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[Illustration: _Frontispiece_--BUILDING AND DOME FROM SOUTH]




  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.

[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]




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.

[Illustration: 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.


Special Features of the Mounting

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.




SECTION 4.--OPTICAL PARTS AND SPECTROGRAPHS


The Principal Mirror

The great mirror is composed of hard plate glass cast in one piece and
after annealing, ground and polished to the correct shape. As received
from the St. Gobain Co., the disc was 73·5 inches diameter, over 13
inches thick with a central hole about 6 inches diameter and weighed
nearly 5,000 pounds. It was first of all ground truly circular to a
diameter of 73 inches and flat on both sides to a thickness of slightly
over 12 inches, while the central hole was enlarged to 10 inches. When
the back was polished approximately flat, the disc was seen to be a
beautiful specimen of the glass makers art, homogeneous and almost
entirely free from bubbles or other defects.

[Illustration: Fig. 4.--PHOTOGRAPH OF RING NEBULA IN LYRA

(_Enlarged 8 diameters_)]

It was now ready for the second stage of the operation the grinding
of the correct shape for the upper reflecting surface. In order to
bring the light of a star to an accurate focus this surface must
be a paraboloid of revolution, the same kind of curve given to the
reflectors of search lights or automobile headlights. The curve for
this reflector of 30 feet focus is very nearly a section of a sphere of
60 feet radius, within one-thousandth of an inch, and consequently
would nearly fit a huge globe 120 feet in diameter. The upper surface
of the disc was fine ground and polished to this spherical surface and
was then ready for the final stage, the “figuring” a continuation of
the polishing process until the centre is deepened about a thousandth
of an inch and the surface becomes accurately paraboloidal. This
“figuring,” an exceedingly delicate and difficult process especially
over such a large surface as the 72-inch, with the added difficulty of
a central hole, occupied about two years and was not completed until
nearly a year and a half after the mounting was ready. When it is
remembered, however, that the surface nowhere deviates from the true
theoretical form more than one four-hundred-thousandth of an inch and
that if one part is accidentally polished too deep, the whole surface
has again to be brought down to this level, the exceeding delicacy of
the operation is evident and the time taken not excessive.

Accurate quantitative tests showed that the final figure is of the
highest order of accuracy and this is further clearly shown by the
practical test of direct photographs at the principal focus. Figure
4, a six fold enlargement of a photograph at the principal focus, of
the Ring Nebula in Lyra shows how sharp and small are the star images.
Actual measurement on the original negative gives a minimum diameter
of two one-thousandths of an inch equivalent to only a second of arc
at the focus. As the images are enlarged considerably by unsteadiness
of the air and errors in guiding the star light reflected from the
whole surface of the mirror is collected into a little disc less than a
thousandth of an inch in diameter indicating the extraordinary accuracy
of the reflecting surface. Mr. J. B. McDowell, head of the firm since
Dr. Brashear’s death, and Mr. Fred Hegemann, his chief optician, are
to be highly congratulated on the perfection of figure obtained under
specially difficult circumstances. Further the fine rendering of the
detail in the ring and the strength of the two bands in the interior
indicate not only perfect figure but exceptionally high polish.


Mounting of Mirror

This mirror, to maintain its accuracy, not only requires careful
mounting in its cell but also protection against temperature changes.
Even though 12 inches thick it would bend under its own weight of 4,300
lbs. sufficiently to affect the figure and consequently it is supported
in the cell by a specially counterweighted lever system so that it is
equably supported at twelve points and there is no tendency to bend.
A similar lever support system around the edge prevents distortion
due to constraint when it is tipped from the horizontal position at
different positions of the tube. Temperature changes can produce much
greater distortion than flexure but Victoria has the advantage of very
low diurnal range and the temperature change around the mirror is made
very small by a lagging of cotton felt about 2 inches thick all round
the sides of the closed section of the tube, laced on with a duck
cover (compare Fig. 3 with Fig. 2) and an equal thickness below and
around the edge of the mirror. By this lagging the temperature rise in
the day-time is only about half a degree while the dome temperature
increases five degrees, hence the figure of the mirror remains good
whatever the temperature changes outside.


