GIANT TELESCOPE

Evening Post, Volume CXXI, Issue 17, 21 January 1936, Page 18

GIANT TELESCOPE

FOR USE IN AMERICA

ADVANTAGES OF POSITION

PLANNING THU >VOEK

The proposal to construct a huge telescope with a mirror 200 inches in diameter has made a great appeal to the popular imagination as well as to ■the professed astronomer, says a writer in the Melbourne "Age." 'This interest has been quickened by a cablegram from New York which stated that the great mirror had been successfully removed from the annealing process at Corning, New York, and that an examination had shown that the glass disc was flawless. What a great relief it must have been to everyone concerned when it was found that the casting and annealing had resulted in a perfect disc. The telescope is still Sax from completion, however; indeed, work is still proceeding on the design for the mounting, so that it is likely to be several years before the mammoth telescope will be finished, and ■work with it commenced. Much interesting information is available about the plans for the Observatory and its equipment, these plans, and the reasons that led to their adoption will be incorporated in the present article. • A full description of the telescope-itself must be held over until details of its construction are published. The year 1904 saw the commencement of the Mount Wilson Solar Observatory, the administrative offices, laboratories, and workshops of which are in Pasadena, in Southern California, the Observatory itself on the summit of Mount Wilson, only a few miles away, at ah elevation of some 6000 feet, a site chosen after extensive search as being specially favourable for observation. This observatory is maintained by the Carnegie Institution of Washing;, ton, although funds have been received from other sources as well, notably the cost of the great 100-inch mirror, which has been in active use there for some years. THIRTY YEARS' GROWTH. Under the direction of Dr. Hale, now honorary director, and his successor. Dr. W. S. Adams, and the band of physicists and astronomers that have collaborated with them, this institution in thirty, years has grown into the Mount Wilson Observatory, from which such important contributions in astrophysics have come. In Pasadena also is the California Institute of Technology. With the appointment of Dr. A. A. Noyes and Dr. R. A. Millikan to this institute, and aided by funds from many sources, notably the Carnegie and the Rockefeller Boards, an extensive research programme was instituted, in intimate association with the Mount Wilson Observatory- Pasadena thus became an active centre of research, and a favourable place, both climatically and intellectually, for further developments of similar undertakings. In 1928 the Kockefeller Board offered funds to the California institute for the construction of a 200-inch reflecting telescope, together with all buildings and equipment necessary to render the instrument as efficient as possible. Two conditions were imposed. First, the assurance of the active co-opera-tion of the Mount Wilson Observatory in this project, and, second, the provision of an adequate endowment for the new observatory. Both conditions were fulfilled without delay,* and early in July, 1928, the trustees of the California institute decided to proceed at once with the undertaking. THE NEW OBSERVATORY. An observatory council of four, with Dr. Hale as chairman, was appointed to take charge of the planning, construction, and operation of the observatory. . With these four was associated a very strong advisory committee, the personnel of which emphasises the close co-operation that is being given by the Mount Wilson. Observatory in planning this great new observatory and astrophysical laboratory. The plan that was decided upon included:— (1) A 200-inch reflecting telescope and other instruments, mounted at the most favourable high-altitude site, within effective working distance of Pasadena. (2) An astrophysical laboratory, on the campus of the California Institute, to serve as the Pasadena headquarters, where the reduction of observations could be carried out, where laboratory experiments for the interpretation of the observations could be performed, and where new instruments and methods of research could be developed. (3) Instrument and optical'shops on the campus of the institute, for the construction of special apparatus and the figuring and testing of the mirrors and lenses required for the 200-inch telescope and other instruments. This plan follows very closely that which.has proved so successful in connection with the Mount Wilson Observatory. THE MIRROR DISCS. The first step was to decide on the material for the 200-inch disc. Experience had shown that ordinary plate glass as used for the 100-inch mirror could not be used for the 200-inch owing to the change of figure; with temperature producing noticeable effects. Fused silica, with a very low coefficient of expansion, was experimented with. The body of the disc was formed by fusing nearly pure quartz sand. This disc, which contained many bubbles, was ground to •the proper curvature, and then sprayed by an oxy-hydrogen flame with pure crystalline quartz in finely-ground form. <"This gave a layer of perfectly transparent fused quartz, free from 'bubbles. Unfortunately, Very large mirrors could not be made by this process. After further experimenting, suitable burners were designed, and finally a solid disc of pure silica 66 inches in diameter and 15 inches thick was obtained. In the final stages of the cooling the cover of the furnace was removed too soon through some misunderstanding, and a radial crack developed. In a second attempt the disc was rendered unsuitable for optical work by pieces of firebrick falling from the cover of the furnace. •Finally, in an effort to recoat one of these discs with silica, a failure at midnight in the electric power supply resulted in another disaster. The attempt was then abandoned, as the estimated cost of the discs was prohibitive. Pyrex glass was next tried, a long series of investigations being carried out in the laboratory of the Corning Glass Works. Special methods of casting, preliminary cooling, and annealing were developed; which proved very successful for a 60-inch disc with a special ribbed back, which gives the requisite stiffness, with a large saving in weight and decrease in the time required for annealing. A special Pyrex glass, with a still lower coefficient of expansion, reduced annealing time, and complete freedom from any danger of devitrification, was also developed.

