INGENIOUS AND controllable way to rig the base of a fairly large telescope which nevertheless must be made portable is shown in Figure 1. Two of the legs have large casters and the third has a loose lift-up lever for towing purposes. This is an 8" telescope and therefore not light. A. J. Walrath, 14024 Archdale Ave., Detroit, Mich., is the maker. It is his first telescope. He writes, "The mirror is larger than recommended for a beginner but no serious difficulty was encountered. With a 1/2 eyepiece, giving a magnification of 144, the rings of Saturn are clear and beautiful. Jupiter, too, with its moons, can be seen in good detail."

SEVERAL readers have inquired within recent months about a mirror grinding problem. Here is a typical question

"I have passed the roughing-out stage with coarsest abrasive and am now trying to obtain contact with abrasive No. 120. However, the mirror, no matter how much grinding I do, or how short the strokes I use, persists in touching the tool at the edge zone alone.''

Remembering a battle with one mirror which persistently acted in the same manner, your scribe dug up a log book for 1930 and from it has worked out the data which follow. It is not, however, claimed that these methods will always work.

Use full-diameter-that is very long -hogging-out strokes on only the first two sizes of abrasive grains. Thereafter, use regular one third strokes. That is, do not change to short strokes.

Grind with No. 220 abrasive a long time, not alone for pit insurance but also to reduce the gap between mirror and tool to about 1/2000". This can be measured with "feelers."

Let the first feeler be a tiny piece of ordinary paper-say a corner torn from a book page. Lay this on the center of the tool and place the mirror on it. Will the mirror now spin as on a turntable? Seize the mirror and tool simultaneously between the thumbs and fingers of both hands, on either side and develop any hidden slight rocking by alternate squeezing. Note an inch of similar book pages, dividing by 2. This gives the feeler's thickness-generally around 1/250" or 1/500". Now you have at least an approximate idea of the space you are dealing with.

To reduce the gap, try painting a ring of wet abrasive in the edge zone.

A method which gave still better results was to invert the tool and mirror and work the tool on the mirror in a series of epicycles limited in width to the center on one side and the intermediate zone on the other (farther might turn the edge).

Another method, which worked up to the hilt on one occasion, was to carry the wet far toward bone dry-10 minutes instead of half that time. This requires muscle and there is risk of a sticking mirror, so don't stop or lose full control. This method brought a persistent central gap into central contact very rapidly.

As finer and finer grades are used in grinding, the gap can be narrowed (this probably cannot be done on the coarser sizes) and the space measured with tissue paper feelers, which are generally about 1/800" thick. One can "guesstimate" the rest, up to about 1/2000", by noting the amount of teeter on such a feeler.

Reduce the gap to about 1/2000" on No. 220 before proceeding to finer sizes of abrasive. The same methods should then further reduce it on the finer sizes.

Since short strokes alone did not solve the problem, while fearless one third strokes with the above methods did, timid fussing with short strokes was thereafter abandoned. (Such timid fussing often leads to other troubles, and grows by what it feeds on. Your scribe recalls futile hours with feeble quarter-inch strokes and no results.)

The carrying - out - to - dryness - and - with - muscle method, described above, was hit on as a result of getting mad at the mirror, one bad day, and not caring what happened to it. On another occasion, this time during polishing, the same method brought about sphericity when short-stroke fussing had failed; vigorous strokes that were longer than orthodox one third strokes (due again to being mad at an evil mirror) brought it to reason in short order (Or was it 14 the accompanying imprecations?)

The moral seems to be, don't let a mirror bluff you-but at times get tough and give it the works.

Is the gap between mirror and tool, dealt with above, on the mirror or is it on the tool? No answer seems available, nor was one necessary since the empirical method got results. But if any amateur scientist wishes to be a martyr and give such a mirror a polish, just to measure its zones in order to find out, he will be awarded a medal.

The above matter was shown to Cyril G. Wates, Edmonton, Alberta, since it is unsafe to hand out advice on a basis of a single experience, because too often some hidden persona factor is involved. Wates commented "My experience has been that a one third stroke applied with vim, vigor, and vitality is a cure for many mirror evils-almost a panacea. I have had mirrors with zones that looked like the Rocky Mountains clear up perfectly after a 15-minute session of 'the works.' I use a machine.

"Yes," he continues, "it is hardly possible to obtain contact with coarse abrasive-I too find it a matter of approach only as finer and finer sizes are used. For, even if you got perfect contact with No. 80 or 120, you couldn't keep it."

One reader to whom the above methods were explained in a letter re- -ported, "I tried the 'spit-on-your-mitts-and-punish-it' method but it wasn't very effective. I then tried inverting the tool and mirror, with elliptical strokes, as suggested, and one wet took the mirror down to almost perfect contact. It solved my problem."

