The Refracting Telescope.—The function of a telescope is twofold. First, to magnify the heavenly bodies, or, what comes to the same thing, to make them look as if they were nearer to us, so that we can see them better. Second, to collect a much greater number of rays of light than the unassisted eye alone can grasp, so that objects too dim to be otherwise perceptible are brought within our range of vision.
There are two forms of telescope, distinguished asRefractorsandReflectors. The simplest form of refracting telescope is exemplified by the common opera-glass, and large refractors are not essentially different. Such instruments depend for their action upon the formation of an image by a lens. One can easily illustrate this by producing upon the wall of a room an inverted image of a candle or gas flame with a spectacle lens (one adapted for a long-sighted person), or with one of the larger lenses from an opera-glass. Having such an image, it may be magnifiedby means of another lens, just as one may magnify a photograph with an ordinary reading glass. Technically, the lens which forms the primary image is called theobject-glassof the telescope, and that which is used to magnify this image is called theeye-piece. The object-glass is usually a large lens, which is placed at one end of a tube, while the eye-piece is a much smaller lens, placed at the other end. Means are provided for adjusting the distance between the two lenses so as to admit of distinct vision.
Matters are, however, not quite so simple as has been stated. There is a very great difficulty introduced by the fact that a lens made out of a single piece of glass gives an image which is surrounded by fringes of color, so that some device has to be adopted in order to destroy, as far as possible, this enemy of good definition. In the early history of the telescope, this so-calledchromatic aberrationwas considerably reduced by making small object-glasses of very great focal length.[22]
Lenses of 100-foot focus, however, are not easy to employ as object-glasses, and astronomy was, therefore, greatly benefited by Dollond’s invention of theachromatic lensin 1760. This is a compound lens, usually consisting of a double convex crown-glass lens and a concavo-convex, or double concave, lens of flint glass. The curvatures of the lenses, and the optical properties of the two kinds of glass composingthem, are such that the color due to one of them is practically neutralized by that due to the other acting in opposition. A section of such an object-glass, with the “cell” in which it rests, is shown in Fig. 20.
Drawing of double lensFig. 20.—The Achromatic Object-Glass
Fig. 20.—The Achromatic Object-Glass
In this way the focal length of the lens, and, therefore, the length of the telescope tube, can be kept within reasonable dimensions, while the definition is improved. There is, however, usually a little outstanding color, due to the imperfect matching of the two lenses, and if one looks through a large refractor, even of a good quality, a purple fringe will be noticed round all very bright objects. This only affects a few of the brighter objects, while millions of others which are dimmer may be seen free from spurious color.
It may be remarked that the curved surfaces of the lenses forming telescopic object-glasses must notbe parts of spheres. If they are, the images will be rendered indistinct byspherical aberration, and the optician has to design his curves to get rid of this defect at the same time as chromatic aberration.
A new form of telescopic objective, consisting of three lenses, which has many important advantages, has been invented by Mr. Dennis Taylor, of the well-known firm of T. Cooke & Sons, York, England.
Such a lens as this illustrates the perfection which the optician’s art has now attained. Six surfaces of glass have to be so accurately figured that every ray of light falling upon the surface of the lens shall pass through the finest pin-hole at a distance of eighteen times the diameter of the lens.
The Reflector.—In a reflecting telescope, the object-glass of the refractor is replaced by a concave mirror. In order that such a mirror may reflect all the rays from a star to a single point, its concave surface must be part of a paraboloid of revolution, that is, a surface produced by the revolution of a parabola on its axis. If a spherical surface be employed, all the rays will not be reflected to a single point and the images which it gives will be ill-defined. Yet it is astonishing to find that the difference between a parabolic and spherical surface, even in the case of a large mirror, is exceedingly small. Sir John Herschel states that in the case of a mirror four feet in diameter, and forming an image at a distance of forty feet, the parabolic only departs from the spherical form at the edges by less than a twenty-one thousandth part of an inch.
