Astronomical telescope

“ It furnishes a compact and powerful instrument, very suitable for terrestrial or minor astronomical uses, like the Tolles’ short-focus hand telescopes already mentioned. When properly designed telescopes of this sort give nearly the field of prism glasses, weigh much less and lose far less light for the same effective power and aperture. They also have under fairly high powers rather the advantage in the matter of definition, other things being equal. ”

“ 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 vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective solely: its light-gathering power. Light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. In the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. ”

“ The only instrument which, as I think, may possibly be called for by the demands of the astronomer or the astronomical public, is a telescope of the largest size, for the observation of faint nebulae and minute double stars. Whether the addition of such an instrument to our apparatus would be an advantage, is, in my opinion, not free from doubt. The line of conduct for the Observatory is sufficiently well traced; there can be no doubt that our primary objects ought to be the accurate determination of places of the fundamental Stars, the Sun, the Planets, and, above all, the Moon. ”

“ Such is its power, that if a star of the first magnitude were removed to such a distance, that its light would be three millions of years in reaching us, this telescope would, nevertheless, show it to the human eye. Is it to be wondered at, then, that, with such an instrument, grand discoveries should be made? It has been pointed to the heavens; and, although in the beginning of its career, it has already accomplished mighty things. There are nebulous spots in the heavens which have baffled all the instruments hitherto constructed, but this telescope resolves their true character completely. ”

“ 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 distinctness with which the surface of the moon is seen does not depend so much on the size of the telescope employed, as it does on the steadiness of the atmosphere when the telescope is being used. When one wishes to examine a very distant body like a star, it is necessary to use a powerful telescope, but in the case of a comparatively near body, like one of the planets or the moon, a big telescope is not necessary. ”

“ 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. ”

“ It appears to have been almost overlooked by opticians and others, that telescopes may be constructed so as to exhibit a beautiful and minute view of very near objects, and to produce even a microscopic effect, without the least alteration in the arrangement of the lenses of which they are composed. This object is effected simply by making the eye-tube of a telescope of such a length as to be capable of being drawn out 12 or 13 inches beyond the point of distinct vision for distant objects. ”

“ To form any adequate idea of the discomfort and even danger attending the nights spent in observing, it is necessary to realise that the great telescopes used were erected in the open air, that for both the Newtonian and Herschelian forms of reflectors the observer has to be near the upper end of the telescope, and therefore at a considerable height above the ground. ”

“ The big telescopes are unquestionably as good optically speaking as the small ones but under the ordinary working conditions, even as good as those a double star observer seeks, the smaller aperture by reason of less disturbance from atmospheric factors does relatively much the better work, however good the big instrument may be under exceptional conditions. ”

“ A small telescope will perform very well on a night when a large one is helpless.
Turn the glass upon β (Rigel) , the white first-magnitude star in Orion's left foot. Observe whether the image with a high power is clear, sharp, and free from irregular wisps of stray light. Look at the rings in and out of focus, and if you are satisfied with the performance, try for the companion. A good three-inch is certain to show it, except in a bad state of the atmosphere, and even then an expert can see it, at least by glimpses. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Since in the examination of objects so minute and so distant as these satellites, it is necessary not only to magnify for the purpose of removing them from the body of the planet itself, but also to collect as much of their light as may be, in order to render the impression they make on the eye sensible, no instrument less than a 20-feet telescope is sufficient for discerning them ”

“ These objects are distinctly seen with a 5 feet achromatic of 38/10 inches aperture, and an illuminating power of 144, while they are scarcely visible in a 31⁄2 feet with an aperture of 23⁄4 inches, and an illuminating power of 72, supposing the same magnifying power to be applied. The illuminating power of a telescope is best estimated, in regard to land objects, when it is tried on minute objects, and such as are badly lighted up ”

“ There is a great charm in watching this pair of stars even with a three-inch telescope—not so much on account of what is seen, although they are very beautiful, as on account of what we know they are doing. It is no slight thing to behold two distant stars obeying the law that makes a stone fall to the ground and compels the earth to swing round the sun. ”

“ 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. ”

“ Our star map No. 1 will serve as a guide to the objects which we are about to inspect. Let us begin operations with our smallest telescope, the three-inch. I may remark here that, just as the lowest magnifying power that will clearly reveal the object looked for gives ordinarily better results than a higher power, so the smallest telescope that is competent to show what one wishes to see is likely to yield more satisfaction, as far as that particular object is concerned, than a larger glass. The larger the object glass and the higher the power, the greater are the atmospheric difficulties. ”

“ Spectroscopic and photographic apparatus are simply additions to the telescope. They do not supersede or render it of less importance. On the contrary, the efficacy of their action depends primarily upon the optical qualities of the instruments they are attached to. Hence the development, to their fullest extent, of the powers of the telescope is of vital moment to the progress of modern physical astronomy, while the older mathematical astronomy could afford to remain comparatively indifferent to it. ”

“ 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 ”

“ There is yet another field in which this new telescope may reap great advantages for astronomy. It is suspected that more than one of the stars, those distant suns, may be attended by opaque, dull planets. Mathematical analysis has already pointed to the existence of these attendants. It remains for the telescope to discover them. If the new Ealing reflector be really of the very highest excellence, it will be with that instrument we ought to look for these attending planets, these members of a foreign solar system. ”

“ But it is a vulgar error to imagine that very large and very expensive telescopes are absolutely necessary for viewing the greater part of the more interesting scenery of the heavens. Most of the phenomena of the planets, comets and double stars and other objects, are visible with instruments of moderate dimensions, so that every one who has a relish for celestial investigations, may, at a comparatively small expense, procure a telescope, for occasional observations, which will show the principal objects and phenomena described in books on astronomy. ”

“ With a well mounted telescope of either sort a star at fair altitude on a fine night gives even better testing conditions than an artificial star, (Polaris is good in northern latitudes) but one may have a long wait.
If the reflector is of good figure and well adjusted, the star image, in focus or out, has quite the same appearance as in a refractor except that with a bright star in focus one sees a thin sharp cross of light centered on the image, rather faint but perfectly distinct. ”

“ It is not too much to say that this discovery has revolutionized the science of astronomy, extending the field of human observation into the realm of the infinite. By the aid of clockwork attachments, the telescope is made to follow the apparent motion of the star to which it may be directed, throughout the night, if desired, and the sensitive photographic plate is exposed to the action of light during a corresponding period. ”

“ In consequence of the thinness of the rings they completely disappear from the range of vision of small telescopes when, as occurs once in every fifteen years, they are seen exactly edgewise from the earth. In a telescope powerful enough to reveal them when in that situation they resemble a thin, glowing needle run through the ball of the planet. ”

“ But as the image formed by the speculum in the aerial telescope is viewed directly, without being exposed to any subsequent reflection, it is not so liable to the tremors which are so frequently experienced in other reflectors. Notwithstanding the length of the arm of the 4 feet telescope above mentioned, a celestial object appears remarkably steady, when passing across the field of view, especially when it is at a moderate degree of altitude; and it is easily kept in the field by a gentle motion applied to the arm of the instrument. ”