"I'm just off Palomar Mountain and had a look at the 200" mirror at the prime focus with a knife-edge, using a 9th magnitude zenith star as light source, a sight never to be forgotten. Its image was so bright and appeared so real that one felt he could take it with his fingers and feel it.

"This was the first time I had ever seen the Foucault shadows from a large mirror with light coming through our atmosphere from a star. The seeing was about 2 on a scale of 5 (so Humason estimated) and showed up remarkably the turbulence of our atmosphere. I would liken the appearance, when the light was about half cut off by the knife-edge, to reeds or eel grass in a rapidly flowing river, weaving back and forth in the stream. This shredded appearance of the atmosphere was thought by Dr. Anderson to have its origin about two miles up.

"Later in the night the seeing improved. The Foucault shadow darkened uniformly all over the mirror with the cut-off from any direction.

"All the men who understand mirror testing-Bowen, Anderson, Hubble, Brown-were highly elated at the mirror's figure.

"The sketch I enclose was made the second day and shows the lower end of the telescope tube, looking up under the cell that supports the mirror. In the many openings at the bottom of the cell may be seen traces of the supporting levers. It was below freezing and this drawing was made in a hurry. After about an hour my hands got so cold I couldn't hold my pencil-poor ol' arctic explorer."

To telescope makers these results sing! Interpreted for others they mean:

As the mirror maker works he stops often to test, throwing on his mirror the light from a tiny illuminated pinhole in a distant screen (artificial star). The rays return to his eye and are examined (1) with a telescope eyepiece (low-power microscope) inside and outside focus (2) with the eye by skillfully cutting them with a knife-edge. The first gives qualitative overall tests, the second highly precise quantitative tests of all areas. Finally the mirror is similarly tested in its telescope on an actual star. "Perfect circles" and "darkening uniformly" mean success. Astronomers celebrate with expansive grins.

Later information: The mirror tests satisfactorily in all positions.

At present, the "Diamond Dust' sold by lapidary supply houses costs $2 a carat, but one carat is enough for quite a few experiments. This material is a by-product of the diamond cutting business and can be seen under the microscope to consist of white flakes of all sizes from powder to fairly large pieces. This is the cheapest way to buy diamond but, naturally, such a mixture cuts and polishes at the same time and leaves the work more or less scratched. To get the fastest cutting or to produce a smooth surface it is necessary to have the abrasive graded into at least coarse, medium, and polishing sizes.

In grading diamonds the larger pieces are broken up in an Abich mortar. These can be obtained from scientific supply stores or easily made after an illustration such as the one in Orford's "Lens Work for Amateurs." The reduced material is then levigated in the usual manner, except that oil is used instead of water. Of course it is impracticable to levigate a small amount, also the presence of the oil makes the whole thing very messy. Diamond dust is perhaps easier to buy prepared than to make. The Elgin National Watch Co., Aurora, Illinois, sell well-graded material mixed in a jelly for easy handling. Needless to say, the finer grades, which are comparable to rouge in particle size, are more expensive but will put a beautiful polish on diamonds, quartz, glass, Stellite, Carboloy, or almost any hard substance.

The following method was used for rapidly making several 1" diameter lenses from a very hard substance. By the process described it takes: about two hours' total time to shape and polish one side of a lens having hardness 9.

First, however, before starting to use diamond the optical worker should be warned of diamond poisoning. If a single flake of this very hard material lands on a smoothing tool or metal lap, the tool will probably have to be remade or thrown away. Unlike softer abrasives it seems never to break down on glass and will continue to scratch as long as the tool is used. Diamond dust on machines and shelves will do wonders in wearing and scratching. Once anything is contaminated with it it seems impossible to get rid of it. For this reason diamond dust is always kept wet with oil to reduce its tendency to fly about. With proper care it can be used like any other material but carelessness may result in a situation so bad that the easiest thing you can do is to abandon your shop.

To make the lens mentioned above a concave lap of the proper radius is cut in soft annealed copper. The lap is then mounted on the spindle and, while slowly rotating, a few drops of olive oil containing a few grains of diamond dust (lapidary grade) are applied with a stick. The diamond powder is then rolled into the surface of the lap with a specially made tool consisting of a handle with a forked extension between whose tines is mounted at right angles a barrel-shaped roller about 1/2" long and 3/16" in diameter turned out of a drill rod. This is drilled axially with a 1/16" hole for its bearing in the implement. It is then polished, hardened as hard as possible, and repolished with crocus paper. The convexity of the barrel-shaped surface of the roller should be slightly greater than that of the lap, so that the barrel will roll on only a small area. The completed roller should turn freely on its axle and have a polished surface.

