ZEISS TELESCOPES, large ones, made prewar, are, or were, to be found in many observatories, not alone in the lands of the late German-Italian-Japanese Axis (now the declination axis) but in many others. None are known to exist in the United States. They have a distinctive appearance shown even in the smaller, tripod size not unknown in this country-white, clean enamel, neat finish, sound mechanics, and in the large instruments a complex mechanism (Figure 1, the 49" Neubabelsberg, reflector near Berlin).

For this complex appearance a peculiar Zeiss design principle is largely responsible, the unique stress-relieving system. Probably few in this country have had opportunity to see a mounting embodying this system and, now that an amateur has built one (Figure 2), it may be time to add the type to our TN repertoire-it is not patented. An attempt to explain this Zeiss principle will follow the description of the telescope shown.

Dr. K. Hermann-Otavsky, 335 Dolní Mokropsy, Praha, Czechoslovakia, writes, "I long ago read 'A.T.M.' and 'A.T.M.A.' with great interest and since the reopening of the national borders I am again following your column in Scientific American. I consider 'A.T.M.' and 'A.T.M.A.' best of all of their kind especially as friends of astronomy from beginners to first-class experts tell in it their experiences gained by their own work and not merely from literature." Dr. Hermann-Otavsky's telescope is neat 400-pound semi-portable type mounted on three little roller trucks so that it may be taken indoors. It is towed out by means of a folding stub tongue which steers the front truck spotted over hollows in three plates fixed permanently in the roof deck, and three vertical screws in the base are run rapidly down to their adjusted stopnuts with a detachable crank like a carpenter's brace. Two to four minutes suffice to complete these preparations.

The knee-shaped trunk of the mounting is a tube of sheet iron with 1.3 mm walls, reinforced inside at the knee with a welded-in plate. The knob- motor car gear lever-at this point is simply a handle for pushing the telescope. The base and diagonal brace are also of welded iron tubing. Rigidity, high. The mounting was built in a repair garage and at home, partly by the owner.

The 2" polar axis turns in Timken conical bearings and carries an hour circle of the slip-ring type. The R.A. driving worm is of tool steel and runs oil on ball bearings and is pressed against the 9" soft iron driving wheel by a strong spring, eliminating play. The two were ground in with emery paste and they run smoothly. The slow motion controls at the eyepiece end work on a push-pull principle, eliminating play here also. In addition to the slow motions in R.A. and Decl. the transmission shaft of the hour circle has a reversing differential gearing, by means of which it is possible to make infinitesimal corrections with a flexible cable. This has proved simple and convenient, and long-exposure photographs have been successfully taken at the focus of the main tube, using a marginal guide star, although the errors of guiding must not here exceed four seconds of arc.

By means of A.C. transformed to 6 volts all lenses may be mildly warmed. The same voltage lights the circles and so on. The drive is a gramophone electromotor combined with a hand-wound spring motor.

To the main tube two heavy plates are attached, with numerous holes permitting attachment of various instruments, since the mounting permits considerable overloading. When the photograph (Figure 2) was taken the mounting carried (left to right) a Zeiss C refractor 3-1/8" which rates as an RFT; a Zeiss 5-1/8" E refractor (f/15) and 2" finder (hidden); a camera with 3" Petzval lens; a monocular field glass (hidden). The optics were obtained from Zeiss and from Srb and Stys, Praha.

The main refractor has a Zeiss binocular eyepiece. Two removable achromatic Shapley lenses, the opposite of the Barlow lens, may be used for shortening the main tube focus.

Other auxiliaries include micrometers, a magnification meter, a planetary prism reflex camera with controlling ocular made by the owner, a focal camera with a side-aiming ocular, an accommodation for narrow-film cinematographic camera with mirror reflex control, and a focus-control microscope.

"Literally," Hermann-Otavsky says, "there is no declination axis in the usual sense," and then mentions the standard Zeiss mountings described by Chief Engineer Franz Meyer of Carl Zeiss, in "Zeitschrift fur Instrumentenkunde," Vol. 50 (1930). With the help of this and an article in "Product Engineering," New York, 1931, July, pages W-94, and the pre-war Zeiss catalog of astronomical instruments, your scribe has tried to study out the principle of the mysterious Zeiss stress relieving system.

