AS THE telescope making hobby continues to spread, the tendency to want permanent housings for the telescope increases; they save a lot of bother, and greatly add to comfort in cool weather. W. L. Moore, Coral Ridge, Ky., is the maker of the one shown in Figure 1. The lower structure serves as a tool house. The dome above it is itself 10' in diameter and 8' high. Its upper or hemispherical part is made of 28-gage sheet metal, with ribs similar to those suggested by Scanlon in "Amateur Telescope Making-Advanced." The vertical or skirt part is made of corrugated metal. Contrary to first appearances, however, the two parts are permanently attached to each other, an arrangement which affords a little more headroom at the sides. This puts the rollers, which are ball bearings from junked cars, down at the level of the observing floor. Moore says he can push the whole dome around with one hand. Inside, at present, is an 8" reflector.

Chief value of studying others' work, before undertaking one's own, lies in finding what mistakes others made and not repeating them. Asked, therefore, what, if anything, he would not do again, Moore stated that he placed his stone pier, 30" square, in the center of the dome, not realizing that if he ever wants to use a German or a fork mounting it will then be off-center. "If I built again," he points out, "I'd also make the seams in the dome a little higher. I turned up 1" on one side and l/2" on the other but I should have made these 1-1/2'' and 1", respectively; so I had some trouble with leaks, having to use a lot of solder at the low places. I also made the frame of my shutter, which carries its two pieces of 3~ sheet metal, too light and it rubs on the dome frame, making it a bit hard to open the shutter. This wooden shutter frame rides on barn-door rollers."

A fat collection of "Things I'd Never do That Way Again" would be a big practical help in amateur telescope making. Who is there who couldn't contribute a few! Open invitation.

FROM Clayton F. Howe, 514 Arthur F Ave., Kalamazoo, Mich., comes this note: "Here are photographs and a sketch (Figures 2, 3, and 4) of a turret eyepiece holder I have recently finished for use with my telescope. Holding a battery of three eyepieces of differing focal lengths, it is the most useful piece of equipment I have, next to my finder.

"The turret, made of aluminum, floats around on a thin film of oil and doesn't upset the aim of the telescope as changing eyepieces in the usual way often does. In freezing weather I plan to try substituting glycerine for oil.

"For convenience of representation, in the drawing, the spring-ball holder is shown opposite the large boss but, as Figure 3 shows, it actually is at one side of it. This permits the holder to be mounted nearer the main telescope tube.

"The eyepiece tubes were threaded into the turret while the whole assembly was mounted on a lathe arbor, consequently they are absolutely perpendicular.

"I am making a couple more for local TNs. I made my own patterns, and the castings from these cost me only 50 cents a set. Brass tubing was two bits and the remainder came from the scrap box."

WHILE turning over files of telescoptical items from previous years, your scribe encountered an article from The Journal of the Royal Astronomical Society of Canada, Sept. 1937, in which H. S. McClung, of Regina, Saskatchewan, described what he calls the ''corneal reflex test." Suppose, in the regular set-up for the Foucault test, that the lamp and pinhole were removed and a small ball were substituted for it, with a strong light somewhere in front but off to the side. Then the tiny reflection of the light on the curved ball would in effect become the pinhole. This, of course, has often been done; sometimes a drop of mercury is used, or a small marble of glass, or any spherical reflecting substance.

The ball used by McClung, however, is the ball of the eye itself. This permits the effective light-source-that is, the reflection of the lamp on this ball-to lie very close to the knife-edge, thus avoiding astigmatism due to separation of pinhole and knife-edge. McClung says the light used should be strong but need not be very small in area. This light may be a disadvantage, shining, as it would, in the eye when the latter was studying the subtle shadows on the mirror. It is also hard to hold the head steady enough to conduct this test, and McClung suggests a headrest. However, some may like it. Have many been using it, since McClung published it?

Years ago in this department there was a note about using the edge of the pupil of the eye as the knife-edge and dispensing with the ordinary one. This, too, required considerable steadiness on the part of the tester. Maybe some readers will want to compound these two. We don't necessarily recommend either one for regular testing but as a variation they are interesting-like riding a bicycle while standing on your head on the seat-that is, a bit tricky.

