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.