Part 1

NEW EDITION OF HINTS ON Silvered-Glass Reflecting TELESCOPES

MANUFACTURED BY

MR. G. CALVER, F.R.A.S.

WITH DIRECTIONS FOR SILVERING, ADJUSTING, &c.

1877.

GEORGE CALVER,

HILL HOUSE, WIDFORD,

CHELMSFORD, ESSEX.

[ Illustration: decorative ]

HINTS ON Silvered-Glass Reflecting Telescopes.

Of the various forms of Telescopes now in use, each has its own peculiar advantages; but the Silvered-Glass Reflector is undoubtedly gaining favour among our practical observers. A well-figured speculum, being perfectly free from chromatic aberration, gives, in a proper condition of the atmosphere, the finest possible definition of the Moon and planets, the markings and colours of these objects being excellently seen; while coloured stars, such as Albireo (β Cygni), or Almaach (γ Andromedæ), are exceedingly well shown, the beautiful contrast of the stars in the former being especially noticeable in a reflector. The advice of “F.R.A.S.” (in the “English Mechanic,” March 21st, 1873) as to the choice of a Telescope, may here appropriately be quoted. After expressing a preference for refractors when measuring close double-stars, he says, “But should the object of your correspondent be merely to regard the wonders and beauties of the Heavens, or notably, should he purpose to devote himself to the study of the physical structure of the Moon and planets, then by all means let him obtain the largest reflector he can afford; its absolute achromatism tells most astonishingly on these last-named objects.” This is the opinion of one who has great practical knowledge of the different forms of Telescopes.

If Achromatic Telescopes of large aperture could be made as cheaply as reflectors, and in as convenient a form, they would doubtless be preferred for general star-work, although the aberrations, especially that of colour, cannot be so perfectly corrected. A silvered-glass reflector is, however, much cheaper than a refractor, and, when the aperture exceeds five or six inches, is much handier to work, and occupies less space, being only about half the length of an achromatic of the same aperture. It is true that a reflector will give less light than an achromatic of equal aperture—but this is, in certain conditions of the atmosphere, a distinct advantage, the extra aperture to give the same light adding to the definition and penetrating power. An example of this is seen in the beautiful definition given by an unsilvered mirror on brilliant objects, as the Sun, Moon, and Venus. In large achromatics, the distressing excess of light has often to be reduced by diminishing the aperture or using a higher power than is convenient; and in such cases a lower and more suitable power can be employed with a reflector.

When the air is unsteady, the definition of Reflectors, owing to their tubes being open, is more liable to fluctuate than that of refractors, although when a reflector does not give good definition on account of the atmosphere, a refractor of the same aperture will certainly not perform satisfactorily. Sir John Herschel has shown that when the air in the tube of even an achromatic is disturbed in turning from one object to another, good definition does not return until it is brought to rest. In order to reduce the vibration of the air in the open tube, and also that of the stand, to a minimum, reflectors require to be very firmly and steadily mounted, and to have iron tubes.

Many of the specula made by me are now in observatories, where they have been compared with achromatics of first-rate quality with the most satisfactory results. For instance, several 6-1/2 inch specula were tried with two excellent achromatics of 4-1/2 and 6 inches aperture, when the planetary definition was considered to be superior with the reflectors; and the appearance of the star-discs, with equal apertures, differed little from the beauty and hardness of the images given by the achromatics. 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.