The Principal Focus

As already indicated, the principal mirror when used alone forms an
image of the star 30 feet above, at the centre of the upper end of
the tube, and an eyepiece could be placed there for visual work or a
photographic plate for direct photographs of nebulae, etc. However, it
is generally more convenient to use a flat mirror at 45° forming the
image at the side of the tube for photographs and so the telescope is
only used in this form with a small spectrograph for the ultra-violet
region of star spectra. The course of the parallel beam of light from
the star to its image on the slit of the spectrograph is graphically
shown in Fig. 5 A and also its passage through the slit prisms and
lenses of the spectrograph. The position of the star image on the slit
of the spectrograph can be observed by a guiding telescope extending
to the edge of the tube and can be kept central by the portable
aluminium switchboard already described. The elevating platform is of
course used in work in this position.


The Newtonian Arrangement

For direct photography or visual observations at the focus of the
72-inch mirror, the reflected cone of star light from the mirror B,
Fig. 5, is intercepted by a plane mirror also silvered on the front
surface, 19·5 inches diameter and 3·25 inches thick placed at 45°. This
form of reflecting telescope was first used by Newton, hence the name.
The focus is then formed, as shown, at the side of the tube, and if a
plate is placed there and accurately guided by small eyepieces with
cross wires, photographs of any desired small region in the sky can be
obtained, Fig. 4 being made in this position, or visual observations
may be made. The oculars can easily be reached from the observing
platform for any position of the telescope.


The Cassegrain Arrangement

The most generally useful arrangement of the 72-inch telescope is,
however, the Cassegrain form, so called from the French astronomer who
first used it. About 7 feet below the focus, the conical reflected
pencil from the 72-inch mirror is intercepted by a convex mirror of
the same size as the Newtonian and of about 10 feet focal length as
seen in C, Fig. 5, and also shown in Fig. 2 and 3. This mirror turns
the light downward and, after passing through the central hole, forms
the image of the star about two feet below the mirror surface on the
slit of the spectrograph or on a visual attachment as shown. The
significant property of this combination is that the focal length is
increased from 30 to 108 feet without changing the tube length in a
somewhat similar manner to the action of a telephoto lens. It has the
same size of image and magnifying power as a refractor with a tube 108
feet long and has the decided advantage of a much shorter tube and
smaller dome. Observations with this arrangement are made at the lower
end of the tube from the observing floor and with much greater ease
and convenience than at the upper end. Changes from the Cassegrain to
the Newtonian or Principal Focus arrangements are readily effected by
a device due to the genius of Mr. Swasey whereby only the mirrors and
attaching tubes require to be handled, instead of the whole upper end
of the tube as in previous reflectors.

[Illustration: Fig. 5.--COURSE OF LIGHT IN TELESCOPE FROM STAR TO FOCUS]


Accessory Optical Parts

A full set of eyepieces giving magnifying powers from 120 to 5,000
diameters and a complete double-slide plate holder with guiding
eyepieces for direct photography at the Newtonian focus are provided.
In addition there is a visual attachment shown in diagram C, Fig. 5,
which enables the telescope to be used visually at the Cassegrain focus
without removing the spectrograph. There are three finders attached
to the telescope tube for picking up and centering the stars. Two of
4 inch aperture and 5 feet focus, of power about 50, one north, one
south on the tube, and a long focus tubeless finder with a lens of 7
inch aperture and 30 feet focus at the top of the telescope tube and an
ocular of power 200 at the bottom.


The Spectrographs

Most of the astronomical work with the 72-inch telescope is
spectroscopic, photographing the spectra of the stars, and so a
description of the principles and operation of spectrographs is
desirable. Stellar spectrographs have evolved into certain definite
forms and the two spectrographs for the 72-inch telescope are examples
of the most recent types. In essence a spectrograph consists of a
narrow slit, one or two-thousandths of an inch wide, on which the
star light is focussed. That passing through the slit falls on the
collimator lens which makes it parallel and then on a prism or prisms,
triangular shaped pieces of glass which change the direction of the
light and decompose it, breaking it up into its constituent rainbow
colours. The spectrum, as it is called, is focussed by a camera lens on
a photographic plate or can be viewed by a small telescope if desired.
The course of the star light from the slit through collimator, prism
and camera lens to the plate is shown in C, Fig. 5, while a view of the
Cassegrain spectrograph showing the interior mechanism and accessories
is given in Fig. 6. The part of the spectrum photographed is usually
only the blue and violet region to which the ordinary plate is most
sensitive, and obviously no colours appear on the negative but only a
narrow dark strip which is crossed by light or dark lines. It is from
the number and position of these lines that we obtain such a remarkable
amount of information about the physical and chemical constitution, the
temperature, motion and distance of the stars. The length of the star
spectrum photographed with one prism is about one and a third inches,
twice and three times that with two and three prisms. Its width is
about one-hundredth of an inch and in order to make it this wide the
star image has to be moved back and forward along the slit. The length
of spectrum with the ultra-violet spectrograph, which only differs from
the other in the prisms and lenses allowing the spectrum below the
violet to pass, is about one inch. A photograph of the spectrum of iron
or brass is made beside the star spectrum to serve as a standard to
determine the positions of the star lines.

