Reflector (optics)

“ A refracting telescope is one of the ordinary pocket type, having an object-lens at one end and an eyepiece at the other. A reflector, on the other hand, has no object-lens, its place being taken by a mirror that gathers the rays entering the tube and reflects them back into the eyepiece, which is situated nearer the mouth end of the tube than the mirror itself.
Each system has its peculiar disadvantages. In reflectors the image is more or less distorted by “spherical aberration.” ”

“ There must, by its insertion, be some loss of light, which can ill be spared with small instruments, and when used to obtain magnifying power there is some disturbance of colour, and this subtracts from the beauty and purity of the definition of a reflector. There is nothing equal to a good eye-piece to obtain power, and flatness of field by the curve of the speculum. ”

“ A collimating lens, placed at its principal focal distance from the incandescent filament of the lamp, makes the illumination of the wires nearly independent of their position on the slide, and a piece of red glass, close to the lens, effectually removes all danger of fogging the plate. The light is varied to suit the requirements of observation by rotating the reflector which throws the light in the direction of the eyepiece. ”

“ There is one great point of advantage in the use of the reflector in physical work,—the absence of secondary spectrum; but it is by no means certain that stellar photography will not be more easy with refractors. ”

“ As many of the rays are thus lost or weakened by transmission through the glass of the reflector, the light is much less brilliant than when reflected from a metallic speculum of a uniform parabolic figure, of a more white and dense body, such as silver. Another objection to mirror-glass reflectors, is the great number of interstices or subdivisions between the pieces of glass, which unavoidably induces a want of cleanliness and uniformity in the reflecting surface as a whole. ”

“ It should be observed that at the surface of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body. The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface. It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture. ”

“ It is therefore necessary, as we shall see presently, to take particular precautions in working with a reflecting telescope, which is, so to speak, materially more tender as regards external conditions than the refractor. As regards light-grasp, the power of rendering faint objects visible, there is more room for honest variety of opinion. It was often assumed in earlier days that a reflector was not much brighter than a refractor of half the aperture, i.e., of one quarter the working area. ”

“ Now there is one enormous advantage in favour of the reflector. We do not take our light to bits and put it together again as with an achromatic lens. But curiously enough, there is a something else which quite lowers the position of the reflector with regard to the refractor. Although, in the main all the light falling in parallel lines on a concave surface is reflected to a focus, this is only true in a general sense, because, if we consider it, we find an error which increases very rapidly 105as the diameter of the mirror increases or as the focal length diminishes. For instance, D I, Fig. ”

“ Isaac Roberts before the International Astrophotographic Congress: “The reflector requires the exercise of great care and patience, and a thorough personal interest on the part of the observer using it. In the hands of such a person it yields excellent results, but in other hands it might be a bad instrument. The reflector gives results at least equal, if not superior, to those obtained with the refractor, if the observer be careful of the centring, and of the polish of the mirror, and keeps the instrument in the highest state of efficiency ”

“ The perfect achromatism of a reflector is, of course, a great advantage, but the chromatic aberration of refractors is now so well corrected that their inferiority in that respect may be disregarded. It must be admitted that reflectors are cheaper and easier to make, but, on the other hand, they require more care, and their mirrors frequently need resilvering, while an object glass with reasonable care never gets seriously out of order, and will last for many a lifetime. ”

“ On the other hand the reflector, on account of its perfect achromatism, is the instrument par excellence for photographic observations, and, as more than three-fourths of modern astronomical work is photographic, it appears to be superseding the refractor. ”

“ If, like many really enthusiastic amateurs, he can get only an occasional evening for observing, and from circumstances has to use a portable mount set up on his lawn, or even roof, when fortune favors an evening’s work, he will find a refractor always in condition, easy to set up, and requiring a minimum of time to get into action. The reflector is much the more tender instrument, with, however, the invaluable quality of precise achromatism, to compensate for the extra care it requires for its best performance. ”

“ If the observer wishes to have as much power as possible in proportion to his expenditure, and is not afraid to take the risk of a small amount of trouble with the adjustments and films, the reflector is probably the instrument best suited to him. If, on the other hand, he is so situated that his telescope has to be much moved, or, which is almost as bad, has to stand unused for any considerable intervals of time, he will be well advised to prefer a refractor. ”

“ Rarely, the mirror is replaced by a totally reflecting right angled prism which saves some light, but unless for small instruments is rather heavy and hard to obtain of the requisite quality and precision of figure. A typical modern reflector by Brashear, of 6 inches aperture, is shown in Fig. 35, complete with circles and driving clock, the latter contained in the hollow iron pier, an arrangement usual in American-made instruments. ”

“ One thing should be particularly remembered, that the power used must not be beyond the illuminating capacity of the instrument, for planetary features appear so faint and shady under excessive magnifiers that nothing is gained. To grasp details49 there must be a fair amount of light. I have seen more with 252 on my 10-inch reflector than with 350 on a 5-1/4-inch refractor, because of the advantage from the brighter image in the former case. ”

“ The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6 feet focus, will be found very suitable, and if its illuminating power is not greater than a 4-1/2 inch refractor, its defining and dividing powers are superior. By choosing the above focal lengths, according to circumstances, the observer can reach the eye-tube, while standing erect for objects in the zenith, and it is a great mistake to suppose that the shorter the tube the more handy it becomes. ”