Drawing of a telescopeFig. 21.—The Newtonian Reflector
Fig. 21.—The Newtonian Reflector
An image being formed by a mirror, it is next tobe viewed with an eye-piece just as in the case of a refracting telescope. Here there is a little difficulty, for if the eye-piece be applied in the direct line of the mirror, the interposition of the observer’s head will block out the light. Several ways of overcoming this have been devised, but the plan most generally followed is that which Newton adopted in the first reflecting telescope which was ever constructed. With his own hands Newton made a small reflector, 6¼ inches long and having an aperture of 1⅓ inches, with which he was able to study the phases of Venus and the phenomena of Jupiter’s satellites. This precious little instrument is now one of the greatest treasures in the collection of the Royal Society of London. The general design of this telescope is shown in Fig. 21. The concave mirror is at the bottom of the telescope tube, and normally it would form an image of a star near the end of the tube. A plane mirror, however, of small size intercepts the rays and reflects them to the side, where they converge to a focus. This image is observed and magnified by an eye-piece, as in the refractor.It is true that in this arrangement the plane mirror, orflat, renders the central part of the principal mirror ineffective, but the loss of light is very much less than would be the case if the eye-piece were placed in position to view the image centrally.
In the hands of Sir William Herschel the reflecting telescope was greatly developed. The great telescope with which he enriched astronomical science had a mirror four feet in diameter, and its tube was forty feet in length. With the view of utilizing the whole surface of the mirror and dispensing with a second reflecting surface, the 4-foot mirror was placed at a small angle to the bottom of the tube, so that its principal focal point was no longer at the centre, but at the side of the tube.
In practice, however, it is found that the Herschelian form of reflector does not give the best definition, and it is now very seldom seen.
Among other forms, the “Cassegrain” is perhaps the most important. During the last years this form has received a great deal of attention, more especially in regard to its special adaptability for photographic purposes.
In the Cassegrain telescope, the plane mirror of the Newtonian form is replaced by a small convex mirror which is part of a hyperboloid of revolution, its axis and focal point being coincident with those of the primary mirror. The rays are in this way reflected back to the mirror at the bottom of the tube, and in order that the image may be seen, it is necessary to cut out the middle part of the mirror to admit the eye-piece.
Although the small mirror must theoretically be hyperbolic, tolerable definition is obtained even if it be spherical or ellipsoidal, and its actual departure from these forms is so slight as to be beyond detection by measurement, so that the figuring of such mirrors can only be tested in the telescope. For photographic purposes this telescope has the very important advantage that a short telescope is equivalent to a very long one of the Newtonian form, or refracting telescope, so that the image of sun, moon, or planets formed at the focus is very large in comparison with the size of the telescope. A modification of this form of telescope, in which the small mirror is out of the path of the rays falling upon the larger one, and no longer obstructing the central part, has been revived by Dr. Common, and has become generally known as the “Skew Cassegrain.”
In reflecting telescopes the mirrors were formerly made of speculum metal (an alloy of copper and tin), and the wordspeculumis even now commonly employed to signify a telescopic mirror, although it is usual to make the mirror of glass, with the concave surface silvered and highly polished.
One is frequently asked for an opinion as to which is the better form of telescope, the reflector or refractor, and it is a question that one finds some little difficulty in answering. On one point, however, all are agreed, namely, that the reflector has the advantage in regard to its achromatism; it is indeed perfectly achromatic, while the so-called “achromatic” refractor is at best only a compromise. For the rest, one can not do better than quote the evidence of Dr.Isaac Roberts before the International Astrophotographic Congress: “The reflector requires the exercise of great care and patience, and a thorough personal interest on the part of the observer using it. In the hands of such a person it yields excellent results, but in other hands it might be a bad instrument. The reflector gives results at least equal, if not superior, to those obtained with the refractor, if the observer be careful of the centring, and of the polish of the mirror, and keeps the instrument in the highest state of efficiency; but when intrusted to an ordinary assistant the conditions necessary for its best performance can not be so well fulfilled as the same could be in the case of the refractor.” One great practical advantage of the reflector is that there are fewer optical surfaces, so that a large reflector may be obtained for the price of a much smaller refractor.
