MOUNTINGS
A steady and convenient mounting is just as necessary to the successful
use of the telescope as is a good objective. No satisfactory
observations for any purpose can be made with a telescope unsteadily
mounted and not provided with adjustments enabling it to be moved
smoothly and easily in following a celestial object.
Broadly, telescope mounts may be divided into two general classes,
alt-azimuth and equatorial. The former class is, as its name suggests,
arranged to be turned in azimuth about a vertical axis, and in altitude
about a horizontal axis. Such a mounting may be made of extreme
simplicity, but obviously it requires two motions in order to follow up
any object in the field, for the apparent motion of the heavenly bodies
is in circles about the celestial pole as an axis, and consequently
inclined from the vertical by the latitude of the place of observation.
Pointing a telescope with motions about a vertical and horizontal axis
only, therefore means that, as a star moves in its apparent path, it
will drift away from the telescope both in azimuth and in altitude, and
require to be followed by a double motion.
Alt-azimuth mounts may be divided into three general groups according
to their construction. The first and simplest of them is the
pillar-and-claw stand shown in Figure 69. This consists of a vertical
pillar supported on a strong tripod, usually of brass or iron, and
provided at its top with a long conical bearing carrying at its upper
extremity a hinged joint, bearing a bar to which the telescope is
screwed as shown in the illustration.
If properly made the upper joint comprises a circular plate carrying
the bar and held between two cheek pieces with means for taking up
wear, and providing just enough friction to permit of easy adjustment
of the telescope, which can be swung in altitude from near the zenith
to the horizon or below, and turned around its vertical axis in any
direction.
When well made a stand of this kind is steady and smooth working,
readily capable of carrying a telescope up to about 3 inches aperture.
It needs for its proper use a firm sub-support for the three strong
hinged legs of the pillar. This is conveniently made as a very solid
stool with spreading legs, or a plank of sufficient size may be firmly
bolted to a well set post.
[Illustration: FIG. 69.—Table Mount with Slow Motion.]
Thus arranged the mount is a very serviceable one for small
instruments. Its stability, however, depends on the base upon which
it is set. The writer once unwisely attempted to gain convenience by
removing the legs of the stand and screwing its body firmly upon a very
substantial tripod. The result was a complete failure in steadiness,
owing to the rather long lever arm furnished by the height of the
pillar; and the instrument, which had been admirably steady originally,
vibrated abominably when touched for focussing.
The particular stand here shown is furnished with a rack motion in
altitude which is a considerable convenience in following. More
rarely adjustable steadying rods attached to the objective end of the
instrument are brought down to its base, but for a telescope large
enough to require this a better mount is generally desirable.
Now and then an alt-azimuth head of just the sort used in the
pillar-and-claw stand is actually fitted on a tall tripod, but such an
arrangement is usually found only in cheap instruments and for such
tripod mountings other fittings are desirable.
[Illustration: FIG. 70.—Alt-azimuth Mount, Clark Type T.]
The second form of alt-azimuth mount, is altogether of more substantial
construction. The vertical axis, usually tapered and carefully ground
in its bearings, carries an oblique fork in which the telescope tube is
carried on trunnions for its vertical motion. The inclination of the
forked head enables the telescope to be pointed directly toward the
zenith and the whole mount forms the head of a well made tripod.
Figure 70 shows an excellent type of this form of mount as used
for the Clark Type T telescope, designed for both terrestrial and
astronomical observation. In this particular arrangement the telescope
lies in an aluminum cradle carried on the trunnions, from which it can
be readily removed by loosening the thumb screws and opening the cradle.
[Illustration: FIG. 71.—Alt-azimuth with Full Slow Motions.]
It can also be set longitudinally for balance in the cradle if any
attachments are to be placed upon either end. Here the adjustment for
the height of the instrument is provided both in the spread of the
tripod and in the adjustable legs. So mounted a telescope of 3 or 4
inches aperture is easily handled and capable of rendering very good
service either for terrestrial or celestial work.
Indeed the Clarks have made instruments up to 6 inches aperture,
mounted for special service in this simple manner. For celestial use
where fairly high powers may be required this or any similar mount can
be readily furnished with slow motions either in azimuth or altitude or
both.
Figure 71 shows a 4¼ inch telescope and mount by Zeiss thus
equipped. Some alt-azimuth mounts are also provided with a vertical
rack motion to bring the telescope to a convenient height without
disturbing the tripod. A good alt-azimuth mount such as is shown in
Figs. 70 and 71 is by no means to be despised for use with telescopes
of 3 or 4 inch aperture.
The sole inconvenience to be considered is that of the two motions
required in following. With well fitted slow motions this is not really
serious for ordinary observing with moderate powers, for one can work
very comfortably up to powers of 150 or 200 diameters keeping the
object easily in view; but with the higher powers in which the field is
very small, only a few minutes of arc, the double motion becomes rather
a nuisance and it is extremely inconvenient even with low powers in
sweeping for an object the place of which is not exactly known.
There are in fact two distinct kinds of following necessary in
astronomical observations. First, the mere keeping of the object
somewhere in the field, and second, holding it somewhat rigorously in
position, as in making close observations of detail or micrometrical
measurements. When this finer following is necessary the sooner one
gets away from alt-azimuth mounts the better.
