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.
[Illustration: Fig. 1.--OBSERVATORY BUILDING FROM THE NORTH]
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.
[Illustration: Fig. 2.--TELESCOPE IN AVERAGE OBSERVING POSITION]