When the roller is first applied to the rotating lap it can be felt to roll over the gritty diamonds, but as it is worked all over the surface several times the abrasive sinks in and the rolling action becomes smooth. With a lap of the size described this should take only a minute. The amount of pressure to be used on the roller is difficult to specify. It should be great enough to embed the abrasive but not great enough to deform the lap.

Once the lap is charged it should cut for a long time without any attention. Whenever it seems to be dull it can be recharged in the same manner.

The lens to be shaped is fastened to a handle, pressed against the rapidly rotating charged lap, and worked with the customary motions. For this small size the lap speed can be several r.p.m. The lap must be kept lubricated at all times with oil, preferably olive oil, applied a drop at a time with a stick. It is important that the pressure used on the work be correct, but a feeling for this soon comes naturally. If the pressure is too light the work will, skate over the lap with no abrasive action, but if it is too great the abrasive particles will be dug out of the lap and it will need to be recharged.

The lenses that were shaped by the above processes were semi-polished but remained covered with fine scratches from the unsorted diamond material used. These scratches were removed with fine emery on a brass lap. Polishing was then done in the usual manner. If finer grades of diamond had been available, fine grinding and polishing could have been accomplished using additional identically curved laps. It is not feasible to change grit on the same lap, as is done with Carbo, because with this type of charged lap the abrasive remains embedded in the metal.

Rouge: It is generally conceded that no substance puts a better polish on glass than plain red iron oxide rouge. There is, however, considerable variety in the quality of rouges, also several means for refining it when the need arises. Some workers remove the larger particles with a bruiser, working a thin red paste between two glass plates and forcing the big grains to the edge where they are broken down and lost. Others levigate rouge with dispersing agents and flocculate the suspended material as was described by Parsons in the wartime Roof Prism Program letters. Almost everyone who does careful work washes rouge in some way, even if only to stir it up with water and pour off for use the part that doesn't settle in a given length of time.

Rouge may be used to polish almost any material except metals, many of which are stained by it.

Cerium Oxide: Long in use in France, this material came into widespread popularity during the late war. Pale pink and much cleaner to handle than rouge, it does not "stain" glass and it polishes much faster-some say two to three time faster. In 1947 cerium oxide cost $2.75 a pound in small quantities, which makes it a good deal more expensive than rouge, but the quantity an amateur would use makes the difference inconsequential. Some mirror-making kits now contain this new abrasive and it is preferred by many hand workers because of its greater speed, though they may go back to rouge for figuring because the latter is easier to control.

Barnesite: During the war this mixture of rare earth oxides was used by many large plants. It contains a large percentage of cerium oxide and has many of the same properties. Most samples are dark brown and have a peculiar soft feel. It is claimed that Barnesite polishes almost as fast as cerium and produces as good a surface as rouge. Further, it absorbs water, does not dry out rapidly, and needs less attention on the machine. At present the cost is less than that of cerium.

Putty Powder: This is the lapidary's name for tin oxide, the white, powdery material that Newton used for polishing his metal mirrors. It is also used for polishing petrified wood and agate specimens and is still the best polishing agent for many metals-Stellite for example, being polished on a beeswax-coated pitch lap with tin oxide. Due to the shortage of tin, putty powder has been hard to get for some time.

Titanium Oxide: So far as I know, no one else uses this substance, which is generally classed as a paint pigment. The duPont grade Ti-Pure R-200 (available only in 50-pound bags) is pure white, easily dispersed, and extremely fine. It does not stain most metals, will polish glass, quartz, and harder gems, and has no peer for polishing plastics.

Finally: Some metals are very difficult to polish and in desperation such unorthodox stuff as lampblack has been successfully used. If you have a grinding or polishing problem you may be forced to discover something new. This list makes no attempt to include even all the commercially available possibilities.

End of article by Holeman. A note on Barnesite, crowded out by the happy report on the 200", will appear later.

Suppliers and Organizations

The Society for Amateur Scientists
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East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

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