The Dominion Astrophysical Observatory, Victoria, B.C.

by J. S. Plaskett

Fig. 3.—TELESCOPE FROM THE WEST

The Driving Clock

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 from the 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.

Electric Motions

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 observing ladder. 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.

Method of Operation

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 time by 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.