Astronomy for Everybody/Part 2/Chapter 1

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There is no branch of science more interesting to the public than that with which the telescope is concerned. I assume that the reader wishes to have an intelligent idea as to what a telescope is and what can be seen with it. In its most complete form, as used by the astronomer in his observatory, the instrument is quite complex. But there are a few main points about it which can be mastered in a general way by a little close attention. After mastering these points, the visitor to an observatory will examine the instrument with much more satisfaction than he can when he knows nothing about it.

The one great function of a telescope, as we all know, is to make distant objects look nearer to us; to see an object miles away as if it were, perhaps, only as many yards. The optical appliances by which this is effected are extremely simple. They are made with large well-polished lenses, of the same kind as those used in a pair of spectacles, differing from the latter only in their size and general perfection. A telescope requires an appliance for collecting the light coming from the object so as to form an image of the latter. There are two ways in which the light may be collected, one by passing the light through a set of lenses, and one by reflecting it from a concave mirror. Thus we have two different kinds ​of telescope, one called refracting, the other reflecting. We begin with the former because it is the more usual.

The lenses of a refracting telescope comprise two combinations or systems; the one an object-glass—or "objective," as it is sometimes called for shortness—which forms the image of a distant object in the focus of the instrument; and the other an eyepiece, with which this image is viewed.

The objective is the really difficult and delicate part of the instrument. Its construction involves more refined skill than that of all the other parts together. How great is the natural aptitude required may be judged from the fact that a generation ago there was but one man in the world in whose ability to make a perfect object-glass of the largest size astronomers everywhere would have felt confidence. This man was Alvan Clark, of whom we shall soon speak.

The object-glass, as commonly made, consists of two large lenses. The power of the telescope depends altogether on the diameter of these lenses, which is called the aperture of the telescope. The aperture may vary from three or four inches, in the little telescope which one has in his house, to more than three feet in the great telescope of the Yerkes Observatory. One reason why the power of the telescope depends on the diameter of the object-glass is that, in order to see an object magnified a certain number of times, in its natural brightness, we need a quantity of light expressed by the square ​of the magnifying power. For example, if we have a magnifying power of one hundred, we should need ten thousand times the light. I do not mean that this quantity of light is always necessary; it is not so, because we can commonly see an object with less than its natural illumination. Still, we need a certain amount of light, or it will be too dim.

In order that distinct vision of a distant object may be secured in the telescope, the one great essential is that the object-glass should bring all the rays coming from any one point of the object observed to the same focus. If this is not brought about; if different rays come to slightly different foci, then the object will look blurred, as if it were seen through a pair of spectacles which did not suit our eyes. Now, a single lens, no matter of what sort of glass we make it, will not bring rays to the same focus. The reader is doubtless aware that ordinary light, whether coming from the sun or a star, is of a countless multitude of different colours, which can be separated by passing the light through a triangular prism. These colours range from red at one end of the scale, through yellow, green, and blue, to violet at the other. A single lens brings these different rays to different foci; the red farthest from the object-glass; the violet nearest to it. This separation of the rays is called dispersion.

The astronomers of two centuries ago found it impossible to avoid the dispersion of a lens. About 1750, Dollond, of London, found that it was possible to correct this defect by using two different kinds of glass, the one crown glass and the other flint glass. The prin​ciple by which this is done is very simple. Crown glass has nearly the same refracting power as flint, but it has nearly twice the dispersive power. So Dollond made an objective of two lenses, a section of which is shown in the figure. First there was a convex lens of crown glass, which is of the usual construction. Combined with this is a concave lens of flint glass. These two lenses, being of opposite curvatures, act on the light in opposite directions. The crown glass tends to bring the light to a focus, while the flint, being concave, would make the rays diverse.
Fig. 10.—Section of the Object-glass of a Telescope. If it were used alone, we should find that the rays passing through it, instead of coming to a focus, diverge farther and farther from a focus, in different directions. Now, the flint glass is made with but little more than half the power of the crown. This half power is sufficient to neutralize the dispersion of the crown; but it does not neutralize much more than half the refraction. The combined result is that all the rays passing through the combination are brought nearly to one focus, which is about twice as far away as the focus of the crown alone.

