TESTED BLUEPRINTS - give us tested blueprints else we perish - was the burden of several frantic letters recently received by this department from a single individual. Amateur telescope makers, these letters urged, should do their work from tested blueprints. Yet tested blueprints didn't seem to be available, so something's wrong.

If amateur telescope makers were the kind who craved tested blueprints and wanted to copy some standard model, even if it might be a bit better, something really would be wrong. Instead, they mainly prefer to learn the working principles of a 1' telescope and then cook up their own concrete expression of them. The result has been an almost infinite and endlessly interesting variety of telescopes, no two alike. This freedom to deviate from a set standard irks some types of men.

Below are a few more of the infinite variety of telescopes, each one of which gave its maker some fun, not alone in making but in planning; and after all, isn't fun the chief commodity sought when a man decides to make a telescope?

CHIEF feature of the instrument shown in Figure 1 is the combination on one axis of an 8" f/4.5 Newtonian RFT the lower telescope, for star fields, and a 12-l/2" Cassegrainian, the upper one, for lunar and planetary observation; that is, one telescope for broad, general views and the other for restricted, particular views. Incidentally, this arrangement on one axis brings both eyepieces about the right height for comfort. The maker is J. F. Simpson, a medical and X-ray technician, Garrison General Hospital, Gastonia, N.C. He states that, in place of the two forks he believes a perhaps better mounting would be a long, heavy double yoke, with the two tubes mounted in tandem within this same simple yoke. He also plans to substitute a 12-1/2" f/5 RFT for the present 8" RFT, and to add a motor drive, so that objects will not move out of the field when the user steps away to permit a visitor to look-one of the annoyances of showing the stars to the totally inexperienced.

LIKE the telescopes themselves, housings for them afford an infinite opportunity for the maker's desire for variety. Figure 2 is a housing of the dome type made by Edison T. Schaefer, Schulenburg, Tex. The concrete pier on which the telescope rests is visible in the photograph, within the wooden fabrication which supports the floor, walls, and dome. The latter has a 30" slot.

The telescope within (Figure 3) is a 10", f/7.6 of the Springfield type and Schaefer describes it as follows:

"It is controlled by means of three electric motors; note the switchboard (Figure 3). The declination motor is turned on by on switch and reversed by another. It turns at 5000 r.p.m. and is geared down to 230, also two slower speeds, by means of a 500ohm resistor and two push-button switches The R.A. motor is the same as the declination motor and is hooked up to drive forward or backward at three different speeds. Maximum speed, 180 degrees in two minutes. For sidereal time a third identical motor is use with rheostat control. This gives sufficient accuracy for photographing the Moon."

IN transmitting the photograph shown in Figure 4, Raul J. Fajardo, Aguilera alta 27, Santiago de Cuba, Cuba, states that he wanted an ordinary astronomical telescope plus equipment for measuring position angles of celestial objects, determining geographical coordinates, and so on. On the eyepiece end of the tube, which rotates, is a circle divided down to 5 degree spaces and opposite on the fixed tube is a vernier. Inside is a reticle in the eyepiece, made of threads. The whole is fairly simple and, while it splits no small hairs, it does what it was built to do.

"To illuminate the reticle a flash-lamp bulb in front of a diaphragm near the middle of the tube throws light forward, some of which is reflected back from the back face of the objective lens, resulting in a perfectly even, yellow illumination of the field of view and of the micrometer, against which the stars stand clearly (The accuracy of this telescope is not so great as to make one consider the lamp's heating effect).

"I have many times found the latitude of my town by the zenith method, also the longitude by means of the Greenwich time transmitted by radio from London. In determining the latitude by the zenith method I used B Cassiopeia and B Cetus which have zenith distances about the same in my latitude. For the latitude of Santiago de Cuba I found 20 degrees 1' 15" 30" which is in close agreement with the latitude found years ago by a commission from U. S. A. I have found that my telescope can work every time within an error limit of 30" in latitude and 12s. in longitude. With this instrument, which costs not over $15 in all, I have been able to practice many problems of astronomy and navigation."

MORE about the Gaviola test: Recently your scribe, in corresponding with Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alta., Canada, said he wished someone would describe the new Gaviola test, alluded to in our last two numbers, in about eleven words of one syllable or less, and here is what Wates wrote in reply, with an interlarded comment by N. J. Schell, 1019 Third Ave., Beaver Falls, Pa., and another by your scribe.

"In the Gaviola test, an optical surface is regarded as being built up of a large number of small surfaces, each having about 1/25 (for example) the diameter of the whole mirror. A series of these small surfaces, each of which is regarded as sensibly spherical, is isolated by suitable masks (Figure 5, I), and the exact position of the center of curvature of each element is determined by a new and very accurate method.

