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Originally appearing in Volume V26, Page 573 of the 1911 Encyclopedia Britannica.
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FOR THE YERKES OBSERVATORY use of an overhanging polar axis the difficulty can be overcome ; it has been successfully adopted by Repsolds for their astrographic equatorials of 13-in. aperture and 11.25-ft. focus, and on a much smaller scale by Warner & Swasey for the Bruce telescope of Io-in. aperture and 50-in. focus, made for the Yerkes Observatory. The From Professor Hale's The Study of Stellar Evolution, by permission of the University of Chicago Press. latter is shown in fig. 19. Stability in this method of mounting can only be secured by excessive weight and rigidity in the support of the overhanging axis. In the case of the Victoria telescope (24-in. aperture and 222-ft. focus) mounted at the Cape of Good dope on this plan, it has been found necessary to add supporting stays where great rigidity is required, and thus to sacrifice continuous circuni-meridian motion for stars between the zenith and the elevated pole. Type D.—The first important equatorial of type D was the 4-ft. 'effecting telescope of Lassell (Mem. R.A.S., xxxvi. I-4), and later Lord Rosse's 36-in. reflecting telescope at Birr Castle (Phil. Trans., clxxi. 153), and A. Common's 36-in. reflecting telescope mounted by him at Ealing (Mem. R.A.S., xlvi. 173-182). In Lassell's instrument (a reflector of the Newtonian type) the observer is mounted in the open air on a supplementary tower capable of motion in any azimuth about the centre of motion of the telescope, whilst an observing platform can be raised and lowered on the side of the tower. In Lord Rosse's instrument (also of the Newtonian type) the observer is suspended in a cage near the eye-piece, and the instrument is used in the open air. Common's telescope presents many ingenious features, especially the relief-friction by flotation of the polar axis in mercury, and in the arrangements of the observatory for giving ready access to the eye-piece of the telescope. Type C seems indeed to be the type of mounting most suitable for reflecting telescopes, and this form has been adopted for the 6o-in. reflector completed by G. W. Ritchey, under the direction of Professor G. E. Hale, for the Mount Wilson Solar Observatory. The instrument is shown in fig. 20, and its design is unquestion-ably the most perfect yet proposed for modern astrophysical re-search. The declination axis is here represented by what are practically the trunnions or pivots of the tube, resting in bearings which are supported by the arms of a very massive cast-iron fork bolted to the upper end of the polar axis. This axis is a hollow forging of nickel steel, of which the accurately turned pivots rest on bearings attached to cast-iron uprights bolted upon a massive cast-iron base plate. The base plate rests upon levelling screws which permit the adjustment of the polar axis to be made with great precision. The combined overhanging weight of the cast-iron fork, the mirror and tube is so great, that without a very perfect relief-friction system the instrument could not be moved in right ascension with any approach to practical ease. But a hollow steel float, Io ft. in diameter, is bolted to the upper end of the polar axis just below the fork. This float dips into a tank filled with mercury so that practically the entire instrument is floated by the mercury, leaving only sufficient pressure on the bearings to ensure that the pivots will remain in contact with them. The 6o-in. silver-on-glass mirror (weighing about one ton) rests at the lower end of the tube on a support-system consisting of a large number of weighted levers which press against the back of the glass and distribute the load. Similar weighted levers around the circumference of the mirror provide the edge support. The telescope is moved in right ascension and declination by electric motors controlled from positions convenient for the observer. The driving clock moves the telescope in right ascension by means of a worm-gear wheel, io ft. in diameter, mounted on the polar axis. The 60-in. mirror is of 25-ft. focus, but for certain classes of work it is desirable to have the advantage of greater focal length. For this purpose the telescope can be used in the four different ways shown in fig. 21. (i) As a Newtonian reflector, fig. 21 (a), the converging rays from the 6o-in. mirror being reflected to the side of the tube where a h _. From Professor Hale's The Study of Stellar Evolution, by permission of the University of Chicago Press. the image is formed, and where it may be photographed or viewed with an eye-piece. In this case the image is formed without secondary magnification and the focal length is 25 ft. (2) Asa Cassegrain reflector, fig. 21 (b), in which case the upper section of the tube bearing the plane mirror is removed and a shorter section substituted for it. This latter carries a hyperboloidal 6o irich Reflecting Telescope Dome and Building, Solar Observatory Mount Wilson,Calif. Designed G.W.Ritchey Atr.tightligofd Joint betustw Dome and Building From Professor Hale's The Study of Stellar Evolution, by permission of the University of Chicago Press. mirror, which returns the rays towards the centre of the large mirror and causes them to converge less rapidly. They then meet a small plane mirror supported at the point of intersection of the polar and declination axes, whence they are reflected down through the hollow polar axis as shown in fig. 2, and come to focus on the slit of the powerful spectroscope that is mounted on a pier in the chamber of constant temperature as shown in fig. 20. In this case the equivalent focal length is 15o ft. (3) As a Cassegrain reflector, for photographing the moon, planets or very bright nebulae on a large scale, as shown in fig. 21 (c), with an equivalent focal length of too ft. (4) As a Cassegrain reflector, for use with a spectroscope mounted in place of the photographic plate, fig. 21 (d); in this case a convex mirror of different curvature is employed, the equivalent focus of the combination being 8o ft. Type E.