It is perhaps unnecessary to remind the reader that, when the defining powers of a telescope are put to the test, as much depends on the acuteness of the observer’s eye and the practice he has had, as upon the perfection of the instrument and the fineness of air. It is a mistake to suppose that when the stars appear to be the brightest to the unaided eye, definition will be at its best, though this may happen; when it does so, the astronomer should make the most possible use of the opportunity, as such nights are very scarce. As a rule, the best nights are those when there is the slightest possible mistiness of the atmosphere, and for the faintest stars absence of bright moonlight. The 5 inch mirror is guaranteed to divide stars one second apart with ease, and closer ones in very fine air. The 8-1/2 will split such difficult pairs as γ^2 Andromedæ, and μ^2 Boötis. An acute eye will master these stars with even a less aperture on very favourable occasions, γ^2 Andromedæ having been seen with a 7-inch stop of a 10-inch mirror, and Mr. Sadler, of Honiton Rectory, has split this star with his 6-1/2-inch telescope. It sometimes happens on favourable nights that the most difficult objects will be seen with the same telescope, which on other occasions had failed to show them as well as a much smaller aperture had done in very fine air. These remarks equally apply to the observation of minute stars and planetary detail. The amateur must follow the advice of the Rev. T. W. Webb, given in “Celestial Objects” (pages 15-17), and must cultivate that virtue applicable to all scientific investigation, namely, patience. The following interesting and difficult objects may be looked for, the powers used for their observation should vary from 150 to 300, and for the very closest stars up to 500, or even still higher.

TESTS FOR SPECULA.

LIGHT TESTS.

94 Ophiuchi. 5″ : 7, 13. 58 Ceti. 3″·5 : 6·5, 14. γ Crateris. 3″ : 4, 14. 15 Monocerotis. 25″, 15″ : 6, 9·5, 15. τ Orionis. 15″, 20″ : 4, 15, 12. υ Ceti. 6″ : 4·5, 15. ε Trianguli. 5″ : 5·5, 15. 179 Piscium. 3″ : 8·5, 15. 110 Herculis. 55″ : 5, 16. μ Andromedæ. 45″ : 4, 16. β Equulei. 35″, 3″, 50″ : 5, 13, 16, 14. 85 Virginis. 30″ : 6, 16. 55 Andromedæ. 25″ : 5·5, 16. 178^a Delphini. 20″ : 7·5, 16. 212 Libræ. 20″, 10″ : 6, 16, 8. 14 Monocerotis. 10″ : 6, 16. 94 Ceti. 5″ : 5·5, 16. δ Aquilæ. 1″·5 : 3·5, 16.

SEPARATING TESTS.

33 Orionis. 2″ : 6, 8. 52 Orionis. 1″·8 : 6, 8. δ Cygni. 1″·8 : 3·5, 9. 2 Camelopardi. 1″·7 : 5·5, 7·5. π Aquilæ. 1″·7 : 6, 7. σ^2 Cancri. 1″·4 : 5·5, 7. 9 Leonis. 1″·2 : 7·5, 7·5. η Orionis. 1″ : 4, 5. ε Arietis. 1″ : 5, 6·5. 4 Lyncis. 1″ : 6, 7·5. 37 Pegasi. 0″·8 : 6, 7·5. 749ξ Tauri. 0″·8 : 7·1, 7·2. 46 Arietis. 0″·8 : 8, 9. λ^a Cygni. 0″·7: 5, 6. β Delphini. 0″·7 : 5, 5·5. 20 Draconis. 0″·7 : 7, 7·5. 287 Draconis. 0″·7 : 7, 8. 178^b Delphini. 0″·6 : 4·5, 6. φ Draconis. 0″·6 : 4·5, 6·5. γ^2 Andromedæ. 0″·6 : 8·5, 9. μ^2 Boötis. 0″·5 : 8, 8·5. ι^a Boötis. 0″·5 : 4·5, 4·5. 7 Tauri. 0″·5 : 6, 6·5. 108 Draconis. 0″·5 : 9, 9. η Herculis. 0″·3 : 3, 8. 42 Comæ Berenices. 0″·3 : 4·5, 5. ω Leonis. 0″·3 : 6·5, 7·5.

Since writing the first edition of my catalogue, the writer has received many gratifying accounts of the success of the “Silvered-Glass Reflector.” Many private letters have been received by him from observers, expressing their delight and satisfaction. This has been so encouraging that no pains have been spared, nor any opportunities neglected, in turning to the best purpose every valuable lesson that continued practice and experience may have suggested in the manipulation of specula from time to time, in order to secure the best means for obtaining the highest excellence of defining power.

