A steady and convenient mounting is just as necessary to the successful use of the telescope as is a good objective. No satisfactory observations for any purpose can be made with a telescope unsteadily mounted and not provided with adjustments enabling it to be moved smoothly and easily in following a celestial object.

Broadly, telescope mounts may be divided into two general classes, alt-azimuth and equatorial. The former class is, as its name suggests, arranged to be turned in azimuth about a vertical axis, and in altitude about a horizontal axis. Such a mounting may be made of extreme simplicity, but obviously it requires two motions in order to follow up any object in the field, for the apparent motion of the heavenly bodies is in circles about the celestial pole as an axis, and consequently inclined from the vertical by the latitude of the place of observation.

Pointing a telescope with motions about a vertical and horizontal axis only, therefore means that, as a star moves in its apparent path, it will drift away from the telescope both in azimuth and in altitude, and require to be followed by a double motion.

Alt-azimuth mounts may be divided into three general groups according to their construction. The first and simplest of them is the pillar-and-claw stand shown in Figure 69. This consists of a vertical pillar supported on a strong tripod, usually of brass or iron, and provided at its top with a long conical bearing carrying at its upper extremity a hinged joint, bearing a bar to which the telescope is screwed as shown in the illustration.

If properly made the upper joint comprises a circular plate carrying the bar and held between two cheek pieces with means for taking up wear, and providing just enough friction to permit of easy adjustment of the telescope, which can be swung in altitude from near the zenith to the horizon or below, and turned around its vertical axis in any direction.

When well made a stand of this kind is steady and smooth working, readily capable of carrying a telescope up to about 3 inches aperture. It needs for its proper use a firm sub-support for the three strong hinged legs of the pillar. This is conveniently made as a very solid stool with spreading legs, or a plank of sufficient size may be firmly bolted to a well set post.

Thus arranged the mount is a very serviceable one for small instruments. Its stability, however, depends on the base upon which it is set. The writer once unwisely attempted to gain convenience by removing the legs of the stand and screwing its body firmly upon a very substantial tripod. The result was a complete failure in steadiness, owing to the rather long lever arm furnished by the height of the pillar; and the instrument, which had been admirably steady originally, vibrated abominably when touched for focussing.

The particular stand here shown is furnished with a rack motion in altitude which is a considerable convenience in following. More rarely adjustable steadying rods attached to the objective end of the instrument are brought down to its base, but for a telescope large enough to require this a better mount is generally desirable.

Now and then an alt-azimuth head of just the sort used in the pillar-and-claw stand is actually fitted on a tall tripod, but such an arrangement is usually found only in cheap instruments and for such tripod mountings other fittings are desirable.

The second form of alt-azimuth mount, is altogether of more substantial construction. The vertical axis, usually tapered and carefully ground in its bearings, carries an oblique fork in which the telescope tube is carried on trunnions for its vertical motion. The inclination of the forked head enables the telescope to be pointed directly toward the zenith and the whole mount forms the head of a well made tripod.

Figure 70 shows an excellent type of this form of mount as used for the Clark Type T telescope, designed for both terrestrial and astronomical observation. In this particular arrangement the telescope lies in an aluminum cradle carried on the trunnions, from which it can be readily removed by loosening the thumb screws and opening the cradle.

It can also be set longitudinally for balance in the cradle if any attachments are to be placed upon either end. Here the adjustment for the height of the instrument is provided both in the spread of the tripod and in the adjustable legs. So mounted a telescope of 3 or 4 inches aperture is easily handled and capable of rendering very good service either for terrestrial or celestial work.

Indeed the Clarks have made instruments up to 6 inches aperture, mounted for special service in this simple manner. For celestial use where fairly high powers may be required this or any similar mount can be readily furnished with slow motions either in azimuth or altitude or both.

Figure 71 shows a 4¼ inch telescope and mount by Zeiss thus equipped. Some alt-azimuth mounts are also provided with a vertical rack motion to bring the telescope to a convenient height without disturbing the tripod. A good alt-azimuth mount such as is shown in Figs. 70 and 71 is by no means to be despised for use with telescopes of 3 or 4 inch aperture.

The sole inconvenience to be considered is that of the two motions required in following. With well fitted slow motions this is not really serious for ordinary observing with moderate powers, for one can work very comfortably up to powers of 150 or 200 diameters keeping the object easily in view; but with the higher powers in which the field is very small, only a few minutes of arc, the double motion becomes rather a nuisance and it is extremely inconvenient even with low powers in sweeping for an object the place of which is not exactly known.

There are in fact two distinct kinds of following necessary in astronomical observations. First, the mere keeping of the object somewhere in the field, and second, holding it somewhat rigorously in position, as in making close observations of detail or micrometrical measurements. When this finer following is necessary the sooner one gets away from alt-azimuth mounts the better.

