Newtonian telescope

“ So he was driven to the only alternative that remained; he must himself build a large telescope. His first step in this direction was to purchase the débris of an amateur’s implements for grinding and polishing small mirrors; and thus, by slow degrees, and by indomitable perseverance, he in 1774 had, as he says, the satisfaction of viewing the heavens with a Newtonian telescope of 6 ft. ”

“ The Newtonian telescope is represented in Fig. 8. A A is a concave mirror whose axis is a a. Parallel rays falling on A A converge on the plane mirror B B, and are thence reflected at right angles to the axis, forming an image in the focus of the eye-piece E. The surface of the large mirror should be a paraboloid of revolution, that of the small mirror a true optical Newtonian. plane. The magnifying power is=F/e. This form is employed in the construction of most modern reflecting telescopes. A glass prism of total reflection is sometimes substituted for the plane mirror. ”

“ In using this kind of telescope the observer is placed with his back to the object he wishes to examine, a position that is even more astonishing to those unaccustomed to the use of a Herschellian telescope than the one assumed when employing an instrument of the Newtonian construction. This position has the defect of causing a small portion of the rays proceeding from the object to be intercepted by the head of the observer, but the amount of light lost is so small in comparison to the size of the mirror that in practice it amounts to nothing. ”

“ Satisfied from the performance of his eighteen-inch Newtonian reflector that it would be possible to successfully construct much larger instruments of this kind, it seems to have been about two years ago that Mr. Common first seriously thought of constructing a very large reflecting telescope with a silver-on-glass speculum. It was obvious that this would be a serious undertaking, and one which would require much thought and ingenuity to carry it out successfully. Many difficulties would require to be boldly faced and successfully overcome. ”

“ In size, as also in character, the observer should be careful to discriminate as to what is really essential. Large instruments and high powers are not necessary to show what can be sufficiently well seen in a small telescope with moderate power. Of course there is nothing like experience in such matters, and practice soon renders one more or less proficient in applying the best available means.
Fig. 13.
3-inch Refracting-Telescope, by Newton & Co.
An amateur who really wants a competent instrument and has to consider cost, will do well to purchase a Newtonian reflector. ”

“ It is light chiefly, accompanied with a moderate magnifying power, that enables us to penetrate into the distant regions of space. Sir W. Herschel, when sweeping the profundities of the Milky way, and the Hand and Club of Orion, used a telescope of the Newtonian form, 20-feet focal length, and 187/10 inches diameter, with a power of only 157. On applying this telescope and power to a part of the Via Lactea, he found that it completely resolved the whole whitish appearance into stars, which his former telescopes had not light enough to effect ”

“ In the course of the first reflection, the immense quantity of rays that the aperture of the telescope had received, may be considered as reduced to half. Nor is the diminution less on the small mirror. Now, half of half is a quarter. Therefore the instrument will send to the eye of the observer only a quarter of the incident light that its aperture had received. These two causes of diminished light not existing in a refracting telescope, it would give, under parity of dimensions, four times more [19] light than a Newtonian or Gregorian telescope gives. ”

“ It had been supposed that the reason why success had not been attained in the construction of a refracting telescope was due to the fact that the object glass, made as it then was of a single piece, had not been properly shaped. Mathematicians had abundantly demonstrated that a single lens, if properly figured, must conduct all rays of light to the same focus, provided all rays experienced equal refraction in passing through the glass. Until Newton's discovery of the composition of white light, it had been taken for granted that the several rays in a white beam were equally refrangible. ”

“ Such were the earliest attempts to construct the reflecting telescope, that noble instrument which has since effected such splendid discoveries in astronomy. Looking back from the present advanced state of practical science, how great is the contrast between the loose specula of Gregory and the fine Gregorian telescopes of Hadley, Short, and Veitch,—between the humble six-inch tube of Newton and the gigantic instruments of Herschel and Ramage. ”

“ 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 search can be made with almost any small telescope, if one is careful to use a very low power. With a four-inch telescope a power not exceeding twenty should be employed. To search with ease, and in the best manner, the observer should have what among astronomers is familiarly known as a "broken-backed telescope." This instrument has the eye-piece on the end of the axis, where one would never think of looking for it. By turning the instrument on this axis, it sweeps from one horizon through the zenith and over to the other horizon without the observer having to move his head. ”

“ One position which very naturally occurs to us is the horizontal one. If we lay the big telescope on the ground, there is no danger of its falling: and it is in other respects convenient to work with. Hence several large telescopes have already been built in this way. A good example is the Snow telescope on Mount Wilson. You see in the picture the long shed which is really the telescope itself: and at one end there is the coelostat mirror, reflecting the light (in this case usually the light of the Sun, not that of a star) into the telescope. ”

