George Calver

“ 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. ”

“ On such nights a wooden tube is preferable to a metal one. It has been frequently my experience, when finally testing an instrument, that when I have been dissatisfied with its performance, and in order to trace the cause of the apparent defect, have removed the mirrors to my wooden testing tube, I have found every defect removed, and imperfect replaced by perfect images. The explanation is simple. Not only is the wooden tube much larger in diameter than the specula, but wood is of itself less sensible than metal to differences of temperature. ”

“ For red stars, which are generally small, the reflector seems especially adapted. I used to be rather sceptical about the tints of red stars, but must confess, after seeing the magnificent tints of scarlet and crimson which are shown by a large aperture, that my doubts have vanished. Definition upon the planets, in good air, is simply perfect. The detail on Jupiter and Saturn reminds one of an exquisite engraving. The far distant Neptune is perfectly defined as a tiny disc, and his moon steadily visible with the 11-1/4 in. ”

“ 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. ”

“ 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. ”

“ It is always advisable to leave the telescope for a short time undisturbed, especially if, on first looking at a star, a flare should appear, as these appendages often vanish when the instrument has been for a short time in the air. These adjustments may at first appear somewhat difficult, but are rendered remarkably easy by observing how the different screw movements alter the positions of the reflections. A useful test as to the correctness of the adjustments may be obtained by viewing a star with a high power eye-piece out of focus. ”

“ The definition of a star should now be perfect, and by daylight observe the adjustments; it may appear that the reflected circles are not concentric, but if the definition on a star is good in still air, consider the appearances correct adjustments, except that the reflection of the black spot on mirror may be made concentric. It sometimes happens that a persistent and rigid centreing of the reflections on face of flat are not the positions for best definition, perhaps from a slight optical eccentricity somewhere in the instrument. ”

“ 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. ”

“ 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. ”

“ An equatorial mounting, with two long and stiff shafts for the axes, has always the advantage of firmness; the holding portions of the instrument being in masses, are not liable to receive injury from blows, and thus be put radically out of order. No means should be resorted to to make an equatorial of light weight; an instrument cannot be light and slim and at the same time firm and steady—no amount of steadying rods, splines or strings, will make a slender tripod for a refractor so steady as a firmly made one, with proper size and weight in the parts. ”

“ I have tested it, and can speak in the highest terms of its performance. Definition in good air is as near perfection as it is possible to imagine. Such tests as ζ Cancri, γ2 Andromedæ are well divided. The 6th star in the Trapezium stands out well clear of its brighter neighbour. On the moon any power may be applied, only lessening the light, but retaining sharpness of outline. I have tried a good many telescopes, but never yet saw these 10 inch mirrors surpassed. The image of a star disc expanded on each side of the focus is of as nearly the same appearance as possible. ”

“ Dear Sir,—I have tried the 18-inch speculum on some tests, and especially the satellites of Uranus, and it appears fine, very fine. What I have done in photography promises well.
Yours truly, A. A. C.
Mr. G. Calver.
Southend on Sea,
March 17th, 1877.
My Dear Sir,—Circumstances have prevented me from making many observations of late, but I am more confirmed in my opinion that my telescope of your make is a very fine instrument. (A 6-1/2-inch.) I heartily wish you success, which I feel sure you will obtain, as you are so painstaking and turn out a thoroughly reliable article. ”

“ Calver,—The 12 inch surpasses all my expectations. It has the perfection in figure of the incomparable 10 inch of yours, which was its immediate predecessor, and, in grasp of light, the difference in favour of the 12 inch over the 10 inch, a most brilliant mirror, is simply astounding.
You may equal the 12 inch—I am sure you will, whenever you make one—but you will not easily beat it. Mr. Sadler, the well-known and most accurate observer, has himself worked with it, and writes of it to me as follows:—“The 12 in. is considerably superior in light grasping power to H2’s 20 ft. ”

“ We will assume that the black spot is nearer the edge of flat nearest the mirror, and that the eyepiece is beyond the focus proper for the star; then unscrew the side screw. If the spot is nearer the other edge of flat, screw it up little by little, taking the hand out of the telescope each time, and see what the effect is. The movement and alteration can be watched while the hand is in the tube. If the spot is seen either the top or bottom of the flat that is in the direction of the diameter of tube, the middle screw must be slightly loosened and the flat revolved. ”

“ 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. ”

“ 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 [Pg 9] 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. ”

“ Every speculum is most carefully figured, and confidently guaranteed perfect to the extreme edge.
The employment of “stops” of any kind is quite unnecessary, except on bad nights, when the aperture must of course be suited to the degree of steadiness of the air. For, whether the instrument be a reflector or a refractor, the aperture and power used must of necessity be limited by the atmospheric conditions under which observations are made. ”

“ Any well-defined terrestrial object at a distance can be employed in the day-time to roughly adjust the “finder,” leaving the final adjustments to be made by a star.
The use of Stops.
A stop 1/4 inch less in diameter than the speculum, is often useful, in order to cut off the internal reflections of the tube.
With mirrors 6-1/2 inches in diameter and under stops are seldom required; with the 6-1/2 inch a 5 inch stop may however sometimes be used with advantage on very bright objects. With an 8-1/2 inch mirror a 7 inch stop often improves planetary definition. ”

“ 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 ”

“ Into the draw tube screw the “adjusting piece,” which is a small brass circle with a hole in its centre about 1/20 of an inch in diameter. (The draw tube should be in about the same position as when at focus with an eyepiece.) Place the large mirror in its cell in the tube or body of the telescope, taking care that the three bayonet-joint pins are correctly placed, that is, with grooves pointing downward. They will be found to drop easily into their corresponding holes; care must however be taken that the grooves have gone well home. ”

“ But 150 on a 3 inch is a high power for the quantity of light obtained, to say nothing of the separate consideration and advantage of long focus. Aperture is a quality or function of the telescope considered separately from light or focus. For instance, suppose a 5 inch speculum is so thinly silvered that it gives the exact degree of light as a 2-1/2 inch refractor, the defining and separating power of the 5 inch aperture would be very superior to the 2-1/2 inch. ”

“ 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. ”

“ 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. ”

“ Both in light grasping power, and in the separation of close double stars, it has far surpassed my highest expectations, and quite removed the prejudice I entertained against reflectors. Though possessing a first-class 4-1/4 in. refractor, equatorially mounted in my observatory, I now use your mirror in preference, in the more difficult and delicate observations, and with it am able to master the most severe tests given for 6-1/2 in. aperture. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Calver,—Since writing to you I have been observing with the 6-1/2 in. equatorial telescope, and I have much pleasure in informing you that I am greatly pleased with its performance. The definition of the planets is beautiful. ”

“ 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. ”

“ When the telescope is in the open air, ice will not infrequently form on the top of the tube, and the effect will be at once obvious in the irregularity of a star image at the edge corresponding to the top of the tube. ”

“ 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. ”

“ More failures are due to unequal temperature than any other cause in the hands of the amateur. One careful and clever amateur informed me that he had failed six times, but on the seventh, by attending to the advice given above, the silvering was a perfect success, and all seemed then as easy and certain as it had before seemed uncertain and difficult.
THE FINAL ADJUSTMENT OF A NEWTONIAN REFLECTOR ON A STAR.
The adjustments being as near correctness as can be seen by the usual method of procedure—of which the fullest details are given in my little book, the telescope is ready to try on a star. ”