Still another form of alt-azimuth mount is shown in Fig. 72 applied for
a Newtonian reflector of 6 or 8 inches aperture. Here the overhung fork
carrying the tube on trunnions is supported on a stout fixed tripod, to
which it is pivoted at the front, and it is provided at the rear with a
firm bearing on a sector borne by the tripod.
At the front a rod with sliding coarse, and screw fine, adjustment
provides the necessary motion in altitude. The whole fork is swung
about its pivot over the sector bearing by a cross screw turned by a
rod with a universal joint.
This mount strongly suggests the original one of Hadley, Fig. 16, and
is most firm and serviceable. A reflector thus mounted is remarkably
convenient in that the eyepiece is always in a most accessible
position, the view always horizontal, and the adjustments always within
easy reach of the observer.
[Illustration: FIG. 72.—Alt-azimuth Newtonian Reflector.]
Whenever it is necessary to follow an object closely, as in using
a micrometer and some other auxiliaries, the alt-azimuth mount is
troublesome and some modification adjustable by a single motion,
preferably made automatic by clockwork, becomes necessary.
The first step in this direction is a very simple one indeed. Suppose
one were to tilt the azimuth axis so that it pointed to the celestial
pole, about which all the stars appear to revolve. Then evidently
the telescope being once pointed, a star could be followed merely by
turning the tube about this tilted axis. Of course one could not easily
reach some objects near the pole without, perhaps, fouling the mount,
but in general the sky is within reach and a single motion follows the
star, very easily if the original mount had a slow motion in azimuth.
This is in fact the simplest form of equatorial mount, sometimes
called parallactic. Figure 73 shows the principle applied to a small
reflector. An oblique block with its angle adjusted to the co-latitude
of the place drops the vertical axis into line with the pole, and the
major part of the celestial vault is then within easy reach.
It may be regarded as the transition step from the alt-azimuth to
the true equatorial. It is rarely used for refractors, and the first
attempt at a real equatorial mount was in fact made by James Short F.
R. S. in mounting some of his small Gregorians.[15] As a matter of
record this is shown, from Short’s own paper before the Royal Society
in 1749, in Fig. 74.
[15] Instruments with a polar axis were used by Scheiner as early
as 1627; by Roemer about three quarters of a century later, and
previously had been employed, using sights rather than telescopes, by
the Chinese; but these were far from being equatorials in the modern
sense.
[Illustration: FIG. 73.—Parallactic Mount for Reflector.]
A glance shows a stand apparently most complicated, but closer
examination discloses that it is merely an equatorial on a table
stand with a sweep in declination over a very wide arc, and quite
complete arrangements for setting to the exact latitude and azimuth.
The particular instrument shown was of 4 inches aperture and about 18
inches long and was one of several produced by Short at about this
epoch.
[Illustration: FIG. 74.—Short’s Equatorial Mount.]
In the instrument as shown there is first an azimuth circle _A A_
supported on a base _B B B B_ having levelling screws in the feet.
Immediately under the azimuth circle is mounted a compass needle for
approximate orientation, and the circle is adjustable by a tangent
screw _C_.
Carried by the azimuth circle on a bearing supported by four pillars
is a latitude circle _D D_ for the adjustment of the polar axis, with
a slow motion screw _E_. The latitude circle carries a right ascension
circle _F F_, with a slow motion _G_, and this in turn carries on four
pillars the declination circle _H H_, and its axis adjustable by the
slow motion _K_.
To this declination circle is secured the Gregorian reflector _L L_
which serves as the observing telescope. All the circles are provided
with verniers as well as slow motions. And the instrument is, so to
speak, a universal one for all the purposes of an equatorial, and when
the polar axis is set vertical equally adapted for use as a transit
instrument, theodolite, azimuth instrument, or level, since the circles
are provided with suitable levels.
This mount was really a very neat and complete piece of work for the
purpose intended, although scarcely suitable for mounting any but a
small instrument. A very similar construction was used later by Ramsden
for a small refractor.
It is obvious that the reach of the telescope when used as an
equatorial is somewhat limited in the mount just described. In modern
constructions the telescope is so mounted that it may be turned readily
to any part of the sky, although often the polar axis must be swung
through 180° in order to pass freely from the extreme southern to the
extreme northern heavens.
The two motions necessary are those in right ascension to follow the
heavenly bodies in their apparent course, and in declination to reach
an object at any particular angular distance from the pole.
There are always provided adjustments in azimuth and for latitude over
at least a small arc, but these adjustments are altogether rudimentary
as compared with the wide sweep given by Short.
The fundamental construction of the equatorial involves two axes
working at right angles positioned like a capital T.
The upright of the T is the polar axis, fitted to a sleeve and bearing
the cross of the T, which is hollow and provides the bearing for the
declination axis, which again carries at right angles to itself the
tube of the telescope.
When the sleeve which carries the upright of the T points to the pole
the telescope tube can evidently be swung to cover an object at any
altitude, and can then be turned on its polar axis so as to follow
that object in its apparent diurnal motion. The front fork of a bicycle
set at the proper angle with a cross axis replacing the handle bars has
more than once done good service
[Illustration: FIG. 75.—Section of Modern Equatorial.]
in an emergency. Figure 75 shows in section a modern equatorial mount
for a medium sized telescope.
