SECTION 3.—MECHANICAL PARTS OF TELESCOPE
It is useful, before describing the mechanical parts or mounting of the telescope, to explain the difference between the two kinds of telescope, refracting and reflecting, employed in astronomical work. The refracting telescope is the most familiar type as the ordinary spyglass or draw-tube telescope and the field or opera glass are all refracting telescopes. The refracting telescope is so called because the light from the distant object is refracted through a lens at the outer end of the tube and forms an image of the object at the inner end, just as a camera forms an image on the ground glass or film, and this image is viewed and magnified by the eyepiece or ocular. The reflecting telescope on the other hand has theupper or outer end of the tube open and the light from the distant object is reflected (hence the name) from a concave mirror at the lower end of the tube, forming the image of the object at the top, where it can be viewed and magnified by the ocular as in the refractor.
Each type of telescope has its astronomical advantages and disadvantages. The refractor is better suited for visual observations such as the measurement of double stars and the study of planetary detail and is less affected by temperature changes than the reflector. On the other hand the reflector, on account of its perfect achromatism, is the instrument par excellence for photographic observations, and, as more than three-fourths of modern astronomical work is photographic, it appears to be superseding the refractor. This advantage is increased by the fact that the refractor has apparently reached the useful limit in size and that it costs at least three times as much as a reflector of the same aperture. Although each type of telescope has its characteristic type of mounting for astronomical purposes, the principles are the same for each and can probably be most easily followed by describing the essential parts of the mounting of the 72-inch telescope.
The tube performs the important function of carrying in relatively invariable position and adjustment the optical parts of the telescope. The tube of the 72-inch telescope is 31 feet long, 7 feet 4 inches outer diameter and weighs 15 tons. Its form and construction are well shown in Figs. 2 and 3. It consists of the main or central section A, Fig. 2 the lower section B which carries the main mirror and the skeleton section C which carries the secondary mirrors. The central section is a cylindrical steel casting heavily ribbed on the inside about 6 feet high and weighs 7 tons. The lower section is securely bolted to it through the flanges shown and with the mirror and its supporting mechanism weighs about 6 tons. The upper skeleton section is built up of structural steel, 3 inch I beams, firmly braced and rivetted together in the manner shown in the figures. Aspecial feature of this skeleton tube, making it more rigid than any previous design, consists of the diagonal tension rods in each rectangular compartment screwed up each to a tension of about 2,000 pounds, so that the whole tube is under tension in every position. This stiffness is essential for the proper performance of the optical parts, as the principal and secondary mirrors at bottom and top of tube respectively should occupy the same relative positions in whatever direction the tube is pointed.
The telescope tube is firmly screwed at right angles to the flanged end of a massive shaft 16 inches in diameter, called the declination axis, extending through the cubical section D of the polar axis NDS, Fig. 2, through the declination sleeve E into the housing F. This declination axis is rotated, carrying the tube with it, on ball bearings in D and F, this rotation being effected by an electric motor with reduction mechanism, gearing into a large spur gear attached to the end of the declination axis, the whole being concealed within the declination housing F. Hence the tube can be turned at the rate of 45 degrees to the minute to any required position up or down, north or south. The position in the sky, the declination, corresponding to latitude on the earth, is read on a large circle graduated into degrees within F and subdivided into 5 minute intervals on the small auxiliary circle H.
As positions north or south are given by turning the tube on the declination axis, so positions east or west are given by rotation on the polar axis, so called because it points to the pole of the heavens and is exactly parallel to the axis of the earth. The Polar axis NDS Fig. 2, which is 21 feet long and weighs 9·5 tons, is built up of three steel castings, a central cubical section D and two conical end sections, all securely bolted together and turning in ball bearings on its ends. The upper, north, bearing is carried in an adjustable pillow block, by which the final parallelism with the earth’s axis is obtained,bolted on the curved cement pier shown at the left or north in Figs. 2 and 3. The lower, south, bearing is carried in a massive cast iron pedestal bolted to the south cement pier. The polar axis is rotated on these bearings, also at the rate of 45 degrees per minute, carrying the declination axis and tube with it to any position east or west in the sky by an electric motor and reduction gearing concealed within the south pedestal. The position east or west in the sky, the right ascension as it is called corresponding to longitude on the earth, is read by means of a graduated circle shown above G, Fig. 2, which is divided into 24 hours and each hour into single minutes. While longitudes on the earth are occasionally expressed as so many hours and minutes east or west of Greenwich, right ascensions in the sky are almost invariably given in hours and minutes rather than degrees.
Fig. 3.—TELESCOPE FROM THE WEST
It is evident, by rotation of the telescope on the declination and polar axes by means of the quick-motion motors, that the tube can be pointed in any direction in the sky, towards any star. But owing to the rotation of the earth on its axis from west to east, which is the cause of the apparent motion of sun, moon and stars from east to west, the telescope will be quickly carried eastward of the star which will only remain for an instant in the field.