—In the Comp/es Rendus for the year 1883, vol. 96, pp. 735-741, Loewy gives an account of an instrument which he calls an " equatorial coude," designed (1) to attain Loewy's greater stability and so to measure larger angles than is equator- generally possible with the ordinary equatorial; (2) to ietttoude enable a single astronomer to point the telescope and make observations in any part of the sky without changing his position; (3) to abolish the usual expensive dome, and to substi- tute a covered shed on wheels (which can be run back at pleasure), leaving the telescope in the open air, the observer alone being sheltered. These conditions are fulfilled in the manner shown in fig. 22. E P is the polar axis, rotating on bearings at F. and P. The object-glass is at 0, the eye-piece at E. There is a plane mirror at M, which reflects rays converging from the object-glass to the eye-piece at E. A second mirror N, placed at 45° to the optical axis of the object-glass, reflects rays from a star at the pole; but by rotating the box which contains this mirror on the axis of its supporting tube T a star of any declination can be observed, and by combining this motion with rotation of the polar axis the astronomer seated at E is able to view any object whatever in the visible heavens, except' circumpolar stars near lower transit. An hour circle attached to E P and a declination circle attached to the box containing the mirror N, both of which can be read or set from E, complete the essentials of the instrument. There must be a certain loss of light from two additional reflections; but that could be tolerated for the sake of other advantages, provided that the the mirrors could be made sufficiently perfect optical planes. By making the mirrors of '\ silvered glass, one-fourth of their diameter in thickness, the Henrys have not only ~`. \ succeeded in mounting them with all \\ necessary rigidity free from flexure `. but have given them optically '. s true plane surfaces, notwith-N standing their large diameters, viz., t t and 15.7 in. Sir David Gill tested the equatorial coude on double stars at the Paris Observatory in 1884, and his last doubts as to the practical value of the instrument were dispelled. He has Equatorial. optical definition in any of the many telescopes he has employed, and certainly never measured a celestial object in such favourable conditions of physical comfort. The easy position of the observer, the convenient position of the handles for quick and slow motion, and the absolute rigidity of the mounting leave little to be desired. In a much larger instrument of the same type subsequently mounted at Paris, and in like instruments of intermediate size mounted at other French observatories, the object-glass is placed outside the mirror N, so that both the silvered mirrors are protected from exposure to the outer air. A modification of Loewy's equatorial coude has been suggested by Lindemann (Asir. Nachr., No. 3935); it consists in placing both the mirrors of Loewy's " equatorial coude " at the top of the. polar axis instead of the. lower end of it. By this arrangement the long cross tube becomes unnecessary, and neither the pier nor the observatory obstruct the view of objects above the horizon near lower transit as is the case in Loewy's form. The reflected rays pass down the tube from the direction of the elevated pole instead of upward towards that pole. The observer is, therefore, at the bottom of the tube instead of the top and looks upward instead of downward. The drawbacks to this plan are (I) the necessarily large size of the upper pivot (viz. the diameter of the tube) and of the lower pivot (which must be perforated by a hole at least equal in diameter to the photographic field of the telescope), conditions which involve very refined arrangements for relief of friction, and (2) the less comfortable attitude of looking upward instead of down-ward. The plan, however, would be a very favourable one for spectroscopic work and for the convenient installation of an under-ground room of constant temperature. The difficulties of relief friction could probably be best overcome by a large hollow cylinder concentric with the polar axis fixed near the centre of gravity of the whole instrument and floated in mercury, on the plan adopted in the Mount Wilson 6o-in. reflector already described, but in this case the floating cylinder would be below and not above the upper bearing. In 1884 Sir Howard Grubb (Phil. Trans. R. Dub. Soc., vol. iii. series 2, p. 61) proposed a form of equatorial telescope of which an The excellent example was erected at Cambridge (Eng). in The 1898. The instrument in some respects resembles the tirubbNal equatorial coude of Loewy, but instead of two mirrors equato at cam- there is only one. A flanged cast-iron box, strongly bridge. ribbed and open on one side, forms the centre of the polar axis. One pivot of the polar axis is attached to the lower end of this box, and a strong hollow metal cone, terminating in the other pivot, forms the upper part of the polar axis. The declination axis passes through the two opposite sides of the central box. Upon an axis concentric with the