MOST OBSERVATORY buildings for telescopes fall into two general categories -the conventional dome and the open-topped rectangular building. Examples of each are shown in illustrations on the present page.

The volume of an observatory dome is directly proportional to the cube of the length of the telescope it houses, hence when B. L. Bradley, 235 North High Street, Salem, Oregon, made a 12-1/2" mirror of f/11-1/2, with a 13' tube (Figure 2), the 20' dome he built (Figure 1) was a logical sequitur. It is a fine, roomy dome, quite unlike some which leave so little room between walls and telescope that a fat man, or even a skinny one, must become a contortionist in order to get around.

Bradley s dome has a fixed base 7' high, of reinforced concrete 5" thick. Rafters are two-by-fours, roofing is 26-gage galvanized iron painted with aluminum. Shutters open a full 6', running on roller-skate wheels. Dome rolls around on ball-bearing assemblies from wrecked cars.

The telescope stands on a heavy, deep concrete pillar. Its mounting has Timken and New Departure bearings, a closely calibrated declination circle, electric lights, and other trimmings. Tube is 14" in diameter Finder is a 6", f/5.5 reflecting telescope. Bradley got the machine work of the mounting done on the well-known "you scratch my back and I'll scratch yours" principle. That is, he made the lads of the Salem High School an 8" mirror while they did this part of the work for him.

IN winter, the user of a telescope likes a source of warmth to which he may occasionally flee, while in summer he may want to flee from the astronomer's enemy the mosquito. Frederick C. Holtz, 2150 Wiggins Ave., Springfield, III., has worked out a combination, shown in part in Figure 3, which provides for both. "The general appearance," he states, "is that of a garden house with a flat roof. The roof is supported on rails and can be rolled back, giving a clear view of the heavens. This has the added advantage of quickly equalizing the temperatures and does away with the turbulance frequently observed when looking through the slit of the hemispherical type of dome. Many other advantages will also occur to those who have had experience with both types." [There has been many an all-night argument between exponents of the two views, it's like that other problem: which are better-blondes or brunettes?-in final analysis insoluble.-Ed.]

Continuing, Holtz says: "The portion over which the roof rests when the observatory is in use has been made into a screened-in porch which provides ample room for entertaining guests while they wait their turn to view the celestial wonders. This is augmented by an outdoor fireplace which provides hot dogs and coffee to keep out the cold.

"The telescope (Figure 4) is a 6" refractor constructed by the writer," Holtz adds, "which ultimately will be replaced by a 20" reflector-though it must be said for the refractor that its fine definition and many good points could hardly be improved upon. I have made a 3", a 4", and this 6" refractor, also two 8" reflectors, one with quartz mirror."

Holtz says Steinheil and Conrady are his bibles, likewise Hastings' "Principles in Geometrical Optics," which he calls a wonder of a book.

UNCOMMONLY trim workmanship is characteristic of things telescoptical made by D. Everett Taylor, 191 Prospect Street, Willimantic, Connecticut, the author of the chapter on the construction of the metal parts of a refracting telescope in "Amateur Telescope Making-Advanced." Other Taylor jobs-eyepieces, for example -have been described in this department. Now, at our request, Taylor describes one of his recent pieces of workmanship, as follows:

"On page 242, 'ATMA,' Haviland shows a spherometer, and on page 250 he recommends a dial-indicating edge or thickness gage. These two items of his are directly responsible for my making the combined spherometer and edge gage shown in three aspects in Figures 5, 6, and 7. For those who aim at a high degree of accuracy in making a lens, this combined instrument demonstrates itself as a necessity instead of 'almost a necessity.' The edge of the thickness gage is for measuring the edge thickness of a lens, while the spherometer gives the sagitta or depth of curve, in decimals of an inch, and is used in connection with the two formulas from page 243, 'ATMA.' The combination gage is made of a 3 5/8" x 3/8" brass disk, 1/4" stainless-steel rods and 1" x 9/16" brass rod holder clamps. The Starrett dial gage is graduated to 0.0005", with a side bezel which simplifies lifting the dial spindle at top. The brass disk is edged, finished on both sides, and a 2" and a 3" V-1ine circle is machined on the face of the disk. These circles are divided into sixths (60 degrees apart), drilled and tapped with 3/16" x 32 thread. The edge of the disk is also drilled and is tapped with the same sized hole thread, the holes 60 degrees and located between or at 30 degrees relative to any hole on the face of the disk. Each rod holder clamp is machined, is drilled for a 1/4" rod, threaded for a screw, then split with a thin hack saw.