[Illustration: Fig. 6.--STELLAR SPECTROGRAPH ARRANGED FOR USE WITH ONE
PRISM

(TEMPERATURE CASE REMOVED)]


Method of Use

Every terrestrial element gives groups of lines in certain positions
in the spectrum and if we find similar groups in the star spectrum
we are sure this element is present in the star. Further if we find
these lines are displaced to red or violet of their normal position
we know that the star is receding from or approaching to us. With
the one-prism spectrograph a speed of one mile per second means a
displacement of the lines of one thirty-thousandth of an inch. Thus
if the lines are shifted to the violet by a thousandth of an inch,
the star is approaching the earth with a speed of 30 miles a second.
These displacements are accurately measured by a microscope and the
measurement of the radial velocities of the stars is one of the main
researches of this observatory. Obviously with such small displacements
to be measured the greatest care must be taken to avoid all sources
of error. The spectrograph must be exceptionally rigid to avoid
differential bending as it moves with the telescope. As change of
temperature can produce spurious shifts of the lines, the temperature
must be kept as constant as possible, this being effected here by
a very accurate electrical thermostatic device called the Calendar
Recorder which maintains the temperature constant to one-hundredth of
a degree. The optical parts must be of the highest quality to give
perfect definition to the spectrum lines and many other precautions
must be taken if accurate work is desired. The spectrographs of the
72-inch telescope have unequalled defining power and are the last word
in convenience of manipulation and accuracy of work.

Owing to the faintness of the star light and to its being spread out
into a spectrum a considerable time is required to photograph the
spectrum of a star, about 20 minutes for the sixth magnitude, the limit
of visibility to the unaided eye, when photographed with one prism,
while three prisms will take nearly five times as long. Hence the
necessity and use of large telescopes is not to get high magnifying
power but to collect sufficient light from the fainter stars to enable
their spectra to be photographed or other observations made.




SECTION 5.--THE WORK OF THE TELESCOPE


Prevalent Misconceptions

The general idea of an astronomer’s work, as gathered from the
questions and remarks of visitors, is that he sits at the eyepiece of
the telescope sweeping the heavens in a search for new planets, comets
or stars. The absolute futility of such a use of the telescope is
evident when it is realized that the main field of the 72-inch covers
only about one hundred-millionth of the sky and if only five seconds
was required to examine each field it would take more than a lifetime
to go over the whole sky once. A second misconception is the idea that
large telescopes are used for visual observations of the planets with
special reference to their habitability. No work is being attempted
at this or other large observatories on planetary detail for which
about an 18-inch refractor gives the best results and such a large
telescope as the 72-inch is quite unsuited. All scientific observations
with the 72-inch are made photographically and it is only arranged for
visual use on Saturday evenings when for two hours visitors are allowed
to observe the heavenly bodies. A third misconception is that the
astronomer only works at night. However true this idea may have been
in the days of visual observations when the measurements were made at
the eyepiece, there is certainly now, when photography is so generally
applied, more day than night work in astronomy. Besides the advantages
of permanency, accuracy of measurement and power of recording objects
beyond the range of the keenest eyesight, the photographic method has
the further great advantage that an hour’s exposure may give sufficient
material for several days’ measurement and discussion.