Drawing of a telescopeFig. 22.—The Cassegrain Reflector
Fig. 22.—The Cassegrain Reflector
Eye-Pieces.—So far we have regarded the eye-piece of a telescope as a simple lens, but it is evident that the spherical and chromatic aberration of such a lens will interfere with its performance. For occasionaluse, however, even a simple lens is very serviceable if the object observed is brought to the centre of the field of view.
Compound eye-pieces are of various forms, each having certain advantages, the desiderata being freedom from color and “flatness of field”—that is, stars in different parts of the field are to be equally well in focus. Those most commonly employed are the Ramsden and Huyghenian eye-pieces. The former consists of two plano-convex lenses of equal focal lengths, having their curved faces toward each other, and being placed at a distance apart equal to two-thirds of the focal length of either lens. Such an eye-piece can be used as a magnifying-glass, and it is therefore placed outside the focal image formed by the telescope with which it is used; on this account it is called apositive eye-piece. This kind of eye-piece is not quite achromatic, but its flat field of view gives it a special value for many purposes.
In the Huyghenian eye-piece there are again two lenses, made of the same kind of glass. That which comes nearest to the eye has a focal length of only one-third that of thefieldlens, and the distance between the two lenses is half the sum of the focal lengths. This form of eye-piece can not be used as a magnifying-glass in the ordinary sense, and as the field lens must be placed on the object-glass or mirror side of the focus, it is called anegative eye-piece. The Huyghenian eye-piece is more achromatic than the Ramsden, and is more widely used when it is only required to view the heavenly bodies. In instruments employed for purposes of measurement, a positiveeye-piece is essential in order that the spider threads may be placed at the focus of the telescope. The images formed by an astronomical telescope are upside down, and neither of the eye-pieces described reinverts them.
A special form of eye-piece is therefore used when a telescope is employed for terrestrial sight-seeing. The desired result is obtained by the introduction of additional lenses, but there is a corresponding reduction of brightness.
For viewing the sun some device is necessary to reduce the quantity of light entering the eye. To look at the sun directly, even with a small instrument, is very dangerous. The arrangement usually adopted is asolar diagonal, in which the light is reflected from a piece of plane glass before entering the eye-piece; the piece of glass is wedge-shaped, so that the reflection from one surface only is effective; if the glass had parallel sides, the solar image would be double.
Magnifying Power.—The magnifying power of a telescope depends upon the focal length of the object-glass, or speculum, and that of the eye-piece. Optically, it is equal to the former divided by the latter, so that the greater the focal length of an object-glass, or the smaller the focal length of the eye-piece, the greater will be the magnifying power. In a given telescope, the object-glass, or speculum, is a constant factor and the magnifying power can only be varied by changing the eye-piece. The focal length of the Lick telescope, for example, is about 600 inches; with an eye-piece which is equivalent to a lens of one-inch focus, the magnifying powerwould be 600; with a lens of half an inch focus, it would be 1,200, and so on.
The magnifying power which can be effectively employed, however, depends upon a great variety of circumstances. First, the clearness and steadiness of the air; then there is the quality of the object-glass, or speculum, to be considered; and also the brightness of the object to be observed, for when the object is very dim, its light will be spread out into invisibility if too high a power be used.
In practice, good refractors perform well with powers ranging up to 80 or 100 for each inch in the diameter of the object-glass. Thus, on sufficiently bright objects, a six-inch telescope will work well with a power of about 500, while a 30-inch may be effectively employed with powers between 2,000 and 3,000.
Illuminating Power.—It has already been pointed out that magnification is not the only function of a telescope. As a matter of fact, the most powerful telescopes in the world fail to produce the slightest increase in the apparent size of a star, for even if these objects be brought to apparently a 3,000th part of their real distances, they are still too far away to have any visible size. But although a star can not be magnified, it can be rendered more visible by the telescope, for the reason that the object-glass collects a greater number of rays than the naked eye. The pupil of the eye may be taken to have a diameter of one-fifth of an inch; a lens one-inch in diameter will have twenty-five times theareaof the pupil, and will therefore collect twenty-five timesthe amount of light from a star; a two-inch lens will grasp one hundred times, and a 36-inch 32,400 times as much light as the pupil alone. Practically all these rays collected by the object-glass, or speculum, of a telescope can not be brought into the eye; some are lost through the imperfect transparency of the glass, or the imperfect reflecting power of the speculum. Still, allowing a considerable percentage for loss, there is an enormous concentration of light when a large telescope is employed.