The mounting shown in Fig. 75, by Zeiss, is thoroughly typical of
recent practice in instruments of moderate size. The general form of
this equatorial comes straight down to us from Fraunhofer’s mounting
of the Dorpat instrument. It consists essentially of two axes crossed
exactly at right angles.
P, the polar axis, is aligned exactly with the pole, and is supported
on a hollow iron pier provided at its top with the latitude block L to
which the bearings of P are bolted. D the declination axis supports the
telescope tube T.
The tube is counterpoised as regards the polar axis by the weight a,
and as regards the declination axis by the weights b b. At A, the upper
section of the pier can be set in exact azimuth by adjusting screws,
and at the base of the lower section the screws at B. B. allow some
adjustment in latitude. To such mere rudiments are the azimuth and
altitude circles of Short’s mount reduced.
At the upper end of the polar axis is fitted the gear wheel g, driven
by a worm from the clockwork at C to follow the stars in their course.
At the lower end of the same axis is the hour circle h, graduated for
right ascension, and a hand wheel for quick adjustment in R. A.
At d is the declination circle, which is read, and set, by the
telescope t with a right angled prism at its upper end, which saves the
observer from leaving the eye piece for small changes.
F is the usual finder, which should be applied to every telescope of 3
inches aperture and above. It should be of low power, with the largest
practicable field, and has commonly an aperture ¼ or ⅕ that of the
main objective, big enough to pick up readily objects to be examined
and by its coarse cross wires to bring them neatly into the field. At m
and n are the clamping screws for the right ascension and declination
axes respectively, while o and p control the respective tangent screws
for fine adjustment in R. A. and Dec. after the axes are clamped. This
mount has really all the mechanical refinements needed in much larger
instruments and represents the class of permanently mounted telescopes
used in a fixed observatory.
The ordinary small telescope is provided with a mount of the same
general type but much simpler and, since it is not in a fixed
observatory, has more liberal adjustments in azimuth and altitude
to provide for changes of location. Figure 76 shows in some detail
the admirable portable equatorial mounting used by the Clarks for
instruments up to about 5 or 6 inches aperture.
Five inches is practically the dividing line between portable and fixed
telescopes. In fact a 5 inch telescope of standard construction with
equatorial mounting is actually too heavy for practical portability
on a tripod stand. The Clarks have turned out really portable
instruments of this aperture, of relatively short focus and with
aluminum tube fitted to the mounting standard for a 4 inch telescope,
but the ordinary 5 inch equipment of the usual focal length deserves a
permanent placement.
In this mount the short tapered polar axis P is socketed between the
cheeks A, and tightened in any required position by the hand screws B.
The stout declination axis D bears the telescope and the counterweight
C. Setting circles in R. A. and Dec., p and d respectively, are carried
on the two axes, and each axis has a worm wheel and tangent screw
operated by a universal joint to give the necessary slow motion.
[Illustration: FIG. 76.—Clark Adjustable Equatorial Mount.]
The worm wheels carry their respective axes through friction bearings
and the counter poising is so exact that the instrument can be quickly
swung to any part of the sky and the slow motion picked up on the
instant. The wide sweep of the polar axis allows immediate conversion
into an alt-azimuth for terrestrial use, or adjustment for any
latitude. A graduated latitude arc is customarily engraved on one of
the check pieces to facilitate this adjustment.
Ordinarily portable equatorials on tripod mounts are not provided
with circles, and have only a single slow motion, that in R. A. A
declination circle, however, facilitates setting up the instrument
accurately and is convenient for locating an object to be swept for in
R. A. which must often be done if one has not sidereal time at hand. In
Fig. 76 a thumb screw underneath the tripod head unclamps the mount so
that it may be at once adjusted in azimuth without shifting the tripod.
As a rule American stands for fixed equatorials have the clock drive
enclosed in the hollow pillar which carries the equatorial head
as shown in the reflector of Fig. 35, and in the Clark mount for
refractors of medium size shown in Fig. 77. Here a neat quadrangular
pillar carries an equatorial mounting in principle very much like Fig.
76, but big enough to carry telescopes of 8 to 10 inches aperture.
It has universal adjustment in latitude, so that it can be used in
either hemisphere, the clock and its driving weight are enclosed in the
pillar and slow motions are provided for finding in R. A. and Dec. The
adjustment in azimuth is made by moving the pillar on its base-plate,
which is bolted to the pier. The convenient connections for accurate
following and the powerful clock make the mount especially good for
photographic telescopes of moderate size and the whole equipment is
most convenient and workmanlike. It is worth noting that the circles
are provided with graduations that are plain rather than minute, in
accordance with modern practice. In these days of celestial photography
equatorials are seldom used for determining positions except with the
micrometer, and graduated circles therefore, primarily used merely for
finding, should be, above all things, easy to read.
All portable mounts are merely simplifications of the observatory type
of Fig. 75, which, with the addition of various labor saving devices is
applied to nearly all large refractors and to many reflectors as well.
There is a modified equatorial mount sometimes known as the “English”
equatorial in which the polar axis is long and supported on two piers
differing enough in height to give the proper latitude angle, the
declination axis being about midway of the polar axis. A bit of the
sky is cut off by the taller pier, and the type is not especially
advantageous unless in supporting a very heavy instrument, too heavy to
be readily overhung in the usual way.
[Illustration: FIG. 77.—Universal Observatory Mount (Clark 9-inch).]