Many modifications and improvements have been made in working and testing mirrors, and special machinery and appliances constructed for large sizes. But to complete my conditions suitable for truly figuring and testing specula of considerable size, I found it necessary to remove from the traffic and tremor of a town to a still and tranquil situation in the country.

The truth of the curve is so sensibly and seriously affected by vibration constantly going on in and near a town, that it is liable to a variety of defects, and the surface becomes wavy and “plucked.”

As an instance, I may mention that my first and most convincing proof of the advantage of the stiller situation was tested by an 18-inch speculum (on which much labour had been bestowed); it was laid aside, but successfully finished after removal, and without undulations or any perceptible defect, and the Observer wrote me, that in good air, he “saw Sirius as a brilliant white dot, without a ray or appendage of any kind, and celestial photographs obtained with it are very fine.” Such results were exceedingly gratifying, as they were obtained with much less labour and uncertainly, and the tedious process of the final touches had not to be repeated so many times.

It is said that the celebrated Alvan Clarke, from the same effects of tremor, never finished an object-glass to his satisfaction above-ground; and Dr. Draper, testing his mirrors at the centre of curvature, to avoid draughts, &c. in an ordinary apartment, resorts to an underground one.

In this little book of “Hints” it may be useful to remind those who possess a speculum of fine quality that they are not produced by a “rule of thumb”—so to speak—and that the difference between _a_ speculum and a really _fine_ one, giving a maximum of defining and illuminating power, is the result of considerable labour and thought, and deserves careful usage.

The Rev. Cooper Key, an amateur of much experience, writes in the “English Mechanic,” that he was eight months (working sometimes eight hours a day), giving his 18 inch speculum its final touches and corrections.

A well known correspondent of the “English Mechanic,” “Hyperion,” tells us he found it impossible to test his 8-1/2 inch mirror in an ordinary room, and had to resort to a tunnel under the clay of his garden. Those who have the means and perseverance to make their own mirror, should be careful not to proceed with the finishing touches until an hour at night when their workshop, if in a town, is free from tremor.

First secure a well-ground and carefully-centred disc, let the polish be as perfect as possible before any attempt is made to figure. Care must be taken that every square or portion of the polisher is of the same consistency and temperature, that the disc may not be acted on irregularly.

To give the pitch this quality it must be well boiled and “pulled,” so that no air bubbles are in the squares, as these cause expansion or contraction as the temperature of the apartment varies, or that of the pitch and glass from friction.

It is much the best plan to keep the workshop to the same uniform temperature as the polisher was made for, allowing no draughts to pass over the mirror while working, nor the gas or lamp to be near. When the polisher is warmed—which it should be after laying aside for any time—it should be warmed _equally_. Neglect of the above in making the polisher, or any incautious handling of either the disc or polisher, will be sure to cause defects, which cannot be cured but by retracing the early steps in the fine grinding with an accurately centered tool.

The polishing successfully accomplished, the process of correcting for parallel rays is next proceeded with. At every step all possible care must be taken to prevent the mirror from running into an irregular curve. The importance of this cannot be too strongly urged, if a speculum capable of doing the _best_ work is desired, as the curve _must_ be true, regular, or uniform, to give the _highest_ defining and illuminating power. An under-corrected mirror, if of a regular curve, will perform much better than a compound correction, exhibiting at the focal point much less lateral aberration.

A brief explanation of this may be of use as far as the limits of these pages will admit. It would be impossible to teach, by a mere description of methods, how to commence and finish off a speculum of good quality, even if every working secret and minute detail were unreservedly explained. The only way is to master sufficient theory, and the rest will come by prolonged care and perseverance. I state this because amateurs who have been desirous of enjoying the gratification of observing with a mirror of their own making have written for information which they have been quite willing even to pay for liberally, but have felt disappointed, and perhaps thought it somewhat discourteous, on being told that what they wanted was impracticable and could not be satisfactorily attempted by the Optician in writing. In some cases I have finished amateurs’ work on agreeable terms, and which has led to a pleasant correspondence or acquaintance. These remarks may prevent some future disappointment.

