As mentioned in Chapter 7, it is best to defer the aluminizing until the mirror cell and other tube parts have been completed; then the mirror and diagonal can be treated at the same time. It seems almost superfluous to add that it is the polished and figured surfaces that are aluminized. The lustrous metallic coating, of the order of a quarter of a wave length of light in thickness, increases their reflectivity some 22 times.
Of course, the finished coating of aluminum should have no foggy areas or other blemishes. Pinholes in the aluminum film can be avoided, although it is quite common to find, upon holding the mirror up to a strong light, at least a sparse scattering of them rather evenly distributed over the surface. If they are not too numerous, the light loss thereby occasioned can be disregarded, but a profusion of pinholes, indicating that a poor vacuum was had in the process, should not be tolerated. It is desirable that the coating be perfectly opaque, but a loss in transmission of as much as one per cent of the light is tolerable.
Sometimes the aluminum has been unevenly deposited, with a considerable variation in its thickness in different areas, giving a spurious and detrimental figure to the mirror. On several occasions I have come across aluminized mirrors that bore no resemblance whatever to the original figures. Of these, one that was originally hyperbolic proved to be an oblate spheroid under test. It was again hyperboloidal after removing the aluminum, so any possibility of flexure was disproved. Another poorly aluminized mirror showed partial astigmatism, and another consisted of a non-descript assortment of shapes.
Accordingly, after your mirror is returned by the aluminizing laboratory, give it another knife-edge test, and for this it may be necessary to make a very tiny pinhole; otherwise the almost blinding reflection may render the shadows indistinguishable. A filter, however, such as colored cellophane, may permit testing with the old pinhole. Needless to say, the figure should faithfully correspond to the one that was originally given to the glass surface.
It may be of interest to record the condition of three aluminized mirrors figured by the author, which have been giving excellent performance. When mirror A was held about three feet in front of an unfrosted 100-watt lamp, a rather even sprinkling of tiny pin-holes was observed. The filament was clearly and sharply outlined, steely-blue in color. As the mirror was moved about, in one or two places the filament became dim, suggesting that in those areas the film was more thickly deposited. When the mirror was held up to the sun, there was considerable irradiation in yellow and blue light, and because of its brightness, the outline of the sun could not be seen. No attempt was made to measure the amount of light
transmitted, but best estimates placed it at under one per cent. Mirror B showed but a thin scattering of pinholes, and was apparently opaque to the electric lamp. The sun was visible through it, quite dim, but hard in outline and whitish in hue. There was little variation in its appearance through different parts of the film, although in one region it became nearly obscured. Except for about half a dozen pinholes, mirror C was absolutely opaque to all visible radiation; not even the shadow of a solid object passed across the path of the sun's rays was observable. On the Foucault stand, none of these mirrors revealed any noticeable departure from its original figure, nor could any criticism be made of the appearance of the extra-focal diffraction rings of a star, despite the fact that, in the cases of A and B, variations in the thicknesses of their respective films were demonstrated.
Because of its relatively small area, there is little likelihood of the figure of the diagonal being affected to any injurious degree, although the figures on larger optical flats have sometimes been altered by aluminizing. In rechecking by the interference test (described in Chapter 8), there is great danger of the aluminized surface being scratched unless suitable precautions are taken in bringing the surfaces together. Any dust particles will be conspicuously visible on the diagonal's surface, and may be blown off with the breath. Or if they still adhere, the smooth skin on the under side of the forearm, after being washed and thoroughly dried, makes an effective "wiper" if passed lightly across the face of the diagonal. The test piece, similarly freed of dust and other foreign matter, is then carefully placed on top of the diagonal, and lightly but firmly pressed down, slowly excluding the air until contact is made. It should not be allowed to slide about in this process, as scratches will almost surely develop.
Neon light is practically useless for this diagonal test, for the almost overpowering reflection from the aluminized surface makes the interference bands so faintly discernible that one may falsely conclude that some contamination prohibits their appearance. A more monochromatic light source is certainly desirable for testing silvered or aluminized plane surfaces.
Magnesium fluoride coatings were mentioned in the first chapter; their purpose is to reduce reflection losses at the surfaces of lenses and prisms. If a prism is used for the secondary reflection, its square faces should be fluoride coated, of course. Or the diagonal flat, if used, as well as the primary mirror in any case, should be similarly treated to prolong indefinitely the maximum efficiency of the aluminum film. The fluoride coating provides a hard, durable surface that withstands abrasion well without impairing the optical qualities of the surfaces.