Still another form of alt-azimuth mount is shown in Fig. 72 applied for a Newtonian reflector of 6 or 8 inches aperture. Here the overhung fork carrying the tube on trunnions is supported on a stout fixed tripod, to which it is pivoted at the front, and it is provided at the rear with a firm bearing on a sector borne by the tripod.

At the front a rod with sliding coarse, and screw fine, adjustment provides the necessary motion in altitude. The whole fork is swung about its pivot over the sector bearing by a cross screw turned by a rod with a universal joint.

This mount strongly suggests the original one of Hadley, Fig. 16, and is most firm and serviceable. A reflector thus mounted is remarkably convenient in that the eyepiece is always in a most accessible position, the view always horizontal, and the adjustments always within easy reach of the observer.

Whenever it is necessary to follow an object closely, as in using a micrometer and some other auxiliaries, the alt-azimuth mount is troublesome and some modification adjustable by a single motion, preferably made automatic by clockwork, becomes necessary.

The first step in this direction is a very simple one indeed. Suppose one were to tilt the azimuth axis so that it pointed to the celestial pole, about which all the stars appear to revolve. Then evidently the telescope being once pointed, a star could be followed merely by turning the tube about this tilted axis. Of course one could not easily reach some objects near the pole without, perhaps, fouling the mount, but in general the sky is within reach and a single motion follows the star, very easily if the original mount had a slow motion in azimuth.

This is in fact the simplest form of equatorial mount, sometimes called parallactic. Figure 73 shows the principle applied to a small reflector. An oblique block with its angle adjusted to the co-latitude of the place drops the vertical axis into line with the pole, and the major part of the celestial vault is then within easy reach.

It may be regarded as the transition step from the alt-azimuth to the true equatorial. It is rarely used for refractors, and the first attempt at a real equatorial mount was in fact made by James Short F. R. S. in mounting some of his small Gregorians.[15] As a matter of record this is shown, from Short’s own paper before the Royal Society in 1749, in Fig. 74.

A glance shows a stand apparently most complicated, but closer examination discloses that it is merely an equatorial on a table stand with a sweep in declination over a very wide arc, and quite complete arrangements for setting to the exact latitude and azimuth. The particular instrument shown was of 4 inches aperture and about 18 inches long and was one of several produced by Short at about this epoch.

In the instrument as shown there is first an azimuth circle A A supported on a base B B B B having levelling screws in the feet. Immediately under the azimuth circle is mounted a compass needle for approximate orientation, and the circle is adjustable by a tangent screw C.

Carried by the azimuth circle on a bearing supported by four pillars is a latitude circle D D for the adjustment of the polar axis, with a slow motion screw E. The latitude circle carries a right ascension circle F F, with a slow motion G, and this in turn carries on four pillars the declination circle H H, and its axis adjustable by the slow motion K.

To this declination circle is secured the Gregorian reflector L L which serves as the observing telescope. All the circles are provided with verniers as well as slow motions. And the instrument is, so to speak, a universal one for all the purposes of an equatorial, and when the polar axis is set vertical equally adapted for use as a transit instrument, theodolite, azimuth instrument, or level, since the circles are provided with suitable levels.

This mount was really a very neat and complete piece of work for the purpose intended, although scarcely suitable for mounting any but a small instrument. A very similar construction was used later by Ramsden for a small refractor.

It is obvious that the reach of the telescope when used as an equatorial is somewhat limited in the mount just described. In modern constructions the telescope is so mounted that it may be turned readily to any part of the sky, although often the polar axis must be swung through 180° in order to pass freely from the extreme southern to the extreme northern heavens.

The two motions necessary are those in right ascension to follow the heavenly bodies in their apparent course, and in declination to reach an object at any particular angular distance from the pole.

There are always provided adjustments in azimuth and for latitude over at least a small arc, but these adjustments are altogether rudimentary as compared with the wide sweep given by Short.

The fundamental construction of the equatorial involves two axes working at right angles positioned like a capital T.

The upright of the T is the polar axis, fitted to a sleeve and bearing the cross of the T, which is hollow and provides the bearing for the declination axis, which again carries at right angles to itself the tube of the telescope.

When the sleeve which carries the upright of the T points to the pole the telescope tube can evidently be swung to cover an object at any altitude, and can then be turned on its polar axis so as to follow that object in its apparent diurnal motion. The front fork of a bicycle set at the proper angle with a cross axis replacing the handle bars has more than once done good service in an emergency. Figure 75 shows in section a modern equatorial mount for a medium sized telescope.

The mounting shown in Fig. 75, by Zeiss, is thoroughly typical of recent practice in instruments of moderate size. The general form of this equatorial comes straight down to us from Fraunhofer’s mounting of the Dorpat instrument. It consists essentially of two axes crossed exactly at right angles.

