On the Construction of Telescopes with Silvered Glass
Part 6
Construction details for large telescopes
Setup of eyepieces
Changing power
Mounting the mirror
New wooden equatorial mounting
After its invention by Newton, the [reflecting] telescope has been redesigned several times by various scholars and artisans. The image formed at the focal point of the mirror in a reflecting telescope does not present itself in a position that is as easy to observe as in a refracting telescope. The various ways that have been devised to make the image of a reflecting telescope more accessible are based on different methods that could be the subject of some debate. From the outset, Newton took the wisest path, which consists of projecting the image out to the side, perpendicular to the axis of the tube, and of observing the image via an eyepiece that is mounted on the exterior of the telescope tube. The cone of the convergent rays of light is reflected by a flat mirror that forms a 45-degree angle with the axis of the tube. The mirror is, of course, located in front of the focal point, at a distance at least as long as the radius of the tube.
In order to avoid the losses of light transmission that would be caused by a second metallic reflection, telescope makers have tried to replace the diagonal mirror by a prism with total internal reflection, which would act on the bundle of light rays without causing any other losses except those from absorption in the prism and from partial reflections from the two perpendicular surfaces. However, in large telescopes such a prism would have to grow to such a size that it would be almost impossible to make. In short-focal-length instruments, such as we had in mind, this prism would have to assume even larger dimensions, and would threaten, by its own internal imperfections, to cause all kinds of distortions in the images that would be formed.
We chose a different method, in placing a small prism in the path of the light cone, close to the focal point, which would leave the image inside the telescope tube. We then put a composite lens with four elements in the path to bring the image outside the tube and make it visible. Whatever the objections observers might have against lenses composed of four different pieces of glass, one cannot ignore the many advantages of this arrangement. Actually, it solves quite a few problems. First of all, by using a prism only as large as needed to not restrict the field of view, one obtains total internal reflection. Secondly, by being small, it is relatively easy to have a high-quality prism with good internal homogeneity and excellent workmanship on the optical surfaces and their angles. The final advantage is that the four-element lens delivers an upright image.
On the other hand, since the composite lens was designed and engineered for refracting telescopes and binoculars, when one links it without modification to short-focus parabolic mirrors, one tends to find a certain amount of spherical aberration. In other words, in this eyepiece, two of the pieces of glass in fact play the role of objective lenses and begin to display the imperfections found with spherical curves. The remedy for this problem is quite simple: one simply performs an additional optical refiguring, which, while sacrificing the mirror image, results in a corrected image vis-a-vis the entire optical system of the mirror and the objective parts of the lens. Using this method, the mirror and the system of amplifying lenses are invariably linked together, and to change the enlargement power of the telescope, one merely has to change the system of the two other lenses, which is similar to the arrangement in an ordinary astronomical eyepiece. Thus, it is no longer actually required to construct mirrors that are exactly parabolic. We feel it is better to end up with an experimental mirror surface, which has purposely the property of acting in concert with the magnifying lenses in the eyepiece, to assure the production of a perfect image.
In speaking of the formation of images, the considerations which we have developed have helped us to evaluate the degree of precision required in performing local refiguring of the mirror. These same considerations determine the limits beyond which accidental deformations of the mirror would begin to harm the quality of the images. If we want the images to appear cleanly, it is required that in all the positions held by the mirror, the various elements of the surface must stay fixed among them to a precision of one ten-thousandth of a millimeter, because any relative displacement which exceeds this minimum quantity would place some of the rays of light out of phase with the others, and would throw them out of the effective group. One can understand from this the extreme importance of the precautions to be taken to remove from the mirror all forces that would tend to alter its figure.
When the mirror is placed at the end of the tube and the instrument is obliquely directed towards a point in the sky, the weight acts along two perpendicular component vectors, one which tends to compress the mirror in the direction of a diameter seen in a vertical plane, and the other which presses the mirror against the resistant parts that its back rests on. These two components, which vary in a sense contrary to the direction of the instrument, must be combated separately. The one that compresses the mirror on its edge can only by opposed by the rigidity of the material, which, under a given weight, takes on a maximum value when one finishes the back of the mirror with a sufficiently convex form. We have found it advantageous to shape the back of the mirror along a curve such that the thickness doubles from the edge towards the center, where it takes on at least one-tenth of the diameter. This is only a palliative which does not radically prevent deformation, but in reality this component of the weight along a diameter is not too much to be feared, since this deformation diminishes as one raises the telescope towards the zenith, and since the flattening which can occur over the totality of all the convergent ray bundles can be corrected easily by using a cylindrical lens.
The other component, whose intensity varies in an inverse manner with the first one, exercises a much more annoying influence on the image. As the instrument is raised up, the solid parts on which the mirror presses make the corresponding parts of the mirror push forwards, and cause undulations which show themselves at the focus via long trails of light. It is necessary to get rid of these local pressures and to spread them out uniformly over the entire surface of the back of the mirror. (To achieve this) we attach to the mirror mount a slab of wood, and we arrange between the two of them a space wherein we slide a circular rubber sac, which will press on the glass once it is inflated. The narrow tube which brings the air into this cushion passes all along the body of the instrument, extends right up to the eyepiece, and ends with a valve. By blowing into the tube with his mouth, the observer can thus, without losing the image in view, regulate at will the pressure and bring it precisely to the point where the mirror floats on its mount, without pressing on either one surface or the other. It is clear that in these conditions, the mirror will escape its own weight as far as the effects of the component which presses entirely on the pneumatic cushion.