Every one is familiar with the fact that the parabola is the only concave surface that can reflect rays of light falling on its surface from an object at an infinite distance—such as the stars and planets practically are—to one common focus, or without aberration. This series or column of rays (which is equal in diameter to the opening of the speculum), reach the mirror without making an angle to it or to each other, they travel side by side, they are all of one length, and are reflected to a point, and are therefore all of the same length at the proper focal distance, viz., half the radius of the curvature of the concave. The properties of the parobola make the nearest possible approach to it of the utmost importance.

It has been said that a considerably under-corrected surface, if of a true and uniform curve, is better than one with less aberration, with zones or sections of various curves. To explain this, let us suppose an artificial star at some short distance, say 50 yards, the parabola would not form an image of this at the eye-piece without aberration; it has for this distance too short a focus for the central rays, and the best disc is inclined to the inner focus, because the rays from the object _diverge_ towards it, instead of travelling parallel; and they reach the surface (the central rays compared with the marginal), at an angle equal to the semi-diameter of the mirror.

But, if instead of a parabolic mirror an elliptical one be used, which has one of its foci at 50 yards’ distance, the image will be perfect. Now place the artificial star at 500 yards, the image will now be attended with perceptible aberration. The longer focus of the ellipse must be worked further and further from the mirror by shortening the focus of the central rays. Correct it for this distance, and again remove the artificial star to a still greater distance, repeating the corrections as before and carrying the outer focus towards the object, and the inner towards the mirror, until the rays from the object become more and more parallel, and the curve is nearer and nearer to the parabola, or that eccentricity of ellipse which acts better and better for distant objects or parallel rays.

It is evident from this that if the ellipse is corrected very considerably towards the parabola, without irregularities, and _every part_ of the surface corrected regularly from the centre to the edge (no part hastening more than another), that such a surface, though under-corrected, is much better than if one portion is fully and another under-corrected, especially if the more imperfect portion is towards the edge. Such a compound correction may show little or no _outstanding_ aberration at the focal point, but the rays do not find a focus at the same regular pace as they would from a regular curve—one edge of a zone will be coming into focus when another would be going out. With the focussing screw they are focussed to the place where they appear to collapse, and are most satisfactory, though, _in reality_, the rays at the best place bend over each other, and the definition is imperfect. From a star, which is only a point, this may be more tolerated, but on the planets, where the image has a sensible diameter, and is perhaps magnified many times, this imperfect curve is very inferior to the regular one, whose error is all of _one kind_. The light is all there _somewhere_, but not with any good effect. There is no _proper_ illumination or definition, as rays are employed which are crossing the optical axis at _varying_ angles, and the result is confusion.

So the amateur who sets himself the pleasant task of making his own speculum (for there are many who can better afford the labour than the capital to purchase, and whose capabilities are thus superior to their means), need not be discouraged and give up the pursuit because he cannot obtain the best results by getting a perfectly parabolic glass. But if he has been successful in removing a considerable amount of the spherical error, and advanced to the elongated ellipse by maintaining a truthful curve, “let well alone” with this disc, and proceed no further, but commence _another_, taking care not to alter the first until the second is _better_, and _then_ an improvement of the first may be attempted.

In the second attempt, should the amateur lose control over the _regularity_ of the surface, let him try it as an experiment against the first on the planets, and he will not fail to appreciate the difference, and will be stimulated by fresh courage to get as near to the parabola as possible with the same accurate curve.

To produce a true and uniform curve is, however, the acme of troubles, whether it is desired to obtain the spherical, or parabola, or any other curve. It is generally supposed that the spherical curve is a very easy matter, so easy indeed that it is difficult to avoid. This is a very great mistake—a spherical curve of undeviating truth is as difficult a problem as a true parabola. The spherical curve is the _only uniform_ curve, it has but _one_ focus, and the polisher must coincide with, and be of the same radius as the glass, at every instant. This is why the optician strives to obtain a semi-polish with the grinder to lessen the risk of losing his curve on the polisher, for the curves of an object glass are spherical. The curve most liable to be obtained by the amateur is the spheroidal, a curve with its marginal rays _shorter_ than the central, or half the radius of curvature.

