"Enclosed are photos of my weird-looking 15-1/2" 'modified' Cassegrain telescope-'tubeless' yet framed by steel tubes. It has turned out to be optically entirely satisfactory and will do all that a l5-1/2" can do, and that at a most comfortable manner. It is modified not alone in respect to the spherical secondary-this is the least important modification. You will recall that I described briefly the main features of the proposed modification in a letter to you early in 1932, also in the Journal of the British Astronomical Association, May, 1932, and for convenience I repeat the description.

"Mainly, it consists in the introduction of a high-grade intermediate lens (erecting lens if you like) between the secondary mirror 1, Figure 1 and the final image. For convenience, compactness and so on, this lens is supported by a tube 2, through the primary perforation. The benefits resulting from the addition of this lens are very great and lie in many directions.

"(1) It enables the sky-flooding diaphragm to be moved from the eyepoint (where it is nothing but an infernal nuisance, has to be fitted to each eyepiece, and is almost impossible to keep in good adjustment because of its minute aperture) to a position between erecting lens and eyepiece where it is quite out of the way. It has a large aperture and always keeps in optical alinement. This permits:

"(2) Wide-field eyepieces with comfortable eyepoint, greatly appreciated by spectacled observers.

"(3) A good iris diaphragm to be used for the sky stop just described, so that the aperture of the telescope can be varied by a small index lever (as in the case of my 15-1/2,") from full aperture down to nothing. This can be operated while actually observing.

"(4) Location of the erecting lens between the mirrors enables the long focus of the normal Cassegrain to be shortened down to a very convenient length-an important feature of this 15-1/2". I can secure the advantages of variability of the distance between lens and secondary and lens and eyepiece, giving me:

"(5) A final image varying in angular aperture from f/10.5 to f/26. This enables me to get a continuously variable power over a range of 1 : 2-1/2 from each eyepiece.

"(6) To accommodate any small thermal variations of spherical aberration of the primary by opposing the aberration introduced by varying the 'tube length' from the mean position for which aberrations are nil.

"(7) It erects the final image, permitting excellent terrestrial views of great brilliance and completely free from any sky-flooding troubles. I have a fine outlook, terrestrially. From my observatory and find I can take full advantage of this unusual terrestrial aperture at all steady air periods, evening and morning, when air clarity is reasonable.

"In the case of the 15-1/2" my erecting lens is 2-3/4" in aperture and enables me to have an eyepiece, 3, of lowest power (X60) of real RFT character, which will include the whole Moon and a large margin to spare. This is probably unheard of in a 15-1/2" ordinary Cassegrain and I get it with a central obstruction of only 20 percent of the primary diameter (4 percent of the light).

"The fully illuminated field is about 0.3 degrees, but cut-off noticeable at the edge of the low-power eyepieces. In order to get this, of course, the erecting lens is somewhat difficult and took me longer to make than the primary. Good corrections are obtained by cemented triplet construction and the residual secondary spectrum in the final image is quite negligible and visible only to a practiced eye.

"I calculated the eccentricity of the ellipsoidal primary and did the final figuring with the pinhole at near focus and the k-e at remote focus, some 120' away, the whole being quite convenient and easy. This method is the one I have always used for my spherical secondary Cassegrains.

"The driving clock, 4, is a Synclock weighing little more than l/2 pound all told. and giving ample power even when I am pulling the telescope backward against the independent friction drive (between the polar axis trunnions, 5, 5, and the worm wheel) .

"One great advantage of such a small motor-probably the smallest ever attempted for this aperture telescope-is that there are only one or two watts of heat to dissipate and the chance of warm air trouble in the observatory is thereby very much reduced.

"I had intended to put a pair of deflector sheets V-fashion below the optical cones to guide any rising warm air out of the optical paths, but have not yet done so-the need is not extreme, but I think it is desirable. I am certainly not troubled wit tube currents, and it seems to me when comparing performance with my earlier 14", open-air, square wooden-tube reflector that I am decidedly better off now. The observatory, shielding the instrument from rapid radiation, helps a lot, despite statements I have sometimes beard to the contrary. The observatory is aluminum painted to retard rapid changes of temperature due to radiation, as per my British Astronomical Association Journal article of January. 1938.

"Focusing is done principally with the little handwheel, 6, and extension shaft which moves the secondary, although additional spiral sleeve focusing can be done at the eyepiece end.

"The mirror lid is hinged and fastens back on the framework where it is out of the way (shown better in Figure 2).

"To save making up a new stand, all the lower part, 7, is an old Calver equatorial stand (date 1882). It originally carried an 8-1/2" mirror in iron tube between the trunnions but, as I couldn't get a 15-1/2" between the same trunnions, I carried it outside and put lead balance weights, 8, 8, on the other side. The overhang is thus more than one would use from unfettered choice, but the whole system is very light-much lighter than the original 8-1/2" Calver. I have added worm drives on the two axes, but I have yet to add circles and a few other improvements.

"The finder, 9, is 3" in aperture and the power 10X with an actual field of more than 5 degrees.

"The photo shows the little clock, 4, but barely shows the RA worm wheel and the anti friction rollers, 10, which I have added at each end of the 2-1/8" polar axis to reduce friction due to the polar tilt. These rollers are spring loaded and press up on the upper and down on the lower roller, respectively, with a force equal to the calculated gravity forces. A ball race takes the thrust down the polar axis.

