Part 1

DEPARTMENT OF THE INTERIOR

Hon. Charles Stewart Minister W. W. Cory, C.M.G. Deputy Minister

The Dominion Astrophysical Observatory

Victoria, B.C.

A sketch of the development of astronomy in Canada and of the founding of this observatory. A description of the building and of the mechanical and optical details of the telescope. An account of the principal work of the institution.

By J. S. Plaskett, F.R.S.

Ottawa F. A. ACLAND, Printer to the King’s Most Excellent Majesty 1923

STAFF OF THE OBSERVATORY:

Director J. S. Plaskett, B.A., D.Sc., F.R.S.C., F.R.S. Research Astronomer W. E. Harper, M.A. Astronomer Reynold K. Young, Ph.D., F.R.S.C. Astronomer H. H. Plaskett, B.A. Secretary Miss H. R. Keay Observing Assistant T. T. Hutchison, Esq.

THE DOMINION ASTROPHYSICAL OBSERVATORY

VICTORIA, B.C.

By J. S. PLASKETT, Director.

SECTION 1.--HISTORY AND CONSTRUCTION

Introduction

This description of the observatory, its equipment and work has been written by the director in response to a need, frequently expressed by the numerous visitors to the institution, of a non-technical account of the principles of the telescope and of the work of the institution. This account will commence by a short historical sketch of the beginnings of the undertaking followed by a description of the observatory and telescope and concluded by a resume of its work.

Historical Sketch

This observatory is a branch of the Department of the Interior of the Federal Government, the department which has charge of the administration of the western lands of the Dominion. In the colonization of these lands, one of the obvious first needs was a survey of the boundaries and subdivision into townships and sections. This need led to the organization of a surveys branch of the department and out of the necessity of accurate astronomical observations to delimit the boundaries and define the position of the base lines for subdivision work arose the astronomical branch. The first Chief Astronomer of Canada, the late Dr. W. F. King, was a man of sterling integrity and remarkable ability and to his genius must be ascribed in large degree the present development of astronomy in Canada. As Chief Astronomer and H. M. Commissioner of International Boundaries, he early realized the need of an observatory for an initial meridian, for housing and standardizing the instruments, and for some astronomical research. He was successful in having the fine Dominion Observatory erected on the Experimental Farm, Ottawa, in 1905 which housed the Astronomical Branch, the staff of the International Boundary Surveys and later the Geodetic Survey of Canada, of which he was the first superintendent.

The Dominion Observatory was equipped with a 15-inch refractor provided with micrometer, photometer, solar and stellar cameras, and spectrograph. The writer was entrusted with the work with this telescope and the spectroscopic work especially was energetically developed and helped, with the other activities of the institution, to bring gratifying recognition from the scientific world. The need of a larger aperture for extending this part of the work was soon realized by the writer and was brought to the attention of the Government. After some delays, owing to various circumstances, it was finally decided early in 1913 to provide a large reflector for extending the work. Enquiries were at once made, specifications prepared and estimates obtained from prospective makers of the instrument. Contracts were finally awarded in October 1913 to the John A. Brashear Co. of Pittsburgh for the optical parts and to the Warner & Swasey Co. of Cleveland for the mechanical parts of a 72-inch reflecting telescope.

Location

It was necessary to know the location of the instrument before the design of the mounting could be completed as the angle of the polar axis depends upon the latitude of the site. When the new telescope was first proposed there was no thought of locating elsewhere than at the seat of government at Ottawa. However, upon further consideration it was decided that the telescope should be located where, in Canada, the best observing conditions prevailed. To determine this location preliminary selection of five likely stations was made by the aid of Sir Frederic Stupart, chief of the Meteorological Service from the meteorological records. These stations, at Ottawa, at Medicine Hat, at Banff, at Penticton, and at Victoria were occupied by Mr. W. E. Harper, astronomer at Ottawa and the astronomical conditions were observed by means of a 4.5-inch telescope. Victoria was unmistakably superior in “seeing” or defining power, in low diurnal and seasonal range of temperature and about equal so far as quantity of clear sky is concerned. For such a large telescope as a 72-inch there could be no question of the marked astronomical advantages of such a location, and it was therefore decided to locate the telescope at Victoria. While the observatory should not be in the city itself it should not be too far away, not only on account of accessibility and facility in obtaining supplies, but also the advantageous conditions of good seeing, low diurnal range of temperature and small rainfall were confined to a relatively small area near Victoria. An isolated monadnock called locally Little Saanich Mountain but now named Observatory Hill, was selected. This hill is about 7 miles north of Victoria and has a main road and an interurban railway passing its base. It has an elevation of 730 feet, sufficient area around the summit for all necessary buildings and was by far the most suitable site available.

Construction

The Provincial Government had generously agreed to give $10,000 for the purchase of a site and to build a road to the summit. This undertaking was fully met and the road, splendidly located and constructed and costing over $25,000, was completed in the spring of 1915. Contracts for the construction of the telescope pier and the circular steel walls of the building were awarded to a local firm and this work was completed in 1916. The revolving dome with accessories for the operation of the telescope was made by the Warner & Swasey Co., the builders of the telescope mounting, and was completed and erected in 1916, thus making the building ready for the telescope.

The design of the mounting was very carefully gone into by the Warner & Swasey Co. in collaboration with the writer and was completed in the autumn of 1914. Construction was at once begun and the mounting was completed and temporarily erected at Cleveland in May 1916. It was then shipped to Victoria and permanently erected in its building by November 1916.