The regular movement of the instrument will never force the mirror to rock back and forth on its mounting, and the addition of the cushion does not augment this instability of the optical axis which people have complained about telescopes even today. The cushion, which cannot move around as a unit, still continues to modify its surface, under pressure, and reacts distinctly on the clean focus of the image. The frame that holds the mirror, the cylinder, and the pneumatic cushion, is attached to the body of the telescope with pushing and pulling screws (??) which act to regulate the optical axis with respect to the prism and to maintain it in a fixed position.
The body of these new telescopes is made of wood; it has the form of an octagonal tube. Diaphragms that are open and fixed inside at various distances give the system a rigidity which is used when mounting it equatorially. At one third of the way from the mirror we attach two small cylindrical axes (see figure 19) that are mounded perpendicular to the axis of the telescope. Elsewhere, we construct a turntable with two columns, rolling via bearings on a plane that is oriented parallel to the equator and maintained in this position by a little wooden frame. The two columns on the turntable are fitted with babbits to receive the axes of the body of the instrument. Also, the two columns maintain the desired height and separation of the telescope so that it can move freely. The telescope being then put in place, is now mounted equatorially, because its two degrees of freedom are in declination around the little side axes and in right ascension around the axis of the turntable. Prolonged observation of a star requires that the instrument be stopped at a certain declination. For that reason, we attach on the turntable a sort of arm whose end is attached at some point of the telescope by a sliding bar that can be tightened, which forms one variable side of a triangle, and which determines the opening of the opposite angle.
A metal disk divided along its circumference and mounted on one of the side axes will serve as a circle of declination, and divisions drawn on the edge of the equatorial platform will serve as the divisions of hours of right ascension. But the positions which they point to will have no more precision than one would need to help find a star which one wants to put into the field of view. This mounting system only constitutes a support mounted in an equatorial manner to aid in observation. The movements are easy, and nothing would prevent one from adding a motor drive if desired.
We are currently constructing a similar mount for the 42-centimeter telescope that has been established for several months at the Imperial observatory. [France was then ruled by Emperor Napoleon III - trans.] The mirror was cast at Saint-Gobain, then roughed out and started off at the Sautter and Company factory, which is devoted especially to the construction of light-house lenses. Afterwards, Mr. Sautter has prepared for the future, for much larger disks, and we have received assurances from him of a
cooperation which would only fall back if there was a material impossibility. Relieved of a preparation which required special tools and setup, the house of Secretan did all the rest, except for the final refiguring which they did not want to be responsible for. By the intelligent care of Mr. Eichens, who directs the workshops, the mechanical part of the work is perfecting itself so that in a little while we will be in possession of the entire apparatus. {not sure whether he means a complete telescope or a complete workshop - trans.)
We are now coming to the conclusion of this series of details, all of which needed to be described, or else we would have left for others the need to discover that which experience had already taught us. We have given those details as information for those who would desire to reproduce the effects which we have obtained. Among the details of execution, there are quite a few that we have obtained in the workshops of Mr. Secretan, and we are pleased to recognize that all of these daily interactions with skilled workmen, intelligent foremen, and a very enlightened head of the operation, have considerably
abbreviated our task.
Of what did this task consist? We had proposed for ourselves, or rather we had received from the Director of the Observatory, the mission of preparing the way for creating objectives of large dimensions. Would it be necessary to rely on the empirical methods that up until now had been seen as sufficient for the work on glass? What results could we anticipate obtaining in exchange for the increases in expenses? How would we judge that we had succeeded? Could our best instruments up until now still be improved? Could it be that in optics, as in mechanics, there is a maximum of useful effects that would come sooner or later to limit our efforts? All of these questions were implicitly contained in the mission which we received from the Director.
In seeking to resolve them, we see now that there was a danger that we might end up going down a path with no way out. Fortunately we took a detour, and leaving refraction aside for the time being, we entrusted to reflection various ways of acting upon light more simply and of correctly forming the focal point where all of the physical theories of light are revealed. Since we only had to deal with a single surface, because of the simple fact of reflection, the experimental line that is folded back upon itself could be contained inside a closed room. Since the point and the image were in proximity to each other, we were able, without leaving a spherical figure, to familiarize ourselves with the methods of acting on glass surfaces, of observing them, and of modifying them based on the demands of optical phenomena. Afterwards applying the same procedures to the cased where the point and the image become farther and farther apart, we have seen the surfaces progressively evolve to various other conic sections, which for a long time have been designated as specially apt for optical uses. Now that this experience has been acquired, we would not hesitate to apply to achromatic objectives a method which has nothing to fear from the analytical complication of surfaces. Nonetheless, these glass mirrors, which were merely accessories, have lent to the silvering process such a remarkable metallic shine, that now they rival objective lenses of the same size.
Without losing sight of the main object of this work, which was to furnish practical results, we have been led in our work to recognize the weaknesses of the purely geometrical approach on which the theory of optical instruments was formerly based. All of the facts we have observed condemn any system in which one takes no account of the periodic nature of light, and wherein one neglects the principal element which comes into play in the mechanics of the formation of images. On the contrary, the facts show that at the focal point of surfaces that precise enough to display the intimate details of light, the rays obey the fundamental principle of interference. The latter results justify a doctrine that the
human spirit has given itself as a guide and which appears to embrace the entire universe of phenomena of optical physics.
<THE END>
Copyright, Guy F. Brandenburg,