IN STARRY SKIES

Evening Post, Volume CVIII, Issue 137, 6 December 1929, Page 18

IN STARRY SKIES

AN ASTROPHYSICAL OBSERVATORY

By "Omega Centauri."

Last week we gave a general view of the second largest telescope in the world, the 72in reflector in the Dominion Astrophysieal Observatory, Victoria, B.C. To-day wo give an enlarged picture of one of its most important accessories, the great stellar speetograph. At the top we see the mounting of tho great mirror. The glass disc as received from Paris weighed nearly 50001b, and in its finished state there are still 43001b of glass. The

back -was first ground to a flat surface and polished. The upper surface was then formed into a basin in the form of a portion of a hollow sphere 120 ft in diameter. But then the most difficult process still remained to be accomplished. This was the "figuring" of the surface. The shape had to be changed from the section of a sphere to that of a paraboloid of revolution. The centre bad to be deepened by about one-thousandth of an inch, and the whole surface had to be true to the theoretical form within one four hundred thousandth of an inch. If any part was accidentally made the slightest bit too deep, the whole had to be brought down to a new level. Thia last operation took about two years, and was effected with such accuracy that the mirror gathers all the light from a star which falla on any part of its surface into a disc less than one thousandth of an inch in diameter. But to keep the surface true, the mirror must be guarded most carefully. In tho course of an evening's work it has to be turned into different positions, and, though a foot in thickness, it would bend under its own- weight. , The distortion of the image which would result from such flexure is prevented by an ingenious system of levers, which support it

equally at twelvo points. Temperature changes would be still more disastrous to its figure, but although tho temperature within the double-walled dome rises five degrees during the day, a lagging of cotton felt prevents that of the disc changing by more than half a degree.

The principal focus of the mirror is 30ft above it, and in the centre of the tube. A small speetograph is actually used in this position for photographing the ultra violet region in the spectra of tho stars. There are, however, two other ways of using the telescope which are more convenient. These are the Newtonian and the Cassegrain arrangements. When used as a Newtonian' telescope, the cone of reflected light is intercepted by a plane mirror l'9.sin in diameter, which turns it to the side of the tube. The image so formed may be used either for visual observation or for photography. In the most useful arrangement of all, a

convex mirror is used instead of a plane one, and this reflects the light through a hole in the centre of the great mirror, and brings it to a focus two feet below it. This method of using a reflector, is due to Cassegrain, a French astronomer. It has two very decided advantages. It brings the image into a far more convenient position, and it greatly increases its size. The focal length of this telescope is changed by adopting the Cassegrain arrangement, from 30ft to 108 ft, and the increased magnifying power is attained without any increase in the length of the tube or the size of the dome.

The telescope has three finders, the two four-inch ones are seen in the former photograph, and one of them appears at the right in this one. The third finder has no tube, and consists of a 7in lens at the top of the great telescope, and an eye piece of power 200 at the bottom.

The chief work of the observatory is photographing the spectra of the stars. The great spectrograph adapted .to the Cassegrain arrangement is shown here placed in position for use ■with one prism. If two or three are used, the plate holder is attached to one of the other projections, tlie light being bent through a greater angle.

With one prism tho length of tho spectrum ia one and a third inches, with two or threo prisms it ia twice or three times as great. With one prism the spectrum of a sixth magnitude star can be photographed in twenty minutes. With three prisms it takes five times as long. Tho chief reason for wishing to increase the size of our telescopes is not to get larger images, but to ensure greater light gathering power, and so to reduce the time of exposure. One of the main researches of this observatory is tho measurement of the radial velocities of the stars. A speed of thirty miles a second relative to the observer displaces the spectral lines about a thousandth part of an inch. The speetrograph must therefore be extremely rigid, and must be guarded from temperature changes. We saw that the great mirror is kept within half a degree of a constant temperature, but this would not be nearly good enough for the great speetrograph. Fortunately, an electric thermostatic device enables the observer to keep the temperature constant within one hundredth part of a degree. The adoption of precautions such as these enable astonishing results to be achieved. Here are some of them: The radial speeds of distant stars are determined in miles per second. The distance apart and the masses of double stars are measured even when, the stars are too close to be seen as double in any telescope. The absolute brightness of a star can be found, and hence its distance can be estimated. The temperatures and pressures in the outer atmosphere of stars and the chemical elements present can be investigated, and light is thus thrown even on the structure of their atoms. Results such as these are obtained at Victoria, even fii the case of stars too faint to see, and thousands of light years away.