"Other details: 11 is a handle for moving the telescope. 12 is the screw for slow motion in declination, 13 the switch for drive, and 14 the connecting box, l5 are bright and dim lamps on the dome, and nearby is the dome drive shown in Figure 4.

"The observatory (Figure 3) is 12' x 12' over the brickwork base, the dome being 11'6" in diameter, with 20 ribs. Except for the mechanical parts, it was planned and built largely by Perry, a neighbor. The dome rides on ball-bearing roller skate wheels attached to the wall, or fixed base, of the structure. The ash rail, which shows as a broad band traversing Figure 4, is on the dome-Perry's idea. This saves a lot of work. One of the skate wheels shows in the extreme right in Figure 4. It is mounted on a bell-crank arrangement-a triangle of iron pivoted at its upper right-hand corner. Attached to its lower corner and to the ash rail is a tension spring from an old-mesh type bedspring. These springs insure that each roller carries its due share of the load, within a few percent-practically impossible with fixed rollers. Incidentally, the observed deflection of the springs in a gale enables me to judge the direction and amount of the forces due to the wind.

"To the left in the same picture is the motor drive for the dome: gears and a rubber-faced wheel. The drive is by friction and works nicely.

"The shutter of the observatory dome has a 36" opening. The flat part of the roof is covered with copper sheet."

ADDENDUM to note on ruling engines for diffraction gratings, in "Amateur Telescope Making," page 466: In an article on Prof. Michelson, published in The Scientific Monthly, January, 1939, Prof. R. A. Millikan says that in 1900 the former "turned his attention to the problem that gave him more trouble and at the same time filled his associates with more admiration for him than any of its predecessors had done; namely, the problem of ruling very high resolution gratings. He had thought he could build a machine in a few months, or at most a few years, which would give him the desired resolution, but he spent the rest of his life without reaching the point at which he was willing to drop the problem. He often said he regretted that he ever 'got this bear by the tail,' but he would not let go, and, in spite of endless discouragements, at the end of about eight years of struggle, he had produced a good 6" grating containing 110,000 lines."

Undoubtedly the note in "ATM," mentioned above, failed to lay enough stress on the supreme difficulty of this problem and a number have planned, therefore, at various times within recent years, to undertake the job. For this, your scribe blames himself in large measure. This is not to say dogmatically that the amateur cannot succeed, but rather to point out the worst; namely, that the job is tough, tough, tough. It was tough even for Professor Michelson, and he was the physicist's best exponent of the methods of ultra-refinement and precision, having a marked native flair for pushing these characteristics to their very utmost. Yet this piece of work took him eight years, hence the amateur, if he undertakes it, should not do so lightly.

The ruling engine as a whole is a nice piece of instrument building but it contains one item that goes far beyond ordinary or even extraordinary varieties of niceness- the lead screw, its very heart. Making a screw is not a hard job but freeing it from errors-there is the nub. A fair glimpse into the nature of this cantankerous, pernickety job and, in fact, the only glimpse your scribe has ever been able to find in print, is contained in a six-page illustrated article in the June, 1917, number of Machinery (New York). Its title is "Making Precision Screws for Scientific Instruments, "and it is based on the method as used by Gaertner. First, the screw is made as good as can be by ordinary methods. The job has then just begun. With a special lathe and special equipment-in other words, a lot of construction has to be done before the screw an even be started-the high places due to irregularity are shaved off, leaving smaller high places. The operation must be viewed with a microscope if the extremely thin cuts are to be seen at all, for the naked eye alone seems to show that the tool is not in action at all. The smaller remaining high places are again shaved off and the same process carried to finer and finer residuals. The remaining steps are too lengthy to describe here. One is the testing, which is done with an interferometer. It takes the observer on month merely to test the accuracy of a screw 3-1/4" long. The article tells how all this is done, but not how to do it. Probably no article can do that. One physicist, famous for his gratings, told your scribe that if a man had it in him he wouldn't need written instructions, while if he hadn't he wouldn't be likely to get to first base with the most detailed instructions. Sounds cynical but is probably about right. Not, however that this will scare off the aspirant-see what Porter says in "ATM,'' page 65!

"SOME may find it difficult to make a Ronchi grating, as described in ATM, page 266 by threading the edge of a brass frame and then winding it. I found that the sharp edges of the frame cut the fine wire. However, no thread is necessary. I made two gratings, using No. 40 enameled wire (at any radio store) The brass frame should have rounded edges and be carefully chucked in the lathe between centers. Mount a simple guide in the tool post, so that the wire is fed to the frame when it-the frame-is vertical. Set the feed at about 150 to the inch, and wind the wire on with slight tension. Before removing from the lathe, paint the edges with Duco cement and a fine camel's hair brush. When dry, put a thick layer of liquid solder on one side only, and when dry, cut away the wire on that side with a razor blade. Do not attempt to cement the other side, as the wires are easily disturbed." - A note contributed by Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alta., Canada.

STRAIN-warped surfaces on a flat-explains the pattern of interference fringes shown in Figure 5, two photographs sent us Horace H. Selby, author of th instruction for making flats in "ATMA." They represent a 12" flat on an 8" disk and the chief symptom is lack of parallelism of the fringes. The appearance is exaggerated in the right-hand photograph, where the fringes are more widely separated.

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