“ In order that objects near the zenith may be observed with ease, a diagonal reflector is sometimes used, so that the eye looks sidewise into the telescope tube instead of directly upwards. This reflector may take the form of two short pieces of tube joined together at right angles, and having a piece of silvered glass or a right-angled prism at the angle, so that when one tube is screwed into the telescope, the rays of light falling on the reflector are sent up the other, in which the ordinary eyepiece is placed. ”

“ The new apparatus—a bewildering arrangement of massive glass prisms—is in striking contrast with its predecessor, the old reflector system of lighting, a system, by the way, now almost obsolete. The following description of the new light is copied from an engraved plate affixed to the new apparatus:—“Combined hyper-radiant and 1st order apparatus, with equiangular dioptric elements and catadioptric back prisms; power of red flash and white flash equalised. White and red flashing light, showing white and red flashes alternately every half minute, the period being one minute. ”

“ If a single-prism reflector is to be made perfect, we have only to make it achromatic, which could be done extempore, by correcting the colour of the fluid prism by another fluid prism of different refractive and dispersive power. ”

“ In the intermittent distinction the light is suddenly obscured by the closing of metallic shades which surround the reflector frame, and on their opening, it is as suddenly revealed to sight, in a manner which completely distinguishes it from the ordinary revolving light, which from darkness, gradually increases in power till it reaches its brightest phase, and then gradually declines until it is again obscured ”

“ It is in fact out of proportion to the degree of refinement attained in other adjustments of the reflector, for a slight pressure of the hand on the draw-tube, or movement of the telescope to a different altitude, instantly destroys the perfection of the adjustment. ”

“ In viewing stars of great altitude, the use of the refractor is extremely inconvenient to the observer, whose position is necessarily very uncomfortable, while with the Newtonian reflector any part of the heavens may be observed with the same ease and comfort. In short, a good speculum, well mounted and situated, is sure to be both pleasing and satisfactory. ”

“ The Reflector.—In a reflecting telescope, the object-glass of the refractor is replaced by a concave mirror. In order that such a mirror may reflect all the rays from a star to a single point, its concave surface must be part of a paraboloid of revolution, that is, a surface produced by the revolution of a parabola on its axis. If a spherical surface be employed, all the rays will not be reflected to a single point and the images which it gives will be ill-defined. ”

“ A mirror is more likely to get out of order than is a lens, however, and any irregularity in the shape of a mirror produces a greater distorting effect than in a lens. A refractor is also more convenient to handle than is a reflector. For these reasons great refractors are still made, but the largest of them, the great Yerkes' refractor, is much smaller than the greatest reflector, the one on Mount Wilson, California. The lens of the Yerkes' refractor measures three feet four inches in diameter, whereas the Mount Wilson reflector has a diameter of no less than eight feet four inches. ”

“ Another man then spoke up, remarking, "My experience does not agree with yours, gentlemen, for when I used a 6-inch refractor I could see some of the lines, yet felt doubtful of their actuality; but since I have used a 12-inch reflector my opinion has entirely changed. The lines are visible whenever the atmospheric conditions are favourable, and are seen with so much certainty that I have long abandoned my doubts of their representing real markings!" "Hear, hear!" said several, "and in a clearer atmosphere you would see still more!" ”

“ The wheel at the foot adds very much to its portability. Tho form of its stand is the best I have seen, giving a complete sweep without any fresh adjustment. The cell bearing the reflector drops into its place without any difficulty, even in the dark. The trials I have made of the instrument have revealed a clearness and sharpness of definition far superior to anything of the kind I had seen before. ”

“ In America the reflector has always kept at least even pace with the refractor. As early as 1830, Mason and Smith, two students at Yale College, enthused by Denison Olmsted, built a 12-inch speculum with which they made unsurpassed observations of the nebulæ. Dr. Henry Draper, returning from a visit to Lord Rosse, began about 1865 the construction of two silver-on-glass reflectors, one of 15 inches diameter, the other of 28 inches, with which he did important work for many years in photography and spectroscopy, and his mirrors are now the property of Harvard College Observatory. ”

“ It will surprise many to learn that even Lord Rosse's great reflector is surpassed in certain respects by some of the exquisite refractors now constructed by opticians. As a light-gatherer the mirror is, of course, unapproachable by refractors. Even if it were possible to construct an achromatic lens six feet in diameter, yet, to prevent flexure, a thickness would have to be given to the glass which would render it almost impervious to light, and therefore utterly useless. But refractors have a power of definition not possessed by large reflectors. ”

“ The adjustments of a reflector are soon familiar and easy, and there is this to be said, the adjustments can be manipulated upon to any amount without the slightest injury to the instrument; there is no danger of any sort, and the instrument can be perfected in its adjustments by daylight, and this makes it both pleasant and convenient to leisurely work at, for experience sake. ”

“ A good 3-inch refractor or 4-inch reflector will reveal an astonishing diversity of structure in the spots, and show something of the complicated minutiæ of the general surface. If the aperture of either instrument is 2 inches more than that stated, so much the better; but further than this it is rarely advisable to go. When the objective or mirror exceeds a diameter of 5 or 6 inches a stop often improves the images, and even smaller instruments will perform better when a little contracted. Definition is here the point to be desired; of light we have a superabundance. ”