Utility of Stops.—There are a good many details connected with observation which, though advice may be tendered in a general way, are best left to the discrimination of observers, who will very soon discover their influences by practical trial and treat them accordingly. The employment of stops or diaphragms to contract the aperture of telescopes is a question on which a diversity of opinion has been expressed. It is often found, on nights of indifferent seeing, that the whole aperture, especially of a faulty instrument, gives bad images, and that, by reducing it, definition becomes immensely improved. But Mr. Burnham, the double star observer, records his opinion that a good glass needs no contraction, and that the whole aperture shows more than a part unless there is defective figuring at the outer zone of the lens, which will be cut off by the stop and its performance thereby greatly improved. He seems to think that a glass requiring contraction is essentially defective, but this is totally opposed to the conclusions of other observers. It is almost universally admitted that, on bad nights, the advantages of a large aperture are neutralized by unsteady definition, and that, by reducing the diameter, the character of the images is enhanced. As regards instruments of moderate calibre the necessity is less urgent. With my 10-inch reflector I rarely, if ever, employ stops, for by reducing the aperture to 8 inches the gain in definition does not sufficiently repay for the serious loss of light. But in the case of large telescopes the conservation of light is not so important, and a 14-inch or 16-inch stop may be frequently employed on an 18-inch glass with striking advantage. The theory that only defective lenses improve with contraction is fallacious, for in certain cases where stops are regularly employed it is found that, under circumstances of really good seeing, the whole aperture gives images which are as nearly perfect as possible. It is clear from this that the fault lies with the atmosphere, and that under bad conditions it becomes imperative to limit its interference consistently with the retention of sufficient light to distinguish the object well. In large reflectors, particularly, the undulations of the air are very active in destroying definition, and the fact will be patent enough to anyone who compares the images given in widely different apertures. The hard, cleanly cut disks shown by a small speculum or object-glass offer an attractive contrast to the flaring, indefinite forms often seen in big telescopes.
Cleaning Lenses.—As to wiping objectives or mirrors, this should be performed not more often than absolute necessity requires; and in any case the touches should be delicate and made with materials of very soft texture. The owner of a good objective should never take the handkerchief out of his pocket and, in order to remove a little dust or dew, rub the glass until the offensive deposit is thought to be removed. Yet this is sometimes done, though frequent repetition of such a process must ultimately ruin the best telescope notwithstanding the hardness of the crown glass forming the outer lens of the objective. It will not bear such “rough and ready” usage and in time must show some ugly scratches which will greatly affect its value though they may not seriously detract from its practical utility. Good tools deserve better treatment. When the glass really wants cleaning, remove it from the tube and sweep its whole surface gently with a dry camel’s-hair brush, or when this is not at hand get a piece of linen and “flick” off the dust particles. Then wipe the lens, as soon as these have been dislodged, with an old silk, or soft cambric handkerchief; fine chamois leather is also a good material, and soft tissue paper, aided by the breath, has been recommended. But whatever substance may be adopted it must be perfectly clean and free from dust. When not in use it should be corked up in a wide-necked bottle where it will be safe from contact with foreign particles. In the case of mirrors there is an obvious need that, when being repolished, the material used should be perfectly dry and that the mirror also should be in the same state. It is unnecessary to say here that in no case must the silver film be touched when it is clouded over with moisture. This must first be allowed to evaporate in a free current of air or before a fire; the former is to be preferred. A suitable polishing-pad may be made with a square piece of washleather or chamois in which cotton-wool is placed and then tied into a bag. This may be dipped into a little of the finest rouge, and its employment will often restore a bright surface to the mirror. But the latter should be left “severely alone” unless there is urgent occasion to repolish it, as every application of the rouged pad wears the film and may take off minute parts of it, especially when dust has not been altogether excluded. The precarious nature of the silvered surface undoubtedly constitutes the greatest disadvantage of modern reflectors. The polish on the old metallic mirrors was far more durable. Some of Short’s, figured 150 years ago, still exist and are apparently as bright as when they were turned out of the workshop! I have a 4-inch Gregorian by Watson which must be quite a century old, and both large and small specula seem to have retained their pristine condition.