TREES make a wonderful support for a mid-summer hammock but often they have virtually prohibited amateur telescope makers from having an observatory dome. Your scribe, for example, lives in a deep "hole" between 90' oaks on neighbors' lands and cannot even mount a telescope. How, then, about a dome atop a tower?

Figure 2 shows how Earl Manbeck, Jr., 3920 Cottage Grove Avenue, Des Moines, Iowa, got above the trees; be built a 23' tower, 10' square, and put the observatory on it.

"The tower," he writes, "is made of wood bolted together, and my neighbor, E. H. Ruby, helped me a great deal. The deck is made of 2" lumber. A 12'' reflector, which I purchased second hand, is mounted on it. The dome is 14'6" in diameter and made of iron ribs and sheet metal."

Asked whether this arrangement was steady, Manbeck at first replied that it was, but after 12 days of observing with the telescope he reported that "I have found that when other persons are present it is necessary for everyone to stand still, otherwise the telescope picks up a lot of vibration. So we are going to put in the suggested central pier, separate from and at no place touching the tower."

This note is published to forewarn readers against towers in which the telescope is an integral part of the tower. Possibly such a tower could be steady enough, at great expense, but the separate pier is probably cheaper.

When your visitor prances around or scratches his ear, your physical senses will hardly detect the vibrations is the tall tower structure-direct. But your eye, at the eyepiece where the vibrations are many times magnified probably will and all too well-for a telescope is a magnifying seismoscope. An amateur in Washington, D. C., once related how his telescope on the roof of an old, lightly built, wooden-framed four story apartment house always told him when the ladies were beating eggs far below in the kitchen. Not, however, that a telescope atop a whole house will necessarily be unsteady-it has been done and has proved good. Much, however, depends on the house. But a tall tower usually lacks the mass of a whole house.

FIVE years is a long time to hide one's light under a modest bushel. Frank A. Jasset, Newton, Mass., assisted by Frederick Richards, Newton, Mass., and Guy E. Gordon, Natick, Mass., five years ago finished a Cassegrainian reflector (Figure 3) of 20" aperture and then failed to write it up for publication.

Learning recently about this telescope we invited Jasset to give us the pertinent data, and these now are published perhaps more briefly than would otherwise have been the case. We wonder how many other larger-than-average telescopes lurk elsewhere, similarly unheralded and unsung.

The observatory is wooden and is 10' x 10', with a square pyramidal roof revolving on a "round-square" track.

Concrete pier separate from floor.

Springfield-type mounting.

Electric drive.

Tube, 21" x 61".

Primary, 1-1/2'' x 20", 47" f.1.

Secondary, 4" diameter.

There is a 2" right-angle prism at the elbow near the eyepiece.

At first the primary was silvered and coated with amyl acetate, which worked very well, but when no amyl acetate was applied to a later coating the silver soon tarnished rather badly. Asked about the unconventional thinness of the mirror, about 1:13, Jasset replied: "Definition very good, no distortion. The mirror rests on a flotation system resting in turn on felt blocks on a disk of 1-1/2" laminated wood sealed with wax and painted. This has proved very satisfactory.

Jasset and Richards, also Gordon, were members of the recent "Roof Prism Gang." Because working only in spare time, of which there was not enough, they were unable to go into actual production but together produced 36 acceptable roof prisms and thus demonstrated that in more favorable circumstances they could have produced many. The initial belief on the part of your scribe, that the amateurs could make these prisms in spare hours, was mainly erroneous. The work proved to be too exacting for tired men who already had done one day's work. With one exception those who produced large numbers-thousands-were those who could arrange to quit their regular vocations and go into prism production whole time (actually, in fact, for most about double time, with sleep taken mainly after the emergency). The exception was the team of Ralph Franklin of Patchogue, N. Y. and Frank Cameron, Inwood, N. Y., who, together, produced 1700 roof prisms in spare hours- and felt about 100 years old during the long period of the doing. Today, all the roof prism work is completed.

LAST MONTH'S Telescoptics department dealt with the effects of temperature in different types of telescope tubes. In addition to the effects described there and previously in astronomical literature, a reader contributes a new one so far as this department knows- the huddle, or "crowd poison" effect. Too many people crowding around a telescope together give off about as big a volume of heat as a small bonfire, One human being alone sheds heat at a rate of about 400 B.T.U., about enough to light a lamp bulb. That's why the poor telescope aches to yell, "Spread out-crowd poison. And quit breathing on my eyepiece or I'll get hot and put on some temperamental aberrations or go into a coma if you persist in ganging up on me this way." A crowd indoors will warm a room; even outdoors it will suffocate a telescope.

COPY of first edition of "A.T.M.," good condition, now rare, has turned up in the hands of a telescope maker's widow. To highest bidder.

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