Spectroscopic Work

As already indicated most of the work with the 72-inch telescope
is spectroscopic, but as also indicated modern spectroscopic
investigations cover so wide a range of research that the actual work
of the observatory is very varied. By aid of suitable spectrographs
attached to a large telescope we can measure the speed of the stars
towards or from us, their radial velocity as it is termed. We can
discover double stars too close ever to be seen double in any telescope
and we can determine the manner in which they revolve around one
another and their distance apart and mass. From the spectra of the
stars we can determine their absolute brightness as compared with the
sun and their parallax or distance. The chemical elements present in
the outer atmospheres of the stars can be determined and the pressure
in these atmospheres. The measurement of temperatures and other
physical conditions in the stars by means of the spectroscope is now
an accomplished fact and one of the most recent developments of the
spectroscopic work here has been to provide evidence of the truth of
a theory of atomic structure and to show that the atomic constants in
the enormous furnaces of the stars are the same as on the earth. Such
a catalogue of information, obtained from an investigation of the mere
quality of the light from stars so faint as to be quite invisible to
the unaided eye and so distant that it may take thousands of years
to travel to us, is sufficiently comprehensive to be treated in more
detail.


Radial Velocities

When the 72-inch telescope was in course of design and construction,
one of the greatest needs in astronomical work was increased data in
regard to the radial velocities of the stars. Although the telescope
was so designed as to be suitable for all kinds of observational
work, special attention was devoted to the spectroscopic end. After
consultation with the most prominent astronomers an observing programme
of about 800 stars whose “proper” or cross motions across the sky were
accurately known but whose radial velocities had not been determined,
was prepared and spectroscopic observations of the stars on this
programme were commenced as soon as the telescope was completed in
May 1918. After slightly over three years’ work, observation and
measurement were completed and Vol. II, No. 1 of the observatory
publications, “The Radial Velocities of 594 Stars,” was published early
in 1922. As hitherto the radial velocities of only about 2,000 stars
had been obtained, this work was a considerable addition to existing
data about the motions of the stars and will be of great use in
extending our knowledge of the structure and motions of the universe.
A second programme of 1,500 stars has been prepared but owing to other
intervening observational work, not much has yet been done on this new
programme.

One of the auxiliary programmes undertaken and nearly completed since
the first programme is the determination of the radial velocities of a
very interesting but limited class of stars, the highest temperature
stars known, the O-type stars. The radial velocities and other
interesting data about 50 of these stars have been completed.


Spectroscopic Double Stars

In the measurement of the radial velocity of the 800 stars on the
first programme it was found that in about 180 stars successive plates
did not give constant velocities, the stars at one time approaching
at another receding from us. This phenomenon is practically certain
proof that we are measuring the velocity of a star revolving around an
invisible companion and such stars are generally called spectroscopic
binaries to distinguish them from visual binaries which can be seen
double in the telescope. Over 200 spectroscopic binaries have been
discovered at this observatory as compared with about 700 discovered
elsewhere, again a considerable addition. In about 20 of these
binaries, observations were continued until the period of revolution,
the form of the orbit, the separation of the two stars and in some
cases, their masses were determined. In a particular class of
spectroscopic binaries, the eclipsing variables, which allow from the
combination of spectroscopic and photometric measurements the absolute
dimensions to be obtained, we were able to determine the separation of
the two stars, their diameters, densities, masses and brightnesses and
the probable distances. Such complete information has been obtained
here about seven systems, while only seven other systems have been
determined elsewhere.


Absolute Magnitude and Distance

A new application of spectroscopic methods is to the determination of
the total brightness of the stars as compared with the sun and their
distance or “spectroscopic parallax” as it is called. This depends not
on the positions but on the relative intensity or strength of the lines
in the spectra of the stars and has only been developed in the last
three or four years. The absolute magnitude and spectroscopic parallax
of about 800 stars is now nearly completed and will soon be published
while as a side line the radial velocities of 125 more stars have been
determined.


Physical Conditions in the Stars

The spectrographs have also been used by one member of the staff in
the determination of the physical and chemical conditions in stellar
atmospheres, a new and difficult problem but one which promises not
only very valuable additions to our knowledge of the constitution of
the stars but may also lead to economic applications of the greatest
importance. A new method depending upon the use of a wedge of dark
glass has been applied to determining the distribution of energy in the
different parts of the spectrum of the stars and to measuring their
temperatures, while an application of the same methods to individual
lines may lead to a great increase in our knowledge of conditions in
the stars. A special investigation of three of the high temperature
O-type stars referred to above has proved the existence in the spectra
of these stars of lines predicted as present from purely theoretical
conditions but never previously identified and has thus remarkably
verified a theory of the structure of the atom. The measurement of the
wave lengths of these hitherto unknown lines has led to an important
independent determination of the fundamental constants of atomic
structure and the dimensions of the atom and has shown that these
constants and dimensions are the same in the tremendous furnaces of
the stars as in our terrestrial laboratories, a verification of the
homogeneity of matter and the uniformity of physical laws throughout
the universe. Further interesting results from this investigation
are the application of a new theory of ionization with the probable
relative abundance of the elements to an independent determination of
the temperature of these stars.