With regard to the duration of the silver-on-glass films, much of course depends upon the care and means taken to preserve them. Calver says that sometimes the deposit does not last so long as expected, though he has known the same films in use for ten years. A mirror that looks badly tarnished and fit for nothing will often perform wonderfully well. With my 10-inch in a sadly deteriorated state I have obtained views of the Moon, Venus, and Jupiter that could hardly be surpassed. The moderate reflection from a tarnished mirror evidently improves the image of a bright object by eliminating the glare and allowing the fainter details to be readily seen. When not in use a tight-fitting cap should always be placed over the mirror, and if a pad of cotton wadding of the same diameter is made to inlay this cap it tends to preserve the film by absorbing much of the moisture that otherwise condenses on its surface. The ‘Hints on Reflecting-Telescopes,’ by W. H. Thornthwaite and by G. Calver, and the ‘Plea for Reflectors,’ by J. Browning, may be instructively consulted by all those who use this form of instrument. The latter work is now, however, out of print, and Mr. Browning tells me that he has quite relinquished the manufacture of reflecting-telescopes. Mr. G. With of Hereford, who formerly supplied the mirrors for his instruments, has recently disposed of his reserve stock and entered an entirely different sphere of labour. In the publications above alluded to amateurs will find a large amount of practical information on the value and treatment of glass mirrors.
Opera-Glass.—A very useful adjunct, and often a really valuable one to the astronomical amateur, is the Opera-Glass, or rather the larger form of this instrument generally known as the Field-Glass. Of certain objects it gives views which cannot be surpassed, and it is especially useful in observations of variable stars and large comets. Whenever the horizon is being scanned for a glimpse of the fugitive Mercury, or when it is desired to have a very early peep at the narrow crescent of the young Moon, or to pick up Venus at midday, or Jupiter before sunset, all one has to do is to sweep over the region where the object is situated, when it is pretty sure to be caught, and the unaided eye will probably reach it soon afterwards. The opera-glass has the dignity of being the first telescope invented, for even its binocular form is not new; it is virtually the same pattern of instrument that was introduced at Middleburg in 1609, though its compound object-glasses are of more modern date. Anyone who entertains any doubts as to the efficacy of the opera-glass or has had little experience in its use will do well to look at the Pleiades and compare the splendid aspect of that cluster, as it is there presented, with the view obtained by the naked eye, and he will acknowledge at once that it constitutes a tool without which the observer’s equipment is by no means perfect. The object-glasses should have diameters of 2 or 2½ inches, and the magnifying power lie between 4 and 6. There is a large field of view and the images are very bright. The observer is enabled to enjoy the luxury of using both his eyes, and when he directs the instrument upon a terrestrial landscape he will be gratified that it does not turn the world upside down! It is not surprising that an appliance, with recommendations so significant, is coming more into favour every day, and for those branches suitable to its means it is doing much useful work. A volume has been recently published dealing expressly with the use of the opera-glass in Astronomy; and in the ‘Journal of the L.A.S.’ vol. vii. p. 120, there is an excellent paper by Major Markwick on the same subject. This instrument will never, of course, by the nature of its construction, be comparable to a modern telescope in regard to power, for Galilei, when he augmented his magnifiers to 30, appears to have practically exhausted the resources of this appliance. But in all those departments requiring an expansive field and little power with a brilliant and distinct image, the larger form of opera-glass is a great desideratum, and its portability is not one of the least of its advantages.
Dewing of Mirrors.—The disposition of mirrors to become clouded over upon rises of temperature is a point meriting comment. When permanently left in a telescope, fully exposed out of doors, the speculum undergoes daily transitions. The heat generated in the interior of the tube by the sun’s action causes a thick film of moisture to form upon the silvered surface of the mirror, which remains in this state for a considerable time, though the moisture evaporates before the evening. The flat is similarly affected, and the result of these frequent changes is that the coating of silver becomes impaired and presents a crackly appearance all over the surface. Sometimes when a marked increase of temperature occurs towards evening the speculum is rendered totally unserviceable until it has been submitted to what Dr. Kitchiner terms a process of “roasting.” The vapour will soon disappear when the mirror is brought indoors and placed before a fire; but it is not till some time after it has been remounted in the tube that it will perform satisfactorily. Those who keep their mirrors in more equable temperatures will not experience these inconveniences, which may also in some measure be obviated by regularly placing a tight-fitting cap, inlaid with cotton-wool, over the speculum at the conclusion of work. This also protects the silver from the yellow sulphurous deposit which soon collects upon it if used in a town. All sudden variations of temperature act prejudicially on the performance of specula, and their best work is only accomplished when free from such disturbing elements. I have rarely found the flat to become dewed in a natural way during the progress of observation. If on a cold night the observer puts his hand upon its supports in order to alter its adjustment it instantly becomes dewed, or if he stands looking down the tube it is almost sure to be similarly affected; but in the ordinary course of work the flat is little liable to become dewed in sensible degree. With refractors dew-caps are very necessary, though they do not always prevent the deposition of moisture on the object-glass, and this occasions frequent wiping or drying, which in either case is very objectionable.
