Transit telescope

“ This instrument is used to measure the declination of stars, or angular Page 64 distance north or south of the equator. Thus a star's place in two directions is exactly fixed. When the telescope is mounted on two pillars instead of the face of a wall, it is called a transit instrument. This is used to determine the time of transit of a star over the meridian, and if the transit instrument is provided with a graduated circle it can also be used for the same purposes as the mural circle. Man's capacity to measure exactly is indicated in his ascertainment of the length of waves of light. ”

“ Similarly, if the pivots of the telescope are precisely on the same level, the centre wire of the right ascension series would coincide with its reflected image. A third point is determined by looking through the eye-piece of the north collimator telescope—that is to say, the telescope mounted in a horizontal position at the north end of the room—at the spider lines in the focus of the south collimator. In order to get this view, the transit telescope has either to be lifted up out of its usual position, or else the middle of the tube has to be opened. ”

“ The field was illuminated by means of a polished ring placed outside of the object-glass. The simple system of cross wires, then, though it has done its work, is not to be found in the telescope now, either to mark the axis of collimation, or roughly to measure small distances. For the first purpose a much more elaborate system than that introduced by Römer is used. We have a large number of vertical wires, the principal object of which is, in such telescopes as the transit, to determine the absolute time of the passage of either a star or planet, or the sun or moon, over the meridian ”

“ When the time of transit actually comes, the chief will be at the equatorial, and will observe the first contact, and record the time on his chronograph, and at once commence measures of the distance of cusps. The assistant astronomer will see that the heliostat which is to throw the image of the sun into the stationary photographic telescope does this properly; and within the dark room the two photographers must be taking negatives as rapidly as possible. ”

“ 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 best of all stands is a solid iron pillar firmly fastened into a brick or stone pier, sunk at least four feet in the ground, and surmounted by a well-made equatorial bearing whose polar axis has been carefully placed in the meridian. It can be readily protected from the weather by means of a wooden hood or a rubber sheet, while the tube of the telescope may be kept indoors, being carried out and placed on its bearing only when observations are to be made. With such a mounting you can laugh at the observatories with their cumbersome domes, for the best of all observatories is the open air. ”

“ In larger observatories a more elaborate arrangement is adopted, the rails, on which the seat moves, are curved to form part of a circle, having the centre of motion of the telescope for its centre; as the seat with its back is moved up or down on the curved slides, its inclination is changed, so that the observer is always in a favourable position for observing. The seat on its frame runs on circular rails round the pier of the telescope, so that the eyepiece can be followed round as the telescope moves in following a star. ”

“ 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. ”

“ The straight line connecting the extreme points of the first station intersects that connecting the two points of the second station, nearly at right angles, and at little distance from the Observatory.—The question of dependence of the measurable amount of sidereal aberration upon the thickness of glass or other transparent material in the telescope (a question which involves, theoretically, one of the most delicate points in the Undulatory Theory of Light) has lately been agitated on the Continent with much earnestness. ”

“ But the fact is when one is dealing with a fine instrument like Fig. 168 of as much as 5 inches aperture, the question of a permanent housing (call it observatory if you like) at once comes up and will not down.
It is of course always more convenient to have the telescope permanently in place and ready for action. Some observers feel that working conditions are better with the telescope in the open, but most prefer a shelter from the wind, even if but partial, and the protection of a covering, however slight, in severe weather. ”

“ Across the eye-piece of the telescope are stretched seven lines of spider-web, dividing the field of view. If his seat requires change, the least motion arranges it to his satisfaction, for it rests upon a railway of its own. ”

“ The present period of coupled eccentric transits will, in the course of ages, be succeeded by a period of single, nearly central transits. The alignments by which transits are produced, of the earth, Venus, and the sun, close to the place of intersection of the two planetary orbits, now occur, the first a little in front of, the second, after eight years less two and a half days, a little behind the node. But when the first of these two meetings takes place very near the node, giving a nearly central transit, the second falls too far from it, and the planet escapes projection on the sun. ”

“ The 100-inch mirror rests at the bottom of the telescope tube on a special support system, so designed as to prevent any bending of the glass under its own weight. Electric motors, forty in number, are provided to move the telescope rapidly or slowly in right ascension (east or west) and in declination (north or south) , for focussing the mirrors, and for many other purposes. They are also used for rotating the dome, 100 feet in diameter, under which the telescope is mounted, and for opening the shutter, 20 feet wide, through which the observations are made. ”

“ Thus, if the east and west parts of the horizon be observed, and that the index glass be uppermost, and telescope pointing to the west, the observer is on the south side, and his face must be turned to the north. When the instrument is inverted, if the observer turn himself round at the same time, so as to face the south, then the telescope will be pointed as before to the west; but since the index glass is now undermost, the inferior arc will now be measured precisely as if his face were to the north, but with the advantage of the same lights seen in the erect position of the instrument. ”

“ 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. ”

“ The driving circle is also much too small, so that a very slight mechanical freedom of the screw in the teeth involves a large angular freedom of the telescope in right ascension, whilst its position at the lower end of a too weak polar axis tends to create instability in right ascension from torsion of that axis. ”

“ These giant telescopes, however, require an enormous object-glass or mirror, for the points of light are so spread out in making the large image that it is very faint unless an enormous number of rays are collected. Lord Rosse's telescope has a reflecting mirror measuring six feet across, and a man can walk upright in the telescope tube. The most powerful telescope yet made is that at the Lick Observatory, on Mount Hamilton, in California. It is fifty-six and a half feet long, the object-lens measures thirty-six inches across. ”

“ Fig. 97.—Hartness Turret Observatory from the N. E.
Circles arc a necessity in all but the small telescopes used on portable tripods, otherwise much time will be wasted in finding. In any event do not skimp on the finder, which should be of ample aperture and wide field, say 1⁄4 the aperture of the main [Pg 133] objective, and 3° to 5° in field. Superior definition is needless, light, and sky room enough to locate objects quickly being the fundamental requisites. ”

“ A new feature in this instrument is the platform at the lower end of the polar axis, where an assistant can view the hour circle by one eye-piece and the declination circle by another (looking up the perforated polar axis) , and where he can also set the telescope to any hour angle by one wheel, or to any declination by a second, with the greatest ease. The observer at the eye end can also read off the hour and declination circles and communicate quick or slow motions to the telescope both in right ascension and declination by conveniently placed handles. ”

“ The object or the part of an object to be observed should be brought as nearly as possible to the centre of the field of view. When there is no apparatus for keeping the telescope pointed upon an object, the best plan is so to direct the telescope by means of the finder, that the object shall be just out of the field of view, and be brought (by the earth's motion) across the centre of the field. Thus the vibrations which always follow the adjustment of the tube will have subsided before the object appears. ”

“ If the observer's dot comes between two clock dots, it is easy, by measuring its distance from them with a divided scale, to tell the instant the star was on the wire to the tenth of a second, or even to a smaller fraction. Whilst, since the transit was taken over ten wires, and the distance of each wire from the centre of the field of view is known, we have practically ten separate observations, and the average of these will give a much better determination of the time of transit than a single one would. ”

“ 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. ”

“ 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. ”

“ The telescope is driven by a most efficient clock, whose motive power is a heavy weight. The rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. A seconds pendulum is mounted in a glass case on the wall of the Observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. ”

“ 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 next stage of evolution is the telescope house with a sliding roof in one or more sections—ordinarily two. In this case the building itself is a simple square structure large enough to accommodate the instrument with maneuvering room around it. The side walls are carried merely high enough to give clearance to the tube when turned nearly flat and to give head room to the observer. ”