IN THIS department, October, 1935, I Joseph A. McCarroll, of Teaneck, N. J., described a penetrometer for reducing the hardness or softness characteristics of pitch to a quantitative science, and, in the October, 1936, number, two more penetrometers were described. Now comes Robert E. Smith, D.D.S., Medico-Dental Bldg., Sacramento, Calif., with the one shown in Figure 5. It is made from sundry pick-me-ups, plus an optician's diopter gage to register the time-depth penetration of the point into the pitch at given temperatures. The point is shown above two pitch facets. Using this, Dr. Smith claims he knows all the time, as he works with pitch where he is "at."

AMONG amateur glass polishers you can always get up an all-night argument about the merits and demerits of various pet fine-finishing abrasives. Here's a comment by D. Everett Taylor, 191 Prospect St., Willimantic, Conn.

"For a final step in fine grinding, before rouge, use extra fine emery in kerosene, as the grains break down easily and smoothly when so used and are much less likely to cause fine scratches."

With regard to Levigated Alumina he says, "This is a buffing flour for polishing metals and, for this purpose, it undoubtedly is tops, but when the microscope revealed that it had an extremely small grain size its use also for superfine finishing of optical surfaces was recommended. ["ATM," 4th edition, pp. 296, 493.—Ed.] If you are intent on using Levigated Alumina for fining glass surfaces, watch out for fine scratches."

Taylor states also that there is a glass removal of about a thousandth of an inch, between the kind of surface left by Carbo 600 and one sufficiently fined with fine flours, to be called ready for polishing. He says also that 0.0003', of glass must be removed in order to eliminate fine scratches of the kind that require magnification in order to be seen. Asked how he determined this, Taylor replied that the statement is based on a measurement of a lens before and afterward, using a Starrett micrometer.

INTEREST in Walkden's Richest-Field Telescope, or "RFT," continues, and many have been made. In the following contribution Walkden, whose address is 46 Cavendish Road, Harringay, London, N.4, England, discusses the matching of RFT eyepieces and objectives.

"If an amateur has an objective and wonders what eyepiece he needs to make it an RFT, he can get the answer from the accompanying diagram without even a calculation; or, if he has a few eyepieces or their lenses lying around and wonders how he could use one, if possible, for an RFT, the diagram again answers without calculation.

"A natural way of designing an RFT is to start with the aperture, a"; then decide on the focal ratio, c, then the focal length, F'', and, finally, proceed to the eyepiece focal length, f ", and the particulars of the lens. But among the eyepieces available is likely to be one not exactly but only nearly what is needed, and then, taking that nearest, we soon find ourselves working back to the main focal length and the aperture, in a matching process.

"Now the diagram of Figure 6 performs this matching process very quickly for every type of RFT-refractor, Newtonian, Herschelian, Cassegrainian and even Gregorian-and it also shows at a glance the various alternatives we might prefer to what we originally thought of constructing.

"Suppose, for example, we have in possession, or in makers' lists, a 1-1/4" RFT eyepiece. Looking at Figure 6, and at the sloping line for the 1-1/4" eyepiece, the eyepiece is seen satisfactorily to complete any Newtonian RFT of from 4" to 12" aperture, the proper focal length being read on the left-hand scale, where it is 4.4 times the aperture, agreeing with the formula near the top. The eyepiece can stretch over the range of 3" to 15" or even 20" aperture, but Newtonian RFTs of less than 4" aperture are not good being too much choked by their flats; and Newtonians of 15" and 20" aperture are better with eyepieces of 1-3/4" and 2" focal lengths and main focal lengths of 92" and 140", and even then they hardly equal large Herschelians. While considerable margin is allowable, especially for eyes not very sensitive to the blind spot (image of the flat) in the center of the Newtonian's Ramsden circle, it is always as well to keep in mind that for an eyepiece of f inches focal length, the best aperture under the minimum-obstruction rule is given by a = 4.9 f² inches. But eyepieces of less than 1" should not, and over 2-1/2 " need not, be considered candidates for Newtonian RFT construction; and the apertures of less than 3", and even 4'', are really not allowable. The "best" Newtonian line, a = 4.9 f² or F = 2.5 a³, may be seen to cross steeply the 1-1/4" eyepiece line at 7.6" aperture.