P, the polar axis, is aligned exactly with the pole, and is supported on a hollow iron pier provided at its top with the latitude block L to which the bearings of P are bolted. D the declination axis supports the telescope tube T.

The tube is counterpoised as regards the polar axis by the weight a, and as regards the declination axis by the weights b b. At A, the upper section of the pier can be set in exact azimuth by adjusting screws, and at the base of the lower section the screws at B. B. allow some adjustment in latitude. To such mere rudiments are the azimuth and altitude circles of Short’s mount reduced.

At the upper end of the polar axis is fitted the gear wheel g, driven by a worm from the clockwork at C to follow the stars in their course. At the lower end of the same axis is the hour circle h, graduated for right ascension, and a hand wheel for quick adjustment in R. A.

At d is the declination circle, which is read, and set, by the telescope t with a right angled prism at its upper end, which saves the observer from leaving the eye piece for small changes.

F is the usual finder, which should be applied to every telescope of 3 inches aperture and above. It should be of low power, with the largest practicable field, and has commonly an aperture ¼ or ? that of the main objective, big enough to pick up readily objects to be examined and by its coarse cross wires to bring them neatly into the field. At m and n are the clamping screws for the right ascension and declination axes respectively, while o and p control the respective tangent screws for fine adjustment in R. A. and Dec. after the axes are clamped. This mount has really all the mechanical refinements needed in much larger instruments and represents the class of permanently mounted telescopes used in a fixed observatory.

The ordinary small telescope is provided with a mount of the same general type but much simpler and, since it is not in a fixed observatory, has more liberal adjustments in azimuth and altitude to provide for changes of location. Figure 76 shows in some detail the admirable portable equatorial mounting used by the Clarks for instruments up to about 5 or 6 inches aperture.

Five inches is practically the dividing line between portable and fixed telescopes. In fact a 5 inch telescope of standard construction with equatorial mounting is actually too heavy for practical portability on a tripod stand. The Clarks have turned out really portable instruments of this aperture, of relatively short focus and with aluminum tube fitted to the mounting standard for a 4 inch telescope, but the ordinary 5 inch equipment of the usual focal length deserves a permanent placement.

In this mount the short tapered polar axis P is socketed between the cheeks A, and tightened in any required position by the hand screws B. The stout declination axis D bears the telescope and the counterweight C. Setting circles in R. A. and Dec., p and d respectively, are carried on the two axes, and each axis has a worm wheel and tangent screw operated by a universal joint to give the necessary slow motion.

The worm wheels carry their respective axes through friction bearings and the counter poising is so exact that the instrument can be quickly swung to any part of the sky and the slow motion picked up on the instant. The wide sweep of the polar axis allows immediate conversion into an alt-azimuth for terrestrial use, or adjustment for any latitude. A graduated latitude arc is customarily engraved on one of the check pieces to facilitate this adjustment.

Ordinarily portable equatorials on tripod mounts are not provided with circles, and have only a single slow motion, that in R. A. A declination circle, however, facilitates setting up the instrument accurately and is convenient for locating an object to be swept for in R. A. which must often be done if one has not sidereal time at hand. In Fig. 76 a thumb screw underneath the tripod head unclamps the mount so that it may be at once adjusted in azimuth without shifting the tripod.

As a rule American stands for fixed equatorials have the clock drive enclosed in the hollow pillar which carries the equatorial head as shown in the reflector of Fig. 35, and in the Clark mount for refractors of medium size shown in Fig. 77. Here a neat quadrangular pillar carries an equatorial mounting in principle very much like Fig. 76, but big enough to carry telescopes of 8 to 10 inches aperture. It has universal adjustment in latitude, so that it can be used in either hemisphere, the clock and its driving weight are enclosed in the pillar and slow motions are provided for finding in R. A. and Dec. The adjustment in azimuth is made by moving the pillar on its base-plate, which is bolted to the pier. The convenient connections for accurate following and the powerful clock make the mount especially good for photographic telescopes of moderate size and the whole equipment is most convenient and workmanlike. It is worth noting that the circles are provided with graduations that are plain rather than minute, in accordance with modern practice. In these days of celestial photography equatorials are seldom used for determining positions except with the micrometer, and graduated circles therefore, primarily used merely for finding, should be, above all things, easy to read.

All portable mounts are merely simplifications of the observatory type of Fig. 75, which, with the addition of various labor saving devices is applied to nearly all large refractors and to many reflectors as well.

There is a modified equatorial mount sometimes known as the “English” equatorial in which the polar axis is long and supported on two piers differing enough in height to give the proper latitude angle, the declination axis being about midway of the polar axis. A bit of the sky is cut off by the taller pier, and the type is not especially advantageous unless in supporting a very heavy instrument, too heavy to be readily overhung in the usual way.

