IF THE FACETS of a pitch lap are arranged symmetrically, a mirror polished on it may show zones. All instructions for making pitch laps therefore specify that the center of the lap must fall in one corner of a facet. But even this method may produce zones, because the channels on either side of the center are equidistant. For that reason J. W. Draper of Warren, Ohio, suggested to Leo J. Scanlon of the Valley View Observatory, Pittsburgh, Pa., that the channels adjacent to the center of the lap be spaced at unequal distances from it, as shown in the left-hand design below.

After using this Draper lap layout in teaching mirror making at the Buhl Planetarium, Scanlon praises it highly. He also finds it well worth while to design laps with care on paper and then transfer the design to the pitch by embossing through to the lap.

A refined, delicate, special-purpose lap originally devised by John A. Brashear and used in modified form by George Croston of Aloha, Ore., is shown in Figure 1. It should be made up on a Pyrex or metal backing because it will be cooled under running water, which would crack plate glass. The ripsaw-toothed profile of its long ridges is shown in the drawing. All work done with this lap is against the grain, with the lap on top of the work.

The purpose of this "scraper" lap is the elimination of small imperfections in the last stages of polishing; it should not be used either for normal polishing or for figuring. "It does its special job perfectly," Croston writes, "but is useless for anything else."

He continues: "The Brashear finishing lap is made of pitch that has been hardened with strained rosin. It may be made of rosin alone if the worker can form that brittle material so thin. Strict adherence to the ridge profiles shown is not essential, but their toys must be narrow and parallel. However, a vertical side or a steep slope provides a better wiping action and also tends to hold better the thin application of rouge that does more work than thick.

"To form the lap to the surface that is being worked it should be wetted with soapy water while it is still warm, worked with short strokes parallel to the ridges until it shows perfect contact, and then cooled under the tap before being charged with rouge and used.

"To remove small imperfections such as HCF prints or wiggly lines on a flat, the lap is rubbed across the glass with firm pressure and about one-sixth strokes and with the ridges at right angles to the direction of travel. After 10 strokes the mirror is rotated 90 degrees and the 10 strokes are repeated. This is usually enough to produce a perfect surface. If it does not, wash off all rouge from lap and glass (important). and dip the tips of the ridges of the lap in lukewarm water for a couple of seconds or, better still, use a heat-ray lamp. Very little rubbing and no cold pressing will be needed to make perfect contact. Cold pressing would transfer the irregularities of the mirror to the lap.

"This lap should be used at once after it is formed; otherwise it will slowly flow and then create new defects on the mirror.

"The nearest relative of this lap is shown in John Strong's Procedures in Experimental Physics, page 48."

The third lap to be described is the invention of Walter W. Puderbaugh of Richland, Wash. He calls it the sintered pitch lap. Others call it the Puderbaugh lap. Puderbaugh was fed up with pitch, which he says is "just plain hell to form. The more rosin in it the meaner it is– in short, a stubborn mess that will do everything possible to resist taking a charge of rouge." He put some pitch in a household refrigerator, chilled it, brought it out, pulverized it while it was cold between a steel rolling pin and plate glass, and fined it further with a spatula. Next he built the conventional paper dam around the tool. He painted the tool with pitch and beeswax cut with carbon tetrachloride and made tacky. Then he poured in the chilled powdered pitch, leveled it off, painted his mirror thickly with rouge paste and laid it on the lap and tool. He set the whole unit, tool up, in rather warm tap water, waited a while, took the assembly apart, added rouge, and started polishing.

He found that cutting channels in this lap with a wet blade is no trouble, and he believes the lap works faster than the common kind. It cold-presses well.

What is the principle of the sintered lap? Sintering is a method of converting powder into a cohesive mass by heating it to considerably below its melting point, generally after compacting by pressure.

Union between the particles results not from fusion but from surface cohesion. The irregular particles touch one another only at their corners. Elsewhere air spaces remain.

Puderbaugh finds that the main secret of success with the sintered lap is to supply enough heat to produce sintering without melting. Sintered pitch has peculiar characteristics. Some that he, kept for months in a refrigerator had not consolidated.

In any case, the Puderbaugh lap is radically different from the fused lap. It was first experimented with more than two years ago, and its fortunes have been watched ever since by this department. It appears to be sound. John M. Holeman of Richland now reports that "more and more local users have tried it and like it very well. They have nothing but good to say for it. Easy to make. Holds figure.