The Altazimuth Mounting.—Having got a telescope, we have next to see how it can be best supported, for unless it be a very small instrument indeed, it will be impossible to hold it in the hand like a spy-glass. However a telescope be mounted, provision must be made for turning it to any part of the sky whatsoever. Very frequently one of the axes on which the instrument turns is vertical, while the other is horizontal. Such a stand for a telescope is called analtazimuth mounting, for the reason that it permits the instrument to be moved in altitude and in azimuth.
As a rule, one finds only small telescopes mounted in this manner. The objection to it is that, as one continues to observe a heavenly body, two independent movements must be given to the telescope in order to follow the body in its diurnal movement across the heavens. If we commence observing a star newly risen, for example, the telescope must trace a star-like path in order to follow it as it ascends into the heavens.
The Equatorial Telescope.—A much more convenientmethod of setting up a telescope is to mount it as anequatorial. The essential feature of this instrument is that one of the axes of movement, instead of being vertical, is placed parallel to the axis of the earth. This is called thepolar axis, and, when the telescope is turned around such an axis, it traces out curves in the sky which are identical with those described by the stars in their diurnal motions. If, then, the telescope be directed to a star or other heavenly body, it can be made to follow the object and keep it in view by a single movement. The axis at right angles to the polar axis is called the declination axis, and is necessary in order that the telescope may be moved toward and from the poles so that all the heavenly bodies above the horizon may be included in its sweep.
One very important advantage of the equatorial is that, as only one motion is required to keep a star in view, so long as it is above the horizon, the necessary movement may be furnished by clockwork. A good equatorial is accordingly provided with a driving-clock, which is regulated so that it would drive the telescope through a whole revolution once a day. Unlike an ordinary clock, the driving-clock of a telescope is regulated by a governor, in order that the instrument may have a continuous and not a jerky movement.
The telescope is also provided with clamps and fine adjustments, one each in R. A. and declination, in order that it may be under the control of the observer. It is evident that the telescope must be capable of moving independently of the driving-gear, sothat it may first be placed in the desired direction; when this is accomplished, the R. A. clamp is used to put the telescope in gear with the clock. The declination clamp is then made to fix the telescope firmly to the declination axis. Fine adjustments in both directions are necessary, because it is impossible to sight a large instrument with such precision as to bring an object exactly to the centre of the field of view.
Some of the driving-clocks fitted to equatorials are very elaborate. As clocks regulated by governors are not such reliable timekeepers as those regulated by pendulums, arrangements are made by which the accuracy of a pendulum can be electrically communicated to a governor clock. One of the best forms of electrically controlled clocks is that devised by Sir Howard Grubb.
Another important feature of an equatorial is that it can be provided with circles which enable the telescope to be pointed to any desired object of known right ascension and declination. One of these is the declination circle, attached to the declination axis and read by a vernier fixed to the sleeve in which the axis turns; this is adjusted so as to read 0° when the telescope points to any part of the celestial equator, and 90° when it is directed to the pole. The other circle is attached to the polar axis, and determines the position of the telescope with regard to the meridian; this is called thehour circle, and is divided into twenty-four hours. When the telescope is on the meridian, the hour circle reads zero, so that its reading in any other position gives the hour angleof the telescope. Having given the right ascension and declination of a heavenly body which it is desired to observe, the telescope is turned until the declination circle reads the proper angle, and the hour circle indicates the hour angle which is calculated for the particular moment of pointing the telescope. [The hour angle is the difference between the right ascension of the object and the sidereal time of observation.] In this way it is easy to find objects of known position which are invisible to the naked eye, and one can even pick up the planets and brighter stars in full sunshine. Conversely, one can determine from the circles the right ascension and declination of any object under observation, but for various reasons only approximate results can be obtained in this way. The chief use of the circles on an equatorial is therefore to provide a means of pointing the telescope.
