White's North Star Observatory, highest in Canada (3,600 feet elevation), is set on a round concrete slab 18 feet in diameter. To this slab eight five inch wheels (adapted from V-belt drive pulleys) and their bearings are bolted. An angle iron track is attached to the dome unit and thus rests on the wheels.

The observatory walls are made of quarter inch, three-ply, resin-bonded fir veneer that is screw-nailed to a base ring made from two layers of overlapping spruce segments and 32 light cedar studs. There is also an upper spruce ring like the first. The dome is made of quarter-inch veneer gores screw-nailed to ribs three inches deep. The ribs, like the rings, are built of overlapped segments.

White's double shutters open and close on eight three-inch roller-bearing tackle block sheaves, using Scanlon's method of endless cable described in Amateur Telescope Making-Advanced. Unlike most others, White's shutters open a full quarter the diameter of the dome, as recommended by Scanlon, and they extend to the zenith and 10 inches beyond. Prospective observatory builders who discount the importance of a wide slit opening and a large observatory diameter (White's is 15 feet) to accommodate whatever telescope they might want to install in the future, might well consider making their first observatory a poor job. This will effect an ultimate saving when, after enough cussing, they finally tear it down and rebuild it right.

BECAUSE polishing is a far slower method of removing glass than fine grinding, amateur telescope makers have long stressed the importance of avoiding half a lifetime's needless work by bringing their concave mirrors to a truly spherical surface before departing from the grinding stages. In 1935 the recommendation "Get a Sphere Before Beginning to Polish" was inserted in Amateur Telescope Making and in an earlier edition (1928) another injunction appeared: "Getting Contact Before Polishing." Amateurs have long tested for sphericity before polishing by the pencil mark test or by the safer method of putting a quick, superficial polish on their mirrors with a temporary pitch lap. The latter permits making a Foucault test for checking the sphere.

The Bausch and Lomb Optical Company now furnish details of their recent adoption of this principle and of a production method of accomplishing it with waxed surfaces and test plates which produce Newton's rings.

"The use of wax to test ground glasses started," they state, "in our development laboratory where a check on the flatness of optical surfaces was required. The fine-ground surface is covered with a hard wax. We have used shoe polish, soft red pencils, Johnson's Floor Wax and different types of liquid wax. The only requirement is that the wax be fast drying and hard.

"After the surface has been coated with a liberal amount of wax it is allowed to dry. Then a hard cloth such as canvas or sheeting is used to wipe up the excess wax and shine the wax that remains between the grindings of the glass. Once one spot of the cloth is used we do not change, for the wax on the cloth tends to fill in any holes not completely filled previously.

"After the piece is completely shined with wax, the test glass is used much the, same as on a polished surface. To get a brilliant pattern of interference fringes a good monochromatic light source is needed, since the reflection from the wax is much lower than from polished glass. The pattern seen will be fuzzy but will give a very good indication of the overall curvature of the lens or flat. We grind and polish until we have a grind fairly close to the test glass and free from zones, and then we clean the wax from the lens and polish.

"By this test we found we could save many hours of polishing by starting with a zone-free grinding Job. The practice has spread to our production department, where we are checking grinding on both planes and weak spheres. This method is good in testing spheres of more than 100 mm radius of curvature but it is difficult to check lenses having a shorter radius."

Amateurs who make several identical mirrors or lenses, as is often the case, may find this use of wax and test plates valuable during fine grinding. It can also be used on single jobs when grinding flats.

One flat which could not be tested against a test plate at any stage was the 120-inch surface made at the California Institute of Technology optical shop some years ago as an accessory for testing the 200-inch mirror at the focus. (Since other tests were applied instead, this, the world's biggest flat, never has been used.) The flat was tested during grinding, Russell W. Porter says, by stretching a piano wire above it, calculating the wire's catenary sag, and measuring the vertical distances from flat to wire. It worked!

Next, after enough fine grinding to give specular reflection at grazing incidence, as shown in the accompanying drawing, a test similar in principle to the one in Amateur Telescope Making, page 242, was used. This was Ritchey's test.

Louis J. Rick of 685 North Ridge Road East, Lorain, Ohio, reports a valuable kink he accidentally stumbled upon while fine-grinding his first mirror. "I obtained a very soft drawing pencil, a 6B, with which I drew a broad 3/16-ineh stripe across the horizontal diameter of the mirror. Under the Foucault test the stripe was a better reflector than a wet unpolished mirror and showed enough of he horizontal section of the Foucault shadow to permit determining the curve."

PROVIDENCE, in the state whose full and official name is Rhode Island and Providence Plantations, has for years been he center of an active group of amateur telescope makers. In wartime, the Providence group helped to train more than 50 persons in the technique of grinding and polishing glass and then made glass master gauges, optical flats and colored glass filters (thin plane-parallels) for bombsights and sextants. One of this group is professor C. H. Smiley, head of the Department of Astronomy at Brown University, who as an amateur telescope maker contributes the following practical aid to the amateur "glass pusher."

