One such job was done by Harold A. Lower, co-author of "Amateur Telescope Making," and his father, Charles A. Lower. Both are skillful old hands at the amateur telescope making art. Lower, junior, writes from 1032 Pennsylvania Street, San Diego, California: "I enclose a photo of the latest member of the Lower family, a seven-inch portable Cass. This little instrument is shielded for daylight use, and performs in daylight just like a refractor. While the optical surfaces are not perfect, I believe the errors do not exceed the theoretical limit for good results. At least, the performance is quite good, as it will stand up well with a one fifth inch ocular and will resolve doubles as close as one second of arc. The grinding, polishing, and figuring of this scope were done on a machine. We just wanted to see whether we could make it without any hand work on the mirrors, and we did. I rather believe that the difficulty of figuring short focus mirrors has been exaggerated, as we had no trouble with this one, although the primary mirror is f/2.7."

We asked the Lowers to send us a drawing of the layout and this is also reproduced on this page. As they state, the shade tubes, shown in heavy line, are the main feature. The primary mirror is held in place by a collar on a short tube which extends through the perforation in it, and the shade tube screws fast to this.

We also show a photograph of the twin Lower telescope, a 12-inch Cassegrainian of f/18 and a 12-inch Newtonian of f/4.5, mounted on the same axis. A little coop mounted on wheels rolls forward and covers the two tubes and their mounting when they are not in actual use.

"When we mounted the new instrument," Harold Lower writes, "we compared the new and the old silver coats, and found that the Cass, which had been silvered 10 months, was almost as bright as the new one. That proves that the can of caustic soda which we place in the telescope on closing it up has really helped to prevent tarnishing of the silver. Before we started using the caustic soda, the mirror would be badly tarnished in six months."

A COUPLE of years ago Lincoln K. Davis, 1351 Main Street, Brockton, Massachusetts, made one of the smoothest non-professional telescopes we have ever laid eyes on-in fact it was smoother than many a professional job, and so when heard that he had more recently completed a vest-pocket Cassegrainian we asked him to describe it. Here is his letter:

"Your threat to run photo of my Cass beside Lower's gives me cold feet, because I wrote you this was not a particularly smooth outfit. It was made to be readily portable, which it is, and it has been thrown around all summer, in the car and on a boat. While I can resolve Epsilon Lyrae plainly there is too much scattered light in the field. At first I suspected a turned edge, but Everest pointed out what is probably the real reason-zonal irregularities. He said that he always smooths the surface of his mirrors with a very soft lap and short strokes, after figuring, with an invariable improvement.

"My primary is a good one, but the secondary was whacked into passable shape just about as my patience was running out, so this fall it will be overhauled an trued up. I cannot agree with Lower that difficulty in figuring short focus mirrors been overemphasized, as I had one devil of a time with mine.

The photograph shows the telescope set-up for terrestrial use with an erecting eyepiece, but minus the equatorial mount, which I carried away from a junk yard for quarter (old movie camera tripod head having a geared friction panorama motion which makes a fine slow motion in right ascension ).

"The primary is of Pyrex, 6" diameter, 13-1/8" focus, giving a relative aperture of f/2.2. Primary has a 1-1/2" hole through it. Secondary is of 3/8" plate, 2" diameter, 10-1/2" radius of curvature, and is placed 9-3/16" in front of primary. Amplification is approximately 4X, so that overall focal length is about 21/2", and effective aperture f/9. Box is of plywood, cloth covered, and is 7-1/4'' square and 13" long. Weight of telescope proper is 7 pounds; the eyepiece and tripod shown add about 5 pounds to this.

"The mirrors were silvered by the dunking method, which is to say, face down. This eliminated all my previous troubles with pinholes, poor coats, and so on. The primary mirror is a fine one, if I do say so (ask Everest), and is within one percent of a parabola, but the secondary is fair only, with the result that the performance is not up to that of my 6" Newt, although in daytime use with a power of about 75X it works well.

"I spent 11 hours in grinding and polishing, and then 10 more figuring to a 60 percent correction, when I succeeded in knocking a big, flat chip out of the surface, and after a brief rest and cuss period, sadly went back to No. 60. In the meantime I built a machine, so the second attempt was made with this, and the total time ran into something like 45 hours, as the machine proved to be much slower than hand work. Final figuring was done by hand, and testing done by the Ronchi test and my modification of it. For obtaining parallel light set up my Newt with a pinhole at infinite focus, and found this to work very well.

"For daytime use I made stops the size of the exit pupil for each eyepiece, and found this successfully cut out unwanted light, as Hindle says, so that the Cass give as brilliant an image as the Newt in day light.

"The hole in the primary was cut within an eighth of an inch of through before fine grinding and then carried all the way after figuring. This, on a drill press, was the easiest part of the whole job."

A TRIM Newt-Greg combination made by Dr. S. H. Sheib, a chemist and testing engineer, Box 737, Richmond, Virginia, is shown in the photograph below. Sheib says he had lots of fun making this telescope, but prefers his old Newtonians when it comes to actual use.

So, there you are: To make compound telescopes or not to-that is the question.