declination axis is carried a plane mirror, which is geared so as always to bisect the angle between the polar axis and the optical axis of the telescope. If then the objective tube is directed to any star, the convergent beam from the object-glass is received by the plane mirror from which it is reflected upwards along the polar axis and viewed through the hollow upper pivot. Thus, as in the equatorial coude, the observer remains in a fixed position looking down the polar tube from above. He is provided with quick and slow motions in right ascension and declination, which can be operated from the eye-end, and he can work in a closed and comfortably heated room. A large slot has to be cut in the cone which forms the upper part of the polar axis, in order to allow the telescope to be pointed nearer to the pole than would otherwise be possible; even so stars within 15° of the pole cannot be observed. An illustrated preliminary description of the instrument is given by Sir Robert Ball (Mon. Not. R.A.S., lix. 152). The instrument has a triple photo-visual Taylor object-glass of 121 in. aperture and 19.3-ft. focal length. Type F.—In all the previously described types of telescope mounting the axis of the instrument is either pointed directly at the object or to the pole; in the latter case the rays from the star under observation are reflected along the polar axis by a mirror or mirrors attached to or revolving with it. Equatorials of types A, B, C and D have the advantage of avoiding interposed reflecting surfaces, but they involve inconveniences from the continual motion of the eye-piece and the consequent necessity for providing elaborate observing stages or rising floors. In those of type E the eye-piece has a fixed position and the observer may even occupy a room maintained at uniform temperature, but he must submit to a certain loss of light from one or more reflecting surfaces, and from possible loss of definition from optical imperfection or flexure of the mirror or mirrors. In all these types the longer the telescope and the greater its diameter (or weight) the more massive must be the mounting and the greater the mechanical difficulties both in construction and management. But if it be possible to mount a fixed telescope by which a solar or stellar image can be formed within a laboratory we give the following advantages:—(I) There is no mechanical limit to the length of the telescope; (2) the clockwork and other appliances to move the mirror, which reflects the starlight along the axis, are much lighter and smaller than those required to move a large telescope; (3) the observer remains in a fixed position, and spectroscopes of any weight can be used on piers within the laboratory; and (4) the angular value of any linear distance on a photographic plate can be determined by direct measurement of the distance of the photographic plate from the optical centre of the object-glass. The difficulty is that the automatic motion of a single mirror capable of reflecting the rays of any star continuously along the axis of afixed horizontal telescope, requires a rather complex mechanism owing to the variation of the angle of reflexion with the diurnal motion. Foucault appears to have been the first to appreciate these advantages and to face the difficulty of designing a siderostat which, theoretically at least, fulfils the above-mentioned conditions. A large siderostat, constructed by Eichens after Foucault's design, was completed in 1868—the year of Foucault's death. It remained at the Paris Observatory, where it was subsequently employed by Deslandres for solar photography. The largest refracting telescope yet made, viz., that constructed by Gautier for the Paris The Parrs exhibition of 1900, was arranged on this plan (type F), h refeactor the stars' rays being reflected along the horizontal axis • x900). of a telescope provided with visual and with photo- graphic object-glasses of 49-in. diameter and nearly Zoo-ft. focal length. Up to 1908 neither the optical qualities of the images given by the object-glasses and reflecting plane nor the practical working of the instrument, have, so far as we know, been submitted to any severe test. It is, however, certain that the Foucault siderostat is not capable, in practice, of maintaining the reflected image in a constant direction with perfect uniformity on account of the sliding action on the arm that regulates the motion of the mirror; such an action must, more or less, take place by jerks. There are farther inconveniences in the use of such a telescope, viz., that the image undergoes a diurnal rotation about the axis of the horizontal telescope, so that, unless the sensitive plate is also rotated by clockwork, it is impossible to obtain sharp photographs with any but instantaneous exposures. In the spectroscopic observation of a single star with a slit-spectroscope, this rotation of the image presents no inconvenience, and the irregular action of a siderostat on Foucault's plan might be overcome by the following arrangement : A B (fig. 23) is a polar axis, like that of an equatorial telescope, rotating in twenty-four hours by clockwork. Its lower extremity terminates in a fork on which is S mounted a mirror C D, capable of turning about A on an axis at right angles to A B, the plane of the f4 mirror being parallel to this latter axis. The mirror C D is set at such an angle as to reflect rays from the star S in the direction of the polar axis to the mirror R and thence to the horizontal telescope T. The mirrors of Lindemann's ~- equatorial coude reflecting light R ` Q- - T- -- downwards upon the mirror R FIG.2 would furnish an ideal siderostat for stellar spectroscopy in conjunction with a fixed horizontal telescope. Coelostat.