Telescopes of four inches aperture and upward are usually provided with a smaller companion called afinder. This has a larger field of view than the main telescope, so that objects which are of sufficient brightness can readily be picked up and brought to the centre of the finder, the adjustments being such that the object is then also at the centre of the field of the large telescope.
There are, of course, many practical details connected with the working of an equatorial with which space does not permit us to deal. It may be remarked, however, that the adjustment of the polar axis is very simply performed by first inclining it at an angle approximately equal to the latitude of theplace where it is set up, and setting it as nearly as possible in the meridian by means of a compass or by observations of the sun at noon. The final adjustment is then made by a series of observations of stars of known position.
Some of the World’s Great Telescopes.—Thanks to the wide public interest taken in astronomical matters, a large number of powerful telescopes have been set up in various parts of the world. To the British Islands belongs the honor of possessing the largest telescope in the world. This is the giant reflector erected by Lord Rosse, in 1842, at Parsonstown, the mirror being six feet in diameter, and the focal length sixty feet. Many very valuable observations were made with this instrument in its early days, but of late years it seems to have fallen into disuse. One reason may be that the mounting is not of the most convenient form, and makes the telescope unsuitable for photographic work.
Coming next in point of size to the Rosse telescope is the reflector erected at Ealing by Dr. A. A. Common. The glass mirror of this telescope is five feet in diameter, five inches thick, and weighs more than half a ton. Dr. Common aimed specially at constructing the largest possible telescope which could be equatorially mounted and provided with a driving-clock, and he was only limited to an aperture of five feet by the impossibility of obtaining a glass disk of larger size. He has attained such great skill in this work that he was able to produce a perfect mirror five feet in diameter in three months’ time, althoughno less than 410,000 strokes of the polishing machine were required.
The telescope is of the Newtonian form, and the mounting is quite unique. The polar axis consists of an iron cylinder, made up of boiler plate, seven feet eight inches in diameter, and about fifteen feet long. From the top of the cylinder, near its outer edge, two horns, each six feet long, project outward, and the tube of the telescope swings on trunnions attached to the ends of the horns. The main part of the telescope tube is square, built up of steel angle iron, and carries the mirror at its lower end; the upper part of the tube, which carries the “flat” and eye-piece, is round, and of tinned steel strengthened by a skeleton framework.
It is evident that such an enormous instrument as this can not be made to travel by clockwork with the necessary uniformity without some very efficient arrangement for reducing friction. Dr. Common’s plan—and it is here that his instrument is unlike others—is to make the hollow polar axis watertight, and to fix it in a tank of water. At the bottom of the polar axis is a ball and socket joint to keep it in position, and at the top is another bearing, which can be adjusted so that the polar axis lies truly in the meridian. It was found necessary to introduce nine tons of iron into the bottom of the hollow polar axis in order to sink it to the proper angle, and to put sufficient weight on the bearings to give stability to the instrument. In this way the great mass is brought into the region of manageability, and the driving-clock, which is driven by a weight of one and a halftons, is able to do its work efficiently. Such, in general outline, is this wonderful telescope, which, although not so large as Lord Rosse’s famous instrument, is undoubtedly its superior in light-grasping power and general utility, and more especially in its adaptability for photographing the heavens.
Among other large reflecting telescopes now in use are the 4-foot reflectors at Melbourne and Paris, and the 3-foot reflectors at South Kensington and the Lick Observatory, California.
The largest refracting telescope yet constructed is one of forty inches aperture for the Yerkes Observatory of the University of Chicago. It is interesting to note here that Professor Keeler, in his report as an expert upon the performance of the object-glass, considers that there is “evidence for the first time that we are approaching the limit of size in the construction of great objectives.” Unlike a mirror, a lens can be supported only upon its circumference, and it is the bending by its own weight that proves detrimental to its defining power. If the lens be made thicker with a view of overcoming this defect, the absorption of light by the glass increases, so that there is in the end no special gain by increasing the size.
