Many inquiries have reached the Naval Observatory in regard to the new telescope which is soon to be installed. Interest in this new telescope is greater because the Observatory has operated for many years without new equipment, while other observatories throughout the world have benefited by recent developments and improvements in instruments. In spite of old equipment, the Observatory has carried on its work in such a manner that it has consistently received and deserved the highest rating in accuracy for its work in declination and right ascension determination.
Fifty years ago there were 43 observatories devoting their time to fundamental astronomy, but newer developments in astronomy have proved more attractive, more stimulating, less monotonous, and less plain drudgery. As a result there are today only three, Greenwich, the Cape, and the Naval Observatory, still maintaining their continuous fundamental astronomical observations.
Fundamental astronomy, or astronomy of position, can be compared to navigation. The navigator determines and indicates his exact position on the terrestrial sphere, the earth. In astronomy of position, or fundamental astronomy, the astronomer determines the exact position of the various celestial bodies on the celestial sphere. If a navigator is told that an island is in latitude 30 North and longitude 20 West, he can locate the island exactly on his chart. In the same way if an astronomer is told that a star has declination 30° North and right ascension one hour and twenty minutes, he too can locate the star immediately. In other words, fundamental astronomy, or astronomy of position, determines the exact position of all celestial bodies by their right ascension and declination, just as navigation determines positions on the terrestrial sphere by longitude and latitude.
The uninterrupted continuity of fundamental astronomical observations is necessary because on these fundamental observations and their reductions are based the tabulated data that navigators must use to navigate ships with accuracy and with safety. But it is this enforced continuity of fundamental astronomy that makes it so monotonous, such drudgery, and so expensive. It is absolutely necessary to maintain enough personnel to make practical the continuous observational work, and, also, a force of computers to reduce the observations.
The Naval Observatory’s geographical position is ideal for this particular branch of astronomical work. Greenwich is well north, the Cape is very far south, while Washington can observe the arc between the two other observatories with an overlap on the limits of both. The first two, supported by the British government, and our Naval Observatory have been furnishing these vital data for years.
A few years ago, Congress appropriated funds for the modernization of the Naval Observatory. One of the principal projects of this modernization program was the building of a large reflector telescope, the largest our money would buy. Many of the observatories throughout the world have had the benefit of reflector telescopes for years. But the Naval Observatory has been one of the few not so equipped. This new telescope will reduce the drudgery and expense of many repeated fundamental observations of innumerable celestial bodies. Photographic records will be made of certain areas. Measurements of these plates will furnish data which applied to accurately determined positions of fundamental stars will give the positions of many others.
The first appearance of spectacles was in 1289. But it took over 300 years for the principle of magnification in spectacles to be appreciated, developed, and used in making the first telescopes. The first definite record of the discovery of telescopic vision dates from 1608 when a spectacle- maker of Middleburg, Jan Lippershey, applied for a patent and demonstrated the ability of his telescope. This instrument was made for use with both eyes.
But it remained for Galileo to grasp and to apply the principles involved in an actual research instrument. While in Venice, a year later, Galileo heard rumors that a Belgian had devised an instrument for making distant objects appear near. Confirming this by letter and fully appreciating its importance, Galileo immediately returned to Padua where he solved the problem the very night of his arrival. He procured a plano-convex and a plano-concave lens, fitting them in a lead pipe. The combination magnified three diameters. The relation between power and foci of lenses was quickly grasped. His next recorded trial showed a magnification of eight diameters.
Galileo returned to Venice and exhibited his discovery to the Senators and to the public, disclosing his secret and presenting the telescope to the Doge sitting in Council. That telescope was 20 inches long with 1 5/8-inch aperture. He returned home, laden with honors, and began the arduous work of development, grinding many lenses, until he finally produced an instrument that magnified 32 times. This telescope was 49 inches long with 1 3/4 -inch aperture.
In 1672, Sir Isaac Newton, pursuing an entirely different method in the development of magnification, presented his 6- inch reflector telescope to the British authorities. About the same time Monsieur Cassegrain designed his reflector, and somewhat later, Sir William Herschel, a prominent figure in astronomy, had a share in the development of celestial telescopes.
Since that early date, vast improvements have been made in both types of telescopes—refractors and reflectors. As the size of the telescopes increased, the difficulties in making and grinding the lenses multiplied, until finally they reach the limit for refractors because of the difficulties in casting large discs of homogeneous glass sufficiently perfect to permit grinding satisfactory lenses. As the lenses grew larger in diameter, their thickness was proportionally increased and the loss of light by absorption in passing through large lenses became a serious question.
