SECTION 2.—THE BUILDING AND DOME
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 internalcovering 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.
Fig. 1.—OBSERVATORY BUILDING FROM THE NORTH
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.
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.
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.
Fig. 2.—TELESCOPE IN AVERAGE OBSERVING POSITION