The length of the Yerkes telescope is 62 feet, and is provided with all accessories pertaining to astrophysical research. The world-renowed Lick Telescope is of thirty-six inches aperture. The story of the foundation of this monster instrument is not much less wonderful than the telescope itself.Brought up in poor circumstances, with few opportunities for intellectual development, James Lick, nevertheless, amassed a fortune in business, and having few relations, he was anxious to dispose of his wealth in such a way as to bring him that fame which he had failed to achieve in other directions. Although it is very probable that he had never looked through a telescope in his life, the idea of a large telescope had taken a very firm hold upon his mind, and, thanks to the influence of his advisers, it was definitely announced in 1873 that Mr. Lick’s bid for immortality was to take this form. Several sites were examined by experts, and finally Mount Hamilton, California, 4,200 feet above sea-level, was selected. An excellent road, twenty-six miles in length, made at the cost of the county authorities, connects the observatory with the nearest town, San José, thirteen miles distant.
Owing to various delays, operations were not commenced until 1880, and five years were consumed in clearing away 72,000 tons of rocks and in erecting the buildings.
Mr. Lick had stipulated for the erection of “a telescope superior to and more powerful than any telescope yet made,” and Messrs. Alvan Clark & Co. contracted to supply a lens of thirty-six inches aperture for the sum of $50,000. It turned out, however, that it was much easier to make such a contract than to fulfil it. To produce large disks of optically perfect glass, even in the rough, requires the greatest possible skill and patience, and this part of the work was undertaken by Feil & Co. of Paris. The flintglass disk was safely delivered in America in 1882, but the crown disk was cracked in packing. The elder Feil having retired from business, the duty of providing a new block of crown glass devolved upon his sons, who, after two years spent in vain attempts, ended in bankruptcy, and it was only through the elder Feil again resuming business that the much-required disk was finally completed in 1885. After the lapse of another year, the rough disks were fashioned, in the workshops of the Clarks, into the most marvelous of telescopic lenses.
The mounting of the object-glass is worthy of the occasion. The tube is no less than thirty-seven feet long, and four feet in diameter in the middle part. An iron pier, thirty-eight feet high, beneath which lie the remains of Mr. Lick, supports the equatorial head, and a winding staircase enables the observer to reach the setting circles. Inside the hollow pier is the powerful driving-clock which turns the telescope to follow the heavenly bodies in their apparent movements. Finders of six, four, and three inches diameter, rods for the manipulation of the instrument, and all necessary accessories, complete what must long remain one of the most perfect instruments at the service of astronomical science. The $200,000 expended upon it have already been amply justified by the work accomplished, while Mr. Lick’s dream of immortality has become a reality.
The following list indicates some of the large refractors now doing active service:
It is right to add, however, that opinion is still greatly divided as to whether these telescopes of large aperture really repay the expense and labor involved in their erection and use. On the very rare occasion when the “seeing” is practically perfect—which occurs perhaps only a few hours in a year—it is probable that the superiority of a large telescope is very marked, but under average conditions there seems to be little advantage over instruments of moderate size for many classes of observations.
Certain it is that a great deal of valuable work is done with comparatively small telescopes, ranging from six to fifteen inches aperture, and this in all departments of astronomical research. Hence, some of the most active observatories do not figure in the above list; among them may be mentioned the observatories of Harvard College, Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh, South Kensington, Stonyhurst College, and the observatory of Dr. Isaac Roberts at Crowborough, England.
Housing of Equatorials.—The building which accommodates an equatorial telescope must evidently be designed to admit of giving a clear opening to any part of the sky. Usually this is accomplishedby making the roof, ordome, with a circular base, provided with wheels, which run on rails. It is then only necessary to open a narrow portion of the dome, extending from top to base, and to turn the dome until this aperture is in the required direction. One of the most elaborate domes now in existence is that built by M. Eiffel for the great refractor of the Nice Observatory. The lower part of the building is in the form of a square, having a side of about eighty-seven feet and a height of about thirty feet. The dome itself is seventy-four feet in diameter, and the moving parts alone weigh ninety-five tons.
There are two shutters, each a little wider than half the possible opening; these run on short rails, and are moved simultaneously by means of an endless rope. The whole of the dome is built up of steel angle iron, covered with very thin sheet steel. In order to facilitate the manipulation of the dome, its great weight is buoyed up by means of a float attached to its base and immersed in a circular tank of water of a little greater size than the base of the dome. If any mishap occurs with this gigantic tank, the dome rests on wheels which run on a circular rail, so that the work need not be interrupted. The whole arrangement is very easily turned with the aid of a winch by one man when the dome is floating, but when resting on the wheels several men are required at the winch.
