THE TOWER TELESCOPE

Evening Star, Issue 20148, 12 April 1929, Page 2

THE TOWER TELESCOPE

EARLIER FORMS DESCRIBED Of recent years (writes “Regains,” in the ‘Age’) the rapid strides that have been made in astrophysics have Jed to the desire for increasingly large attachments to be made to the telescopes. In the ordinary form of telescope mounting the telescope, with its attachments, has to move round to follow the object which is being investigated. _ Tins loads at once to serious difficulties, and places a limit to the power of the instruments that can be employed. These difficulties can be overcome, or very considerably reduced, if the whole installation can lie designed so that the telescope and the instruments used with it can remain stationary. EARLY DORMS OF FIXED TEt/Ej SCOPES. , The first step towards a fixed telescope seems to have been the siderostat made after Foucault’s designs in 1868. This is a mirror, rotated by clockwork, which reflects the light from the sun or a star in any fixed direction, so that the light can be caught by a fixed telescope. It has the disadvantage that the image formed by the telescope rotates in an irregular manner, rendering this arrangement quite useless for long exposure photographs. Some thirty years later the coelostat was employed for solar eclipse purposes, at first by some of the British parties, and later by astronomers from all parts of the world. ’The coelostat can be used to reflect the light from a star in a fixed direction, without rotation of the image, but there is not free choice of this direction—it is determined by the angle the star is north or south of the equator. If solar observations arc to be continued throughout the year, as is tho case at a solar observatory, the direction of the reflected beam is constantly changing, and the total change is large. It would be inconvenient, even if the

telescope were horizontal, to swing round the long telescope tube. Consequently a second mirror is introduced. The coelostat mirror can be moved until the light falls on the second mirror, and then the latter ran be turned so as to reflect the light into tho telescope, which is fixed. THE HORIZONTAL TELESCOPE.

In the earlier fixed telescope used with a coelostat—of which tho Snow telescope at the ‘Mount Wilson Observatory is an example—the telescope was horizontal, or nearly so. The different instruments used with it were mounted on fixed piers, the light from the sun being reflected to any of the instruments by rotating slightly the curved telescope mirror which forms the image of the sun. This telescope proved remarkably convenient, as it was possible to pass rapidly from one instrument to another, and thus to study solar phenomena nearly simultaneously by various methods. Very soon after the completion of the Snow horizontal telescope (be advantages of a vertical beam of light instead of a horizontal were pointed out.

THE 60ft tower telescope. Acting on this suggestion, and making use of the experience gained with the Snow telescope, Hale, ol’Mount Wilson, in 1907 designed a tower'telescope 65it in height, in which the eoelostat and second mirror were placed at the top of a galvanised steel tower, not unlike a windmill tower. The sunlight is reflected by the eoelostat to the second _ mirror, which reflects the light vertically downward through a 12iu lens, also mounted at the top of the tower. This lens brings the light to a focus and forms an image of tho .sun 60ft below, a little above ground level. Here the observer has the various controls close to In’s hand. Tho light then pusses through the slit of a spectrograph, the optical parts of which arc eontained_ in a cylindrical concrete well Rjft in diameter and ,‘j()ft deep. This well forms a. subterranean laboratory winch is very nearly constant in temperature. a most important factor in spectrographie work. The photographic plate on which the, spectrum is focused is close to the slit, and conveniently placed lor the observer. the 150 FT TOWER TELESCOPE. The 60ft tower telescope proved so successful that plans for a Tsoft tower telescope were commenced in 1900. in this telescope the eoelostat, the second mirror, and the object glass are supported no less than 160 ft above the ground, and therefore tho question of stability is very important. Tho method used lor protecting the tower I rom the wind is very interesting. The tower is, in fact, double, one towel' serving as a support for the optical parts. .Each ot the steel members of this tower is entirely enclosed within the corresponding hollow member of a second skeleton tower, mounted on independent foundations. This second toner does not, touch the inner tower anywhere, so that even when tho wind velocity reaches twenty miles an hour m difficulty is experienced from vibration,

The outer tower carries the dome which covers the instruments, and also supports the small electric elevator which renders the mirrors and object class accessible to the ’observers. Vertically below the tower is a well, with concrete walls, 4 Oft inside diameter and 78ft deej).

DETAILS OF TELESCOPE. 'Die dome consists of two portions of spheres of slightly different radius, one of which may be rotated inside the other, thus affording a very large opening, through which the sun shines on the coelostat, and promoting free circulation of the air about the mirrors. The mirrors arc no Jess than 12in thick, this great thickness being given to reduce distortion duo to heating. ’The sides of the mirrors are encased in closely-fitting water jackets, through which a stream of water may he kept circulating. The light is reflected from the. second mirror vertically downwards through the lens forming the object glass, which is 12in in diameter and luUft in focal length. A tube AJft square in section, built of angle iron and lined with sheet iron, extends the full height of the tower, enclosing the beam of light from-the lens and protecting the beam from disturbing air currents. To prevent heating of this lube, it is covered dtitside with canvas louvres. The image of the sun, formed by the object glass at a convenient height above the floor of the laboratory, is 17in in diameter, the large scale enabling small portions of the sun’s surface to bo investigated separately. This image is formed on the plate which carries the slit of the spectrographic ajiparatns. A Jens 7,jin in focal length and the diffraction grating which produces the spectrum are at the bottom of the well, and so are maintained at a constant temperature. The lens brings the spectrum to a focus on the photographic plate near the slit, so that all the manipulation can he done h,7 the observer from his position in the laboratory.

EASE OF M A NT P L' L AT lON. 'The ease with which I ho parts of the instrument can bo changed is remarkable. Object glasses of either •30, 60, or 150 ft focal length cau be

swung into position, giving a solar image of either 31, 7, or 17in diameter. The spectrograph may he used either with a focal length of 75 or of 30ft, the latter giving a shorter spectrum, but requiring a much shorter exposure time. The grating which produces the spectrum may also be changed, thus giving further (hoico. The variety of work which may ho carried out with this telescope is very great. For instance, a photograph may be obtained of a group of sun spots, or of the hydrogen clouds in the neighbourhood of the spots, showing their vortex structure. Tho upward or downward movement of these clouds may he measured, and the magnetic field in the spots may he determined. The pressure at different levels of the spots may lie found. and_ the radial motion of tho spot vapours investigated. These form a portion only of the types of research that- may ho carried out rapidly one after the other. In actual use tho 15Qlt tower telescope has proved most satisfactory, and since its completion iu 1912 lias been busily employed in increasing om- knowledge ot the phenomena of the solar surface, the large solar imago, and the high dispersion of tbo 75ft spectrograph greatly facilitating many of the invest igations.