In such case some form of the “English” mounting is very important to
securing freedom from flexure and thereby the perfection of driving
in R. A. so important to photographic work. The 72 inch Dominion
Observatory reflector and the 100 inch Hooker telescope at Mt. Wilson
are thus mounted, the former on a counterpoised declination axis
crosswise the polar axis, the original “English” type; the latter on
trunnions within a long closed fork which carries the polar bearings at
its ends.
[Illustration: FIG. 78.—English Equatorial Mount (Hooker 100-inch
Telescope).]
Figure 78 shows the latter instrument, of 100 inches clear aperture
and of 42 feet principal focal length, increased to 135 feet when used
as a Cassegrainian. It is the immense stability of this mount that has
enabled it to carry the long cross girder bearing the interferometer
recently used in measuring the diameters of the stars. Note the
mercury-flotation drum at each end of the polar axis. The mirrors were
figured by the skillful hands of Mr. Ritchey.
[Illustration: FIG. 79.—English Equatorial Mount (72-inch Dominion
Observatory Telescope).]
Figure 79 gives in outline the proportions and mounting of the
beautiful instrument in service at the Dominion Observatory, near
Victoria, B. C. The mirror and its auxiliaries were figured by Brashear
and the very elegant mounting was by Warner and Swasey. The main mirror
is of 30 feet principal focus. The 20 inch hyperboloidal mirror extends
the focus as a Cassagrainian to 108 feet. The mechanical stability
of these English mounts for very large instruments has been amply
demonstrated by this, as by the Hooker 100 inch reflector. They suffer
less from flexure than the Fraunhofer mount where great weights are
to be carried, although the latter is more convenient and generally
useful for instruments of moderate size. It is hard to say too much of
the mechanical skill that has made these two colossal telescopes so
completely successful as instruments of research.
[Illustration: FIG. 80.—Astrographic Mount with Bent Pier.]
The inconvenience of having to swing the telescope tube to clear the
pier at certain points in the R. A. following is often a serious
nuisance in photographic work requiring long exposures, and may waste
valuable time in visual work. Several recent forms of equatorial mount
have therefore been devised to allow the telescope complete freedom of
revolution in R. A., swinging clear of everything.
One such form is shown in Fig. 80 which is a standard astrographic
mount for a Brashear doublet and guiding telescope. The pier is
strongly overhung in the direction of the polar axis far enough to
allow the instrument to follow through for any required period, even
to resuming operations on another night without a shift of working
position.
[Illustration: FIG. 81.—Open Fork Mounting.]
Another form, even simpler and found to be extremely satisfactory even
for rather large instruments, is the open polar fork mount. Here the
polar axis of an ordinary form is continued by a wide and stiff casting
in the form of a fork within which the tube is carried on substantial
trunnions, giving it complete freedom of motion.
The open fork mount in its simplest form, carrying a heliostat mirror,
is shown in Fig. 81. Here _A_ is the fork, _B_ the polar axis, carried
on an adjustable sector for variation in latitude, _C_ the declination
axis carrying the mirror _D_ in its cell, _E_ the slow motion in
declination, and _F_ that in R. A. Both axes can be unclamped for quick
motion and the R. A. axis can readily be driven by clock or electric
motor.
The resemblance to the fully developed English equatorial mount of
Fig. 78 is obvious, but the present arrangement gives entirely free
swing to a short instrument, is conveniently adjustable, and altogether
workmanlike. It can easily carry a short focus celestial camera up to 6
or 8 inches aperture or a reflector of 4 or 5 feet focal length.
In Fig. 173, Chap. X a pair of these same mounts are shown at Harvard
Observatory. The nearer one, carrying a celestial camera, is exposed
to view. It is provided with a slow motion and clamp in declination,
and with an electric drive in R. A., quickly unclamped for swinging the
camera. It works very smoothly, its weight is taken by a very simple
self adjusting thrust bearing at the lower end of the polar axis, and
altogether it is about the simplest and cheapest equatorial mount of
first class quality that can be devised for carrying instruments of
moderate length.
Several others are in use at the Harvard Observatory and very similar
ones of a larger growth carry the 24 inch Newtonian reflector there
used for stellar photography and the 16 inch Metcalf photographic
doublet.
[Illustration: FIG. 82.—Mounting of Mt. Wilson 60-inch Reflector.]
[Illustration: FIG. 83.—The 60-inch as Cassegrainian, F = 100′.]
In fact the open fork mount, which was developed by the late Dr.
Common, is very well suited to the mounting of big reflectors. It was
first adapted by him to his 3 ft. reflector and later used for his
two 5 ft. mirrors, and more recently for the 5 ft. instrument at Mt.
Wilson, and a good many others of recent make. Dr. Common in order
to secure the easiest possible motion in R. A. devised the plan of
floating most of the weight assumed by the polar axis in mercury.
Figure 82 is, diagrammatically, this fork mount as worked out by
Ritchey for the 5′ Mt. Wilson reflector. Here A is the lattice tube, B
the polar axis, C the fork and D the hollow steel drum which floats the
axis in the mercury trough E. The great mirror is here shown worked as
a simple Newtonian of 25 ft. focal length. As a matter of fact it is
used much of the time as a Cassegranian.
