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.