This brief description will serve to illustrate some of the problems which confront the possessor of a very large telescope. For smaller instruments, the observatories follow pretty nearly the same plan, exceptthat it is unnecessary to provide an arrangement for floating the dome.
The observatory which shelters a reflecting telescope need not differ very greatly from one which contains a refractor. If the instrument be a Newtonian, it is generally convenient to sink the polar axis below the level of the floor in order that the observer may not be at too great a height from the ground, and in that case, the dome, or its equivalent, is all that is necessary. For his five-foot reflector, Dr. Common designed an observatory which is not of the ordinary form, but gives the necessary opening partly by means of large shutters and partly by a revolution of the whole house. It is not every one who is able to lay out $40,000 on such a dome as that erected at Nice by M. Bischoffeim.
The varying position of the eye end of a telescope, when it is turned to different parts of the sky, makes it necessary to provide comfortable and safe seating accommodation for the observer, more especially when the telescope is a very large one. In the case of the Yerkes telescope, the eye-piece is thirty feet higher when observing near the horizon than when observing near the zenith, and the observer must necessarily follow the telescope. The most convenient arrangement in such a case is to raise or lower the floor of the observatory as occasion demands. The floor of the Yerkes Observatory is seventy-five feet in diameter, and by means of electric motors it can be given a vertical motion of twenty-two feet. A similar arrangement was provided for the Lick telescope from the designs of Sir HowardGrubb. With smaller instruments, observing ladders and adjustable chairs of various forms are employed.
The Equatorial Coudé.—A form of equatorial telescope which has possibly a great future before it is one introduced at Paris under the name of theequatorial coudé, or elbowed telescope. Its practical advantage is that the observer remains in a constant and comfortable position, so that revolving domes and elevating floors, or other arrangements serving similar purposes, are no longer necessary. The telescope tube is of two parts of nearly equal length, and what is ordinarily the lower half of the tube forms part of the polar axis, while the other half is attached to it at right angles. At the point of intersection of the two halves of the tube is a plane mirror, and there is another mirror in front of the object-glass. If the latter mirror were removed, such a telescope would only enable the observer to see objects lying along the celestial equator, but by its means objects in all parts of the heavens can be brought within range to an observer gazing down the hollow polar axis. The largest instrument is that at the Paris Observatory, which has an object-glass 23½ inches in diameter for visual observations, and another of the same size for photographic purposes.
Fixed Telescopes.—There is still another method of using a telescope. The telescope itself may be fixed, and the light of the heavenly bodies may be reflected into it by means of a mirror which is made to revolve so as to keep pace with their movements. Foucault devised an instrument called thesiderostatfor this purpose, and although it is not largely employed for telescopic observations, it is very widely utilized for spectroscopic work, where the spectroscope is of a kind not readily attached to a telescope.
Another instrument used for the same purpose has recently been brought forward under the name of thecœlostat. This is simply a mirror which is made to turn on a polar axis in its own plane, and since a reflected ray of light moves through twice the angle that the reflecting surface turns through, the mirror is made to revolve at the rate of one revolution in two days. As the name indicates, the whole heavens appear stationary in such an instrument, whereas in a siderostat only one star at a time appears at rest, while its neighbors slowly revolve round it.
Photographic Telescopes.—The application of photography to the study of the heavenly bodies marks one of the greatest advances of the present century. The instruments which are employed for this purpose range from the ordinary tourist camera to the largest telescope. Unlike a person sitting for a portrait, the heavenly bodies can not be made to stand still for the purpose, and as instantaneous photographs can only be obtained in the case of the sun and moon, it is usually necessary to make the camera follow the stars very exactly during the time of exposure in order that the images may fall on precisely the same parts of the photographic plate.