A disc 120 inches in diameter was cast and annealed, and is now being ground and figured in the optical shop in Pasadena. The 200-inch disc was cast on March 25, 1934, many hours being required to fill the mould. The extremely high temperature of the glass caused some of the supports of the mould material to break down near

the end of the pouring process, so that some portions of the mould rose in the molten disc. After, reheating, these were scooped out, and the disc recooled. This disc is being retained for possible use in case of any accident to its successor. The second 200-inch disc was poured with perfect success on December 2, 1934. As we have learned from cable reports, the annealing is now complete, and the disc has proved flawless. It will be transported to Pasadena, where, after the lengthy process of grinding and figuring to the correct shape, it will be coated with aluminium. TELESCOPE MOUNTING. After a careful study of several possible designs for the mounting, it has been decided to adopt the yoke type, illustrated in one form by the 100-inch reflector. There is to be a cartridgeshaped house in the tube at the principal focus, thus eliminating the Newtonian mirror. There are two Cassegrain mirrors, either of which can be turned into 'position by a motor-driven worm gear. One of these gives, just below the hole in the 200-inch mirror, a large and sharply defined star field. A stellar spectrograph with one, two, or three prisms will be mounted here, or a double-slide plate carrier can be quickly substituted for the spectrograph. A plane mirror inclined at 45 degrees to the axis of the tube can send the converging beam through the hollow declination axis to a totally reflecting prism, mounted before the slit of a long focus grating or prism spectrograph, hung in a large hollow cylinder, supported parallel to the polar axis, and so geared that the slit remains vertical. A second cylinder on the opposite side of the polar axis carries a radiometer or any other auxiliary instrument which must re^ main in a vertical plane. Another Cassegrain mirror with plane mirror suitably orientated will send the converging beam through the hollow polar axis into a constant temperature chamber. This will permit the use of a fixed spectrograph or other auxiliary apparatus of any focal length desired. The question has sometimes been raised whether the 200-inch telescope would not be more useful in the Southern Hemisphere^ A very large telescope should never be used for work within the ready range of smaller instruments. It must be devoted almost exclusively to the study of selected objects. Thus, the vastly greater knowledge of stars and nebulae acquired during centuries of bbservation north of the equator is precisely what is needed in making adequate selections for more intensive study. The Southern Hemisphere can at present furnish no co-operating research institutions comparable in staff . and equipment with the California institute and the Mount Wilson Observatory. It is, however, very desirable that much work be done south of the equator with ,large telescopes, thus gradually preparing the way, as in the north, for another instrument as large as the 200-inch telescope. MANY ADVANTAGES. The south-western corner of the United States offers many advantages as a site for a great telescope. Its latitude gives access to the whole of the northern sky, as well as a broad belt south of the equator. Its mountain ranges rise above the dense air of the lower atmosphere, which not only produces poor star images, but absorbs and scatters much light. In some of the mountains the daily range of temperature is remarkably low, and the annual range moderate, so that the observer never suffers from severe cold. The average wind velocities are very low, while comparative freedom from cloud permits observations on a large percentage of nights during the year.

A site easily accessible from Pasadena was desired so as to permit the readiest co-operation between the staffs of the Mount Wilson Observatory and the California institute. After a fiveyear study of the most promising mountain sites, all exceptionally free from local atmospheric disturbances and the effect of city lights, a nearly level tract on the summit of Mount Palomar (altitude about 6100 feet), 93 miles southeast of Pasadena and 50 miles north of San Diego, has been provisionally selected.

The San Diego county authorities have promised a broad and vwell-sur-faced road, so that the motor trip from Pasadena can be made quickly over perfect concrete routes. If other necessary conditions are fulfilled, and there is permanent protection from all appreciable sources of disturbance, the 200----inch telescope will presumably be erected on Palomar Mountain. But other excellent sites are available.