“ So that 'tis a wonder, that Telescopes represent Objects so distinct as they do. But were all the Rays of Light equally refrangible, the Error arising only from the Sphericalness of the Figures of Glasses would be many hundred times less. For, if the Object-glass of a Telescope be Plano-convex, and the Plane side be turned towards the Object, and the Diameter of the [Pg 95] Sphere, whereof this Glass is a Segment, be called D, and the Semi-diameter of the Aperture of the Glass be called S, and the Sine of Incidence out of Glass into Air, be to the Sine of Refraction as I to R ”

“ A small telescope is almost as good for this purpose as a large one, because the atmospheric difficulties in the way of getting a good view of the planet cannot be lessened by an increase of telescopic power.
Uranus and Neptune are so distant that telescopes of considerable size and high magnifying power are necessary to show their disks. In small telescopes they have the appearance of stars, and the observer has no way of distinguishing them from the surrounding stars unless he can command the best astronomical appliances, such as star maps, circles on his instrument, etc. ”

“ Will it be used to increase our knowledge of astronomy, or will it be allowed to rest in idleness like so many other fine instruments? It is to be trusted, and it may be safely anticipated, that the former will be its fate. It will wear out, not rust out. There is much in astronomy which this grand telescope can do. It can be used for observing the faint and difficultly visible satellites of Mars, Saturn, Uranus, and Neptune. All these pressingly want observing and measuring, and there are few telescopes of sufficient power and excellence to do the work wanted. ”

“ When they are at their greatest angular distance apart as seen from the earth one of them will evidently be approaching at the same moment that the other is receding. The spectral lines of the first will therefore be shifted toward the violet, and those of the second will be shifted toward the red. Then if the stars, when at their greatest distance apart, are still so close that the telescope can not separate them, their light will be combined in the spectrum ”

“ Imagine an astronomer at the base of this huge metallic structure, having at his finger's ends a dozen wire strings intricately connected with the oval system, and by the proper use of which he can increase or decrease the magnifying power of the ponderous telescope. The highest magnifying power of a telescope of this size is so great that the Milky Way is penetrated and its solar systems revealed. What an accomplishment it would be if a telescope of this magnitude could be mounted, a thing that these creatures never attempted to do. ”

“ They appeared three times nearer, and nine times larger in surface, than to the naked eye. He soon after made another, which represented objects above sixty times larger; and, sparing neither labour nor expense, he finally constructed an instrument so excellent, as “to show things almost a thousand times larger, and above thirty times nearer to the naked eye.”
There is, perhaps, no invention that science has presented to man so extraordinary in its nature, and so boundless in its influence, as that of the telescope. ”

“ Both theory and experiment have demonstrated the extreme sensitiveness of the "interference fringes," on the observation of which the method depends, and it was generally supposed by astronomers that disturbances in the earth's atmosphere would prevent them from being clearly seen with large telescopes. Furthermore, a very large interferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double stars could have been easily studied by this device with several of the large telescopes of the early nineties. ”

“ His ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "space-penetrating power" by increasing their light-gathering surface. These, as he was the first to explain, [308] are in a constant proportion one to the other. For a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. ”

“ In a paper on the “Aspects of American Astronomy,” Prof. Newcomb says, “A great telescope is of no use without a man at the end of it, and what the telescope may do depends more upon this appendage than upon the instrument itself. The place which telescopes and observatories have taken in astronomical history are by no means proportional to their dimensions. Many a great instrument has been a mere toy in the hands of its owner. ”

“ 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. ”

“ It measures 32 inches in diameter, and has a focal length of a little more than 16 feet, and is put in motion by clockwork of a very perfect description, so that when once pointed at a star or planet it follows the object, which would otherwise disappear on account of the rotation of the earth. The path taken by the rays is precisely the same as in Newton’s telescope, the eye-piece being placed at the side of the tube, which is provided with a movable platform and staircase for the observer. ”

“ And consequently, the object-glass of any Telescope cannot collect all the rays, which come from one point of an object, so as to make them convene at its focus in less room then in a circular space, whose diameter is the 50th part of the Diameter of its Aperture; which is an irregularity, some hundreds of times greater, then a circularly figured Lens, of so small a section as the Object-glasses of long Telescopes are, would cause by the unfitness of its figure, were Light uniform. ”

“ We shall take the five-inch for this, and a steady atmosphere and sharp seeing will be necessary on account of the wide difference in the brightness of the component stars. Amateurs frequently fail to make due allowance for the effect of such difference. When the limit of separating power for a telescope of a particular aperture is set at 1" or 2", as the case may be, it is assumed that the stars composing the doubles on which the test is made shall be of nearly the same magnitude, or at least that they shall not differ by more than one or two magnitudes at the most. ”

“ The naked eye has its limit of vision in the stars of the sixth magnitude. The light of fainter stars than these does not affect the retina enough for them to be seen. A very small telescope penetrates to smaller, and, in general, without doubt, to more distant stars. A more powerful one penetrates deeper into space, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions. Whoever has followed the history of the series of Herschel's telescopes will have observed this. ”