"If we have only the 1-1/4" eyepiece we should not think of any other RFT than a Newtonian of moderate aperture; but suppose the RFT eyepiece is of 3" focal length. This 3" eyepiece can be seen to complete any refractor RFT of over 2-1/4'' aperture, of about the focal length to be read on the left-hand scale, 10.5 a inches, corresponding to the formula near the top. Or, paying regard to the thick Herschelian curve, the eyepiece will complete good Herschelian RFTs of about 5" to 10" or even greater apertures, all of the same focal lengths as refractors, or like the formula near the top.

"Should our preference be for a Cassegrainian RFT, the 3" eyepiece will complete any such RFT of 22" main focal length, ranging in aperture from about 5-3/4" to 14-1/2", chosen larger according to experience and skill in figuring mirrors of small focal ratios when aided by Kirkham's scheme of using a spheroidal small convex mirror (see Scientific American, June, 1938). The formulas in the right hand lower corner may also aid the constructor; and the peculiarity of the Cass RFT may throughout be noticed, that the focal length of the eyepiece primarily determines only the focal length of the main mirror, leaving the aperture to be chosen from independent considerations.

"Should a Gregorian RFT be chosen (although it gives little reason for being liked), the 3" eyepiece will complete such an instrument, ranging from a 4-1/4 " of 11 " focal length do a 10" of 16" focal length.

"In another use often required of the diagram, suppose we have already decided on 6" aperture, and have the choice of several RFT eyepieces. Proceeding upward from the bottom of the diagram at 6" aperture, we can evidently have a 6" Cass RFT of 9.8" main focal length, employing an eyepiece of 2" focal length. And the Cass rules in the right-hand lower corner tell how the convex and the hole are both to be of 1.40" diameter; also how the convex is to be 1.85" within the main focal point, and be of 2.42" focal length. The alternative Cass RFTs' of greater focal lengths, employing larger eyepieces, are easier to make, but they are less and less efficient.

"In 6" Gregorian RFTs we can have one of 10" main focal length, employing a 2-1/2 " eyepiece, but it is not a very good instrument. The alternatives of longer focal lengths, employing larger eyepieces, while easier to construct, are even less efficient.

"The 6" Newtonian RFT is one of 26" focal length, employing a 1-1/4 " eyepiece, and is the useful handy kind of RFT which is perhaps the most r liked, until the virtues of the Herschelians come to be further realized. Again, in connection with Newtonians, the 6" mirrors can be matched by others than the recommended 1-1/4" eyepieces. The 'best' eyepiece corresponding to minimum-obstruction design is always of focal length equalling 0.45a inches, which is 1.10" for a 6" mirror, but the insensitiveness of some eyes to the blind spot in the Newtonian's Ramsden circle allows of considerable variation in the eyepiece focal length. When f is decided upon, the mirror must always be made of focal length F = 3.5 fa inches, and the flat be arranged according to the rule given in 'ATMA'. An aperture' less than 3" or even 4" should not be r assumed for a Newtonian RFT-and an aperture greater than 25" perhaps need not be assumed.

"The 6" Herschelian RFTs begin to be quite good at 63" focal length, employing a 3" eyepiece, but 72" focal length, using a 3-1/2" eyepiece, proves better in definition. Only the fastidious might want still greater focal length employing a still larger eyepiece. It is curious to notice that the larger eyepieces go with the smaller Herschelian RFTs, contrary to the rule for the Cassegrainian and even the Newtonian RFTs."

As no 3" eyepiece is known to be available on the market, the amateur must make his own. For this, Walkden gives the following data on a Ramsden of the usual two plano-convex lenses. Focal length of each lens, 4". Distance of lenses apart, 2.67". Diam. of field lens, 2.31", of eyelens, 1.3". Eye-hole distance suiting smallest need, 7/8". Eye-hole diam., 1/2".

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