Other Investigations

Direct photographs have been made of some nebulae and clusters but
this work is not being definitely followed at present. Investigations
into the phenomena accompanying some short period binaries have been
made and into the behaviour of the two strong calcium lines H and K
in the spectra of the high temperature stars from which interesting
and valuable results are expected. Since the observatory commenced
work two bright novae or new stars have appeared, which have been
fully observed spectroscopically here and the results discussed. The
plates of the nova in Aquila have been loaned to the observatory at the
University of Cambridge, England, for fuller discussion and analysis.


Value of Astronomical Work

This brief sketch of the work of the observatory naturally leads to
the question frequently asked of astronomers:--What use is the work
done at observatories and what practical value can a knowledge of the
stars have in everyday life? While astronomy has obvious practical
applications to navigation and surveying yet nine-tenths of modern
astronomical research is devoted to the more or less abstract question
of the constitution and motion of the stars and the structure of the
universe. Indeed most physical and chemical as well as astronomical
research is undertaken for the purpose of increasing our knowledge
and of investigating the secrets and laws of nature and has generally
no direct practical economic application. But it is now generally
recognized by the layman as well as the scientist that without abstract
there can be no applied science and that all the great economic and
industrial applications of science have had to be preceded by the
abstract and apparently non-practical investigations of pure science.
The Great War perhaps made more evident than ever before the absolute
dependence of applied science upon the unselfish and abstract work of
pure science.


Economic Value

In view of past experience in science it would hence be a rash
prediction to assert that the investigation of the conditions in
distant stars can have no practical application upon earth. It may be
of interest to point out one possible application of astrophysical
research.

It is generally agreed that one of the most important economic
problems of the not far distant future will be the provision of
sources of energy to replace our rapidly depleting supplies of coal
and oil. It appears now that the most probable solution of this
problem will consist in the development of some method for utilizing
the inexhaustible stores of energy contained in the atoms of matter.
Modern research on conditions in the stars has made it practically
certain that the enormous supply of energy, which has been radiated
into space for aeons of time from these bodies, can only be maintained
undiminished by the energy released by the transformation of atoms
in the interior of the stars, where conditions of temperature and
pressure prevail at present unattainable in terrestrial laboratories.
The most hopeful line of attack upon this tremendously important
economic problem hence seems to lie in the systematic astrophysical
investigation of conditions in the stars supplemented by physical and
chemical researches on the structure of the atom.


Ethical Value

While astronomers and scientific men generally fully realize the value
of the practical applications of science, their main purpose is the
search for truth and the extension of our knowledge of nature. While it
is possible that investigation of the stars may have immense economic
value, it is certain that it has tremendous ethical value giving us a
clearer knowledge of the laws of nature and of our relations to the
wonders of creation. Astronomy is the oldest and in many respects the
most important of the sciences and its study, through the ages has
been one of the most elevating influences on human character. Poincaré
has well said that if the earth had been so continuously covered with
clouds that the heavenly bodies could not be seen, mankind would still
be in a primitive state and under the domain of superstition. The main
superiority of modern over ancient civilization does not consist in the
greater abundance of the necessities and luxuries of life, although
this is undoubtedly due primarily to scientific research, but to the
elevating influences of the truer conceptions of nature made possible
by the abstract study of astronomy and other sciences.

It has been truly said that the degree of civilization of a country may
be judged by the support it gives to the study of astronomy. By the
establishment and maintenance of the Dominion Observatory at Ottawa
and of the Dominion Astrophysical Observatory at Victoria with the
second largest telescope in the world, Canada has a just claim on this
criterion to the favourable estimation of the scientific world.

  VICTORIA, B.C.,
  May, 1923.




TRANSCRIBER’S NOTES:


  Italicized text is surrounded by underscores: _italics_.

  Obvious typographical errors have been corrected.