To this end the upper section of tube carrying the oblique mirror is
removed and a shorter tube carrying any one of three hyperboloidal
mirrors is put in its place. Fig. 83 is the normal arrangement for
visual or photographic work on the long focus, 100 ft. The dotted lines
show the path of the rays and it will be noticed that the great mirror
is not perforated as in the usual Cassegrainian construction, but that
the rays are brought out by a diagonal flat.
Figure 84 is a similar arrangement used for stellar spectroscopy with
a small flat and an equivalent focus of 80 ft. In Fig. 85 a radically
different scheme is carried out. The hyperboloidal mirror now used
gives an equivalent focus of 150 ft., and the auxiliary flat is
arranged to turn on an axis parallel to the declination axis so as to
send the reflected beam down the hollow polar axis into a spectrograph
vault below the southern end of the axis. Obviously one cannot work
near the pole with this arrangement but only through some 75° as
indicated by the dotted lines. The fork mount is not at all universal
for reflectors, as has already been seen, and Cassegrainian of moderate
size are very commonly mounted exactly like refractors.
[Illustration: FIG. 84.—The 60-inch as Cassegrainian, F = 80′.]
[Illustration: FIG. 85.—The 60-inch as Polar Cassegrainian, F = 150′.]
We now come to a group of mounts which have in common the fundamental
idea of a fixed eyepiece, and incidentally better protection of the
observer against the rigors of long winter nights when the seeing
may be at its best but the efficiency of the observer is greatly
diminished by discomfort. Some of the arrangements are also of value in
facilitating the use of long focus objectives and mirrors and escaping
the cost of the large domes which otherwise would be required.
Perhaps the earliest example of the class is found in Caroline
Herschel’s comet seeker, shown in Fig. 86. This was a Newtonian
reflector of about 6 inches aperture mounted in a fashion that is
almost self explanatory. It was, like all Herschel’s telescopes, an
alt-azimuth but instead of being pivoted in altitude about the mirror
or the center of gravity of the whole tube, it was pivoted on the
eyepiece location and the tube was counterbalanced as shown so that it
could be very easily adjusted in altitude while the whole frame turned
in azimuth about a vertical post.
Thus the observer could stand or sit at ease sweeping in a vertical
circle, and merely had to move around the post as the azimuth was
changed. The arrangement is not without advantages, and was many years
later adopted with modifications of detail by Dr. J. W. Draper for
the famous instrument with which he advanced so notably the art of
celestial photography.
The same fundamental idea of freeing the observer from continual
climbing about to reach the eyepiece has been carried out in various
equatorially mounted comet seekers. A very good example of the type
is a big comet seeker by Zeiss, shown in Fig. 87. The fundamental
principle is that the ocular is at the intersection of the polar and
declination axis, the telescope tube being overhung well beyond the
north end of the former and counterbalanced on the latter. The observer
can therefore sit in his swivel chair and without stirring from it
sweep rapidly over a very wide expanse of sky.
This particular instrument is probably the largest of regular comet
seekers, 8 inches in clear aperture and 52½ inches focal length
with a triple objective to ensure the necessary corrections in
using so great a relative aperture. In this figure 1 is the base
with corrections in altitude and azimuth, 2 the counterpoise of the
whole telescope on its base, 3 the polar axis and R. A. circle, 4
the overhung declination axis and its circle, 5 the counterpoise in
declination, 6 the polar counterpoise, and 7 the main telescope tube.
The handwheel shown merely operates the gear for revolving the dome
without leaving the observing chair.
The next step beyond the eyepiece fixed in general position is so
to locate it that the observer can be thoroughly protected without
including the optical parts of the telescope in such wise as to injure
their performance.
One cannot successfully observe through an open window on account of
the air currents due to temperature differences, and in an observatory
dome, unheated as it is, must wait after the shutter is opened until
the temperature is fairly steadied.
Except for these comet seekers practically all of the class make use of
one or two auxiliary reflections to bring the image into the required
direction, and in general the field of possible view is somewhat
curtailed by the mounting. This is less of a disadvantage than it would
appear at first thought, for, to begin with, observations within 20°
of the horizon or thereabouts are generally unsatisfactory, and the
advantages of a stable and convenient long focus instrument are so
notable as for many purposes quite to outweigh some loss of sky-space.
[Illustration: FIG. 86.—Caroline Herschel’s Comet Seeker.]
The simplest of the fixed eyepiece group is the polar telescope of
which the rudiments are well shown in Fig. 88, a mount described by Sir
Howard Grubb in 1880, and an example of which was installed a little
later in the Crawford Observatory in Cork. Here the polar axis A is
the main tube of the telescope, and in front of the objective B, is
held in a fork the declination cradle and mirror C, by which any object
within a wide sweep of declination can be brought into the field and
held there by hand or clockwork through rotating the polar tube.
[Illustration: FIG. 87.—Mounting of Large Comet Seeker.]
Looked at from another slant it is a polar heliostat, of which the
telescope forms the driving axis in R. A. The whole mount was a
substantial casting on wheels which ran on a pair of rails. For use the
instrument was rolled to a specially arranged window and through it
until over its regular bearings on a pier just outside.
A few turns of the wheel D lowered it upon these, and the back of the
frame then closed the opening in the wall leaving the instrument in the
open, and the eye end inside the room. The example first built was of
only 4 inches aperture but proved its case admirably as a most useful
and convenient instrument.