In such case some form of the “English” mounting is very important to securing freedom from flexure and thereby the perfection of driving in R. A. so important to photographic work. The 72 inch Dominion Observatory reflector and the 100 inch Hooker telescope at Mt. Wilson are thus mounted, the former on a counterpoised declination axis crosswise the polar axis, the original “English” type; the latter on trunnions within a long closed fork which carries the polar bearings at its ends.

Figure 78 shows the latter instrument, of 100 inches clear aperture and of 42 feet principal focal length, increased to 135 feet when used as a Cassegrainian. It is the immense stability of this mount that has enabled it to carry the long cross girder bearing the interferometer recently used in measuring the diameters of the stars. Note the mercury-flotation drum at each end of the polar axis. The mirrors were figured by the skillful hands of Mr. Ritchey.

Figure 79 gives in outline the proportions and mounting of the beautiful instrument in service at the Dominion Observatory, near Victoria, B. C. The mirror and its auxiliaries were figured by Brashear and the very elegant mounting was by Warner and Swasey. The main mirror is of 30 feet principal focus. The 20 inch hyperboloidal mirror extends the focus as a Cassagrainian to 108 feet. The mechanical stability of these English mounts for very large instruments has been amply demonstrated by this, as by the Hooker 100 inch reflector. They suffer less from flexure than the Fraunhofer mount where great weights are to be carried, although the latter is more convenient and generally useful for instruments of moderate size. It is hard to say too much of the mechanical skill that has made these two colossal telescopes so completely successful as instruments of research.

The inconvenience of having to swing the telescope tube to clear the pier at certain points in the R. A. following is often a serious nuisance in photographic work requiring long exposures, and may waste valuable time in visual work. Several recent forms of equatorial mount have therefore been devised to allow the telescope complete freedom of revolution in R. A., swinging clear of everything.

One such form is shown in Fig. 80 which is a standard astrographic mount for a Brashear doublet and guiding telescope. The pier is strongly overhung in the direction of the polar axis far enough to allow the instrument to follow through for any required period, even to resuming operations on another night without a shift of working position.

Another form, even simpler and found to be extremely satisfactory even for rather large instruments, is the open polar fork mount. Here the polar axis of an ordinary form is continued by a wide and stiff casting in the form of a fork within which the tube is carried on substantial trunnions, giving it complete freedom of motion.

The open fork mount in its simplest form, carrying a heliostat mirror, is shown in Fig. 81. Here A is the fork, B the polar axis, carried on an adjustable sector for variation in latitude, C the declination axis carrying the mirror D in its cell, E the slow motion in declination, and F that in R. A. Both axes can be unclamped for quick motion and the R. A. axis can readily be driven by clock or electric motor.

The resemblance to the fully developed English equatorial mount of Fig. 78 is obvious, but the present arrangement gives entirely free swing to a short instrument, is conveniently adjustable, and altogether workmanlike. It can easily carry a short focus celestial camera up to 6 or 8 inches aperture or a reflector of 4 or 5 feet focal length.

In Fig. 173, Chap. X a pair of these same mounts are shown at Harvard Observatory. The nearer one, carrying a celestial camera, is exposed to view. It is provided with a slow motion and clamp in declination, and with an electric drive in R. A., quickly unclamped for swinging the camera. It works very smoothly, its weight is taken by a very simple self adjusting thrust bearing at the lower end of the polar axis, and altogether it is about the simplest and cheapest equatorial mount of first class quality that can be devised for carrying instruments of moderate length.

Several others are in use at the Harvard Observatory and very similar ones of a larger growth carry the 24 inch Newtonian reflector there used for stellar photography and the 16 inch Metcalf photographic doublet.

In fact the open fork mount, which was developed by the late Dr. Common, is very well suited to the mounting of big reflectors. It was first adapted by him to his 3 ft. reflector and later used for his two 5 ft. mirrors, and more recently for the 5 ft. instrument at Mt. Wilson, and a good many others of recent make. Dr. Common in order to secure the easiest possible motion in R. A. devised the plan of floating most of the weight assumed by the polar axis in mercury.

Figure 82 is, diagrammatically, this fork mount as worked out by Ritchey for the 5' Mt. Wilson reflector. Here A is the lattice tube, B the polar axis, C the fork and D the hollow steel drum which floats the axis in the mercury trough E. The great mirror is here shown worked as a simple Newtonian of 25 ft. focal length. As a matter of fact it is used much of the time as a Cassegranian.