"Two of the problems which beset the new hand at telescope making are determining whether tool and mirror are making good contact at a given stage of grinding, and estimating the quality of a given fine grind, especially the last one before polishing. If mirror and tool suddenly seize, it probably will be because of poor contact combined with inadequate lubrication. (If you do have such a seizure, be very careful to wash both mirror and tool after separation before going on, because in such an event some glass is almost always removed from one of the disks, usually at an edge, and bad scratches often result.

"Hours of polishing may be saved by a little extra time and care spent on the last fine grind before polishing. It is proposed here to outline a method by which one can determine whether contact is being made and by which one can measure approximately the quality of a given fine grind.

"If you hold a mirror on your hand almost horizontally at arm's length in front of you, even in the coarser grades of grit you will be able to get a reflection of an electric light down the hall or diagonally across the room. The light ought to be the only bright light in that direction and there should be relatively little light coming in from the sides. An old-fashioned clear glass bulb with the filament showing is perhaps best for this purpose although a good bright frosted bulb will work almost as we]l. With the mirror well out in front of you, rock it back and forth in such a way that the reflected image of the light moves alternately toward you and away from you. Slowly lower the mirror. As this is done the reflected image grows reddish and dim, eventually is lost. Just before it goes, rock the mirror back and forth very slowly, watching to see whether the image grows dim at one spot more than at another. A dull gray area with poor reflection indicates a zone on the mirror which is not in good contact with the tool; you can judge better on the portion of the mirror away from you than on the nearer part. This is particularly true as the hand drops lower and lower with the finer grades. As a check on your judgment, rock the mirror sidewise a bit and see whether the gray areas seem to come in the same zones as when the image is rocked toward and away from you.

"This test does not, however, tell you whether the given grade of abrasive has been used long enough. One still needs a magnifying glass to see whether the pits that remain are essentially of uniform size.

"Now, as the fine grinding proceeds, one finds that the hand may be dropped lower and lower before the reflected image is lost. Also the region in which a red image is seen increases in size so that there is a generous warning before the image is lost entirely.

"If now the angle from the light down to the position of the mirror at which the image is lost is measured, the quality of the fine grind can be judged fairly well. A simple and rapid method by which the free hand may be used as a caliper to measure this angle is as follows.

"Suppose the reflected image has finally been lost when the mirror, held in the left hand, is seen in line with a particular crack in the floor or opposite a particular pattern in the carpet. The right arm is now extended in front, full length, parallel with the left. Its fingers and thumb are opened as widely as possible, as in thumbing your nose. The wrist is rotated so that the thumb is straight down, the little finger straight up. With the little finger in line with the light, the point at which the tip of the thumb lines up with the floor is now noted, and the arm is lowered until the little finger on top lines up with the same point. Only one eye is used and the head is, of course, held steady. This spacing off process is repeated on down to a point level with the center of the mirror and the number of 'hands' is counted.

"One then estimates that the mirror is good for about one, three, or so many hands down, as the case may be. Fractional hands may be counted but it is hardly worth while to estimate nearer than a half hand. In general, the number of hands measured by one person will agree fairly well with the number found by another person of radically different dimensions.

"Our experience has been that a good grind with 500 Carbo or Aloxite should show about three hands down; 303 emery will give four hands, and 303 1/2 emery should give five and a half or six hands, but the average beginner usually wants to quit at five hands down and start polishing. In terms of time and energy this is a very expensive procedure. He should go back to 3031/2 for a final wet, adding only a little water from time to time and slowly working out the abrasive. The pressure of the hands should be lessened in the later stages of fine grinding until, in this final step, it should be just the weight of the mirror applying the pressure. If you hold your ear down close to the mirror as you move it back and forth in this final stage you will hear a faint swish reminiscent of silk. This 'repeated dilution of the final wet' resembles the 'Ferson technique' of drying up the final wets, adding only a very little water, and working the emery out to the finest possible state. The latter is described in detail by Ferson in Amateur Telescope Making-Advanced. [In printings subsequent to June 1944.-Ed.]

"When you get down to four or five hands you should take care that no oil or grease is applied to the mirror as you wipe the last bit of moisture from it. Only a small amount of grease is enough to mislead one seriously as to the quality of the fine grind. Just rubbing the hand over a mirror several times appears to provide the grease which misleads.

"Although this note is not meant to cover polishing, one of the commonest troubles encountered in polishing is the 'Mexican Hat.' Under the Foucault test, with the mirror in an intermediate position, there appears to be a turned up edge (the brim of the hat) and a hill in the middle (the crown of the hat). As long as the hat is a fairly flat one, in comparison with its diameter, this is not a serious matter. Care taken in pressing the mirror on the lap each time before starting polishing, and in using a very short stroke, will usually prevent the appearance of a Mexican Hat and many times will remove one after it has appeared. It seems likely that the hat is a result of the natural curvature of a pitch lap near its edges due to surface tension."

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

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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