This mount with various others of the fixed eyepiece class may be
regarded as derived from the horizontal photoheliographs used at the
1874 transit of Venus and subsequently at many total solar eclipses.
Given an equatorially mounted heliostat like Fig. 81 and it is evident
that the beam from it may be turned into a horizontal telescope placed
in the meridian, (or for that matter in any convenient direction) and
held there by rotation of the mirror in R. A., but also in declination,
save in the case where the beam travels along the extension of the
polar axis.
[Illustration: FIG. 88.—Grubb’s Original Polar Telescope.]
For the brief exposure periods originally needed and the slow variation
of the sun in declination this heliostatic telescope was easily kept
in adjustment. The original instruments were of 5 inches aperture and
40 ft. focal length, and the 7 inch heliostat mirror was provided with
ordinary equatorial clockwork. Set up with the telescope pointing along
the polar axis no continuous variation in declination is needed and the
clock drive holds the field steadily, as in any other equatorial.
Figure 89 shows diagrammatically the 12 inch polar telescope used for
more than twenty years past at the Harvard Observatory. The mount was
designed by Mr. W. P. Gerrish of the Harvard staff and contains many
ingenious features. Unlike Fig. 88 this is a fixed mount, with the
eye-end comfortably housed in a room on the second floor of the main
observatory building, and the lower bearing on a substantial pier to
the southward.
[Illustration: FIG. 89.—Diagram of Gerrish Polar Telescope.]
In the figure, _A_ is the eye end, _B_ the main tube with the objective
at its lower end and prolonged by a fork supported by the bearing _C_
and _D_ is the declination mirror sending the beam upward. The whole
is rotated in R. A. by an electric clock drive, and all the necessary
adjustments are made from the eye end.
A view of the exterior is shown in Fig. 90, with the mirror and
objective uncovered. The rocking arm at the objective end, operated by
a small winch beside the ocular, swings clear both mirror and objective
caps in a few seconds, and the telescope is then ready for use. Its
focal length is 16 ft. 10 inches and it gives a sweep in declination of
approximately 80°. It gives excellent definition and has proved a most
useful instrument.
A second polar telescope was set up at the Harvard Observatory station
in Mandeville, Jamaica, in the autumn of 1900. This was intended
primarily for lunar photography and was provided with a 12 inch
objective of 135 ft. 4 inches focal length and an 18 inch heliostat
with electric clock drive.
[Illustration: FIG. 90.—Gerrish Polar Telescope, Harvard Observatory.]
Inasmuch as all instruments of this class necessarily rotate the image
as the mirror turns, the tail-piece of this telescope is also mounted
for rotation by a similar drive so that the image is stationary on the
plate both in position and orientation. As Mandeville is in N. lat. 18°
01′ the telescope is conveniently near the horizontal. The observatory
of Yale University has a large instrument of this class, of 50 feet
focal length, with a 15-inch photographic objective and a 10-inch
visual guiding objective working together from the same heliostat.
Despite its simplicity and convenience the polar telescope has an
obvious defect in its very modest sweep in declination, only to be
increased by the use of an exceptionally large mirror. It is not
therefore remarkable that the first serious attempt at a fixed eyepiece
instrument for general use turned to a different construction even at
the cost of an additional reflection.
This was the _equatorial coudé_ devised by M. Loewy of the Paris
Observatory in 1882. (Fig. 91.) In the diagram A is the main tube
which forms the polar axis, and B the eye end under shelter, with all
accessories at the observer’s hand. But the tube is broken by the box
casing C containing a mirror rigidly supported at 45° to the axis of
the main tube and of the side tube D, which is counterbalanced and is
in effect a hollow declination axis carrying the objective E at its
outer end.
[Illustration: FIG. 91.—Diagram of Equatorial Coudé.]
In lieu of the telescope tube usually carried on this declination
axis we have the 45° mirror, F, turning in a sleeve concentric with
the objective, which, having a lateral aperture, virtually gives the
objectives a full sweep in declination, save as the upper pier cuts it
off. The whole instrument is clock driven in R. A., and has the usual
circles and slow motions all handily manipulated from the eye end.
The _equatorial coudé_ is undeniably complicated and costly, but
as constructed by Henry Frères it actually performs admirably even
under severe tests, and has been several times duplicated in French
observatories. The first _coudé_ erected was of 10½ inches aperture
and was soon followed by one of 23.6 inches aperture and 59 ft. focus,
which is the largest yet built.
Still another mounting suggestive of both the polar telescope and the
_coudé_ is due to Sir Howard Grubb, Fig. 92. Here as in the _coudé_
the upper part of the polar axis, _A_, is the telescope tube which
leads into a solid casing _B_, about which a substantial fork, _C_, is
pivoted. This fork is the extension of the side tube _D_, which carries
the objective, and thus has free swing in declination through an angle
limited by the roof of the observing room above, and the proximity of
the horizon below.
Its useful swing, as in the polar telescope, is limited by the
dimensions of the mirror _E_, which receives the cone of rays from the
objective and turns it up the polar tube to the eyepiece. This mirror
is geared to turn at half the rate of the tube _D_ so that the angle _D
E A_ is continually bisected.
[Illustration: FIG. 92.—Grubb Modified Coudé.]