To this end the upper section of tube carrying the oblique mirror is removed and a shorter tube carrying any one of three hyperboloidal mirrors is put in its place. Fig. 83 is the normal arrangement for visual or photographic work on the long focus, 100 ft. The dotted lines show the path of the rays and it will be noticed that the great mirror is not perforated as in the usual Cassegrainian construction, but that the rays are brought out by a diagonal flat.

Figure 84 is a similar arrangement used for stellar spectroscopy with a small flat and an equivalent focus of 80 ft. In Fig. 85 a radically different scheme is carried out. The hyperboloidal mirror now used gives an equivalent focus of 150 ft., and the auxiliary flat is arranged to turn on an axis parallel to the declination axis so as to send the reflected beam down the hollow polar axis into a spectrograph vault below the southern end of the axis. Obviously one cannot work near the pole with this arrangement but only through some 75° as indicated by the dotted lines. The fork mount is not at all universal for reflectors, as has already been seen, and Cassegrainian of moderate size are very commonly mounted exactly like refractors.

We now come to a group of mounts which have in common the fundamental idea of a fixed eyepiece, and incidentally better protection of the observer against the rigors of long winter nights when the seeing may be at its best but the efficiency of the observer is greatly diminished by discomfort. Some of the arrangements are also of value in facilitating the use of long focus objectives and mirrors and escaping the cost of the large domes which otherwise would be required.

Perhaps the earliest example of the class is found in Caroline Herschel’s comet seeker, shown in Fig. 86. This was a Newtonian reflector of about 6 inches aperture mounted in a fashion that is almost self explanatory. It was, like all Herschel’s telescopes, an alt-azimuth but instead of being pivoted in altitude about the mirror or the center of gravity of the whole tube, it was pivoted on the eyepiece location and the tube was counterbalanced as shown so that it could be very easily adjusted in altitude while the whole frame turned in azimuth about a vertical post.

Thus the observer could stand or sit at ease sweeping in a vertical circle, and merely had to move around the post as the azimuth was changed. The arrangement is not without advantages, and was many years later adopted with modifications of detail by Dr. J. W. Draper for the famous instrument with which he advanced so notably the art of celestial photography.

The same fundamental idea of freeing the observer from continual climbing about to reach the eyepiece has been carried out in various equatorially mounted comet seekers. A very good example of the type is a big comet seeker by Zeiss, shown in Fig. 87. The fundamental principle is that the ocular is at the intersection of the polar and declination axis, the telescope tube being overhung well beyond the north end of the former and counterbalanced on the latter. The observer can therefore sit in his swivel chair and without stirring from it sweep rapidly over a very wide expanse of sky.

This particular instrument is probably the largest of regular comet seekers, 8 inches in clear aperture and 52½ inches focal length with a triple objective to ensure the necessary corrections in using so great a relative aperture. In this figure 1 is the base with corrections in altitude and azimuth, 2 the counterpoise of the whole telescope on its base, 3 the polar axis and R. A. circle, 4 the overhung declination axis and its circle, 5 the counterpoise in declination, 6 the polar counterpoise, and 7 the main telescope tube. The handwheel shown merely operates the gear for revolving the dome without leaving the observing chair.

The next step beyond the eyepiece fixed in general position is so to locate it that the observer can be thoroughly protected without including the optical parts of the telescope in such wise as to injure their performance.

One cannot successfully observe through an open window on account of the air currents due to temperature differences, and in an observatory dome, unheated as it is, must wait after the shutter is opened until the temperature is fairly steadied.

Except for these comet seekers practically all of the class make use of one or two auxiliary reflections to bring the image into the required direction, and in general the field of possible view is somewhat curtailed by the mounting. This is less of a disadvantage than it would appear at first thought, for, to begin with, observations within 20° of the horizon or thereabouts are generally unsatisfactory, and the advantages of a stable and convenient long focus instrument are so notable as for many purposes quite to outweigh some loss of sky-space.

The simplest of the fixed eyepiece group is the polar telescope of which the rudiments are well shown in Fig. 88, a mount described by Sir Howard Grubb in 1880, and an example of which was installed a little later in the Crawford Observatory in Cork. Here the polar axis A is the main tube of the telescope, and in front of the objective B, is held in a fork the declination cradle and mirror C, by which any object within a wide sweep of declination can be brought into the field and held there by hand or clockwork through rotating the polar tube.

Looked at from another slant it is a polar heliostat, of which the telescope forms the driving axis in R. A. The whole mount was a substantial casting on wheels which ran on a pair of rails. For use the instrument was rolled to a specially arranged window and through it until over its regular bearings on a pier just outside.

A few turns of the wheel D lowered it upon these, and the back of the frame then closed the opening in the wall leaving the instrument in the open, and the eye end inside the room. The example first built was of only 4 inches aperture but proved its case admirably as a most useful and convenient instrument.