"In the case of a truly spherical mirror, the center of curvature of all elements is situated at one and the same point-the C of C of the whole mirror (Figure 5, II), but in the case of a paraboloid (or any other shaped surface), the centers of curvature of the elements do not coincide. Instead, they form a more or less irregular conic surface in space. A cross-section of this spatial surface is a curved line on either side of the axis, which Gaviola calls a caustic." [The envelope, or general shape, traced out by combining or linking up parts of the several reflected rays shown in the lower part of the drawing on page 283 of "A.T.M." is a caustic. It looks like a big C, rather small at the top. It is true this particular caustic happens to be made by the envelope of parallel rays reflected by a sphere but if, instead, one were to use rays from a pinhole-that is, diverging rays-and reflect them from the parabola in the upper part of the same drawing, one would get a caustic instead of the focus shown. The heart of Gaviola's test is that it is done along the caustic, and is therefore more nearly exact. Exactness in practice is a relative matter. Strictly speaking, our everyday assumption that our reflected rays come to foci along a straight line is not quite correct-though most of us will have to exhaust the full degree of exactness and skill contained by the old test before we venture into this more exalted realm of precision. Even if we never get there a all we shall, however, be curious to know what the test is all about, hence this little discussion.-Ed.]

"By the application of suitable formulas, this caustic may be reduced to a graph of the actual surface of the mirror, either with reference to a true sphere, or with reference to any one of a family of paraboloids, jus as we do with the Foucault test, by placing the knife-edge at various positions, inside or outside of the mean center of curvature.

"In Figure 5, I, is the surface of a mirror with a series of small isolated elements. In the Foucault test these elements or zone are measured directly on the axis in pairs A, BB, CC, DD, and so on. In the Gaviola test they are measured individually, but along the caustic, and their relative displacement is thus determined.

"In the Foucault test, with any non-spherical mirror, it is assumed that the C of C of any given pair of elements or zones occupies a position somewhere on the optical axis of the mirror, but this is not absolutely true, as may be seen in Figure 6, at III.

"In the Foucault test, the knife-edge is made to cut the portion of the two cones of rays, common to both (shown blacked out), and the operator tries to make the shadows reach the centers of the two zones simultaneously. This is made more difficult by the fact that the shadows move at different speeds.

"In the Gaviola test, the knife-edge is placed at A and B, these being the points at which each separate element, a and b appears spherical, that is, darkens evenly all over. By means of very accurate micrometers, the distances 2x and y are measured. The fact that this must be done with an accuracy of about 1/20,000 of an inch makes the test unsuitable for most amateur workers. But I have no doubt that someone will overcome this difficulty." [The test consists of measuring a lateral displacement smaller than the usual longitudinal displacement but provides means for doing this with an accuracy 25 or more times better than was possible with the older method. Its usefulness over the older method applies particularly to mirrors of focal ratio shorter than f/6. Very careful construction of tester, and very rigid supports, are required. Gaviola recommends masonry supports. As an indication of the values to be measured, with stationary light source, the y figure is approximately three times the familiar , and the 2x figure is approximately divided by the focal ratio. Both of these are, of course, to be taken from the mean of each zone under test.-N. J. Schell.]

"Although the test as described can be done with the conventional knife-edge, Gaviola prefers to use a double knife-edge in the form of a thin wire. In his article in the November Journal of the Optical Society of America, he describes three methods of observing the image of this wire, with and without an eyepiece, and comments that any of these methods is more sensitive than the accuracy of any micrometer screw.

"Having measured x and y for all zones, these measurements are then applied to the formulas, and a graph drawn of the surface of the mirror, which may be done within less than 1/100 of a wavelength.

"The surfaces of the zonal elements may (and do) vary in two ways. They may change their curvature, and they may tilt. These two changes cause corresponding movements in the C of C-longitudinal and transverse, as shown by the arrows in Figure 5, at IV. The Gaviola test provides for accurate determination of both of these variations-the Foucault test does not.

"The beauty of the Gaviola test lies in the fact that individual judgment of shadows passes out of the picture. One is testing 3 series of spheres, and the sphere is the easiest surface of all to measure. Whether some T.N. will develop a modification of the test suitable for amateurs who do not possess expensive micrometer apparatus, remains to be seen. In measuring a 6-inch mirror (f/3), the maximum value of x is less than one tenth of an inch, which means pretty delicate technique! One of my brainwaves is the suggestion to try two plates as a condenser, in an oscillating circuit, in place of a micrometer, to measure x.

JOINING two sheets of HCF is a necessity in cases where the desired lap is wider than the standard widths (8" and 10-1/8") HCF will afford, and in "ATM," p. 367, a method of welding these sheets edge to edge is described. An alternative method which requires less of the art of legerdemain has been communicated by Horace H. Selby. First, he trims the two sheets back from the edge a little way, in order to get into uniform material, so that a careful crosswise match between the cells is possible. Then he slides one of the pieces along the other till he has a precise lengthwise match between cells. The sheets are now ready for permanent joining. This he does with an ice pick, very hot, which he touches very lightly and quickly to the cell slopes at their respective junctions, skipping alternate slopes (the down slopes) and filling these in later by turning the sheets around and repeating the process (Figure 7). He says it takes only ten minutes to join pieces of HCF for a 15" lap.

PITTSBURGH is to be the mecca for all amateur astronomers and telescope nuts on Friday, Saturday, and Sunday, July 5-7, and there will be plenty to do and see while there. This is in addition to the annual Stellafane get together, to be announced here later.

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.