Refracting telescope

“ For photographic purposes this telescope has the very important advantage that a short telescope is equivalent to a very long one of the Newtonian form, or refracting telescope, so that the image of sun, moon, or planets formed at the focus is very large in comparison with the size of the telescope. A modification of this form of telescope, in which the small mirror is out of the path of the rays falling upon the larger one, and no longer obstructing the central part, has been revived by Dr. ”

“ Only a small part of the spectrum of a heavenly body, for instance, can be distinctly seen at one time; and a focal adjustment of half an inch is required in passing from the observation of a planetary nebula to that of its stellar nucleus. A refracting telescope loses, besides, one of its chief advantages over a reflector when its size is increased beyond a certain limit. ”

“ 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. ”

“ In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. These telescopes were of a construction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular, however, that we hear no more about the construction of reflectors till more than half a century afterwards. It was not till the year 1723, that any reflectors were known to have been made, adapted to celestial observations. ”

“ When photographs are to be taken the observing telescope is unscrewed and a photographic lens and camera put in its place. If the brightness of the object permits, three prisms are installed, turning the beam 180° into a camera braced to the same frame alongside the slit.
For purely photographic work, too, the objective prism used by Fraunhofer for the earliest observation of stellar spectra is in wide use. It is a prism fitted in front of the objective with its refracting faces making equal angles with the telescope and the region to be observed, respectively. ”

“ 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. ”

“ There would be no limit to the rigidity which could be given to such a telescope, as great ease of motion would not have to be considered, and we should abolish all complicated anti-friction apparatus for the declination axis, retaining it only for the polar axis to save wear in the teeth of the driving arc. ”

“ 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. ”

“ It suffers more than the refractor, as a rule, from scattered light, for imperfect polish of the film gives a field generally presenting a brighter background than the field of a good objective. After all the preference depends greatly on the use to which the telescope is to be put. For astrophysical work in general, Professor George E. Hale, than whom certainly no one is better qualified to judge, emphatically endorses the reflector. Most large observatories are now-a-days equipped with both refractors and reflectors. ”

“ Every increase above this magnifies all the optical faults of the telescope and the atmospheric difficulties as well, beside decreasing the diameter of the emergent pencil which enters the eye, and thereby causing serious loss of acuity. For the eye like any other optical instrument loses resolving power with decrease of effective aperture, and, besides, a very narrow beam entering it is subject to the interference of entoptic defects, such as floating motes and the like, to a serious extent. ”

“ Which is the most powerful telescope in existence? There are the great refractors of Poltava and of Cambridge, United States, each of fifteen inches in diameter and twenty-three feet in focal length. There is the still larger refractor of Chicago, with an aperture of eighteen inches and a focal length of twenty-three feet. All these instruments are of high excellence in defining power, the essential point where Lord Rosse's breaks down. There is the reflector of Mr. Russels, with its metal speculum of two feet in diameter and its tube twenty feet in length. ”

“ The effective discovery of the achromatic telescope was, accordingly, reserved for John Dollond, whose method of correcting at the same time chromatic and spherical aberration was laid before the Royal Society in 1758. Modern astronomy may be said to have been thereby rendered possible. Refractors have always been found better suited than reflectors to the ordinary work of observatories. They are, so to speak, of a more robust, as well as of a more plastic nature. They suffer less from vicissitudes of temperature and climate. ”

“ Reflectors are easily given a much greater relative aperture than is practicable in a single achromatic objective. In fact they are usually given apertures of F/5 to F/8 and now and then are pushed to or even below F/3. Such mirrors have been successfully used for photography; [9] and less frequently for visual observation, mounted in the Cassegranian form, which commonly increases the virtual focal length at least three or four times. A telescope so arranged, with an aperture of a foot or more as in [Pg 51] some recent examples, makes a very powerful and compact instrument. ”

“ 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. ”

“ This steadily decreasing ratio is probably due to an actual thinning out of the stars toward the boundaries of the stellar universe, as the most exhaustive tests have failed to give any evidence of absorption of light in its passage through space. But in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the 60-inch reflector. Here is one of the chief sources of interest in the possibilities of a 100-inch reflecting telescope. ”

“ Since time is limited, it is necessary to provide all operations with automatic arrangements as far as possible, by using such an apparatus as that described. What can be applied successfully to a forty-foot lens can serve for shorter telescopes. In combination with spectroscopes, polariscopes, and special instruments for photographing, an immense amount of work can be compressed into the few seconds allowed by mounting them all on such a movable frame as the triangle. The old-fashioned method of putting one observer to one telescope ought to be abandoned. ”

“ 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. ”

“ I trust it has been shown that expensive telescopes are not necessarily required for practical work. My advice to an intending purchaser would be to put into the objective for a refractor, or into the mirror for a reflector, all the money he feels warranted in spending, leaving the mounting to be done in the cheapest possible manner consistent with accuracy of adjustment, because it is in the objective or in the mirror that the value of the telescope alone resides. ”

“ To view Fraunhofer’s lines on the solar spectrum it is only necessary to turn the knife edges towards a white cloud, and make the slit formed by the knife edges very narrow by turning the screw at the side of them. In every instance the focus of the telescope must be adjusted in the ordinary way, by sliding the draw tube until it suits the observer’s sight, and distinct vision is obtained. ”

“ 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. ”

“ 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. ”

“ An instrument quite uncovered suffers from gusts far more than one housed under a dome, which is really the sum of the whole matter, save that a [Pg 243] dome to a slight extent does shelter the observer in extremely cold weather.
Even very large reflectors can be housed in similar fashion if suitably mounted. For example in Fig. 176 is shown the 36 inch aperture reflector of the late Dr. Common, which was fitted with an open fork equatorial mounting. Here the telescope itself, with its short pier and forked polar axis, is shown in dotted lines.
Fig. 175.—Sliding Housing for 11-inch Refractor. ”

“ The effect, for instance, of a portrait, made life-size, is very striking. Some astronomers have supposed that advantages would arise from taking original lunar negatives of larger size in the telescope, that is, from enlarging the image two or three times by a suitable eyepiece or concave achromatic, before it reached the sensitive plate. But apart from the fact that a reflector would then have all the disadvantages of an achromatic, the atmospheric difficulties, which in reality constitute the great obstacle to success, would not be diminished by such means. ”

“ The fact is, that most of the time over most of the world seeing conditions are bad, so that the telescope does not have a fair chance, and on the whole the bigger the telescope the worse the chance. One famous English astronomer, possessed of a fine refractor that would be reckoned large even now-a-days, averred that he had seen but one first class night in fifteen years past. ”

“ 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. ”

“ 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. ”