This mount with various others of the fixed eyepiece class may be regarded as derived from the horizontal photoheliographs used at the 1874 transit of Venus and subsequently at many total solar eclipses. Given an equatorially mounted heliostat like Fig. 81 and it is evident that the beam from it may be turned into a horizontal telescope placed in the meridian, (or for that matter in any convenient direction) and held there by rotation of the mirror in R. A., but also in declination, save in the case where the beam travels along the extension of the polar axis.

For the brief exposure periods originally needed and the slow variation of the sun in declination this heliostatic telescope was easily kept in adjustment. The original instruments were of 5 inches aperture and 40 ft. focal length, and the 7 inch heliostat mirror was provided with ordinary equatorial clockwork. Set up with the telescope pointing along the polar axis no continuous variation in declination is needed and the clock drive holds the field steadily, as in any other equatorial.

Figure 89 shows diagrammatically the 12 inch polar telescope used for more than twenty years past at the Harvard Observatory. The mount was designed by Mr. W. P. Gerrish of the Harvard staff and contains many ingenious features. Unlike Fig. 88 this is a fixed mount, with the eye-end comfortably housed in a room on the second floor of the main observatory building, and the lower bearing on a substantial pier to the southward.

In the figure, A is the eye end, B the main tube with the objective at its lower end and prolonged by a fork supported by the bearing C and D is the declination mirror sending the beam upward. The whole is rotated in R. A. by an electric clock drive, and all the necessary adjustments are made from the eye end.

A view of the exterior is shown in Fig. 90, with the mirror and objective uncovered. The rocking arm at the objective end, operated by a small winch beside the ocular, swings clear both mirror and objective caps in a few seconds, and the telescope is then ready for use. Its focal length is 16 ft. 10 inches and it gives a sweep in declination of approximately 80°. It gives excellent definition and has proved a most useful instrument.

A second polar telescope was set up at the Harvard Observatory station in Mandeville, Jamaica, in the autumn of 1900. This was intended primarily for lunar photography and was provided with a 12 inch objective of 135 ft. 4 inches focal length and an 18 inch heliostat with electric clock drive.

Inasmuch as all instruments of this class necessarily rotate the image as the mirror turns, the tail-piece of this telescope is also mounted for rotation by a similar drive so that the image is stationary on the plate both in position and orientation. As Mandeville is in N. lat. 18° 01' the telescope is conveniently near the horizontal. The observatory of Yale University has a large instrument of this class, of 50 feet focal length, with a 15-inch photographic objective and a 10-inch visual guiding objective working together from the same heliostat.

Despite its simplicity and convenience the polar telescope has an obvious defect in its very modest sweep in declination, only to be increased by the use of an exceptionally large mirror. It is not therefore remarkable that the first serious attempt at a fixed eyepiece instrument for general use turned to a different construction even at the cost of an additional reflection.

This was the equatorial coudÉ devised by M. Loewy of the Paris Observatory in 1882. (Fig. 91.) In the diagram A is the main tube which forms the polar axis, and B the eye end under shelter, with all accessories at the observer’s hand. But the tube is broken by the box casing C containing a mirror rigidly supported at 45° to the axis of the main tube and of the side tube D, which is counterbalanced and is in effect a hollow declination axis carrying the objective E at its outer end.

In lieu of the telescope tube usually carried on this declination axis we have the 45° mirror, F, turning in a sleeve concentric with the objective, which, having a lateral aperture, virtually gives the objectives a full sweep in declination, save as the upper pier cuts it off. The whole instrument is clock driven in R. A., and has the usual circles and slow motions all handily manipulated from the eye end.

The equatorial coudÉ is undeniably complicated and costly, but as constructed by Henry FrÈres it actually performs admirably even under severe tests, and has been several times duplicated in French observatories. The first coudÉ erected was of 10½ inches aperture and was soon followed by one of 23.6 inches aperture and 59 ft. focus, which is the largest yet built.

Still another mounting suggestive of both the polar telescope and the coudÉ is due to Sir Howard Grubb, Fig. 92. Here as in the coudÉ the upper part of the polar axis, A, is the telescope tube which leads into a solid casing B, about which a substantial fork, C, is pivoted. This fork is the extension of the side tube D, which carries the objective, and thus has free swing in declination through an angle limited by the roof of the observing room above, and the proximity of the horizon below.

Its useful swing, as in the polar telescope, is limited by the dimensions of the mirror E, which receives the cone of rays from the objective and turns it up the polar tube to the eyepiece. This mirror is geared to turn at half the rate of the tube D so that the angle D E A is continually bisected.

In point of fact the sole gain in this construction is the reduction in the size of mirror required, by reason of the diminished size of the cone of rays when it reaches the mirror. The plan has been very successfully worked out in the fine astrographic telescope of the Cambridge Observatory of 12½ inches aperture and 19.3 ft. focal length.