Celestial Globe.—This forms another extremely useful addendum to the appliances of the amateur. It enables a great many problems to be solved in a very simple manner, and helps the young student to a lucid comprehension of the apparent motions and positions of the fixed stars. With ‘Keith on the Globes’ as a reference-book he may soon acquire the method of determining the times of rising, southing, and setting of any celestial object the place of which is known. He can also readily find the height (altitude) and bearing (azimuth) at any time. The distance in degrees between any two stars or between a star and the Moon, a planet, or a comet may be found at a glance by laying the quadrant of altitude on the pair of objects and reading off the number of degrees separating them. If a new comet has been discovered, its position should be marked in pencil upon the globe; and the observer, after having noted its exact place relatively to neighbouring stars, may proceed to identify the object with his telescope. If a large meteor is seen, its apparent path amongst the constellations should be projected on the globe and the points, in R.A. and Dec., of beginning and ending of the flight read off and entered in a book. In many other practical branches of astronomy this instrument will prove highly serviceable, and is far preferable to a star-atlas. But the latter is the most useful to the beginner who is just learning the names of the stars and the configuration of the chief groups, because on the globe the positions are all reversed east and west. The surface of the globe represents the entire star-sphere reduced to a common distance from the earth, and as seen from outside that sphere. The observer, therefore, must imagine his eye to be situated in the centre of the globe, if he would see the stars in the same relative places as he sees them in the heavens. The reversion of the star-positions to which we have been alluding is very confusing at first, and no doubt it provokes mistakes, but a little experience will practically remove this objection. The one great recommendation to a star-atlas is that it displays the stars in the natural positions in which they are discerned by the eye, thus enabling the student to become readily acquainted with them, whereas the celestial globe affords no such facility. But in other respects the latter possesses some valuable functions, and the amateur who devotes some of his leisure to mastering the really useful problems will attain a knowledge that will be of great benefit to him in after years. A globe of 12-inches diameter will be large enough for many purposes, but one of 18-inches will be the most effective size. It should be mounted on a tall stand with single body and tripod base. The stands, fitted with three parallel legs, in which the globe is supported in the middle by weak connections from them, are not nearly so durable. I have used several 18-inch globes mounted in this manner, and the supports have quite given way under the pressure of constant use; but this is impossible with the strong single body, which is capable of withstanding any strain. Globes are frequently to be obtained second-hand, and at trifling cost; but the observer must allow for precession if he uses an old article. Many of the stars will be 1° or 2° east of the positions in which they are marked on the globe; and it will be necessary to remember this if the appliance is to be employed for exact results.
Observatories.—Massive and lofty buildings have long gone out of fashion, and lighter, drier structures have properly supplanted them. Instruments of size are generally placed on or near the ground and solidly supported to ensure stability, while the other erections are made consistent with the necessity for pretty equable temperature and freedom from damp. Amateurs will ordinarily find that a simple wooden enclosure for the telescope, with suitable arrangements for opening the top in any direction, is sufficient for their purpose and very inexpensive. Some observers have, indeed, secured the desired shelter for themselves and their telescopes by means of a canvas tent provided with ready means for obtaining sky-room. Berthon has given a good description of an amateur’s observing-hut in ‘The English Mechanic’ for October 13th and 20th, 1871; and Chambers supplies some information about amateur observatories in ‘Nature’ for November 19th, 1885[8]. Mr. Thornthwaite’s. ‘Hints on Telescopes’ may be usefully consulted for details of the Romsey Observatory, which, like the Berthon model, seems peculiarly adapted to the necessities of the amateur. The great requirements in such structures are that they should be dry and not obstruct any region of the firmament. They should also be large enough to allow the observer perfect freedom in his movements and during the progress of his observations. They are then decided advantages, and will materially add to that comfort and convenience without which it is rarely possible to accomplish really good work. When an observatory is to be dispensed with it becomes necessary to erect a small wooden house near the instrument, especially if placed at the far end of a garden, in which the observer may keep certain appliances, such as a lantern, celestial globe, step-ladder or observing-seat, oil, &c. Here also he may record his seeings, complete his sketches, and consult his working-list, star-charts, and ephemerides. A shelter of this sort, apart from its practical helpfulness, avoids any necessity for the observer to go in and out of doors, up and down stairs, &c., to the annoyance of the rest of his family, who, on a frosty night, are decidedly not of an astronomic turn, and vastly prefer house-warming to stargazing!
[CHAPTER IV.]
NOTES ON TELESCOPIC WORK.
Preparation.—Working-Lists.—Wind.—Vision.—Records.—Drawing.—Friendly Indulgences.—Open-Air Observing.—Method.—Perseverance.—Definition in Towns.—Photography.—Publications.—Past and Future.—Attractions of Telescopic Work.