In point of fact the sole gain in this construction is the reduction
in the size of mirror required, by reason of the diminished size of
the cone of rays when it reaches the mirror. The plan has been very
successfully worked out in the fine astrographic telescope of the
Cambridge Observatory of 12½ inches aperture and 19.3 ft. focal
length.
As in the other instruments of this general class the adjustments are
all conveniently made from the eye end. The Cambridge instrument has a
triple photo-visual objective of the form designed by Mr. H. D. Taylor
and the side tube, when not in use, is turned down to the horizontal
and covered in by a low wheeled housing carried on a track. The sky
space covered is from 15° above the pole to near the horizontal.
It is obvious that various polar and _coudé_ forms of reflector
are quite practicable and indeed one such arrangement is shown in
connection with the 60 inch Mt. Wilson reflector, but we are here
concerned only with the chief types of mounting which have actually
proved their usefulness. None of the arrangements which require the
use of additional large reflecting surfaces are exempt from danger of
impaired definition. Only superlatively fine workmanship and skill in
mounting can save them from distortion and astigmatism due to flexure
and warping of the mirrors, and such troubles have not infrequently
been encountered.
To a somewhat variant type belong several valuable constructions which
utilize in the auxiliary reflecting system the cœlostat rather than
the polar heliostat or its equivalent. The cœlostat is simply a plane
mirror mounted with its plane fixed in that of a polar axis which
rotates once in 48 hours, i.e., at half the apparent rate of the stars.
[Illustration: FIG. 93.—Diagram of Snow Horizontal Telescope.]
A telescope pointed at such a mirror will hold the stars motionless
in its field as if the firmament were halted à la Joshua. But if a
change of view is wanted the telescope must be shifted in altitude or
azimuth or both. This is altogether inconvenient, so that as a matter
of practice a second plane mirror is used to turn the steady beam from
the cœlostat into any desired direction.
By thus shifting the mirror instead of the telescope, the latter can be
permanently fixed in the most convenient location, at the cost of some
added expense and loss of light. Further, the image does not rotate as
in case of the polar heliostat, which is often an advantage.
An admirable type of the fixed telescope thus constituted is the Snow
telescope at Mt. Wilson (Cont. from the Solar Obs. #2, Hale). Fig.
93 from this paper shows the equipment in plan and elevation. The
topography of the mountain top made it desirable to lay out the axis of
the building 15° E. of N. and sloping downward 5° toward the N.
At the right hand end of the figure is shown the cœlostat pier, 29 ft.
high at its S end. This pier carries the cœlostat mirror proper, 30
inches in diameter, on rails _a a_ accurately E. and W. to allow for
sliding the instrument so that its field may clear the secondary mirror
of 24 inches diameter which is on an alt-azimuth fork mounting and also
slides on rails _b b_.
The telescope here is a pair of parabolic mirrors each of 24 inches
aperture and of 60 ft. and 145 ft. focus respectively. The beam from
the secondary cœlostat mirror passes first through the spectrographic
laboratory shown to the left of the main pier, and in through a long
and narrow shelter house to one of these mirrors; the one of longest
focus on longitudinal focussing rails _e e_, the other on similar rails
_c c_, with provision for sliding sidewise at _d_ to clear the way for
the longer beam.
The ocular end of this remarkable telescope is the spectrographic
laboratory where the beam can be turned into the permanently mounted
instruments, for the details of which the original paper should be
consulted. The purpose of this brief description is merely to show the
beautiful facility with which a cœlostatic telescope may be adapted to
astrophysical work. Obviously an objective could be put in the cœlostat
beam for any purpose for which it might be desirable.
Such in fact is the arrangement of the tower telescopes at the Mt.
Wilson Observatory. In these instruments we have the ordinary cœlostat
arrangement turned on end for the sake of getting the chief optical
parts well above the ground where, removed from the heated surface,
the definition is generally improved. To be sure the focus is at or
near the ground level, but the upward air currents cause much less
disturbance than the crosswise ones in the Snow telescope.
The head of the first tower telescope is shown in Fig. 94.[16] A is the
cœlostat mirror proper 17 inches in diameter and 12 inches thick, B
the secondary mirror 12¾ inches in the shorter axis of the ellipse,
22¼ inches in the longer, and also 12 inches thick. C is the 12
inch objective of 60 ft. focus, and D the focussing gear worked by a
steel ribbon from below.
[16] Contributions from the Solar Obs. #23, Hale, which should be seen
for details.
[Illustration: FIG. 94.—Head of 60-inch Tower Telescope.]
This instrument being for solar research the mirrors are arranged for
convenient working with the sun fairly low on either horizon where the
definition is at its best, and can be shifted accordingly, to the same
end as in the Snow telescope. There is also provision for shifting the
objective laterally at a uniform rate from below, to provide for the
use of the apparatus as spectro-heliograph.
The tower is of the windmill type and proved to be fairly steady
in spite of its height, high winds being rare on Mt. Wilson. The
great thickness of the mirrors in the effort to escape distortion
deserves notice. They actually proved to be too thick to give thermal
conductivity sufficient to prevent distortion.
[Illustration: FIG. 95.—Porter’s Polar Reflector.]
In the later 150′ tower telescope the mirrors are relatively less
thick, and a very interesting modification has been introduced in
the tower, in that it consists of a lattice member for member within
another exterior lattice, so that the open structure is retained, while
each member that supports the optical parts is shielded from the wind
and sudden temperature change by its corresponding outer sheath.