As in the other instruments of this general class the adjustments are all conveniently made from the eye end. The Cambridge instrument has a triple photo-visual objective of the form designed by Mr. H. D. Taylor and the side tube, when not in use, is turned down to the horizontal and covered in by a low wheeled housing carried on a track. The sky space covered is from 15° above the pole to near the horizontal.

It is obvious that various polar and coudÉ forms of reflector are quite practicable and indeed one such arrangement is shown in connection with the 60 inch Mt. Wilson reflector, but we are here concerned only with the chief types of mounting which have actually proved their usefulness. None of the arrangements which require the use of additional large reflecting surfaces are exempt from danger of impaired definition. Only superlatively fine workmanship and skill in mounting can save them from distortion and astigmatism due to flexure and warping of the mirrors, and such troubles have not infrequently been encountered.

To a somewhat variant type belong several valuable constructions which utilize in the auxiliary reflecting system the coelostat rather than the polar heliostat or its equivalent. The coelostat is simply a plane mirror mounted with its plane fixed in that of a polar axis which rotates once in 48 hours, i.e., at half the apparent rate of the stars.

A telescope pointed at such a mirror will hold the stars motionless in its field as if the firmament were halted À la Joshua. But if a change of view is wanted the telescope must be shifted in altitude or azimuth or both. This is altogether inconvenient, so that as a matter of practice a second plane mirror is used to turn the steady beam from the coelostat into any desired direction.

By thus shifting the mirror instead of the telescope, the latter can be permanently fixed in the most convenient location, at the cost of some added expense and loss of light. Further, the image does not rotate as in case of the polar heliostat, which is often an advantage.

An admirable type of the fixed telescope thus constituted is the Snow telescope at Mt. Wilson (Cont. from the Solar Obs. #2, Hale). Fig. 93 from this paper shows the equipment in plan and elevation. The topography of the mountain top made it desirable to lay out the axis of the building 15° E. of N. and sloping downward 5° toward the N.

At the right hand end of the figure is shown the coelostat pier, 29 ft. high at its S end. This pier carries the coelostat mirror proper, 30 inches in diameter, on rails a a accurately E. and W. to allow for sliding the instrument so that its field may clear the secondary mirror of 24 inches diameter which is on an alt-azimuth fork mounting and also slides on rails b b.

The telescope here is a pair of parabolic mirrors each of 24 inches aperture and of 60 ft. and 145 ft. focus respectively. The beam from the secondary coelostat mirror passes first through the spectrographic laboratory shown to the left of the main pier, and in through a long and narrow shelter house to one of these mirrors; the one of longest focus on longitudinal focussing rails e e, the other on similar rails c c, with provision for sliding sidewise at d to clear the way for the longer beam.

The ocular end of this remarkable telescope is the spectrographic laboratory where the beam can be turned into the permanently mounted instruments, for the details of which the original paper should be consulted. The purpose of this brief description is merely to show the beautiful facility with which a coelostatic telescope may be adapted to astrophysical work. Obviously an objective could be put in the coelostat beam for any purpose for which it might be desirable.

Such in fact is the arrangement of the tower telescopes at the Mt. Wilson Observatory. In these instruments we have the ordinary coelostat arrangement turned on end for the sake of getting the chief optical parts well above the ground where, removed from the heated surface, the definition is generally improved. To be sure the focus is at or near the ground level, but the upward air currents cause much less disturbance than the crosswise ones in the Snow telescope.

The head of the first tower telescope is shown in Fig. 94.[16] A is the coelostat mirror proper 17 inches in diameter and 12 inches thick, B the secondary mirror 12¾ inches in the shorter axis of the ellipse, 22¼ inches in the longer, and also 12 inches thick. C is the 12 inch objective of 60 ft. focus, and D the focussing gear worked by a steel ribbon from below.

This instrument being for solar research the mirrors are arranged for convenient working with the sun fairly low on either horizon where the definition is at its best, and can be shifted accordingly, to the same end as in the Snow telescope. There is also provision for shifting the objective laterally at a uniform rate from below, to provide for the use of the apparatus as spectro-heliograph.

The tower is of the windmill type and proved to be fairly steady in spite of its height, high winds being rare on Mt. Wilson. The great thickness of the mirrors in the effort to escape distortion deserves notice. They actually proved to be too thick to give thermal conductivity sufficient to prevent distortion.

In the later 150' tower telescope the mirrors are relatively less thick, and a very interesting modification has been introduced in the tower, in that it consists of a lattice member for member within another exterior lattice, so that the open structure is retained, while each member that supports the optical parts is shielded from the wind and sudden temperature change by its corresponding outer sheath.