All this time the British astronomers and telescope makers were concentrating on an entirely different type of telescope, the reflector telescope. For years they were considered the great reflector builders of the world. In fact, some years ago, when Professor Ritchey was discussing reflectors with a friend, the latter said,
I do not wish to discourage you but to warn you. Your faith and your hope in great reflectors are misplaced. The British astronomers say that, and they know, for they are the great reflector builders. They have had the experience of centuries, from the time of Newton.
In spite of this discouraging advice, Professor Ritchey continued year after year investigating each of the fundamental parts of reflector telescopes. He has built a number of the largest and the most famous reflector telescopes in the world, and his work with these instruments has brought him international fame not only as one of the foremost in his specialty, but also as the premier celestial photographer in the world.
The loss of light in a refractor telescope has always been a very serious question. Also, in refractor telescopes where the light has to pass through a series of lenses, it is necessary to determine whether the telescope is to be used for visual work or for photographic work. As that part of light which is most active photographically has a different focal length than that used for visual work, a telescope used photographically is very inefficient for visual work, and vice versa. Then, too, it is very difficult to insure that the lenses shall be achromatic.
Our 26-inch equatorial telescope has a record of historic achievement, including the discovery of the moons of Mars. Yet, frequently, visitors who look through this telescope speak of the beautiful blue colors, and these blue colors interfere with its use. In contrast, the principle of the reflector telescope eliminates this trouble. White light is reflected in its pure value. The light does not pass through any lenses, it merely touches the silvered surface of the large reflector, or mirror. It is then projected to the second reflector mirror, and from there is projected through the axial hole in the big mirror on to the photographic plate; or, when the telescope is used for visual work, it is projected to the eye of the observer.
The reflectors are capable of more universal use than refractors, and they are free from losses due to absorption of light passing through the lenses. The difficulty in making the lenses achromatic is also eliminated, and the only loss in the reflector telescope is the small percentage of loss at each reflecting surface.
Due to the super-refinements which Dr. Ritchey has incorporated in his new telescope, he claims that this loss amounts to not more than 10 or 12 per cent; that is 5 or 6 per cent loss at each reflecting surface. In contrast to this are the losses in the refractor telescope which amount to 50 or 60 per cent, and in the case of poor glass, as high as 70 per cent. In the latter case, only 30 per cent of the light from the celestial body ever reaches the eye of the observer, or the photographic plate if the refractor is ground for photographic work. In the case of the reflector, 88 or 90 per cent of the light will reach the plate or the eye of the observer.
The Naval Observatory’s new 40-inch reflector telescope is, of course, much smaller than the famous 100-inch at Mount Wilson, which was also built by Dr. Ritchey. Neither is it as large as the new telescope being built for the University of Texas, nor the new one in Canada. But Dr. Ritchey explains that his new telescope, due to its extra refinements, will exceed the others in unit efficiency.
Dr. Ritchey’s familiarity with the big telescopes at Mount Wilson and his actual experience in operating them permit him to make an intelligent comparison between what they can do and what he expects the new one to do. The new instrument is the result of a life time of study, careful planning, expert designing, and minute refining. New and better curves for the mirrors were conceived in order to give more concentrated and more symmetrical telescopic images of the celestial bodies. The telescope structure is designed to eliminate all flexures. Minute care is evident in the protection of the telescope from the effect of temperature changes. The rotation of the telescope is designed to be frictionless by supporting the entire weight of the telescope in a bath of mercury, and, as Dr. Ritchey says, making it turn as smoothly and as accurately as the earth revolves on its axis.
The photograph on page 8, the 40-inch dome from the northeast, shows shutters, tube extension, and the curtain which cuts off all extraneous light. The height of the dome is about 40 feet. Its exterior diameter is 35 feet. The dome is constructed of light metal, only .04 of an inch thick, electrically welded to the members of the structure of the dome. Every effort has been made to insure quick radiation and prevent heat being stored up in the material of the dome. The dome is designed with 25-foot space between the inner and the outer shells. Ventilating louvers top and bottom and four electric blowers insure circulation of air.
The driving clock is mounted on the second deck and is completely housed in to prevent injury to the driving mechanism. The third deck is the observing platform where all operations of the telescope are controlled.
The photograph on page 7 shows the base, the north and south columns, the polar axis, the worm wheel and bevel gear, the float and the reservoir, the fork, and the lower section of the tube for the support of the big mirror.
