The following was read before the Louisville Astronomical Society on May 16, 1939.




Charles K. Bruce

During the winter season of 1935-36, the Louisville Astronomical Society cooperating with the Department of Education of the University of Louisville, sponsored a series of lectures on astronomy, selling tickets for the series at $2.00, with the intent of raising money to purchase a piece of glass for constructing a reflector. Dr. W. L. Moore was the lecturer, and presented a series of eight extremely interesting lectures, giving in condensed form a comprehensive outline of the essentials of astronomy. The lectures were well attended both by members and by friends of the Society.

Upon casting up the accounts it was found that the telescope fund stood credited with about $125.00 after deducting city, county, state, federal, galactic and extra galactic income taxes. Thereupon, Dr. Moore who was then, as now, the"man behind the plow", began making inquiries about prices and sizes of pyrex glass discs, and after several months, placed an order with Corning for a twenty inch disc, which arrived about September or October 1936, as I recollect. The then president of the Society, Mr. Robert Burke, appointed a committee of eight or ten members, most of whom had made smaller mirrors, as an instrument committee, with Mr. Thorpe as chairman.

Upon opening the box containing the glass, the committee received a rude shock. The twenty inch glass was much larger, and tons heavier, than the sixes, eights and tens to which they were accustomed. The very thought of pushing this mountain of glass back and forth sufficiently to see stars in it, made the committee turn pale. After much talk of subdiameter glass tools, subdiameter cast iron tools, and sundry other pertinent and profane suggestions, it was decided to call upon the Louisville Gas and Electric Company to assist, they to furnish the electricity and the committee to furnish the "gas".

Seriously, the best practicable solution seemed to be to build a machine to do the grinding and polishing. This in itself proved to be somewhat of a chore. Some of the materials were purchased from the fund, others were donated by members of the committee, and still others, solicited or possibly "borrowed in the interest of science". I am not going to torture you with pages of calculations on strokes per minute, revolutions of the tool and other technical data, as I don't know them myself. Any of you who wish to see the machine are welcome on any Thursday night in the basement of the Physics Building, east side.

Having a mirror and a machine to push it, we still lacked one essential, namely a cheaper quality glass disk the same diameter as the mirror, to serve as a tool. To those unfamiliar with mirror work, I might say that when two glass discs are placed together and the upper one is moved back and forth over the stationary lower one, at the same time maintaining a very slow relative rotation and using suitable abrasives between the two, the two surfaces gradually become truly and accurately spherical, or within the boundaries of discs, become portions of an imaginary sphere. By using this double glass disc arrangement, the formation of the desired concave spherical surface on one face of the mirror becomes a simple matter, not requiring any expensive and exacting work in machine shops such as is necessary to fabricate accurately curved cast iron tools for forming the mirror.

Now, in our hour of need - need of another twenty inch glass disk - our now president, Mr. J. J. Burke, gazed long (and longingly) into the crystal sphere, muttered an 'abracadabara' and, presto - there was the tool, a nice twenty inch disc of green bottle glass - just what the doctor ordered.

However, before we could begin the major operation of forming the curve, there were several preliminary operations that needed attention. It was decided to grind the edges of the mirror to make it truly circular, as the rough cast glass was some 3/8 inch longer on one diameter than on the other. This operation called for setting up an auxiliary cast iron grinding wheel to operate against the periphery of the turntable of the main machine. Two or three nights work with this apparatus gave us a truly circular edge with a rough ground surface. Next, several nights were devoted to smoothing up both faces of the mirror, and bringing these two planes to approximate parallellism - that is, to a condition where the finished mirror would not be thicker on one end of a diameter than on the the other. It was felt that these preliminary operations devoted to truing up the roughly cast blank would be of some advantage in mounting the mirror as well as to give it a workmanlike appearance.

There was still another preliminary operation to be performed - namely the perforation of the central hole which is advisable where it is intended to make a Cassegrain type of instrument. While it is possible to make a compound reflector without perforating the primary, it is then necessary to introduce a third reflecting surface with its inevitable loss of light, whereas with the perforated primary, only two reflectors are made. So it was decided to perforate the mirror, which operation was simplified by Mr. J. J. Burke's kind offer to lend us the use of equipment at the Central Glass Company's plant. Accordingly, on the appointed night, Mr. Thorpe and Mr. Steele loaded the mirror into the 'chevie' and carried it down to the Central Plant on Broadway. During the process of unloading, it was dropped and broken into three pieces - or so I was informed upon my later arrival. however, Mr. Thorpe and Mr. Steele, with great skill and ingenuity, had welded the three broken pieces together with such perfection that I was unable to locate the places where it had been broken, with a magnifying glass. So it was decided that no harm had been done to the mirror, although several reputations for veracity had been stained, and we therefore proceeded to watch one of the skillful glass workers at the plant, who had kindly offered his services, make a perforation. Perhaps it would interest some of you to know how this was done. A hollow brass tube about three inches in diameter was mounted in a drill press so as to rotate about its central axis, located vertically, and the mirror blank was placed below the tube with its center coinciding with the axis of the tube. The tube was then set in rotation and pressed against the face of the mirror, at the same time applying water and carborundum grains. This apparatus cut a circular groove in the glass, so that when the cut was completed, the central plug was lifted out complete.

Now, having cut the hole through the mirror blank, we immediately plugged it up again by cementing back in place the piece just removed. The object of this apparently illogical procedure is to allow any changes of shape, which might be caused by perforation, to take place before, rather than after, the formation of the reflecting surface. While such changes of shape due to perforation would be extremely minute, they would be inadmissable on a good mirror.

All of these preliminaries having been gotten out of the way, we were ready to begin forming the curve.

Previous discussions had settled on a focal ratio of five as being a good compromise value to permit satisfactory use of the completed instrument both as a Newtonian and as a Cassegrain. With this point settled, the depth to which the glass must be scooped out becomes known, and in our case amounted to a little over 1/4 inch. It became then, a question merely of patience and perseverance until this was accomplished. However, the committee had to be on its guard constantly to avoid the consequences of that law of nature which I like to call "The Principle of the Perversity of Inanimate Matter" - or in other words, if you don't watch out, something will happen and it won't be nice.

Finally the curve was found to be of the desired depth when measured by several different methods, and fine grinding was begun and carried to completion with only one unfavorable incident caused by a tiny flake of glass breaking off of the edge of the tool and producing two scratches which were too deep to remove except at the loss of a couple of month's progress. As they will mean nothing in the performance of the telescope, they are being ignored.

During this last month the lap has been cast and applied to the tool. After a short spell of polishing we shall find out with the optical tests whether the curve is sufficiently true to continue the polishing, or whether a return to fine grinding will be necessary.

During the last stages of the fine grinding, too rapid or too long continued working usually results in a so called deep hyperbola on the mirror. This is impossible to detect until the surface is partially polished so that its figure can be seen, and there is little that can be done to avoid this except to carry on the last stages of fine grinding in short spells of work alternated with much longer spells of resting to permit equalization of temperature and conduction of the heat generated during the working speed to all parts of the mirror and tool. Also, the temperature fluctuations of the work room can cause difficulties of this type, and, during the winter months our workroom had its full share of fluctuations.

And so this brings us up to the present, and the end of my story.

                                      Read before the LAS on May 16,1939.

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