Still another form of mounting to give the observer access to a fixed eyepiece under shelter is found in the ingenious polar reflector by Mr. Russell W. Porter of which an example with main mirror of 16 inches diameter and 15 ft. 6 inches focal length was erected by him a few years ago. Fig. 95 is entirely descriptive of the arrangement which from Mr. Porter’s account seems to have worked extremely well. The chief difficulty encountered was condensation of moisture on the mirrors, which in some climates is very difficult to prevent.

It is interesting to note that Mr. Porter’s first plan was to use the instrument as a Herschelian with its focus thrown below the siderostat at F', but the tilting of the mirror, which was worked at F/11.6, produced excessive astigmatism of the images, and the plan was abandoned in favor of the Newtonian form shown in the figure. At F/25 or thereabouts the earlier scheme would probably have succeeded well.

Still another fixed eyepiece telescope of daring and successful design is the turret telescope of the Hon. J. E. Hartness of which the inventor erected a fine example of 10 inch aperture at Springfield, Vermont. The telescope is in this case a refractor, and the feature of the mount is that the polar axis is expanded into a turret within which the observer sits comfortably, looking into the ocular which lies in the divided declination axis and is supplied from a reflecting prism in the main beam from the objective

Figure 96 shows a diagram of the mount and observatory. Here a is the polar turret, bb the bearings of the declination axis, c the main tube, d its support, and e the ocular end. Optically the telescope is merely an ordinary refractor used with a right angled prism a little larger and further up the tube than usual. The turret is entered in this instance from below, through a tunnel from the inventor’s residence. The telescope as shown in Fig. 96 has a 10 inch Brashear objective of fine optical quality, and the light is turned into the ocular tube by a right angled prism only 2¾ inches in the face. This is an entirely practicable size for a reflecting prism and the light lost is not materially in excess of that lost in the ordinary “star diagonal” so necessary for the observation of stars near the zenith in an ordinary equatorial. The only obvious difficulty of the construction is the support of the very large polar axis. Being an accomplished mechanical engineer, Mr. Hartness worked out the details of this design very successfully although the moving parts weighed some 2 tons. The ocular is not absolutely fixed with reference to the observer but is always conveniently placed, and the performance of the instrument is reported as excellent in every respect, while the sheltering of the observer from the rigors of a Vermont winter is altogether admirable. Figure 97 shows the complete observatory as it stands. Obviously the higher the latitude the easier is this particular construction, which lends itself readily to large instruments and has the additional advantage of freeing the observer from the insect pests which are extremely troublesome in warm weather over a large part of the world.

This running account of mountings makes no claim at completeness. It merely shows the devices in common use and some which point the way to further progress. The main requirements in a mount are steadiness, and smoothness of motion. Even an alt-azimuth mount with its need of two motions, if smooth working and steady, is preferable to a shaky and jerky equatorial.

Remember that the Herschels did immortal work without equatorial mountings, and used high powers at that. A clock driven equatorial is a great convenience and practically indispensable for the photographic work that makes so large a part of modern astronomy, but for eye observations one gets on very fairly without the clock.

Circles arc a necessity in all but the small telescopes used on portable tripods, otherwise much time will be wasted in finding. In any event do not skimp on the finder, which should be of ample aperture and wide field, say ¼ the aperture of the main objective, and 3° to 5° in field. Superior definition is needless, light, and sky room enough to locate objects quickly being the fundamental requisites.

As a final word see that all the adjustments are within easy reach from the eyepiece, since an object once lost from a high power ocular often proves troublesome to locate again.

REFERENCES

• Chambers’ Astronomy, Vol. II.
• F. L. O. Wadsworth: Ap. J., 5, 132. Ranyard’s mounts for reflectors.
• G. W. Ritchey: Ap. J., 5, 143. Supporting large specula.
• G. E. Hale: Cont. Solar Obs. # 2. The “Snow” horizontal telescope.
• G. E. Hale: Cont. Solar Obs. # 23. The 60 ft. tower telescope.
• J. W. Draper: Smithsonian Contrib. 34. Mounting of his large reflector.
• G. W. Ritchey: Smithsonian Contrib. 35. Mounting of the Mt. Wilson 60 inch reflector.
• Sir H. Grubb: Tr. Roy. Dublin Soc. Ser. 2. 3. Polar Telescopes.
• Sir R. S. Ball: M. N. 59, 152. Photographic polar telescope.
• A. A. Common: Mem. R. A. S., 46, 173. Mounting of his 3 ft. reflector.
• R. W. Porter: Pop. Ast., 24, 308. Polar reflecting telescope.
• James Hartness: Trans. A. S. M. E., 1911, Turret Telescope.
• Sir David Gill: Enc. Brit., 11th Ed. Telescope. Admirable summary of mounts.