The polar axis is 10 feet long and 10 inches in diameter. It is hollow and weighs about three-quarters of a ton. The cast iron base is 10 feet long, 4 ½ feet wide, and weighs about 2 tons. The total height of the telescope above the pier when it is pointed at the zenith is 18 ½ feet, and the total weight of the telescope is about 9 tons. The length of the telescope tube is only 10 ½ feet. The equivalent focal length of this telescope is 272 inches. The large mirror has a clear aperture of 40 inches. It is about 6 inches thick and has a 10- inch central hole. The secondary mirror is 16 inches in diameter and is 2 ½ inches thick. Both are made of special low expansion glass with a coefficient of expansion about one-third that of ordinary glass. The distance between the two mirrors is only 8 feet.
The large mirror is supported by a mechanical flotation system. The back support consists of 3 rigid pads and 15 lever supports with counterpoises. The edge support consists of 3 fixed arcs, 3 movable arcs, and 12 lever supports with counterpoises. This is a very ingenious mechanical arrangement for distributing the weight of the big mirror and insuring its being held in a constant position. Every time gravity tends to move this big mirror one way, these lever supports and counterpoises exert an opposite and an equal force which counterbalances the effect of gravity.
The secondary mirror has its edge grooved and is supported by 3 fixed arcs and 3 lever supports and counterpoises.
The upper end of the telescope tube is prevented from bending or flexing under its own weight by 4 counterpoise levers located inside the 4 tubes which form the square frame outside of the octagonal telescope frame. This is another very ingenious method of counteracting the effect of gravity and insuring that the 2 mirrors are in exact alignment regardless of the inclination of the telescope from the vertical. All flexures are thus prevented.
There are also 2 counterpoise levers in the arms of the fork to prevent flexures from disturbing the adjustment of the declination axis bearings.
The mercury reservoir is bolted to the upper face of the north column. This reservoir contains mercury and the big hollow steel float attached to the polar axis dips into this mercury bath and acts as a thrust bearing. It supports the entire weight of the telescope and frees the bearings from even the slightest pressure, permitting absolute smoothness of revolution.
The driving clock is controlled by a centrifugal governor and is run by a heavy weight which is kept wound up automatically. On the governor shaft, which revolves twice a second, is a worm which a meshes with a worm wheel having 320 teeth. This wheel is on the same shaft with the large worm which meshes with the large worm wheel on the polar axis. Only two worm and worm-wheel reductions are necessary to secure the motion is necessary to counteract the earth’s rotation.
Dr. Ritchey has stated that this is the first telescope he has been able to build where he has been permitted to carry out his ideas. He has been unhampered and has been encouraged to exercise his inventive genius in solving the intricate and difficult problems. His solutions of many of these problems are simple, fundamental, and foolproof.
One of the most ingenious accessories is the observer’s carriage (see page 1). This carriage raises the observer absolutely independently of the telescope but the effect is the same as if the observer were carried on the end of the telescope tube. If is equatorially mounted, and is operated by synchronous motors which keep step with the telescope so that without any attention on the part of the observer, his chair mounted on the carriage moves exactly as the telescope itself moves. This in ls an enormous advantage over the ordinary type of observer’s carriage which has to be shifted intermittently when the observer can stretch no further. The Psychological effect of the elimination of these annoying interruptions will improve the quality of the observer’s work.
When the telescope is not in use, it is completely covered by a heavy felt canopy lie which is lowered into place over the entire telescope structure and is secured to the deck. After the heavy folding canopy is lowered into place, a small frigidaire unit is connected and the temperature of the telescope, its mirrors and all accessories, is maintained automatically at the expected night temperature. This accessory will prevent the mirrors from becoming heated during the day and will reduce to a minimum the change of temperature that they must undergo. Mirrors are equally good in warm or cold temperatures, but it is the change in temperature which distorts them temporarily and until they have had time to assume the new temperature. Therefore, this refinement was incorporated so that when the astronomer finishes his night’s work, he sets the temperature for the mean of that night and if the next night follows the law of averages, he will find the instrument at the temperature of the outside air when he uncovers the telescope the next night. Without this refinement, it would require several hours for the large mirror to readjust itself to the change in temperature.
Every possible refinement has been very carefully developed and it is hoped that in the very near future we may be able to show the citizens of the United States that they have a telescope at the Naval Observatory in which they can take great pride. As Dr. Ritchey recently said, “This telescope is where it should be—in the national observatory.” We all hope that the future work of this telescope will be so outstanding as to reflect great astronomical credit on the United States.
Voyages of discovery are becoming increasingly difficult, as the remaining spaces on the earth to be discovered or investigated are becoming increasingly smaller. But there is an infinite space which has not yet been visited and which can only be visited by means of the most powerful celestial telescopes.