BY now the reader may have noticed that an extra page is given to this department. This addition, we hope, will be permanent, and is a reflection of the fact that the amateur telescope making hobby is still on the make.

TO continue, Mr. Davis mentioned above that he had a modification of the Ronchi test and we therefore asked him to tell the world about it. Here is what he says:

"I ran across the fact that the Ronchi may be conducted in broad daylight, or with any ordinary unshielded lamp. I placed a piece of ruled celluloid over a white card having a hole in it, to look through. Then I held the card at the center of curvature of the mirror under test, with the ruling facing the mirror, and a light source arranged to illuminate it rather brightly. By looking at the mirror through the hole in the card the Ronchi bands may be seen.

"As I figure it, light is reflected from the white card between the lines of the ruled screen, thus forming a multiplicity of slits or slit sources, and the returning rays are examined through the grating in the usual manner.

"I made another grating which works somewhat better by following Kirkham's suggestion ("A.T.M.," 266, Fig. 4) and threading a piece of brass having a hole in it, winding some fine wire around it, soldering the edges, and then cutting away the wires on one side. The wires facing the mirror were polished, with the side to face the eye blackened. In this case the polished wires act as very bright sources, with the added advantage that these sources and the point of observation are practically coincident, eliminating parallax effects, which with short focus mirrors are to be reckoned with."

There is some difference of opinion whether this method will give as good contrast as the single slit, also whether it will give periods of confusion ("A.T.M.," page 270, line 2). Try it and see what you think of it.

THE use of small or sub-diameter tools for amateurs is a heresy but amateurs, ever since this hobby took hold of them, have been dealing in heresies and the result has been an advance in the art of telescope making. So, avast with dogmas: Here is what two amateurs have written about small tools, at our suggestion. Believe it or not, a thing that works, works, even if it is all wrong. First, testimony from Harold A. Lower, who writes:

"Ellison mentions that it is easy to grind and polish with small tools, but does not say how to do it. The 12-inch Pyrex mentioned above was rough ground face down for nine hours over a nine-inch tool. At the end of that time the curve had reached full depth at the center, as determined by measuring the sagitta, but lacked about an inch and a half of reaching the edge of the disk. (This first grinding face down leaves the edge of the mirror untouched- not even scratched.) When the curve had reached full depth in the center, the mirror was turned face up and the same tool used on top. One simply makes large epicycles all around the mirror, working mainly on the edge of the hollow, until the curve reaches the edge, at which time the curve should have become spherical. This grinding with the mirror face up also required nine hours, but did not deepen the center the slightest bit, so it is important to go to full depth in the first grinding while the mirror is face down.

"The fine grinding is all done with the mirror face up. The strokes used are large epicycles around the mirror, alternated with a zigzag stroke across the mirror. Do not permit the edge of the tool to overhang the edge of the mirror more than an inch or so, or a turned edge may result. One can tell when the surface is spherical, as the tool will slide freely in all directions. If it binds at any point, the surface is not spherical, and must be made so by working on the zone that binds until the tool will slide easily.

"Polishing was done with a 9-inch tool, with the mirror face up. The strokes used were large epicycles, alternated with the zigzag stroke. No difficulties with turned edge or zones were encountered, but it should be understood that good contact is just as important when using small tools as with full size. The handle of the small tool must be low and well centered. Never apply any pressure on the edge of the tool. Pressure may be applied to the center of the tool, and will merely hasten polishing.

"Figuring was done with a 6-inch tool, working with a variety of strokes, mainly over the center. The figure is easily controlled, as one simply applies more abrasion at the points that seem to need it. I would not recommend this small tool method for any except short focus mirrors. For f/6 or shorter, it works fine."

Paul Linde (see "A.T.M.," 138, 228) of Crossville, Tenn., submitted a 12-inch mirror to us for test and its figure proved up beautifully. On inquiring we found out he had figured it with small tools, so we asked him also to write a word about that method. This is what he says:

"The first time I tried a smaller-than-mirror tool was on a 12-inch of f/4.5. I used a 7-inch tool merely because it was one I happened to have. The facets were graduated to fine points at the outer edge. The strokes should be of about the same length and of an elliptical or circular nature while making one round around the pedestal, and should be changed with every round to prevent formation of zones. As a general rule the center of the tool should travel more often over that zone or diameter of the mirror which needs deepening most. Starting with short, circular strokes close to the edge of the mirror and keeping away from the center, the strokes should be lengthened with every other round until they are quite long, after which they can be gradually shortened.

"If there is a hill in the center it can easily be reduced by going with the tool across the mirror with slightly elliptical strokes, beginning with short ones and gradually increasing them with every second round. Turned up edge can be gotten rid of easily by pulling the tool farther over the edge of the mirror.

"Of course, there can be no fixed rules for using the small polisher and one simply has to experiment and test often to see the results. Any mistakes made by the small tool can be corrected in comparatively short time by the use of the full-sized tool to bring the figure back to flat. I find the small polisher method by far the easiest way to get all the zones right, especially with mirrors of short focal length. One word of warning: The mirror, when face up, is much more likely to be scratched."

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