TO ELIMINATE five of Russell Porter's "seven devils" of pitch, Kenneth Falor of Bay City, Mich., simmers pure rosin until the froth and bubbles, which form in large numbers and need close watching, subside. He then adds high-grade no-detergent automobile oil of SAE 80 or 40 to obtain the desired temper. This mixture keeps its temper almost indefinitely despite frequent melting. For his inspiration Falor credits the 200-inch telescope job. He finds advantage in dropping the test samples on metal instead of glass; the metal conducts their heat away much faster than glass and saves much waiting.

IN THE PAST many amateur telescope makers have independently discovered that mirrors may be ground and polished without the conventional handle, and have reported their apostasy with quite unnecessary apology. In a debate on handles v. no handles much could be said on both sides. The correct way is the one you like best.

If a handle is preferred, it is likely to receive much use. A good one is thus worth while. Provided it does not cover too much of the mirror, its correct design is again the one you like best. The handle shown in the drawing in Figure 2, pitched to a 6-inch mirror, is a somewhat aristocratic transparent handle made by Holeman. The shaft was turned from a rod of Lucite. It is screwed to a Lucite disk 3/8-inch thick and 2 1/2 inches in diameter to receive the thrust of the hands.

With less trouble a short button may be used as a pickup handle as in the second illustration, redrawn from a little booklet by T. J. Mulligan of 1 Airthwaite, Kendal, Westmorland, England, entitled Making, a Telescope Reflector. With A. J. S. McMillan of 5 Oakfield Road, Bristol, Somerset, England, Mulligan is attempting to revive the telescope-making hobby in Britain by supplying mirror-making kits there.

The thermal shield shown above the Mulligan handle is devised not by Mulligan but by the editor of this department, and has been used for many years without causing any of the scratches that armchair theorists asserted it would create. It can easily be lifted to inspect the lap through the mirror.

AMATEUR telescope makers have often been disappointed because their mirrors would not pass that part of the diffraction-ring test described in Amateur Telescope Making in the final paragraph on page 434. This states that in a perfect mirror the out-of-focus images of a star should be alike at equal distances inside and outside focus. If there are differences the nature of these differences gives useful information about the aberrations.

For some of those who have been unable to obtain this much-desired identity of images F. J. Hargreaves, F.R.A.S., has supplied a happy alibi in The Journal of the British Astronomical Association, Volume 59, No. 3. He points out that the metal mounting of the telescope's diagonal support is usually colder than the atmosphere and that it chills the air in its immediate neighborhood. This chilled, dense and therefore more highly refractive air acts as a weak lens. Thus outside of focus the inner ring adjacent to the black central shadow of the diagonal becomes bright, as shown in Hargreaves' second drawing in Figure 3, and has a hairy or spiky inner fringe.

If there is aberration in the mirror the bright ring is absent on one or the other side of focus. The "air lens" surrounding the cold metal diagonal support has exactly the same effect, and many mirror makers may have blamed their workmanship because of it.

It remains to be explained why the metal parts of a telescope should be colder than the atmosphere. We would expect this if a telescope had just been brought into the open air from a cold cellar. But even if the telescope starts with its metal parts at exactly air temperature they will be found after a time to have a lower temperature, provided the night is clear. How can we account for this?

Others have given more than Hibbard's hint of the explanation. All bodies simultaneously radiate and receive heat in amounts depending on their temperatures. The telescope tube radiates to cold interstellar space at a rate so much greater than space radiates to the telescope that the radiation is practically all in the outward direction. This constant radiant heat leak explains why the telescope does not warm up to air temperature– unless clouds cut off the leak, in which event they also cut off the observing.

In his Meteorology the late Professor Willis I. Milham, an astronomer-meteorologist, discussed this effect which hampers meteorologists by causing thermometers to give readings from 1 to 7 or 8 degrees too low on clear nights. Meteorologists reduce this error by placing their thermometers in shelters having diagonal sloping slats like Venetian blinds that admit the air but obstruct radiation to the sky. In an article in Monthly Weather Review, a publication of the U. S. Weather Bureau, for July, 1905, Professor Milham described long, painstaking experiments which proved that night after night two matched thermometers only 20 feet apart registered 3, 4 and sometimes more degrees difference. One was exposed to the clear sky the other was within an unheated, unprotected but radiation-stopping thermometer shelter.

