NOW THAT 16" telescopes are being tackled by more and more amateurs, advice about ways to work large mirrors is being sought. A 16" may be worked by hand atop a full-size tool. Porter did this years ago and Wm. Buchele, Toledo, in 1939, worked a 100-pound 21" mirror that way but was very muscular. Plate glass tools are satisfactory against Pyrex. To fore-fend against sticking, no joke if on a 16", the tool should be channeled and the lap sub-faceted as described by Ferson in "A.T.M.A.," (printings since 1944, June), pages 101 and 99. Better than the full-size-tool, face-down method may be the sub-diameter hand tool, face-down method described in "A.T.M.A.," page 49. Not all have had happy results with this method; it is hard to confine strokes to desired zones and to apply even pressures. More experience letters that would help others are needed concerning it.

Probably a 16" mirror calls for a machine. The Hindle machine is analogous with full-size-tool, mirror-on-top hand work. Neither method scientifically provides what Dévé in "Optical Workshop Principles" calls the "condition of uniform wear," hinted at by Driscoll in this space 1945, November. Referring to the common mode of working he wrote: "On a concave-surface a polisher having the same diameter as the work will have a tendency continually to shorten the radius of curvature polishing the center more than the edge. Therefore a hyperbola develops. In trying to lengthen the radius and get out of this hyperbola the worker resorts to very short strokes and thus, by polishing the edge more than the center, he lengthens its radius and winds up with two foci."

Suppose now, we substitute a machine (or analogous hand methods) using sub-diameter tools with such correctly chosen (1) size (2) stroke length (3) endwise decentering (4) sidewise offset, that the curve will sink as a whole-no change of radius, no zones of differing radii, just uniformly thin slices off the mirror, curved of course. This is the condition of uniform wear and you can learn the needed combination of the four factors named by experimenting a long time with an old mirror and connecting causes and effects; or by digesting Ritchey's classic "On the Modern Reflecting Telescope, and the Making and Testing of Optical Surfaces" (out of print, rare); or by reading this department next month. Meantime, let's look at two modified Draper machines.

There is no officially correct, ordained form for the Draper but the one shown in Figure 1, reproduced by permission from Strong, "Procedures in Experimental Physics" (Prentice-Hall, New York) is well regarded by many, though its trappy top hammer and slippy vertical crankshaft belt are regarded by others as details not to imitate. The machine from which this drawing was made is still in use at the Mt. Wilson Observatory Optical Shop and a tracing of Figure 1 was sent to Ralph Dietz, a former amateur then working there, to elicit comment. He replied: "The machine in Strong's book is the best type as far as I am concerned; I have used many but like this one best."

On specific details of this drawing he commented: "The belt over the arm is used only in roughing out or fast grinding." Some find Drapers, like Hindle's, are not fast excavators and suggest an auxiliary for hogging out, perhaps a hand lever spindle like "A.T.M.," page 163, Figure 3, C, with 5/6 diameter channeled glass tool or half-diameter ring tool (circle of glass blocks pitched to metal backing). Broadhead temporarily belts the motor directly to the turntable pulley, substitutes for the turntable a convex sub-diameter iron tool, face up, and holds the mirror by hand. Or the Draper may be used normally but with as fast strokes as it will stand and the tool forcibly rotated. (Note: Universal joint in Figure 1 is used only for hogging out.)

Dietz again commented: "Arrange machine so that driving pin can be shifted along arm-very important and useful." This calls for a slot adjustment in the arm; see "A.T.M.," page 165, original Draper. This in turn nullifies belt atop arm. A cone pulley there would, it is true, afford four positions but entire freedom would be still better. For ordinary grinding and polishing, also figuring, Broadhead favors an entirely free tool, as does Ferson for all purposes. Ritchey insisted strongly on forced tool rotation. Numerous advanced amateurs today can match Ritchey in skill. Take your pick; it's a free country. An effect similar to decentering along slot may be had by off-centering mirror on turntable. Ferson often uses this on his Draper machine.

Dietz further wrote: "If afraid of wrist-breaking crank at table level in the Strong drawing, substitute the crank I sketch." This is re-drawn in Figure 3, at A. Central bolt might enter from beneath and have big wing nut. Better than set-screw, weld round eccentric on vertical crankshaft. Sketches in Figure 3 are your scribe's and your groans are excusable. Would that Porter were handy. A draftsman who himself is a telescope maker and therefore knows what he is drawing, and also is an artist, is a combination devoutly to be desired. Many have written and spoken glowingly of Porter's pencil-skilled help to our hobby.

Dietz comments, finally: "I never used fancy counterweight lever on top but piled lead weights on collar on driving pin." This reminds one of Harold Lower's comment about his simple hogger-outer in "A.T.M.," page 410:

"Our machine was so damn' simple that fellows who copied it added fancy complications, and then it was out of order half the time." Perhaps Lower overlooked "Yankee ingenuity;" that is, never leave simple what can be possibly be complicated.

