Mountings are usually painted or lacquered and either surface can be renewed eventually at no great trouble. Bright parts may be lightly touched with oil as an ordinary rust preventive.
Reflecting telescopes are considerably more troublesome to keep in order than refractors owing to the tender nature of the silvered surface. It may remain in good condition with fairly steady use for several years or it may go bad in a few months or a few weeks. The latter is not an unusual figure in telescopes used about a city where smoke is plentiful. The main thing is to prevent the deposit of dew on the mirror, or getting it wet in any other way, for in drying off the drops almost invariably leave spots.
Many schemes have been proposed for the prevention of injury to the mirror surface. A close fitting metal cover, employed whenever the mirror is not in use, has given good results in many places. Where conditions are extreme this is sometimes lined with a layer of dry absorbent cotton coming fairly down upon the mirror surface, and if this muffler is dry, clean, and a little warmer than the mirror when put on, it seems to be fairly effective. Preferably the mirror should be kept, when not in use, at a little higher temperature than the surrounding air so that dew will not tend to deposit upon it.
As to actual protective measures the only thing that seems to be really efficient is a very thin coating of lacquer, first tried by Perot at the Paris Observatory. The author some ten years since took up the problem in protecting some laboratory mirrors against fumes and moisture and found that the highest grade of white lacquer, such as is used for the coating of fine silverware in the trade, answered admirably if diluted with six or eight volumes of the thinner sold with such commercial lacquers. It is best to thin the lacquer to the requisite amount and then filter.
If now a liberal amount of the mixture is poured upon the mirror surface after careful dusting, swished quickly around, and the mirror is then immediately turned up on edge to drain and dry, a very thin layer of lacquer will be left upon it, only a fraction of a wave length thick, so that it shows broad areas of interference colors.
Treated in this way and kept dry the coating will protect the brilliancy of the silver for a good many months even under rather unfavorable circumstances. After trying out the scheme rather thoroughly the treatment was applied to the 24 inch reflector of the Harvard Observatory and has been in use ever since. The author applied the first coating in the spring of 1913, and since that time it has only been necessary to resilver perhaps once in six months as against about as many weeks previously.
The lacquer used in this case was the so-called “Lastina” lacquer made by the Egyptian Lacquer Company of New York, but there are doubtless others of similar grade in the market. It is a collodion lacquer and in recent years it has proved desirable to use as a thinner straight commercial amylacetate rather than the thinner usually provided with the lacquer, perhaps owing to the fact that difficulty of obtaining materials during the war may have caused, as in so many other cases, substitutions which, while perfectly good for the original purpose did not answer so well under the extreme conditions required in preserving telescope mirrors.
The lacquer coating when thinned to the extent here recommended does not apparently in any way deteriorate the definition as some years of regular work at Harvard have shown. Some experimenters have, however, found difficulty, quite certainly owing to using too thick a lacquer. The endurance of a lacquer coating where the mirror is kept free from moisture, and its power to hold the original brilliancy of the surface is very extraordinary.
The writer took out and tested one laboratory mirror coated seven years before, and kept in a dry place, and found the reflecting power still a little above .70, despite the fact that the coating was so dry as to be almost powdery when touched with a tuft of cotton. At the start the mirror had seen some little use unprotected and its reflection coefficient was probably around .80. If the silver coating is thick as it can be conveniently made, on a well coated mirror, the coat of lacquer, when tarnish has begun, can be washed off with amylacetate and tufts of cotton until the surface is practically clear of it, and the silver itself repolished by the ordinary method and relacquered.
There are many silvering processes in use and which one should be chosen for re-silvering a mirror, big or little, is quite largely a matter of individual taste, and more particularly experience. The two most used in this country are those of Dr. Brashear and Mr. Lundin, head of the Alvan Clark Corporation, and both have been thoroughly tried out by these experienced makers of big mirrors.