Some guiding arrangement is, therefore, essential, and generally the photographic camera or telescope is attached to an ordinary equatorial which is drivenby clockwork, or very carefully by hand if the camera be a small one. In the guiding telescope are two spider-threads at right angles to each other, and it is by constantly keeping the image of a star at the intersection of these “wires” that the operator ensures the images remaining in a constant position upon the sensitive plate.
An ordinary portrait camera, in the hands of a skilled observer, yields very beautiful pictures, but they are naturally on a small scale. The field of view of such an instrument is so large that a whole constellation may be photographed with a single exposure.
Portrait lenses of six inches aperture in the hands of Dr. Max Wolf and Professor Barnard have given magnificent delineations of the Milky Way, and of the extremely faint nebulosities which are to be found in many parts of the heavens.
For many purposes, however, telescopes of greater power are required, and here it may be remarked that the distance between the images of any two adjacent stars will vary in direct proportion to the focal length of the telescope. In the same way the size of the image of a planet, the moon, or a comet, increases as the focal length of the objective is increased.
Refracting telescopes which are employed for photography require object-glasses which are specially “corrected” for the photographic rays. White light is compounded of light of all colors, but it is the blue and violet constituents which are effective in producing photographic action on an ordinarysensitive plate. Now, an object-glass which is intended for visual purposes is made to focus at the same point as many as possible of the rays which are most effective to the human eye, that is the green, yellow, and red, and usually there is a blue or purple halo round the images of the brighter objects, which is, however, too feeble as a rule to interfere with visual observations. This blue halo will evidently result in defective definition if the lens be employed for photography. By putting the plate at the point where the blue rays are most nearly focused, a better image is obtained; but for really good work a photographic object-glass must be so designed that all the blue and violet rays are brought to one and the same focus. Such a lens will consequently be a very poor one for visual observations.
The new “photo telescopic” object-glass now manufactured by Messrs. Cooke appears to be full of promise. In this lens all the colors of the spectrum are brought to almost exactly the same focal point, so that it serves equally well for photographic or visual purposes.
This difficulty in regard to achromatism does not exist in the case of the reflecting telescope, since rays of light of every color are reflected at precisely the same angles. For this reason reflectors, when properly managed, give the best photographic results. Dr. Isaac Roberts and Dr. Common are especially identified with the application of the reflecting telescope for celestial photography. The instrument employed by the former consists of a 20-inch reflector and a 7-inch guiding telescope ofthe refracting form. The two telescopes are mounted on the extreme ends of the declination axis of an equatorial.
Dr. Common does not employ a guiding telescope at all. The photographic plate which he places at the focus of the reflector is smaller than the field of view, so that by means of an eye-piece fitted with a cross wire at the side of the dark slide, he is able to watch a star near the edge of the field. Both eye-piece and dark slide are attached to a frame which can be controlled by two screws at right angles to each other. If the guiding star leaves the cross wire through errors in driving, or other causes, the eye-piece and dark slide are bodily moved after it by means of the adjusting screws. This method not only has the advantage of saving the cost of a guiding telescope, but reduces the effects of vibration consequent upon the correction of errors by moving the whole telescope.
For photographing the sun a special instrument called aphotoheliographis usually employed. This differs only from an ordinary photographic telescope in being provided with a secondary magnifier, by which means the focal image formed by the object-glass is amplified before falling upon the photographic plate. On a bright, clear day pictures of the sun eight inches in diameter can be taken with an exposure of about1/500th of a second, and such a photograph will frequently record more facts as to the state of the solar surface than a whole day’s observation. Lenses or mirrors of very long focus are also occasionally employed in solar photography,and in this way a large image is obtained without the use of a secondary magnifier.
Photographs of the moon and planets may be taken either with or without a secondary magnifier, but in either case the exposures are longer than for the sun.
Finally, it may be added that the sensitive plates and processes used in astronomical photography do not differ from those employed by ordinary photographers.
FOOTNOTES:[22]The focal length of a lens is the distance from its centre at which an image of a very distant object, such as the sun, is formed.
[22]The focal length of a lens is the distance from its centre at which an image of a very distant object, such as the sun, is formed.
[22]The focal length of a lens is the distance from its centre at which an image of a very distant object, such as the sun, is formed.