—If a mirror is mounted on a truly adjusted polar axis, the plane of the mirror being parallel to that axis, the normal to that mirror will always be directed to some point on the celestial equator through whatever angle the axis is turned. Also, if the axis is made to revolve at half the apparent diurnal motion of the stars, the image of the celestial sphere, viewed by reflection from such a moving tnieror, will appear at rest at every point—hence the name coelostat applied to the apparatus. Thus, any fixed telescope directed towards the mirror of a properly adjusted coelostat in motion will show all the stars in the field of view at rest; or, by rotating the polar axis independently of the clockwork, the observer can pass in review all the stars visible above the horizon whose declinations come within the limits of his original field of view. Therefore, to observe stars of a different declination it will be necessary either to shift the direction of the fixed telescope, keeping its axis still pointed to the coelostat mirror, or to employ a second mirror to reflect the rays from the coelostat mirror along the axis of a fixed telescope. In the latter case it will be necessary to provide means to mount the coelostat on a carriage by which it can be moved east and west without changing the altitude or azimuth of its polar axis, and also to shift the second mirror so that it may receive all the light from the reflected beam. Besides these complications there is another drawback to the use of the coelostat for general astronomical work, viz., the obliquity- of the angle of reflection, which can never be less than that of the declination of the star, and may be greater to any extent. For these reasons the coelostat is never likely to be largely employed in general astronomical work, but it is admirably adapted for spectroscopic and bolometric observations of the sun, and for use in eclipse expeditions. For details of the coelostat applied to the Snow telescope—the most perfect installation for spectroheliograph and bolometer work yet erected—see The Study of Stellar Evolution by Prof. G. E. Hale, p. 131. The Zenith Telescope The zenith telescope is an instrument generally employed to measure the difference between two nearly equal and opposite zenith distances. Its original use was the determination of geographical latitudes in the field' work of geodetic operations; more recently it has been extensively employed for the determination of variation of latitude, at fixed stations, under the auspices of the International Geodetic Bureau, and for the astronomical determination of the constant of aberration. The instrument is shown in its most recent form in fig. 24. A is a sleeve that revolves very freely and without shake on a vertical steel cone. This cone is mounted on a circular base b which rests on three levelling screws, two of which are visible in the figure. The sleeve carries a cross-piece on its upper extremity to which the bearings of the horizontal axis c are attached. A reversible level d rests on the accurately turned pivots of this axis. The telescope is attached to one end of this axis and a counterpoise e to the other. The long arm f serves to clamp the telescope in zenith distance and to communicate slow motion in zenith distance when so clamped. On the side of the telescope opposite to the horizontal axis is attached a graduated circle g, and, turning concentrically with this circle, is a framework h, to which the readers and verniers of the circle are fixed. This frame carries two very sensitive levels, k and 1, and the whole frame can be clamped to the circle g by means of the clamping screw in. The object-glass of the telescope is, of course, attached by its cell to the upper end of the telescope tube. Within the focus of the object-glass is a right-angled prism of total reflection, which diverts the converging rays from the object-glass at right angles to the axis of the telescope, and permits the observing micrometer n to he mounted in the very convenient position shown in the figure. A small graduated circle p concentric with A is attached to the circular base b and read by the microscopes q r, attached to a. The instrument is thus a theodolite, although, compared with its other dimensions, feeble as an apparatus for the measurement of absolute altitudes and azimuths, although capable of determining these co-ordinates with considerable precision. In practice the vertical circle is adjusted once for all, so that when the levels k and I are in the centre of their run, the verniers read true zenith distances. When the instrument has been set up and levelled (either with aid of the cross level d, or the levels k and I), the reading of the circle p for the meridional position of the telescope is determined either by the method of transits in the meridian (see TRANSIT CIRCLE), or by the observation of the azimuth of a known star at a known hour angle. This done, the stops s and t are clamped and adjusted so that when arm r comes in contact with the screw of stop t the telescope will point due north, and when in contact with s, it will point due south, or vice versa. A pair of stars of known declination are selected such that their zenith distances, when on the meridian, are nearly equal and opposite, and whose right ascensions differ by five or ten minutesof time. Assuming, for example, that the northern star has the smaller right ascension, the instrument is first, with the aid of the stop, placed in the meridian towards the north; the verniers of the graduated circle g are set to read to the reading 4–10s+o ) where is the approximate latitude of the place and b., O. the declinations of the northern and southern star respectively; then the level frame h is turned till the levels k and l are in the middle of their run, and there clamped by the screw m, aided in the final adjustment by the adjoining slow motion screw shown in the figure. The telescope is now turned on the horizontal axis till the levels read near the centres of these scales and the telescope is clamped to the arm f. When the star enters the field of view its image is approximately bisected by the spider web of the micrometer n, the exact bisection being completed in the immediate neighbour-hood of the meridian. The readings of the levels k and l and the reading of the micrometer-drum are then entered, and the observation of the northern star is complete. Now the instrument is slowly turned towards the south, till the azimuth arm is gently brought into contact with the corresponding stop s, care being taken not to touch any part of the instrument except the azimuth arm itself. When the southern star enters the field the same process is repeated. Suppose now, for the moment, that the readings of the levels k and I are identical in both observations, we have then, in the difference between the micrometer readings north and south, a measure of the difference of the two zenith distances expressed in terms of the micrometer screw; and, if the " value of one revolution of the micrometer screw " is known in seconds of arc we have for the resulting latitude =2t(i —i ) +(S,. T&,)1, where i',, is the difference of the micrometer readings converted into arc—it being assumed that increased micrometer readings correspond with increased zenith distance of the star. If between the north and south observation there is a change in the level readings of the levels k and 1, this indicates a change in the zenith distance of the axis of the telescope. By directing the telescope to a distant object, or to the intersection of the webs of a fixed collimating telescope (see TRANSIT CIRCLE) ,it is easy to measure the effect of a small change of zenith distance of the axis of the telescope in terms both of the level and of the micrometer screw, and thus, if the levels are perfectly sensitive and uniform in curvature and graduation, to determine the value of one division of each level in terms of the micrometer screw. The value of " one revolution of the screw in seconds of arc " can be determined either by observing at transit the difference of zenith distance of two stars of known declination in terms of the micrometer screw, the instrument remaining at rest between their transits; or by measuring at known instants in terms of the screw, the change of zenith distance of a standard star of small polar distance near the time of its greatest elongation. The reason why two levels are employed is that sometimes crystals are formed by the decomposition of the glass which cause the bubble to stick at different points and so give false readings. Two levels are hardly likely to have such causes of error arise at exactly corresponding points in their run, and thus two levels furnish an independent control the one on the other. Also it is impossible to make levels that are in every respect perfect, nor even to deter-mine these errors for different lengths of bubble and at different readings with the highest precision. The mean of two levels there-fore adds to the accuracy of the result. Attempts have been made to overcome the difficulties connected with levels by adopting the principle of Kater's floating collimator (Phil, Trans., 1825 and 1828). On this principle the use of the level is abolished, the telescope is mounted on a metallic float, and it is assumed that, in course of the rotation of this float, the zenith distance of the axis of the telescope will remain undisturbed, that is, of course, after the undulations, induced by the disturbance of the mercury, have ceased. S. C. Chandler in 1884 constructed an equal altitude instrument on this principle, which he called the almucantar, and he found that after disturbance the telescope recovered its original zenith distance within of a second of arc. R. A. Sampson at Durham (Monthly Notices R.A.S. Ix. 572) and H. A. Howe (Ast. Jahrb. xxi. 57) have had instruments constructed on the same general principle. It is, however, obviously impossible to apply a micrometer with advantage to such instruments, because to touch such an instrument, in order to turn a micrometer screw, would obviously set it into motion. The almucantar was therefore used only to observe the vertical transits of stars in different azimuths over fixed horizontal webs, without touching the telescope. By the use of photography, however, it is possible to photograph the trail of a star as it transits the meridian when the telescope is directed towards the north, and another trail be similarly photo-graphed when the telescope is directed towards the south. The interval between the true trails, measured at right angles to the direction of the trails, obviously corresponds to the difference of zenith distance of the two stars. This principle has been applied with great completeness and ingenuity of detail by Bryan Cookson to the construction of a " photographic floating zenith telescope," 4s- which he has erected at Cambridge (Eng.) and applied to an investigation of the change of latitude and a determination of the constant of refraction. A description of the instrument, and some preliminary results obtained by it, is given by him (Monthly Notices R.A.S., lxi. 314). [D. GI.]
End of Article: FOR THE YERKES

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