Astronomical instrument

“ The book will be distinguished from other volumes on popular astronomy by a somewhat novel system of mapping, and by an unusually full discussion of the uses of the simpler astronomical instruments. Here too, however, the treatment is definite and practical. Questions of optical theory and construction are subordinated to the pointing out, by the author, of the objects that can be seen, and of the satisfactions that may be obtained, first with the unaided eye (the fundamental optical instrument) , and then with the opera-glass, field-glass, and telescope. ”

“ I have fitted up several instruments of the above description with specula of 16, 27, 35, and 49 inches focal distance. One of these having a speculum of 27 inches focal length, and an astronomical eye-piece, producing a magnifying power of about 90 times, serves as a good astronomical telescope. By this instrument the belts and satellites of Jupiter, the ring of Saturn, and the mountains and cavities of the moon, may be contemplated with great ease and distinctness. With a magnifying power of 35 or 40 times, terrestrial objects appear remarkably bright and well-defined. ”

“ But the telescope and microscope enable us at present to measure celestial arcs much more accurately than was done by the older astronomers. In the best instruments, the measurements extend to a single second of space, or one thirty-six hundredth part of a degree,—a space, on a circle twelve feet in diameter, no greater than one fifty-seven hundredth part of an inch. To divide, or graduate, astronomical instruments, to such a degree of nicety, requires the highest efforts of mechanical skill. ”

“ The astrolabe of Ptolemy, the telescope of Galileo, the pendulum of Galileo and Huygens, the micrometer of Horrocks, and the telescopic sights and micrometer of Gascoyne and Picard, Roemer’s transit instrument, Newton’s and Hadley’s quadrant, Dollond’s achromatic lenses, Harrison’s chronometer, and Ramsden’s dividing engine—such were some of the principal additions to astronomical apparatus. ”

“ Also, in 1848, the ‘Meteoroscope,’ a kind of altitude-and-azimuth instrument in the form of a theodolite, designed for ascertaining the varying dimensions and positions of the zodiacal light, for measuring auroral arches, and determining rapidly the points of appearance and disappearance of shooting-stars (Report Brit. Assoc. 1848, pt. ii. p. 13) .
Challis published, 1832–64, twelve volumes (ix–xx.) of ‘Astronomical Observations made at the Observatory of Cambridge,’ each with an elaborate introduction, the first two containing descriptions of instruments and methods. ”

“ As the telescope is the most important of all astronomical instruments, it is necessary to understand its construction and adjustment and how the astronomer uses it. Telescopes are optical instruments, and nothing but optical parts would be requisite in making them, if only the optical conditions of their perfect working could be obtained without other mechanical accessories. ”

“ The real advantage of these enormous instruments lies in their great 'light-grasp,' which enables observers to see very faint points of light, such as small satellites of planets, faint stars, double stars, distant comets, or nebulæ, which could not be seen with a smaller instrument necessarily having less 'light grasp.' Yet this very excess of light, which is the great advantage of a large instrument, is one of the things that spoils the definition of faint planetary details ”

“ The completely equipped telescope, with its object-glass and mounting, aside from being a triumph of the highest optical and mechanical skill, is certainly the noblest instrument that man has yet constructed, and it is difficult to decide which is the most sublime and elevating to contemplate—the universe, which the telescope enables us to see, measure, weigh, and, combined with the spectroscope, to analyze ”

“ When the power of a telescope is required, the person must measure the clear aperture of the object-glass, then holding the pearl dynameter next the eye-glass, let him observe how many divisions the small circle of light occupies, when the instrument is directed to a bright object. ”

“ Every astronomer wanted a larger telescope than his neighbors, with which to measure double stars. If he could not get such an instrument, he measured the positions of the stars with a transit circle. Then came astrophysics, including photography, spectroscopy and photometry. The study of the motion of the stars along the line of sight, by means of photographs of their spectra, is now the favorite investigation at nearly all the great observatories of the world. ”

“ We cannot judge accurately of the excellence of any telescope by observing objects with which we are not familiarly acquainted. Opticians generally try an instrument at their own marks, such as the dial-plate of a watch, a finely engraved card, a weather-cock, or the moon and the planet Jupiter, when near 399 the meridian. ”

“ In this way I can easily point this instrument to Jupiter or Saturn, or to any of the other planets, visible to the naked eye, even when a power of 160 or 170 times is applied. When high magnifying powers, however, are used, it may be expedient to fix, on the upper part of the short tube in which the speculum rests, a 316 Finder, such as that which is used in Newtonian telescopes. When the moon is the object intended to be viewed, she may be instantly found by moving the instrument till her reflected image be seen from the eye-end of the telescope on the face of the mirror. ”

“ When a telescope is used out of doors a 'dew-cap,' that is, a tube of tin or pasteboard, some ten or twelve inches long, should be placed on the end of the instrument, so as to project beyond the object-glass. For glass is a good radiator of heat, so that dew falls heavily upon it, unless the radiation is in some way checked. ”

“ Superb meridian instruments adorn our great observatories, and are nightly devoted to those measurements upon which the great truths of astronomy are mainly based. These instruments have been constructed with refined skill; but it is the duty of the painstaking astronomer to distrust the accuracy of his instrument in every conceivable way. ”

“ In an ordinary telescope he is accustomed to find a tube with lenses of glass at either end, while the large telescopes that we see in our observatories are also in general constructed on the same principle. At one end there is the object-glass, and at the other end the eye-piece, and of course it is obvious that with an instrument of this construction it is to the lower end of the tube that the eye of the observer must be placed when the telescope is pointed to the skies. ”