I say brought nearly to one focus. It happens, unfortunately, that the combined action of the two glasses is such that it is impossible to bring all the rays of the various colours absolutely to the same focus. The divergence, in the case of the brighter rays, can be made very small indeed, but it cannot be cured entirely. The larger the telescope, the more serious the defect. If you look ​at a bright star through any large refracting telescope, you will see it surrounded by a blue or purple radiance. This is produced by the blue or violet light which the two lenses will not bring to one focus.

By the action of the objective, in thus bringing rays to a focus, the image of a distant object is formed in the focal plane. This is a plane passing through the focus at right angles to the axis or line of sight of the telescope.

What is meant by the image formed by a telescope can be seen by looking into the ground glass of a camera with the photographer, as he sets his instrument for a picture. You there see a face or a distant landscape pictured on the ground glass. To all intents and purposes the camera is a small telescope, and the ground glass, or the point where the sensitive plate is to be fixed to take a picture, is the focal plane. We may state the matter in the reverse direction by saying that the telescope is a large camera of long focus, with which we can take photographs of the heavens as the photographer takes ordinary pictures with the camera.

Sometimes we can better comprehend what an object is by understanding what it is not. In the celebrated moon hoax of half a century ago or more, there was a statement which illustrates what an image is not. The writer said that Sir John Herschel and his friend finding that, when they used enormous magnifying power, there was not light enough for the image to be visible, the ​friend suggested that the image should be illuminated by artificial light. This was done with such brilliant success that animals in the moon were made visible through the telescope. If many people, even those of the greatest intelligence, had not been deceived by this, I should hardly deem it necessary to say that the image of an object formed by a telescope is such that, in the very nature of things, extraneous light cannot aid in its formation. Its effectiveness does not proceed from its being a real image, but only from the fact that all the rays from any one point of a distant object meet in a corresponding point of the image, and there diverge again, just as if a picture of the object were placed in the focal plane. The fact is that the term picture is perhaps a little better one than image to apply to this representation of the object, only the picture is formed by light and nothing else.

If an image or picture of the object is thus formed so as to stand out before our eyes, one may ask why an eyepiece is necessary to view it; why the observer cannot stand behind the picture, look toward the objective and see the picture hanging in the air, as it were. He can really do so if he holds a ground glass in the focal plane, as the photographer does with the camera. He can thus see the image formed on the glass. If he looks into the object-glass he can see it without any eyepiece. But only a very small portion of it will be visible at any one point, and the advantage over looking directly at the object will be slight. To see it to advantage an eyepiece must be used. This is nothing more than a little eye ​glass, essentially of the same kind that the watchmaker uses to examine the works of a watch. The smaller the eyepiece, the more closely the examination can be made, and the greater the magnifying power.

The question is often asked, how great is the magnifying power of some celebrated telescope. The answer is that the magnifying power depends not only on the object-glass but on the eyepiece. The smaller the latter the greater the magnifying power. Astronomical telescopes are supplied with quite a large collection of eyepieces, varying from the lowest to the highest power, according to the needs of the observer.

So far as the geometric principle goes, we can get any magnifying power we please on any telescope, however small. By viewing the image with an ordinary microscope, such as is used by physicians, we might give a little four-inch telescope the magnification of Herschel's great reflectors. But there are many practical difficulties in carrying the magnification of any instrument above a certain point. First there is the want of light in seeing the surface of an object. If we looked at Saturn with a three-inch telescope, using a magnifying power of several hundred times, the planet would seem dim and indistinct. But this is not the only difficulty in using a high magnifying power with a small telescope. The effect of light having a wave length is such that as a general rule we can get no advantage in carrying the magnification above fifty, or one hundred at the ​most, for each inch of aperture. That is to say, with a three-inch telescope we should gain no advantage by using a power much above one hundred and fifty, and certainly none above three hundred.

But a large telescope also has its defects, owing to the impossibility of bringing all the light to absolutely the same focus. There is a limit to the magnification which can be used, rather difficult to define exactly, but of which the observer will be very sensible when he looks into the instrument and sees the blue aureole already mentioned.

