SECTION 4.—OPTICAL PARTS AND SPECTROGRAPHS
The great mirror is composed of hard plate glass cast in one piece and after annealing, ground and polished to the correct shape. As received from the St. Gobain Co., the disc was 73·5 inches diameter, over 13 inches thick with a central hole about 6 inches diameter and weighed nearly 5,000 pounds. It was first of all ground truly circular to a diameter of 73 inches and flat on both sides to a thickness of slightly over 12 inches, while the central hole was enlarged to 10 inches. When the back was polished approximately flat, the disc was seen to be a beautiful specimen of the glass makers art, homogeneous and almost entirely free from bubbles or other defects.
Fig. 4.—PHOTOGRAPH OF RING NEBULA IN LYRA(Enlarged 8 diameters)
It was now ready for the second stage of the operation the grinding of the correct shape for the upper reflecting surface. In order to bring the light of a star to an accurate focus this surface must be a paraboloid of revolution, the same kind of curve given to the reflectors of search lights or automobile headlights. The curve for this reflector of 30 feet focus is very nearly a section of a sphere of 60 feet radius, withinone-thousandth of an inch, and consequently would nearly fit a huge globe 120 feet in diameter. The upper surface of the disc was fine ground and polished to this spherical surface and was then ready for the final stage, the “figuring” a continuation of the polishing process until the centre is deepened about a thousandth of an inch and the surface becomes accurately paraboloidal. This “figuring,” an exceedingly delicate and difficult process especially over such a large surface as the 72-inch, with the added difficulty of a central hole, occupied about two years and was not completed until nearly a year and a half after the mounting was ready. When it is remembered, however, that the surface nowhere deviates from the true theoretical form more than one four-hundred-thousandth of an inch and that if one part is accidentally polished too deep, the whole surface has again to be brought down to this level, the exceeding delicacy of the operation is evident and the time taken not excessive.
Accurate quantitative tests showed that the final figure is of the highest order of accuracy and this is further clearly shown by the practical test of direct photographs at the principal focus. Figure 4, a six fold enlargement of a photograph at the principal focus, of the Ring Nebula in Lyra shows how sharp and small are the star images. Actual measurement on the original negative gives a minimum diameter of two one-thousandths of an inch equivalent to only a second of arc at the focus. As the images are enlarged considerably by unsteadiness of the air and errors in guiding the star light reflected from the whole surface of the mirror is collected into a little disc less than a thousandth of an inch in diameter indicating the extraordinary accuracy of the reflecting surface. Mr. J. B. McDowell, head of the firm since Dr. Brashear’s death, and Mr. Fred Hegemann, his chief optician, are to be highly congratulated on the perfection of figure obtained under specially difficult circumstances. Further the fine rendering of the detail in the ring and the strength of the two bands in the interior indicate not only perfect figure but exceptionally high polish.
This mirror, to maintain its accuracy, not only requires careful mounting in its cell but also protection against temperature changes. Even though 12 inches thick it would bend under its own weight of 4,300 lbs. sufficiently to affect the figure and consequently it is supported in the cell by a specially counterweighted lever system so that it is equably supported at twelve points and there is no tendency to bend. A similar lever support system around the edge prevents distortion due to constraint when it is tipped from the horizontal position at different positions of the tube. Temperature changes can produce much greater distortion than flexure but Victoria has the advantage of very low diurnal range and the temperature change around the mirror is made very small by a lagging of cotton felt about 2 inches thick all round the sides of the closed section of the tube, laced on with a duck cover (compare Fig. 3 with Fig. 2) and an equal thickness below and around the edge of the mirror. By this lagging the temperature rise in the day-time is only about half a degree while the dome temperature increases five degrees, hence the figure of the mirror remains good whatever the temperature changes outside.
As already indicated, the principal mirror when used alone forms an image of the star 30 feet above, at the centre of the upper end of the tube, and an eyepiece could be placed there for visual work or a photographic plate for direct photographs of nebulae, etc. However, it is generally more convenient to use a flat mirror at 45° forming the image at the side of the tube for photographs and so the telescope is only used in this form with a small spectrograph for the ultra-violet region of star spectra. The course of the parallel beam of light from the star to its image on the slit of the spectrograph is graphically shown in Fig. 5 A and also its passage through the slit prisms and lenses of the spectrograph. The position of the star image on the slit of the spectrograph can be observed by a guiding telescope extendingto the edge of the tube and can be kept central by the portable aluminium switchboard already described. The elevating platform is of course used in work in this position.
For direct photography or visual observations at the focus of the 72-inch mirror, the reflected cone of star light from the mirror B, Fig. 5, is intercepted by a plane mirror also silvered on the front surface, 19·5 inches diameter and 3·25 inches thick placed at 45°. This form of reflecting telescope was first used by Newton, hence the name. The focus is then formed, as shown, at the side of the tube, and if a plate is placed there and accurately guided by small eyepieces with cross wires, photographs of any desired small region in the sky can be obtained, Fig. 4 being made in this position, or visual observations may be made. The oculars can easily be reached from the observing platform for any position of the telescope.