There are no means with the _telescope_ of telling the spheroidal, approximating to the sphere, _from_ the sphere. There are _no_ means of analyzing the exact character of a curve _equal_ to certain methods at the centre of the curvature, but to accomplish this requires much practice and observation, with “surroundings” perfectly free from vibration. If the amateur can overcome this, and lives in or very near a town, he should only work at the polishing and figuring during the late hours of the night, when traffic, &c. have subsided. Then, by carefully preventing any draughts in the apartment, and with the mirror of the same temperature as the air in the room, he will then be able to see how varied and numerous are the chances of error in working a mirror, and the great care necessary to avoid or cure them. He will find the surface exceedingly prone to receive zones and irregularities during work, and much more so than to “work true.”

The necessity for avoiding incautious handling or heating may be realized by the following little experiment when one can manage and understand it:—Place the tip of one finger on the surface, as it hangs in the dark room ready for testing, and with very gentle pressure let it remain long enough to spell one’s name; it will then be seen that the feeble heat of the finger has, by expansion, raised a mound on the surface of the glass, and though this amount of swelling must be _very_ small, yet it is enough to cast a shadow across the surface, as if something were laid on it, and quite ten minutes will elapse before the heat will leave this spot and the surface again become level. Now if the polisher were placed on the glass while this hillock was there, a permanent hollow would be the result. For a full account of these methods (of which Foucault was the discoverer) the reader is referred to Sir John Herschel’s and Dr. Draper’s works on the telescope.

Care must be taken not to leave too much aberration, as then the central disc is formed too positively outside the focus, and the rest of the light from the object appears as obtrusive rings and false light. The over-correction is bad, and acting as a negative lens the disc is formed too near the mirror. Such a correction, besides being objectionable on almost all classes of objects, prevents the use of the “Barlow” lens, and acts badly with all kinds of positive or Ramsden eye-pieces, which improve _under_-corrected, but “make bad worse” with over-corrected surfaces.

If these few and brief hints should stimulate the industrious and persevering student to make his own telescope, and thus enjoy the fruits of his own labours (which may add a relish and a pleasure to his astronomical work), they will have served the purpose for which they are written.

After a “Hint” on the choice of focus for the mirror, it only remains to say a few words about the plane, as this, with the large speculum, are the only parts that need be home-made as far as the optical work is concerned. Never adopt a “dumpy” for general use where high powers are sometimes wanted, for small and moderate sizes, say 8 to 12 diameters, and for large sizes not less than about 6 diameters of the mirror, as the larger ones practically admit of a shorter focus. The short ones can be mounted somewhat cheaper, but never choose them on _this_ account, they will not make so satisfactory an instrument, and no adaptation of “Barlow’s” will make it so.

To make the plane, provide three well-turned metal discs of 7 inches or 9 inches diameter. Iron is the best, as the work will go down closer, and the plates or discs may be very thick—say an inch—and not so liable to “spring.” These turned discs must be scraped and ground perfectly flat on each other, until they are in good contact all over, so that there is nothing between the faces when testing them. In the earlier stage of “truing,” oil and colour can be used. When these discs are proved true, a disc of plate glass, same size, is cemented with pitch on one of them, or on a thick disc of glass, and care must be taken that it is not strained while on the block during working. Truly grind this plate on one of the tools to a fine semi-polish. Polish on the same tool with a piece of thick silk or alpaca, laid over and cemented down by a solution of resin dissolved in turpentine (as much resin as the turpentine will dissolve), then, with the tip of the finger, thinly smear over the tool and bind round the edge with cord, and the silk will keep in place. Fill up the texture of the silk or alpaca evenly with damp rouge, keep it uniformly damp, and never let the rouge and water work about. It may be polished on very hard strained pitch, but pitch for the amateur is not so safe, as it is liable in his hands to alter its form and destroy the truth of the plane, but it is the quickest and handiest if it can be managed.