that despite the great size of the telescope, with 4000 magnifications scarcely the whole of one of the large lunar craters would be shown. This disadvantage the telescope has compared with the naked eye. If, however, Trafalgar Square happened to lie within the crater it would be readily distinguished, and the night illumination of London would make our metropolis a prominent object on the moon.
In practice it rarely happens that these high powers can be employed, owing to the disturbance of the atmosphere. For not only is the object observed magnified, but every movement in the atmosphere as well, and unless the night is quite exceptionally still, the image becomes blurred and jogs about, to the dismay of the unhappy astronomer. The "twinkling" of a star is entirely an atmospheric effect.
Where a great telescope is valuable is not, after all, so much in these very high powers of magnification which can be applied, as in its wide grasp of light. Visually, the operation of a telescope is much like that of the eye. All the light that enters the "pupil" of the eye—a small circle generally of about one-eighth of an inch, and never more than three-eighths of an inch, in diameter— is brought to focus on the retina. The Yerkes telescope is a great eye which admits light, not in a small circle of one-eighth of a single inch, but in a great circle of forty inches. Light collected in this great area is brought into a small space, and thence passes into the eye, so that faint objects become visible with comparatively low powers.
In daytime the big telescope will be used for systematic investigation of the sun, in which its unrivalled powers may be expected to be of especial value. Two nights each week are to be devoted to spectroscopic studies of stars and nebulas, with especial view to the detection of variable stars and the motions of the sidereal system; and other nights to the observation of planets, comets, etc., and the measurement of double stars. Its work will be supplemented by two smaller telescopes, one of 16in. and one of 12in. aperture, the domes of which are situated at the eastern end of the long arm of the observatory buildings. The Observatory complete is a munificent gift by Mr. Yerkes to the University of Chicago.
Thorough in all things, Mr. Yerkes has gathered around him the most distinguished astronomers on the American continent. Professor Hale, so well known for his studies on the sun, will direct the labours which are to he undertaken with the great telescope. Professor Barnard, the discoverer of Jupiter's fifth satellite, is his assistant, as also is Mr. Burnham, whose keen sight has brought many new systems of double and triple stars to our knowledge. Their work will be mainly on the lines of the "New Astronomy," which has given us the marvellous discoveries of the spectroscope and the camera, that have made the last thirty years so memorable.
Special thanks to Roger Todd for pointing out this article - Marcus L. Rowland
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