Collimating a Springfield telescope, with its extra reflection, is only a little harder than collimating a common telescope. We asked Russell Porter, who originated the Springfield, if this weren't true, and he replied that it's easier than falling off a log. From this, we sort of gather that there is a slight discrepancy somewhere. Was he pulling our leg?

Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alta., Canada, has shown interest in collimating problems that wreck less agile intellects, and so we recently showed him a wail received from a Springfield mounting maker who was on the verge of death from collimitis. In return he showed us the reply he had thoughtfully sent to the sufferer. It was so lucid that we asked him to dish it up as an article. Here it is:

"I often find that it helps to visualize the problems involved in collimation if one thinks of the various optical axes as thin steel rods. If these 'rods' will turn freely in imaginary bearings rigidly attached to the mechanical parts of the telescope, collimation is bound to be perfect.

"In Figure 1, D represents the declination axis, and a, b, c, are cross-threads. I assume that the main mirror has been lined up with a and b. Now if the diagonal P is inserted and adjusted so that cross-wire c lines up with a, when viewed along the center of the declination axis, the telescope can be rotated on this axis without throwing the cross-hairs out of line, because all the parts involved are solidly connected together. However, the moment prism H is put into the path of the rays, another factor is involved, because H is not connected to the rest of the optical parts; it stands still while everything else turns around.

"Note that it does not make the slightest difference to the collimation whether the declination axis is at right angles to the tube or not, so long as the optical axis of the light cone coincides with the mechanical axis of the declination axis. It might be like Figure 2 without affecting the collimation adversely, since all the parts are united, and once having got the cone of rays truly centered in the declination axis, you may throw the rest of the parts, tube, mirror and diagonal, away, and forget them. This does not mean, of course, that the declination axis need not be at right angles; simply that its rectangularity has no bearing on the collimation.

"Now, let's bring the prism H and eyepiece into the picture. These also are fastened together. Consider the point Q where the ray cd cuts the diagonal surface of the prism. If this point is exactly in the center of the declination axis, it makes no difference, as far as collimation is concerned, if the prism and eyepiece are all cockeyed.

"The whole thing simmers down to this: the ray cd must be lined up exactly in the center of the declination axis, and after that condition is secured, all adjustments can be made on the prism H and eyepiece without in any way affecting the collimation of the main tube.

"There is, however, a way in which the relationship between the declination axis and main tube affects the collimation. Picture the declination axis connected to the tube by a universal joint. Also picture the mechanical center of the declination axis as a thin rod protruding into the tube, and the optical axis of the tube as another thin rod. Obviously, these rods must intersect. Assume that they do intersect. Then the declination axis could be moved on its joint longitudinally without breaking the intersection (Figure 3). One rod slides along the other.

"If, however, the declination axis is moved crosswise the intersection will break at once. (Figure 4). If the intersection is broken, it obviously will be impossible to reflect the central ray so that it follows the center of the declination axis, and I imagine that this is the commonest cause of trouble in collimating a Springfield. The remedy is to shim the tube in its cradle until the declination axis is square-on, but it is more difficult to design a method by which this squaring on may be accurately tested.

"One way would be by means of a wooden rod, turned in the lathe to fit snugly inside the declination axis and having a tiny pin in the center. This could be combined with a plumb-bob hanging axially in the tube (Figure 5). A simpler way is the old one of marking the point a (Figure 4) with great exactness, and sighting at it by means of cross-wires in the declination axis.

"Both these methods assume that the inside of the declination axis is concentric with the outside, which may not be the case; also they lack refinement. No pains should be spared to get this adjustment accurate, as everything else depends upon it. If it is not correct, one of two things will happen: either the central ray will be reflected parallel to, but not coincident with, the mechanical axis of the declination bearing, or the central ray will be cockeyed, like one of the lines c c (Figure 4). Either condition is clearly fatal to collimation.

"The best method is probably a modification of one described in 'A.T.M.' for lining up the declination axis, that is, by sighting at fixed posts and reversing the tube. Assuming that this already has been done, new sighting blocks are provided as near to the ends of the tube as will permit the head to be inserted at either end. The set-up is shown diagrammatically in Figure 6.

"In addition to the cross-wires, provide a disk of cardboard which can be attached to either end of the tube, with a 1/16" hole accurately punched to coincide with the intersection of the cross-wires. (I should make the disk first, and attach the cross-wires to intersect in the hole). Putting the disk at C, sight toward a, and put a dot on the block where the wires fall. Without moving the tube, put the disk at D and sight toward b, which has a scale fastened to it. Note the reading en the scale.

"Without removing the disk, reverse the tube and line up the cross-wires on the dot, at a. Transfer the disk to the other end, and sight at the scale. If the reading is not the same as before, it is because the central line of the declination axis does not intersect the line joining the cross-wires (the axis of the tube). It must be made to do so by shimming under the saddle.

"It may be found that, when reversing the tube, it will be impossible to line up the intersection of the cross-wires with the dot. It may fall above or below the dot. In such event, disregard the crosswire which is at right angles to the declination axis, and merely make the wire that is parallel to the declination axis intersect the dot.

"The adjustment of the tube in the cradle to make both wires coincide with the dot involves shimming in both directions. The best way to accomplish this is a bit of a problem, which may be left to the ingenuity of the worker. Really, it is quite unnecessary to adjust longitudinally by shimming at the ends of the cradle (see Figure 2). All that is required is shims at one side or the other to 'roll' the tube in the cradle and bring its axis into coincidence with the axis of the declination bearing.