The "adjustable guide" in Figure 1, which permits changing offset of stroke without stopping is important. If you must stop you will not fully exploit such a feature, which Ritchey regarded as highly valuable. This, too, might be complicated but the piece of gas pipe shown, running in a plain open metal or wooden notch, is as good.

In the Draper the stroke is slightly elliptical which, as Broadhead points out, brings the lap to two zones at ends. Of strokes instead of to same zone at either end. This would not be the case if decentering were gained by off centering mirror on turntable.

As it is difficult to avoid high extension of crank pin above bearing at tabletop this shaft should be rugged and a little more than generous. Majority of machines made by amateurs have had too light shafts-whippy, some scandalously so. The bearing may be made from a common cast-iron, plumber's floor flange bored out in a lathe. No babbit is needed since cast-iron is porous and absorbs oil.

No scaled blueprints for Figure 1 are to be had. Original drawing in Strong's book is twice present scale, also includes detail of turntable shaft, its head and plate. But most advanced amateurs own this book or should.

One of the best machines your scribe has seen is a modified Draper (Figure 2) built by Dave Broadhead, Wellsville, N. Y. Frame is a double A-frame a big saw horse of two-by-fours Length 44", leg spread 36". Height to table top 38" (make it to suit your own legs or high stool). Lower part of legs does not show in illustration. Table top is two two-by-sixes.

Same illustration shows motor with step pulley, lower right, and long transverse belt (sag helps adhesion if pull is on lower member). At left is a worm gear speed reducer, vertical type, 36: (belts are less satisfactory for high torque at low speeds). Directly couple above reducer is a Ford transmission Belt for side drive to turntable has pulley hidden within housing. Crank is at top left. Arm is a two-by-four ending with 3/8" gas pipe in metal notch.

Turntable shaft (full 1-1/4") has 7/8-14 SAE right hand thread at top and is shouldered. Most commercial machines have 3/4" 10-pitch National Coarse threads, at least for work up to 10", and spindles which rotate counter clockwise.

Arm slides freely up or down around drive pin (Figure 3 D), its weight not on tool. Additional weights, if used (but see Broadhead's "Moist" technique, this department, 1947, April), are added to top of single weight shown.

Figure 3, E, shows two simple vertical thrust bearings for shafts. Omitted in the sketch is some kind of lateral retainer; otherwise the shaft might some time hop off the ball.

End of gear shift handle does no show in Figure 2-bent up for easy reach. Transmission gives three speeds and plenty of torque. Broadhead say his 1/10 h.p. motor at 1750 r.p.m. give 600-pound thrust, in low gear. He uses slow speed for more exacting work- getting smooth spheres, figuring. Nine speeds permit choices that work out in similar lineal speeds regardless of diameter of mirror.

A car light-dimmer floor switch in the power line has been useful as a quick stopper for emergencies.

Action of the polishing lap, free to rotate automatically, is interesting to watch. If the initial contact is poor the lap rotates retrograde from the arm but after a minute or two as contact improves it slows, stops a moment, then reverses; thereafter rotating in the same direction as the arm.

Nearly all manufactured machines for use in the optical industry-that is, commercial machines-have free tools but Ritchey insisted strongly on driven tools, writing "the slow rotation of the grinding and polishing tools is rigorously controlled by pulleys and belts." Which, then, is the correct way? Proponents of each argue heatedly but the cynic punching the type mill on which these comments are written suspects that the correct way is largely the one you happen to adopt and get used to using. If this is the case, may it not be best to fall back on the principle, when in doubt plump for the lesser complication and resist temptations to exploitation of Yankee ingenuity.

Arc stroke machines like "A.T.M.," page 163, B, are basically Drapers. Their "adjustable guide" feature shifts automatically each stroke, spreading the work over two zones. The otherwise elliptical stroke is deformed into a kind of bent hot dog shape.

Triangular (German) machines like A, same figure, are still more versatile. Not shown in sketch is adjustable speed drive for secondary arm, also sliding attachment of that arm along main arm. Note that secondary arm may be lengthened, shortened. Gadget above letter A is a bevel gear screw for altering spindle level. On this German, Zeiss, or triangular machine the "adjustable guide" shifts automatically but not so often as in the arc-stroke. Back geared. Ritchey chose to shift this (his "transverse slide") by hand and often, but he usually worked on such big mirrors that this may have seemed to be a better way to distribute the work widely over the zones. Anyone who knew Ritchey might be likely to say that, with his meticulous nature, it would have been difficult for him to sit and watch the machine do it all. He would have to have a frequent hand in it. The transverse slides of his machines were equipped with a screw to perform this function which he did by means of a hand wheel (lathe type). This may have resulted in smoother progression across the zones but, avast, mention of this may lead some reader into the temptation to substitute a screw and wheel for the simple adjustable slide.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.