The most generally useful arrangement of the 72-inch telescope is, however, the Cassegrain form, so called from the French astronomer who first used it. About 7 feet below the focus, the conical reflected pencil from the 72-inch mirror is intercepted by a convex mirror of the same size as the Newtonian and of about 10 feet focal length as seen in C, Fig. 5, and also shown in Fig. 2 and 3. This mirror turns the light downward and, after passing through the central hole, forms the image of the star about two feet below the mirror surface on the slit of the spectrograph or on a visual attachment as shown. The significant property of this combination is that the focal length is increased from 30 to 108 feet without changing the tube length in a somewhat similar manner to the action of a telephoto lens. It has the same size of image and magnifying power as a refractor with a tube 108 feet long and has the decided advantage of a much shorter tube and smaller dome. Observations with this arrangement are made at the lower end of the tube from the observing floor andwith much greater ease and convenience than at the upper end. Changes from the Cassegrain to the Newtonian or Principal Focus arrangements are readily effected by a device due to the genius of Mr. Swasey whereby only the mirrors and attaching tubes require to be handled, instead of the whole upper end of the tube as in previous reflectors.
Fig. 5.—COURSE OF LIGHT IN TELESCOPE FROM STAR TO FOCUS
A full set of eyepieces giving magnifying powers from 120 to 5,000 diameters and a complete double-slide plate holder with guiding eyepieces for direct photography at the Newtonian focus are provided. In addition there is a visual attachment shown in diagram C, Fig. 5, which enables the telescope to be used visually at the Cassegrain focus without removing the spectrograph. There are three finders attached to the telescope tube for picking up and centering the stars. Two of 4 inch aperture and 5 feet focus, of power about 50, one north, one south on the tube, and a long focus tubeless finder with a lens of 7 inch aperture and 30 feet focus at the top of the telescope tube and an ocular of power 200 at the bottom.
Most of the astronomical work with the 72-inch telescope is spectroscopic, photographing the spectra of the stars, and so a description of the principles and operation of spectrographs is desirable. Stellar spectrographs have evolved into certain definite forms and the two spectrographs for the 72-inch telescope are examples of the most recent types. In essence a spectrograph consists of a narrow slit, one or two-thousandths of an inch wide, on which the star light is focussed. That passing through the slit falls on the collimator lens which makes it parallel and then on a prism or prisms, triangular shaped pieces of glass which change the direction of the light and decompose it, breaking it up into its constituent rainbow colours. The spectrum, as it is called, is focussed by a camera lens on a photographic plate or can be viewed by a small telescope if desired. The course of the star lightfrom the slit through collimator, prism and camera lens to the plate is shown in C, Fig. 5, while a view of the Cassegrain spectrograph showing the interior mechanism and accessories is given in Fig. 6. The part of the spectrum photographed is usually only the blue and violet region to which the ordinary plate is most sensitive, and obviously no colours appear on the negative but only a narrow dark strip which is crossed by light or dark lines. It is from the number and position of these lines that we obtain such a remarkable amount of information about the physical and chemical constitution, the temperature, motion and distance of the stars. The length of the star spectrum photographed with one prism is about one and a third inches, twice and three times that with two and three prisms. Its width is about one-hundredth of an inch and in order to make it this wide the star image has to be moved back and forward along the slit. The length of spectrum with the ultra-violet spectrograph, which only differs from the other in the prisms and lenses allowing the spectrum below the violet to pass, is about one inch. A photograph of the spectrum of iron or brass is made beside the star spectrum to serve as a standard to determine the positions of the star lines.
Fig. 6.—STELLAR SPECTROGRAPH ARRANGED FOR USE WITH ONE PRISM(TEMPERATURE CASE REMOVED)
Every terrestrial element gives groups of lines in certain positions in the spectrum and if we find similar groups in the star spectrum we are sure this element is present in the star. Further if we find these lines are displaced to red or violet of their normal position we know that the star is receding from or approaching to us. With the one-prism spectrograph a speed of one mile per second means a displacement of the lines of one thirty-thousandth of an inch. Thus if the lines are shifted to the violet by a thousandth of an inch, the star is approaching the earth with a speed of 30 miles a second. These displacements are accurately measured by a microscope and the measurement of the radial velocities of the stars is one of the main researches of this observatory. Obviously with such small displacements to be measured the greatestcare must be taken to avoid all sources of error. The spectrograph must be exceptionally rigid to avoid differential bending as it moves with the telescope. As change of temperature can produce spurious shifts of the lines, the temperature must be kept as constant as possible, this being effected here by a very accurate electrical thermostatic device called the Calendar Recorder which maintains the temperature constant to one-hundredth of a degree. The optical parts must be of the highest quality to give perfect definition to the spectrum lines and many other precautions must be taken if accurate work is desired. The spectrographs of the 72-inch telescope have unequalled defining power and are the last word in convenience of manipulation and accuracy of work.
Owing to the faintness of the star light and to its being spread out into a spectrum a considerable time is required to photograph the spectrum of a star, about 20 minutes for the sixth magnitude, the limit of visibility to the unaided eye, when photographed with one prism, while three prisms will take nearly five times as long. Hence the necessity and use of large telescopes is not to get high magnifying power but to collect sufficient light from the fainter stars to enable their spectra to be photographed or other observations made.