In conventional mountings, precision-that is, freedom from flexure in all positions-is sought by making the axes stiff. This makes them heavy. This, in turn, increases friction in the drive, or So Zeiss claims. More than 40 years ago Engineer Meyer therefore swung to almost the opposite extreme. If you can't entirely eliminate bending-and theoretically you can't-then "let 'er bend" and then neutralize it. This you can do entirely. It calls for some added mechanism.

Figures 3 and 4, from the "Product Engineering" article, exhibit the principle. Each of the two axes consists of an inner part which carries the weight-all of it-and around it but nowhere touching it an outer part which maintains the alinement and moves the telescope. The inner, weight-bearing parts, shown only by theoretical lines in Figure 3, are sizable shafts, not by any means flimsy but not designed to prevent all bending.

The entire weight of the tube, its two pairs of spherical counterweights H,H, declination axis shaft D, and polar axis shaft itself including big weight K (below floor level on large telescopes) is delicately balanced on a single ball bearing pivoted horizontally on rockers on ledges in the pedestal (A, Figure 4).

Rotating with this weight-carrying polar axis shaft is the assembly consisting of the forked yoke (F, Figure 3); its lower straight extension (but not the surrounding casting that clasps the pedestal head) with R.A. worm; also tubular part Do (concentric with declination axis shaft); and bridge S connecting with and moving the main tube. These parts move as a unit in R.A.

Now, you inquire, you have given us two systems-somewhere they must connect. Yes and no. A light contact-just a kiss-occurs at O in Figure 3 where a cross disk is diagrammatically indicated at the middle of the declination axis (is in line with the top of the polar axis). Actually, Figure 5 is that disk and there you see how it works: Bulge O on declination axis shaft Di ball bearing Q; spherical surface in spherical seat (not lettered); and enough looseness at R to prevent cramping when the polar axis shaft bends. Because this ball bearing keeps the two systems apart, yet transmits the desired drive, the claim can truthfully be made that the two systems do not touch each other at any point. Let's not quibble over this since, while the actual parts do touch through an intermediary, the stresses, which are what matter, don't.

Figure 1 shows an annular space between the hollow tubular element and the weight-bearing declination axis. The same space similarly exists in Figure 2 but does not there show.

Figure 4, A, was drawn to show that if the bridge part between the tubular element and telescope tube were purposely to be disengaged, as a demonstration stunt, it and the tube still would stay put wherever put; which shows that this assembly puts no stress on the driving element. In the same figure, sketches B,C,D,E show how the counterweight arms isolate the stresses; there is no hidden composition of forces. It is claimed that freedom from cramping and increased precision are thus obtained.

This, then, is a scientific design. Just why it never made headway in this country your scribe knows not. Perhaps some reader does. Yet the fact that, over several pre-war decades, Zeiss made and sold many like it to large observatories seems to say that it must have points. Amateurs desiring to experiment with it may borrow photographs of the Hermann-Otavsky telescope, and of other Zeiss telescopes, not here reproduced. They should also study the two elevations in the German article cited above, these being on "Tafel 3" near page 74 (in case a photostat is sought from some library).

In the pre-war Zeiss catalog, today scarce if not rare, is the following summary of the Zeiss stress-relieving system and telescopes.

"The movable parts of the equatorial mounting are divided up into two distinct equatorial components, viz.-

1) an equatorial directing system comprising the optical parts and enabling the telescope to be directed upon astronomical objects in terms of the coordinates (hour angle and declination);

2) an equatorial system of carriers for taking up any stresses in the directing system.

The catalog continues: "The equatorial telescope mountings with stress relieving system fulfill the following requirements:

1) They obviate flexure in the telescope and in the polar and declination axes, and ensure easy movement of the axes by reason of all stresses being completely relieved in the carrying system.

2) Several telescopes may be assembled side by side on one mounting and any mutual tendency to deformation compensated by the stress relieving system.

3) The tubes are freely movable through all ranges of the hour and declination angles without encountering obstruction by any part of the mounting or stand.

4) The tubes are so mounted as to continue the movements of the eye within a very small range, the eye being applied at a point near the intersection of the polar and declination axis.

5) The design affords easy access from the observer's position at the eye and to all attachments."

Close study of Figure 2 reveals the telescope described does not em body all the standard details of the Zeiss mounting. For example, the count weight K of Figure 3 is absent. The telescope was too small to justify a Chinese copy of the original. But the main feature is embodied; note the two knobs in Figure 2. These correspond to 2,2 of Figure 3 and emerge like wrists through loose cuffs which do not touch them, just as in Figure 1.

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