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.
The Spectrographs
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 light from 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)
Method of Use
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 greatest care 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.