The order for the large disc for the 72-inch mirror and for an auxiliary flat of 55 inches diameter for testing the 72-inch was given to the St. Gobain Glass Works of Paris by the Jno. A. Brashear Co. as soon as the contract was awarded. The 72-inch disc was cast and annealed by June 1914 and was fortunately shipped at once without waiting for the 55-inch disc. It left Antwerp only about a week before war was declared and it was only by this small margin that Canada now has a 72-inch telescope. Grinding and polishing were at once begun but the lack of the large flat and other difficulties delayed the completion and it was not until April 1918, about a year and a half after the completion of the mounting, that the figuring was finally completed and the telescope ready for work. Nevertheless, for an undertaking of such magnitude the work was completed in record time, four and a half years after the awarding of the contracts.

SECTION 2.--THE BUILDING AND DOME

The Observatory Building

The building for housing a large reflecting telescope requires to be of special design for the best results. It should not rise above the shade temperature during the day and should rapidly assume and follow the external temperature at night. Such materials as brick or stone are obviously not suitable and all recent telescope buildings are entirely of metallic construction in order to assume quickly the night temperature, and of double-walled, ventilated type to prevent overheating from the sun’s rays. The building for the 72-inch telescope is entirely of steel construction, circular in form, 66 feet in external diameter and with vertical walls 32 feet high. A view from the south is given in the Frontispiece and from the north in Fig. 1, showing the city of Victoria and the straits of Juan de Fuca in the background. An external and internal covering of galvanized iron separated by about 16 inches allows free circulation of air from a peripheral opening at the base up through a similar double walled dome and out of louvres at the top. The ground floor of Terrazo is laid directly on the rock base and the observing floor 22 feet above this is formed of steel girders and checkered steel plate. In the centre of the ground floor rises the massive pier to support the telescope, of reinforced concrete and symmetrical tapering form. The pier is actually double, united below the observing floor by a massive reinforced arch and extending above the floor as two piers (see Fig. 2) one for each end of the polar axis. Temporary partitions on the ground floor provide a dark room, sleeping room, and temporary office quarters.

The Dome

This circular steel building is capped by a circular plate supported by massive girders on which is placed a curved railroad iron rail turned and adjusted perfectly level and circular. The hemispherical dome turns on this rail on 24 massive wheels mounted on roller bearings and is rotated by an electric motor mechanism operated by reversing switches on each side of the south pier. The framework of this dome, which is 66 feet in external diameter and about 38 feet high, the lower 5 feet being cylindrical, consists essentially of a circular base to which the bearing wheels are attached and two double, very deep and rigid main ribs, one of which can be seen in Fig. 3, 16 feet apart in the clear and extending parallel to each other entirely across the dome. These are united 6 feet beyond the zenith by a cross girder and this opening, which can be revolved to any azimuth and can be closed by double motor operated shutters, enables the telescope to observe at any part of the sky. Auxiliary circular ribs reaching from the base ring up to the main ribs all around the dome except at the shutter opening, form the support for the double steel covering of 12 inches separation. This ventilating space is united through suitable weather guards with the space between the building walls and forms the protection against overheating by the sun in the day time.

Dome Accessories

The shutter opening 16 feet wide and extending from the base of the dome to 6 feet beyond the zenith is closed by a double shutter also double walled and ventilated. Canvas screens mounted on tubes and operated by motor can be run up from the base of the shutter opening, this one being shown in Fig. 3, and down from the top so as to limit the opening to the width of the tube of the telescope. These screens are necessary when the wind is blowing to prevent shaking of the tube and consequent jumping of the star image. For enabling the observer to reach conveniently the upper end of the telescope tube in any position a counterweighted elevating platform, moved up and down the shutter opening by motor and drum with lifting cables is provided. This platform is 20 feet long and 4 feet wide with a wing at each end, extending inwards about 6 feet, on which the observer can stand and can move it and himself by a hand wheel up to and partly encircling the telescope tube. The platform and movable wings, which are well shown in Fig. 3, can be brought to any desired height by an operating switch at one side and can be reached by an auxiliary stairway from a small stationary platform attached to the base ring of the dome. This stationary platform is reached from the observing floor by a permanent stairway moving with the dome and extending to a foot from the floor. Platform wings and stairways are made safe for observers by guard and hand rails 30 inches high. This moving platform, an essential accessory for direct photography and other work at the principal focus of the telescope, meets, in a more convenient and satisfactory manner than any other previous device for the purpose, all the observing requirements for work at the upper end of the tube.

Mechanism for Silvering

In order to renew the silver coating on the upper surface of the mirror, which is necessary about three times a year, mechanism has to be provided for handling the mirror and its cell, the lower section of the telescope tube, with ease and safety. The mirror weighs over 2 tons and the cell 4 tons, so 6 tons have to be removed from and replaced on the telescope tube. This is effected by means of the silvering car, which can be seen at the left of Fig. 2, a massive framework of structural steel rolling on four flanged wheels on flush tracks in the observing floor. With the tube turned to a vertical position, the car is rolled from its normal position at the east side of the dome directly under the tube and a motor-operated screw-jack surmounted by a triangular rocking arm can be brought up against the bottom of the cell. On removing the attaching bolts, cell and mirror can be lowered and rolled on the car to the east out of the way. The removal of 6 tons from one side throws the telescope out of balance and so the outboard end of the declination axis is supported by a counterweighted strut run up through the floor and the upper end of the tube tied to rings at the top of the dome. With a band of paraffined paper tied around the edge of the mirror and a plug in the central hole, the silvering solution can be poured on and evenly flowed over the surface by rocking on a steel ball at the top of the jack-screw. When the silvering is complete, cell and mirror are replaced in the reverse order and the car rolled back out of the way. The operation takes about a day and is performed with perfect safety and ease.

SECTION 3.--MECHANICAL PARTS OF TELESCOPE

Introduction

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 the upper 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 Telescope Tube

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. A special 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 Declination Axis

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.

The Polar Axis

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.

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