But there is still another trouble, which annoys the astronomer more than all others, but which the public rarely understands.

We see a heavenly body through a thickness of atmosphere which, were it all compressed to the density that it has around us, would be equal to about six miles. We know that when we look at a body six miles away, we see its outlines softened and blurred. This is mainly because the atmosphere through which the rays have to pass is constantly in motion, thus producing an irregular refraction which makes the body look wavy and tremulous. The softened and blurred effect thus produced is magnified in a telescope as many times as the object itself. The result is that as we increase the magnifying power we increase a certain indistinctness in the vision in the same proportion. The amount of this indistinctness depends very much on the condition of the air. The astronomer having this in mind tries to find a perfectly clear air, or, rather, air which is very ​steady, so that the heavenly bodies will look sharp when seen through it.

We frequently see calculations showing how near the moon can be brought to us by using some high magnifing power. For example, with a power of one thousand we see it as if it were two hundred and forty miles away; with about five thousand, as if it were forty-eight miles away. This calculation is quite correct so far as the apparent size of any object on the moon is concerned, but it takes no account either of the imperfections of the telescope or the bad effect produced by the atmosphere. The result of both of these defects is that such calculations do not give a correct idea of the truth. I doubt whether any astronomer with any telescope now in existence could gain a great advantage, in the study of such an object as the moon or a planet, by carrying his magnification above a thousand, unless on very rare occasions in an atmosphere of unusual stillness.

Those who have never used a telescope are apt to think that the work of observing with it is simply to point it at a heavenly body and examine the latter through it.^[1] ​But let us try the experiment of pointing a great telescope at a star. A result which perhaps we have not thought of would be immediately presented to our sight. The star, instead of remaining in the field of view^[2] of the telescope, very soon passes out of it by the diurnal motion. This is because, as the earth revolves on its axis, the star seems to move in the opposite direction. This motion is multiplied as many times as the telescope magnifies. With a high power, the star is out of the field before we have time to examine it.

Then it must also be remembered that the field of view is also magnified in the same way, so that it is smaller than it appears, in proportion to the magnifying power. For example, if a magnification of one thousand be used, the field of view of an ordinary telescope would be about two minutes in angular measure, a patch of the sky so small that to the naked eye it would look like a mere point. It would be as if we were looking at a star through a hole one eighth of an inch in diameter in the roof of a house eighteen feet high. If we imagine ourselves looking through such a hole and trying to see a star we shall readily realise how difficult will be the problem of finding it and of following it in its motion.

This difficulty is overcome by a suitable mounting of the telescope, so as to turn on two axes, at right angles to each other. By the mounting is meant the whole system of machinery by the aid of which a telescope is pointed ​at a star and made to follow it in its diurnal motion. In order not to distract the attention of the reader by beginning a study of the instrument with a view of all the details, we first give an outline, showing the relation of the axes on which the telescope turns. The principal axis, called the polar axis, is adjusted so as to be parallel to the axis of the earth, and therefore to point at the celestial pole. Then, as the earth turns from west toward east, a clockwork connected with this axis turns the

instrument from east toward west, with an equal motion. Thus the rotation of the earth is neutralized, as it were, by the corresponding rotation of the telescope in the opposite direction. When the instrument is pointed at a star and the clockwork set going, the star when once found will remain in the field of view.

In order that a telescope may be directed at any point of the heavens at pleasure, there must be another axis, at right angles to the polar axis. This is called the ​declination axis. It passes through a sheath fixed to the upper end of the polar axis so as to form a cross like the letter T. By turning the telescope on the two axes, it can be pointed wherever we choose.

Owing to the polar axis being parallel to that of the earth, its inclination to the horizon is equal to the latitude of the place. In our latitudes, especially in the southern portions of the United States, it will be nearer horizontal than vertical. But in the observatories of northern Europe, it is more nearly vertical.