“ It is well known that the most difficult test-objects on which a telescope can be directed are some of those double stars of which the components have a suitable distance. If the two stars be so close together that they subtend at our system an angle not more than a few tenths of a second, then the telescopic separation of the two components is a feat to tax the powers of the most perfect instrument, and the eye of the most accomplished observer. ”

“ I answer, that the achromatic telescope, when large and well constructed, is a more perfect and more durable instrument than the reflecting telescope. Much more of the light that falls on the mirror is absorbed than is lost in passing through the object-glass of a refractor ”

“ It is the effect produced by illuminating power that renders telescopes, furnished with comparatively small magnifying powers, much more efficient in observing comets and certain nebulæ and clusters of stars, than when high powers are attempted. Every telescope may be so adjusted, as to produce different space-penetrating 391 powers. If we wish to diminish such a power, we have only to contract the object-glass or speculum, by placing circular rims, or apertures of different degrees of breadth, across the mouth of the great tube of the instrument. ”

“ The telescope is directed to any one star, and then by turning the instrument round the axis, it is found, that without any alteration in the position of the telescope in relation to the axis, the telescope will follow the star from its rising to its setting. ”

“ The mounting of large telescopes demands the highest engineering ability. The whole instrument, with its vast weight of a twenty-six-inch glass lens, with its accompanying tube and appurtenances, must be pointed as nicely as a rifle, and held as steadily as the axis of the globe. To give it the required steadiness, the foundation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observatory is allowed to touch this support. When a star is once found, the earth swiftly rotates the telescope away from it, and it passes out of the field. ”

“ The second class of instruments consists of those which are designed principally to measure the angular distances of the heavenly bodies from each other, and their time of passing the meridian. The transit instrument, the meridian circle, the mural circle, the heliometer, and the sextant, belong to this class. The brilliant discoveries of astronomy are, for the most part, made with the first class of instruments ”

“ A rival method to that of the heliometer has been discovered in the photographic telescope. The plan of this instrument, and its application to such purposes as this, are extremely simple. We point a telescope at a star and set the clock-work going, so that the telescope shall remain pointed as exactly as possible in the direction of the star. We place a sensitized plate in the focus and leave it long enough to form an image both of the particular star in view and of all the stars around it. ”

“ We must not, however, assume that for the general work in an observatory a colossal instrument is the most suitable. The mighty reflector, or refractor, is chiefly of use where unusually faint objects are being examined. For work in which accurate measurements are made of objects not particularly difficult to see, telescopes of smaller dimensions are more suitable. The fundamental facts about the heavenly bodies have been chiefly learned from observations obtained with instruments of moderate optical power, specially furnished so as to enable precise measures of position to be secured. ”

“ It fortunately happens that astrophysical research is now at the very apex of its development, founded as it is upon many centuries of astronomical investigation, rejuvenated by the introduction into the observatory of all the modern devices of the physicist, and strengthened with instruments of truly extraordinary range and power. These instruments bring within reach experiments that are in progress on some minute region of the sun's disk, or in some star too distant even to be glimpsed with ordinary telescopes. ”

“ The whole story turns on optical rather than on astronomical considerations; but every astronomer of the least skill is acquainted with the principles on which the construction of optical instruments depends. ”

“ Now, passing over the consideration that a small telescope may afford its possessor much pleasure of an intellectual and elevated character, even if he is never able by its means to effect original discoveries, two arguments may be urged in favour of independent telescopic observation. In the first place, the student who wishes to appreciate the facts and theories of astronomy should familiarize himself with the nature of that instrument to which astronomers have been most largely indebted. ”

“ The instruments used in modern astronomy are amongst the finest triumphs of mechanical skill in the world. In a great modern observatory the different instruments are to be counted by the score, but there are two which stand out pre-eminent as the fundamental instruments of modern astronomy. These instruments are the telescope and the spectroscope, and without them astronomy, as we know it, could not exist. ”

“ And here it is important to remark that in all standard observations in Astronomy, the instrument is not turned to stars in any part of the heavens, but we have to wait until the stars come upon the meridian. We must so manage as not to be too late, or we lose our observation. The transit instrument must be adjusted in reference to our notion of what we want to observe. ”

“ Anything beyond 8 to 10 diameters is highly troublesome, and requires a rudimentary mount or at least steadying the hand against something in order to observe with comfort.
The longer the instrument the more difficult it is to manage, and the best results with hand telescopes are to be obtained with short instruments of relatively large diameter and low power. The ordinary field glass of Galilean type comes immediately to mind and in fact the field glass is and has been much used. ”

“ The telescope is adjusted so that a portion of the sun's limb, usually near a group of active sun-spots, shall be presented before the opened slit of the spectroscope. The light of the chromosphere thus admitted along with some diffused sun-light from the earth's atmosphere, produces a spectrum of intensely bright lines, widely separated, on the fainter background of the strongly dispersed spectrum of sun-light. ”

“ The great difficulty with all instruments of this general type is the formation of an artificial star the image of which shall very closely resemble the image of the real star in appearance and color.
Obviously either the real or artificial star, or both, may be varied in intensity by wedge or Nicols, and a very serviceable modification of the Zöllner instrument, with this in mind was recently described by Shook (Pop. Ast. 27, 595) and is shown in diagram in Fig. 151. ”