Still another form of mounting to give the observer access to a fixed
eyepiece under shelter is found in the ingenious polar reflector by
Mr. Russell W. Porter of which an example with main mirror of 16
inches diameter and 15 ft. 6 inches focal length was erected by him
a few years ago. Fig. 95 is entirely descriptive of the arrangement
which from Mr. Porter’s account seems to have worked extremely well.
The chief difficulty encountered was condensation of moisture on the
mirrors, which in some climates is very difficult to prevent.
[Illustration: FIG. 96.—Diagram of Hartness Turret Telescope.]
It is interesting to note that Mr. Porter’s first plan was to use the
instrument as a Herschelian with its focus thrown below the siderostat
at _F′_, but the tilting of the mirror, which was worked at F/11.6,
produced excessive astigmatism of the images, and the plan was
abandoned in favor of the Newtonian form shown in the figure. At F/25
or thereabouts the earlier scheme would probably have succeeded well.
Still another fixed eyepiece telescope of daring and successful design
is the turret telescope of the Hon. J. E. Hartness of which the
inventor erected a fine example of 10 inch aperture at Springfield,
Vermont. The telescope is in this case a refractor, and the feature of
the mount is that the polar axis is expanded into a turret within which
the observer sits comfortably, looking into the ocular which lies in
the divided declination axis and is supplied from a reflecting prism in
the main beam from the objective
Figure 96 shows a diagram of the mount and observatory. Here _a_ is the
polar turret, _bb_ the bearings of the declination axis, _c_ the main
tube, d its support, and _e_ the ocular end. Optically the telescope is
merely an ordinary refractor used with a right angled prism a little
larger and further up the tube than usual. The turret is entered
in this instance from below, through a tunnel from the inventor’s
residence. The telescope as shown in Fig. 96 has a 10 inch Brashear
objective of fine optical quality, and the light is turned into the
ocular tube by a right angled prism only 2¾ inches in the face. This
is an entirely practicable size for a reflecting prism and the light
lost is not materially in excess of that lost in the ordinary “star
diagonal” so necessary for the observation of stars near the zenith in
an ordinary equatorial. The only obvious difficulty of the construction
is the support of the very large polar axis. Being an accomplished
mechanical engineer, Mr. Hartness worked out the details of this design
very successfully although the moving parts weighed some 2 tons. The
ocular is not absolutely fixed with reference to the observer but is
always conveniently placed, and the performance of the instrument is
reported as excellent in every respect, while the sheltering of the
observer from the rigors of a Vermont winter is altogether admirable.
Figure 97 shows the complete observatory as it stands. Obviously the
higher the latitude the easier is this particular construction, which
lends itself readily to large instruments and has the additional
advantage of freeing the observer from the insect pests which are
extremely troublesome in warm weather over a large part of the world.
This running account of mountings makes no claim at completeness. It
merely shows the devices in common use and some which point the way to
further progress. The main requirements in a mount are steadiness, and
smoothness of motion. Even an alt-azimuth mount with its need of two
motions, if smooth working and steady, is preferable to a shaky and
jerky equatorial.
Remember that the Herschels did immortal work without equatorial
mountings, and used high powers at that. A clock driven equatorial is
a great convenience and practically indispensable for the photographic
work that makes so large a part of modern astronomy, but for eye
observations one gets on very fairly without the clock.
[Illustration: FIG. 97.—Hartness Turret Observatory from the N. E.]
Circles arc a necessity in all but the small telescopes used on
portable tripods, otherwise much time will be wasted in finding. In any
event do not skimp on the finder, which should be of ample aperture and
wide field, say ¼ the aperture of the main objective, and 3° to 5°
in field. Superior definition is needless, light, and sky room enough
to locate objects quickly being the fundamental requisites.
As a final word see that all the adjustments are within easy reach from
the eyepiece, since an object once lost from a high power ocular often
proves troublesome to locate again.
REFERENCES
CHAMBERS’ Astronomy, Vol. II.
F. L. O. WADSWORTH: _Ap. J._, =5=, 132. Ranyard’s mounts for
reflectors.
G. W. RITCHEY: _Ap. J._, =5=, 143. Supporting large specula.
G. E. HALE: Cont. Solar Obs. # 2. The “Snow” horizontal telescope.
G. E. HALE: Cont. Solar Obs. # 23. The 60 ft. tower telescope.
J. W. DRAPER: Smithsonian Contrib. =34=. Mounting of his large
reflector.
G. W. RITCHEY: Smithsonian Contrib. =35=. Mounting of the Mt. Wilson
60 inch reflector.
SIR H. GRUBB: Tr. Roy. Dublin Soc. Ser. 2. =3=. Polar Telescopes.
SIR R. S. BALL: _M. N._ =59=, 152. Photographic polar telescope.
A. A. COMMON: Mem. R. A. S., =46=, 173. Mounting of his 3 ft.
reflector.
R. W. PORTER: _Pop. Ast._, =24=, 308. Polar reflecting telescope.
JAMES HARTNESS: _Trans. A. S. M. E._, 1911, Turret Telescope.
SIR DAVID GILL: Enc. Brit., 11th Ed. Telescope. Admirable summary of
mounts.