A silver coating is not as durable as aluminum, but it may be put on by the amateur himself. Instructions for silvering by the Brashear and other processes may be found in optics, telescope making, and physics laboratory texts.
Packing for Shipment
Protection of the surface of the mirror from abrasion during transportation to the laboratory is imperative and easily accomplished. Place two or three thicknesses of absorbent tissue over the concave face of the mirror, and on top of that place a clean thin board, such as plywood, about 6 1/2" square. On a second similar board spread some absorbent cotton, and lay the mirror on top of that. Cut out some strips of corrugated paper, less than an inch wide, and wrap them around the mirror's edge. Then bind the whole together with adhesive tape, holding the mirror immovable in a compact package. This is then placed in a container of metal, wood, or corrugated board, surrounded by ample cushioning material. If the diagonal is to be included, cut the center out of a piece of corrugated board and place it on top of the package containing the mirror; wrap the diagonal in cotton and put it in the center hole, place another sheet of corrugated on top, tie all together with string and place in the container. As a precautionary measure, an addressed tag should be attached to the inner parcel.
Care of the Optical Parts
Because of the nature of the reflecting telescope, its owner is powerless to do anything in the way of protecting the mirrors during exposure, with the result that dust, dirt, and the elements get in their licks unopposed. But a reasonable amount of protection can be given to the optical parts when they are not in use. Of course, a perfectly designed cell is one that permits the mirror to be quickly removed from the telescope and replaced again when needed, without altering the direction of its axis. In this way the mirror can be given complete protection between observations. The diagonal can be equally well protected by designing a dust-tight cap to fit over it. Such arrangements have been devised by a number of amateurs and have proved to be most practical. But since the tube can be brought indoors (not to be stored in a damp cellar), the mirrors may remain in their places, and at the same time can be protected with tight covers over each end of the tube. Cap-like refrigerator dish covers, made of rubberized fabric with hemmed-in elastic bands, are serviceable for this purpose. The eyepiece opening should be plugged up or similarly covered.
Although it is not a regular occurrence, dewing of the mirrors may take place whenever the temperature falls below the dew point of the night, and ordinarily there is no way of preventing it. Dewing will first be noticed on the eyepiece lenses or on the secondary mirror, at which time it will be wise to seal up the mirror end of the tube with a tightly fitting insulated cover. This will probably forestall condensation on the mirror long enough to enable the observer to conclude the evening's program. Eyepiece lenses, and the surfaces of a prism (if employed for the secondary reflection) can be safely wiped dry with a soft cloth or absorbent tissue, but it is not practical to attempt this in the dark on an aluminized diagonal. Pieces of blotting paper, cut to the size and shape of the diagonal, may be kept on hand for such an emergency, and a piece can be pressed lightly against the dewed surface. If any dirt is present no scratches will result, and the dirt and any lint left by the blotter can be removed at a more favorable time.
It is not necessary to bathe the mirrors each Saturday night. At long intervals, most of the thin layer of dust that will slowly accumulate despite the best of care can be blown off. Washing should be resorted to as infrequently as possible, as each washing-further thins the metallic film. A mild face soap and distilled water or freshly trapped rain water should be used for the bath. Immerse the mirror in a soapy lukewarm solution and allow it to remain there for a time, so the surface dirt can be dissolved. Then, with the mirror still submerged, swab its surface lightly with sterile absorbent cotton, renewing the latter if necessary. A gray surface film resulting from chemical action of the elements may be present on the mirror, and this can be pretty well removed by rubbing a little more briskly with the cotton after all of the coarse dirt particles have been washed off. Finally, flush all the soapy water away in clear distilled water, and spread a clean handkerchief over the surface to dry it.
Occasionally, you will hear of a camel's-hair brush being recommended for cleaning the mirror. Now, the manufacturing optician often finds it necessary, in such operations as testing, cementing of lenses, or sealing up of lenses or prisms, to remove every last particle of dust, and the camel's-hair brush is an admirable tool for the purpose. But the reflecting telescope owner is not interested in isolated dust particles; he waits until there is a sufficient accumulation and then a bath is the only safe method of removal. Furthermore, unless the brush is drawn across the mirror very gently, and held at a low angle in so doing, the ends of the bristles are almost certain to leave sleeks (fine hairline scratches) in the soft aluminum film.
Eyepiece lenses, when soiled, should also be removed from their cells and given a bath. In replacing them, avoid leaving finger marks on the surfaces, as the perspiration oils contain an acid that has a corrosive action on the glass.