It will be seen that the contrivance we have described does not solve the problem of bringing a star into the field of view of the telescope, or as we commonly say, of finding it. We might grope round for minutes or even hours without succeeding in this. There are two processes by which a star may be found:

Every telescope for astronomical purposes is supplied with a smaller telescope fastened to the lower end of its tube, and called the finder. This finder is of low magnifying power, and therefore has a large field of view. By sighting along the outside of it, the observer, if he can see the star, can point the finder at it so nearly that it wdll be in the field of view of the latter. Having found it there, he moves the telescope so that the object shall be seen in the centre of the field. Having brought it there, it is in the field of view of the main telescope.

But most of the objects which the astronomer has to observe are totally invisible to the naked eye. He must, therefore, have a system by which a telescope can be pointed at a star, without any attempt on his part to see ​the latter. This is clone by graduated circles, one of which is attached to each axis. One of these circles has degrees and fractions of a degree marked upon it, so as to show the declination of that point in the heavens at which the telescope is pointed. The other, attached to the polar axis, and called the hour circle, is divided into twenty-four hours, and these again into sixty minutes each. When the astronomer wishes to find a star, he simply looks at the sidereal clock, subtracts the right ascension of the star from the sidereal time, and thus gets its "hour angle" at the moment, or its distance east or west of the meridian. He sets the declination circle at the declination of the star, that is, he turns the telescope until the degree on the circle seen through a magnifying aparatus is equal to the declination of the star; and then he turns the instrument on the polar axis until the hour circle reads its hour angle. Then, starting his clockwork, he has only to look into the telescope and there is the object.

If all this seems a complicated operation to the reader, he has only to visit an observatory and see how simply it is all done. He may thus in a few minutes gain a practical idea of sidereal time, hour angle, declination, etc., which will make the whole subject much clearer than any mere description.

Let us return to some interesting matters, mostly historical, connected with the making of telescopes. The great difficulty, which requires special native skill of the ​rarest kind, is, as we have already intimated, that of constructing the object-glass. The slightest deviation from the proper form—a defect consisting in some part of the object-glass being too thin by a hundred thousandth part of an inch—would spoil the image.

The skill of the optician who figures the glass, that is to say, who polishes it into the proper shape, is by no means all that is required. The making of large disks of glass of the necessary uniformity and purity is a practical problem of equal difficulty. Any deviation from perfect uniformity in the glass will be as injurious to its performance as a defect in its figure.^[3]

A century ago it was found especially difficult to make flint glass of the necessary uniformity. This substance contains a considerable amount of lead, which, during the process of melting the glass, would sink toward the bottom of the pot, thus making the bottom portion of greater refracting power than the upper portion. The result was that, at that time, a telescope of four or five inches aperture was considered of great size. Quite early in the century, Guinand, a Swiss, found a process by which larger disks of flint glass could be made. He professed to have some secret process of doing this, but there is some reason to believe that his secret consisted only in the constant and vigorous stirring of the melted glass ​while it was being fused in the pot. However this may have been, he succeeded in making disks of larger and larger size.

To utilize these disks required an optician of corresponding skill to grind and polish them into proper shape. Such an artist was found in the person of Fraunhofer, of Munich, who, about 1820, made telescopes as large as nine inches aperture. He did not stop here, but, about 1840, succeeded in making two objectives, each of fourteen German inches, or about fifteen English inches in diameter. These, far exceeding any before made, were at the time regarded as marvellous. One of these instruments was acquired by the Pulkova Observatory in Russia; the other was acquired by the Harvard Observatory at Cambridge, Mass. The latter, after a lapse of more than half a century, is still in efficient use.