"These are the preliminary adjustments. When these are complete, I should start collimating from both ends, and meet in the middle. Insert the main mirror and line up so that the optical axis coincides with the cross-wires. Insert accurate cross-wires at the inner end of the declination axis (c, Figure 1). A cap should be provided for the eyepiece adapter tube, with a small hole in the exact center. Such a cap can easily be made of tin, and is a very useful gadget.

"Sighting through the cap, see whether the inner end of the declination axis, where the cross-wires are, lines up centrally on the face of the prism. If not, adjust the prism to make it so. I regard this adjustment as the least important of all, since a slight error here will not affect the collimation. It will simply cause a slight chromatic effect on bright images.

"Now insert the diagonal in the main tube. Adjust this until the cross-wires at a (Figure 1) coincide with the crosswires at c. It should now be possible to rotate the tube on the declination axis without separating a and c. If this is so, collimation is complete.

"Note, by the way, that the first adjustment of the diagonal should be to slide it longitudinally in the spider until the eclipse appears truly circular and centered in the declination axis, as viewed through the hole in the cap. Next, adjust by the push-pull screws until the crosswires line up as nearly as possible. If this adjustment cannot be made exactly, it may be necessary to alter the first adjustment slightly. This is obvious when you consider that the face of the diagonal must coincide with the j unction point of the two axes, as seen in Figure 7. Wherever I have spoken of the 'diagonal,' 'prism' may, of course, be substituted.

"Summarizing, the various steps are as follows:

1 ) Make cardboard disk and punch central hole.

2) Stretch cross-wires in main tube, centering by means of disk.

3) Square up declination axis by sighting blocks (Figure 6).

4) Insert and adjust main mirror.

5) Insert and adjust prism H.

6) Provide cross-wires in declination axis.

7) Insert diagonal (or prism) and adjust to make cross-wires coincide."

ROTOSCREEN: Before he joined the Army, R William Waldeyer, of San Francisco, offered the following to other amateurs: "When an image of the Sun is thrown on cardboard (or any kind of screen, I suppose), the grain of the screen interferes with the sharpness of the image. But if 11 that screen is rotated or moved, the granulations or grains of the screen become invisible, while the image immediately becomes sharp and clear. The image, of course, is thrown on succeeding portions of the rotating screen, and the persistence of vision causes the image to clear up and become sharp. Sunspots, as well as the white flocculi of the Sun, become clearly visible on the screen. The idea seems so obvious that I don't doubt it's already in common usage-only I've never come across it."

Waldeyer made his rig from the motor-driven propeller shaft of a 15-cent toy electric boat.

TEST never tried but offered for what I it may prove to be worth, by J. R. Haviland, author of the long treatise on the refracting telescope in "A.T.M.A.", is shown in Figure 8. Theory, says Haviland, suggests that it would be very accurate, but, he asks, will it work practically ? He continues:

"Unless the center of the sphere of mercury, mounted on a micrometer screw, is placed accurately at the center of curvature, the image of the pinhole will not be sharp-a few thousandths of an inch of shift will blur it. Use a stop exposing one zone at a time, as sketched. However, as sketched, the returning focus falls concentric with the pinhole in the silver, and there would be difficulty in distinguishing which was which in the eyepiece. Perhaps it would prove necessary to use a pinhole in an opaque screen back of the glass.

"The accuracy would be better than one eighth of a wavelength at the peripheral zones. For large mirrors this test would obviate the use of the customary flats."

IF YOUR local hardware dealer's truck, I loaded with a shipment of Pyrex oven-ware, should get into an accident and 200 pounds of Pyrex were broken, would you not, as a telescope maker, have the thought: "I could do things with that much Pyrex if only it were in the form of mirror disks" ? Wilbur E. Gemmill, chemical engineer, 434 North Beaver St., York, Pa., ran true to amateur form when this happened in his community. He already owned equipment for melting down the fragments and he obeyed his TN instincts. It proved successful.

Into a graphite crucible he put 15 pounds of broken Pyrex and set this in his laboratory furnace. This he raised to 2500°, F., allowed the bubbles to rise through the viscous mass, poured the melt into a mold (Figure 9) which had been pre-heated to 1500°, F., in a muffle. He left the poured glass in the muffle for 38 hours, during which the temperature was gradually allowed to fall by changing transformer taps. Next, it was left another 16 hours without heat, and removed at 110°, F.

"The disk made a fine mirror," he writes, "with no evidence of strains when tested with two Polaroids. It was ground, polished, and figured, and has been in use for over a year with beautiful results."

Did it pay? Of course not. Gas-electricity-time: a new Pyrex disk could be bought for less. Yet it did pay out-in fun. "It was done," Gemmill says, "simply because of the intriguing nature of the problem."

"Subsequently," he adds, "I have melted down a local enthusiast's disk in which there was a crack caused by accidental dropping. The large chip thus outlined welded in- and the cracks disappeared at about 2200°, F. I am also saving my broken laboratory ware to make two 10" disks to make flats.

"A local amateur made an 8" disk from scrap window glass. The pieces were washed with soap, rinsed, dried, fed, piece by piece, into an iron crucible, raised to 1800°, F., and held several hours for bubbles to rise. Poured into an asbestos-lined iron mold and covered with asbestos paper and iron, the glass was left to cool till no longer plastic. It was then buried in about 15 pounds of house insulating material and allowed to anneal for 24 to 36 hours. It annealed quite well, as checked by two Polaroids, and seemed very uniform. The mirror took and held a good paraboloid."

Suppliers and Organizations

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