The mechanism by which the rotation of the earth is compensated for is called the driving clock and is contained in the case L, Fig. 2, at the north side of the south pier. In the lower half of the case a governor similar to the governor of a steam engine is driven once per second by a train of gears in the upper section actuated by a weight of 300 pounds below the floor. If the speed of the governor tends to increase the balls raise by centrifugal force and bring increased friction to bear thus reducing the speed to normal while if the speed tends to decrease, the balls drop and reduced friction quickly allows it to accelerate to normal speed. A shaft with a coarse screw thread on it, called technically a “worm” and situated at the top of the case, is driven by intermediate gearing fromthe governor at the rate of one revolution every two minutes. The thread on this shaft engages into teeth cut in the worm wheel G, Fig. 2, which is 9 feet in diameter. As there are 720 teeth very accurately spaced in this worm wheel, it is driven around by the worm in 2 × 720 = 1,440 minutes, 24 hours, the same rate as the earth. This worm wheel, normally loose on the polar axis on which it turns on ball bearings, allowing the axis to be moved freely to any position, can be rigidly clamped to it by pressing a button. When this is done, it will evidently turn the polar axis and hence the tube at the same rate as the earth but in the opposite direction, on an axis parallel to the axis of the earth, thus exactly compensating for the rotation of the earth. Hence any star at which the telescope is pointed will automatically remain central in the field. Owing to the great magnification all this mechanism requires the highest grade of workmanship, else there will be wandering of the image, a most annoying and troublesome defect. Few telescopes are entirely free from periodic error and that the 72-inch drives so regularly and smoothly is a great advantage and evidence of the perfection of workmanship throughout.
It has already been described how the telescope can be moved by motors north or south and east or west at the rate of 45 degrees per minute. These motors are operated from small switchboards on each side of the south pier, the one at the west being seen in Figs. 2 and 3. The left-hand reversing switch moves the telescope east or west, the centre switch north or south and the right-hand switch revolves the dome east or west. In addition to these quick motions of the telescope for rapidly bringing it to the approximate position, much finer and slower motions are required for bringing the image exactly central and for guiding. These slow motions are also operated by electric motors actuated by two small aluminium switchboards attached by flexible cables to the top and bottom of the tube. These switchboards can be carried in the hands of the observer or rested on the observingladder. Pressure on suitable buttons moves the telescope north or south, east or west at either one of two different speeds, a speed of one revolution in 36 hours for centering the image and a speed of one revolution in 30 days for guiding, correcting for slight irregularities due to air disturbance or other causes. Although these speeds may seem excessively slow, the motion of the image even with the monthly rate is at once evident on pressing the button and faster speeds would make accurate guiding difficult. In addition to the two quick and two slow motion motors there are two clamping motors and one for automatically rewinding the clock weight, seven in all. These with the three motors operating the dome are all continuous current motors which can be started and reversed more readily and have greater initial torque than alternating motors. Each motor is supplied with an automatic control, so that all that is necessary is to throw the switch or press the button to start or reverse. Current is supplied by a motor generator set on the ground floor.
A description of the method of setting upon the required star, when, for example, photographing the spectra of the stars, will help to make the operation of the telescope more clearly understood. It is easily possible to pull the telescope around by hand to the required star identified by eye among the constellations. Although the moving parts of the telescope weigh nearly 45 tons, so perfect are the ball bearings in which it turns that a weight of 3 pounds at the upper end of the tube is sufficient to set it in motion. However the settings can be much more quickly and certainly made by turning the telescope to the right ascension and declination of the star by the electrical motions. A programme of the stars to be observed with their right ascensions and declinations is prepared beforehand. The observing assistant stands beside the small switchboard on the south pier and rapidly moves the telescope east or west and north or south until the indexes on the graduated circles point to the tabulated positions, while the dome can be turned to the required position at the same timeby means of the third operating switch. By pressing two buttons the telescope is then firmly clamped and the driving clock starts the telescope automatically following the star. In the meantime the observer has inserted the plate holder in the spectrograph and drawn the slide and by means of the aluminium switchboard brings the star, which is generally near the centre, exactly to the centre of the finder, when it will be visible on the slit of the spectrograph through a guiding eyepiece and can quickly be brought central and the exposure commenced. The time required from the end of one exposure to the beginning of the next, unless the stars are far apart in the sky, does not generally exceed two minutes, a shorter time than usually required for even quite small telescopes. This rapid operation is due to special care in design and construction and markedly increases the efficiency and capacity of the instrument.
The mounting of the 72-inch telescope has several new features not hitherto used and sets a new standard for convenience and accuracy of operation. The observatory is much indebted to the Warner & Swasey Co., who have made most of the large mountings in America, for the spirit in which they undertook and carried through this work. Their sole object was to produce the best possible mounting regardless of cost and no suggestion of the writer looking to improvement was refused. To Mr. Swasey, the president, are due many of the original features of the mounting and the beauty and harmony of the design, while Mr. Burrell, the works manager, is responsible for the simplification of the mechanism and the beautiful co-ordination of the details. No greater testimony to the perfection of design and construction can be given than to say that after five years use there is no feature the director would wish changed, and no single defect of construction has been revealed.
It may be of interest to note the principal improvements in this mounting.
1. All parts of the sky can be readily reached. This is not possible with all types of reflecting telescopes.
2. The elimination of cylindrical bearings with cumbrous friction-relieving devices, formerly considered necessary for maintaining collimation and adjustment on declination and polar axes, and the use of ball bearings for both friction-relieving and collimating purposes has resulted in remarkable ease of movement of the telescope.
3. Freedom from periodic or other errors in driving and smoothness and freedom from “backlash” in slow motions.
4. Ease, speed and accuracy with which settings can be made due to careful design and original features in setting motors and setting circles.
5. Great stiffness of tube and improvements in method of attaching and changing secondary mirrors.
6. Beauty and harmony of design and appearance.