Reflecting telescope

“ 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. ”

“ Upon two-thirds of the clear nights of a year the finest telescopes cannot be used to their full advantage, because the minute flutterings resulting from the passage of the rays of light through moving strata of air of different densities are magnified just as the image in the telescope is magnified, till all minute details are blurred and confused, and only the grosser features are left visible. And supposing the telescope and atmosphere in good state, there is still an important point, the state of the observer’s eye, to be considered. ”

“ But in an instant the disturbing undulations pass, the air steadies, the image shrinks and sharpens, and two points of piercing brightness, almost touching one another, dart into sight, the more brilliant one being surrounded by an evanescent circle, a tiny ripple of light, which, as it runs round the star and then recedes, alternately embraces and releases the smaller companion. The wash of the light-waves in the atmosphere provokes many expressions of impatience from the astronomer, but it is often a beautiful phenomenon nevertheless. ”

“ 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. ”

“ 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. ”

“ One of the most serious difficulties in the way of further progress in the investigation of the telescopic characteristics of the planets and of the constitution of star systems, is what is called bad "seeing." It must be remembered that light, in order to reach a telescope, must pass through from forty to one hundred miles of atmosphere. ”

“ 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. ”

“ Yet when we come to consider what it really is which is described as being seen, when we calmly examine the various drawings which have been made by the aid of one or the other of these great telescopes, then we find that they show nothing which can not be distinctly seen and drawn by the smallest astronomical telescope of high excellence. An enormous blaze of light is gathered by the telescopes, but all this light reveals nothing which can not be seen with far greater ease in a far smaller telescope. ”

“ 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. ”

“ 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. ”

“ You may know something of the witchery of the heavens even with a little telescope of three to five inches aperture!" "Pleasures of the Telescope" is popular in style rather than technical. For setting forth "the chief attractions of the starry heavens," a complete set of star-maps is included, showing "all the stars visible to the naked eye in the regions of sky represented, and in addition some stars that can only be seen with optical aid." ”

“ 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 ”

“ If you ask me how many can be seen with the help of the telescope, I cannot tell you, because more powerful glasses are constantly being made, only to discover worlds beyond worlds, ever new and more distant, strewn in space like golden dust, while stars hitherto invisible through the most powerful telescope can now be made to leave the impress of their rays upon the photographic plate—so that a great astronomer of our time can show us pictures of "invisible stars." ”

“ 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. ”

“ It is true that we can never see them, but it is impossible to believe that space is utterly void and empty where it lies beyond the view of our telescopes. Are we to say that inside this circle stars, worlds, nebulæ, and clusters are crowded, and that outside there is nothing? Everything teaches us that this is not so. We occasionally gain accession to our power by adding perhaps an inch to the diameter of our object-glass, or by erecting a telescope in an improved situation on a lofty mountain peak, or by procuring a photographic plate of increased sensibility. ”

“ Generally they become visible in the form of a bright point of light which traverses in a straight line a longer or shorter path across the heavens, and then vanishes, sometimes leaving behind it for a second or two a faintly luminous train. The shooting-stars are of all degrees of brightness, from the extremely faint streaks which sometimes flash across the field of the telescope, up to brilliant objects, brighter than any of the planets or fixed stars, and sometimes lighting up the whole landscape with a light like that of the full moon. ”

“ To create a sharply defined image the telescope must not only bring light where there ought to be light, but it must also bring darkness where there ought to be darkness. The latter task is the more difficult. Light-waves tend to spread in all directions, and the telescope cannot prevent individual wavelets from straying on to parts of the picture where they have no business. ”

“ This is managed, as in the sextant, by means of the screw, which raises or lowers the telescope. When the telescope is brought nearer to the plane of the instrument, the reflected horizon becomes dark and distinct, but when screwed off it becomes fainter, and is not so well defined. Practice alone can teach the degree of intensity which is most favourable. In general it is best to have one horizon dark, and the other light ”

“ It is well known that when a telescope is used to observe a star, it is not generally pointed exactly in the direction of the star, because the light which it is sought to observe has been bent in, its passage through the atmosphere surrounding the earth, so that the direction in which the light enters the telescope is somewhat different from the direction which the light had when it first encountered the atmosphere at an altitude of some hundreds of miles. ”

“ 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. ”

“ It is more convenient for viewing celestial objects at a high altitude, than other telescopes. When we look through a Gregorian reflector or an achromatic telescope of 4 or 5 feet in length, to an object elevated 50 or 60 degrees above the horizon, the body requires to be placed in an uneasy and distorted position, and the eye is somewhat strained, while the observation is continued. ”

“ 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. ”

“ If a particular star is just visible to the naked eye, it will be quite bright if viewed with this special telescope, which makes it one hundred times more brilliant in appearance. If we could move the star bodily away from us to a distance ten times its present distance, we could thus reduce its brightness, as seen with the telescope, to what it was at first, as seen with the eye alone, i.e., to bare visibility. ”

“ 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. ”

“ 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. ”

“ By making use of moderate magnifying powers, what we see will often convey the impression of a multitude of white points on a black network. This is very apparent during the first few moments that the telescope is brought to bear on the Sun, but its clearness quickly passes away because the eye gets fatigued, and the lenses becoming warm the air in the telescope tube gets disturbed because also warmed. ”

“ The astronomer will not be contented with a mere naked-eye inspection of a world so interesting as the moon. He will get a telescope to help his vision. The word “telescope” means a contrivance for looking at objects which are a long way off. We have explained that the further an object is, the smaller it appears to be. The telescope enables us to largely overcome this inconvenience. It has the effect of making a distant object look larger. ”

“ Bad striæ are of course seen easily, as they would be in a window pane, but such gross imperfections are often in reality less damaging than the apparently slighter ones which must be searched for. The simplest test is to focus a good telescope on an artificial star, remove the eyepiece and bring the eye into its place.
When the eye is in focus the whole aperture of the objective is uniformly filled with light, and if the disc to be tested be placed in front of it, any inequality in refraction will announce itself by an inequality of illumination. ”

“ 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. ”