The metal-chilling effect of radiated heat has a direct relation to the selection of materials and forms for telescope tubes. Hibbard and J. C. Vaughan, a Virginia telescope maker, tested telescope tubes of different materials for two years. They found that:
In a solid metal tube the chilled air that constantly drained down next to the metal produced irregular refraction of light and resulted in poor images.

Lining the tube with cork, asbestos or paper improved this performance somewhat.

Holes bored in the upper part of the tube to cause air drainage there admitted cool air that streaked the image.

A solid metal tube with its central two-thirds portion removed and the ends connected only by six lengthwise angle irons did not give the expected beneficial mixing of outside and inside air. Instead, the chilled iron streaked the images. Wrapping the irons with thick black paper improved the images, but at times they were intolerable. By inserting a cardboard lining inside the skeleton tube it was proved that the chilled metal was the cause of this: the streaking vanished and the images became good. This experiment was tried over and over until the conclusion was inescapable that the metal had an entirely different temperature from that of the air, and that this difference persisted hour after hour.

In the entire two years of the tests only a single night afforded first-rate seeing with an open or skeleton tube of metal.

Hibbard and Vaughan's final experiment was to line a solid metal tube with cork a quarter-inch thick and cut a 20-square-inch hole in the tube just above the mirror. This opening, adjustable in area, ventilated and cooled the mirror and gave excellent results, especially if the hole was covered to steady the air inside just before actual observation.

After all these experiments, conducted with the understanding of underlying factors that meteorological physics affords, Hibbard stated that to build any telescope tube of aperture up to 24 inches he would use sheet metal lined with cork, balsa or Douglas fir. "The 24-inch limit," he added, "came from the late J. W. Fecker, who said he had known for many years that skeleton tubes produce troublesome images that decrease as that aperture is approached. At that point the tube effects are hardly noticeable, especially since telescopes that large are placed in domes.

Hargreaves' discovery that the fringed bright ring of his outside-focus image is caused by the refraction of chill dense air next to the metal diagonal support is clearly an example of this phenomenon of cooling by loss of radiant heat. He states that he had been puzzled by the bright ring and its vagaries for 13 years, during which he had tested a great many mirrors in his telescope. He knew that if spherical aberration is present the ring is absent on one side of the focus and that this absence might also indicate a narrow defective zone just outside the area of the shadow of the diagonal. Reviewing the problem carefully he was struck by the fact that "in every case when other tests had shown that a mirror is free from aberration, the inner bright ring had been absent from the image inside focus and abnormally bright outside focus, and never the other way round."

At last, one very windy night a clue presented itself. "Every time a strong gust struck the telescope the bright ring became temporarily one-sided–weaker on the windward side and stronger on the leeward side. The explanation," he continues, "is simple. The cell and mounting of the diagonal become colder than the atmosphere and chill the air immediately surrounding them. There is therefore a sheath or 'tube' of dense air, densest at the surface of the mounting and becoming progressively less dense radially outwardly, which acts as a weak collective lens. The effect of this air lens is precisely the same as that of a zone on the surface of the mirror; of shorter focal length than the rest of the surface and just outside the area of the shadow of the diagonal. I had been looking for such zones for years, and failing to find them."

Hargreaves put this diagnosis to the test by holding his hands on the mounting of the diagonal for a few seconds to warm it, taking care not to warm the diagonal also. "The immediate effect," he states, "was to reverse the appearance, the bright ring being transferred to the inside-focus image. After a few minutes the two images were exactly the same for a short time and then the original appearance returned" as the metal cooled.

"It must be borne in mind" he emphasizes, "that the abnormally bright ring may be due to spherical aberration or to a real defective zone on the mirror.

"In warm weather when the telescope is first opened up in the evening, the inside-focus image invariably shows a very strong bright ring round the central shadow. I now have no doubt that the effect is due to the diagonal mount being temporarily warmer than the surrounding alt."

Hargreaves is a member of the recently organized firm of Cox, Hargreaves and Thomson' of Kingswood, Surrey, England. All three partners began years ago as amateur telescope makers and after much advanced experience, turned professional. Such has been the history of most professional telescope makers. The three partners retain their amateur status as astronomers. They have received a spate of work, including a 25inch Schmidt camera, regrinding and refiguring the 80-inch mirror made 60 years ago by Common for Greenwich observatory, and a 51-inch mirror for the old Melbourne reflector in Australia.

Suppliers and Organizations

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