"The use of steel balls in the 45 degree tapered depressions has been abandoned. Steel balls are not sufficiently accurate. A variation of 0.0007 has been in the diameter of a steel ball. Therefore, the tapered depressions are not required. Three steel posts threaded for the holes in the disk, and having highly polished domes, are used. Each of the radiating rods has one threaded end, while the guide rods have highly polished ends. Used as an edge gage, this device will take lenses from 2" to 6" in diameter.

"Figure 7 shows the instrument set up as a spherometer on a 6" optical flat, and adjusted to zero. To use it as a spherometer, three points of contact should be in line That is, a domed post should be screwed on the under side of the disk, into a hole on a V-line circle at each end of the 2" diameter or 3" diameter, with the spindle of the dial gage passing through the center hole from above, as shown in the photograph. This arrangement locates the three contacts in line, thus giving a reading of greater accuracy than would be obtained if three posts were located 120 degrees apart on a circle, with the spindle in the center. It may facilitate in adjusting the spherometer on a flat if, in addition to the three contacts in line, as just described, a third post is screwed into a hole at either side, to steady the instrument while setting the dial. The extra post is to be removed when the spherometer is being used.

It is doubtful whether a lens can be brought to a complete polish on a machine, without showing some degree of turned-edge; which in equal measure would discount the performance of the finished objective if it were not in some manner eliminated. The late world-famous Alvan Clark declared, on the authority of an old lens maker, that the outer 10 percent annulus of a lens should not be allowed to function. Which is the better practice, to grind the turn-down away or to cover it up?

"If Clark's dictum is followed, the required size of glass blank for a lens would be the same in either case. After doing the fussy work required in centering a lens on a mandrel, to grind off a 10 percent annulus would take probably less time than expected. While this grinding would add to the scratch hazard, it would assure a crisp edge and improve an otherwise perfect lens. If the outer 10 percent annulus is to be covered, this should be done with authority and permanence, by machining the inner flange of the cell to the required width or depth.

"It pays handsomely to start with flat parallel faces on crown and flint blanks. These blanks should be pitched or balsamed into close contact, then precisely edged as a combination, to the desired diameter. By grinding the curves on crown or flint so that the edge thickness does not vary more than 0.0005", the optical center of the lens will automatically be established-and probably with greater accuracy-compared with the result of the tedious conventional practice which depends on reflection and visual judgment."

HAVING purchased a 2-1/2" Pyrex mirror blank, the Springfield (Mass.) Telescope Makers Carl F. Alsing, president, N. Main St., N. Wilbraham, Mass.-cast about for ways of parallel facing it. One of them owned an old safe for which he had no use because he had nothing to keep in it, and this was converted into a steady support for the grinding spindle. Figure 9 shows the disk on this rough rig while its faces were being ground parallel, as it was 3/8" thicker on one edge than the other, and Figure 8 the 20" by 4-1/2" finished blank with upper face roughed out to approximately the ultimate focal ratio, or 5.5. "We did the edge with the blank pitched to plywood and mounted up grindstone fashion," Alsing states.

TN his new book about the 200" telescope, l entitled "The Glass Giant of Palomar," David O. Woodbury discusses the methods used for rotating observatory domes. At Yerkes and Lowell the domes were pulled around by cables wound on a drum. At Mt. Wilson the big dome rests on wheels geared to motors. At Palomar, however, there are two pairs of 5 h.p. motors, one pair of which is shown in Figure 10, a photograph by Major Martin D. McAllister of the Municipal University of Wichita, Wichita, Kansas. The vertical drive shafts carry solid rubber truck tires held to the bottom band of the dome by means of springs and friction. As shown in Figure 10, they make contact with the dome through a narrow opening in the top band of the fixed base. This dome drive also acts as a brake or clamp for holding the dome otherwise the unbalanced leverage of the opened shutters with a breeze blowing would turn it, even if it does weigh as much as a 12-car train of passenger cars (1000 tons), for the two rings on which it rotates, made of railroad rails, were ground precisely level.

A MONTH ago we hinted here a few facts about the new Gaviola test for optical surfaces. Before offering a description of it we propose to wait to see how it shakes down in actual practice in the hands of the average worker. Since it deals in amounts about as much smaller than the ordinary Foucault test as the latter deals in amounts smaller than precision machine work, its use obviously will be confined pretty largely to amateurs who have developed high skill. If fine machine work today involves precisions of 1/20,000 of an inch, ordinary mirror work has dealt in precisions of about 1/400,000 of an inch. The Gaviola test deals in precisions of about 1/4,000,000 of an inch. Its user must be expert enough, not alone to perform the measurements, but to control the actual surface in the same degree of precision-all of which hath perhaps a supercilious sound to it. Hence, since the amateur has intellectual curiosity, he will want to know how the new test works, anyway.

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