After Fraunhofer's death it was doubtful whether his skill had died with him, or had passed to a successor. The latter appeared where none would have thought of looking for him, in the person of an obscure portrait painter of Cambridgeport, Mass., named Alvan Clark. The fact that such a man, with scarcely the elements of technical education and without training in the use of optical instruments, should have done what he did, illustrates in a striking way what an important element native talent is in such a case. He seemed to have an intuitive conception of the nature of the problem, coupled with extraordinary acuteness of vision in solving ​it. Moved by that irrepressible impulse which is a mark of genius, he purchased in Europe the rough disks of optical glass necessary to make small telescopes. Having succeeded in making one of four inches aperture to his satisfaction, the problem was to make his skill known to astronomers. I regret to say that he found this a very difficult part of his task. The director of the Harvard Observatory would not believe that Mr. Clark could make a really good telescope. When the optician took his first instrument up to the observatory to be tested, the astronomer called his attention to the fact that it showed a little tail attached to the star, which, of course, had no real existence, and was supposed to arise from a serious defect in the figure of the glass. Mr. Clark saw it, but was sure it had not been there before. He could not explain it at the time, but afterwards found that it was caused by the unequal temperature of the air in the tube of the telescope when it was exposed under the sky at night.

Unable to secure any effective recognition at home, he determined to try abroad. He made a larger instrument, scanned the heavens with it and discovered several close and difficult double stars. He wrote out descriptions of these objects and sent them to Rev. W. R. Dawes, an amateur astronomer in England, devoted to this branch of the science. Mr. Dawes was a lovely character. He looked at the objects described by Clark and found great difficulty in making them out. Yet the descriptions were so accurate that it was evident to him that Mr. Clark's instrument must be of the highest class. ​He wrote asking him to look at some other objects and describe them. When the description was received it was found to be exact. No doubt could remain. The result was a further correspondence, the purchase by Mr. Dawes of the largest and best instrument that Mr. Clark could then make, and a friendship which continued as long as Mr. Dawes lived.

Mr. Clark now secured recognition in his own country and became ambitious to make the largest refracting telescope that had ever been known. This was one of eighteen inches diameter, which was completed about I860 for the University of Mississippi. While testing it at his workshop, a discovery of a most interesting character was made with it by Mr. George B. Clark, the son. This was a companion of Sirius, which had been known to exist by its attraction on Sirius, but had never been seen by human eye. The breaking out of the Civil War prevented the University of Mississippi from taking the telescope, and the latter was acquired by citizens of Chicago. It is now mounted at the Northwestern University in Evanston, Ill.

The making of disks of glass of larger and larger size was continued by the great glass works of Chance & Company, in England. But they found the work too delicate and too troublesome, and allowed it to pass into the hands of Feil of Paris, son-in-law of Guinand. With the glass supplied by these two parties, Mr. Clark made larger and larger telescopes. First was the twenty-six-inch telescope for the Naval Observatory at Washington and a similar one for the University of Virginia. ​Then followed a still larger instrument, thirty inches in diameter, for the Observatory of Pulkova, Russia. Next was completed the thirty-six-inch instrument of the Lick Observatory, which has done such splendid work.

After the death of Feil, the business was taken up by Mantois, who made optical glass of a purity and uniformity that no one before him had ever approached. He furnished the disks with which the Clarks figured the objective for the Yerkes telescope of the University of Chicago. This is about forty inches in diameter, and is the largest refracting telescope now in actual use for astronomical purposes.

Our readers have doubtless been interested in the great telescope of the Paris Exposition of 1900, which is yet larger than that of Chicago, being of forty-seven inches aperture. This instrument is of such immense size that it cannot be mounted and pointed at the heavens in the usual way. It is therefore fixed in a horizontal, north and south position, and the rays of the object to be observed are reflected into it by an immense plane mirror. The question whether this contrivance has been successful with so large an instrument is one that is not yet settled with astronomical precision. Nothing has yet been done with this instrument, which, it is feared, is so imperfect in make as to serve no better purpose than that of a toy.

The engineering problem of mounting a great telescope is by no means a simple one. It was one in which Mr. Clark was less successful than in the construction of his object-glasses. In the case of the later telescopes the ​

​mountings of the great instruments were made by other parties. That of the Pulkova telescope was made by the Repsolds of Hamburg, the most noted makers of fine astronomical instruments in Europe. The Lick and Chicago telescopes were mounted by Warner & Swazey, of Cleveland, Ohio, who are gaining the highest reputation in this class of work. In the case of the Chicago telescope, arrangements were devised by them which surpass all ever before thought of. The observer has only to touch electric buttons to have all the work of pointing and moving the telescope performed by electricity.