FROM THE DAY when the book "Amateur Telescope Making-Advanced" was first published, with its detailed chapter on Walkden's Richest-Field Telescope, this compact type of instrument, nicknamed the "RFT," became popular. The Richest Field Telescope is designed on the basis of scientific data to show at one Milky Way view the greatest possible number of stars in one field, and the viewing in the Milky Way is very milky indeed-myriads of stars. Such a telescope is quite small and stubby-it is demonstrable both in theory and fact that a large telescope would show fewer, not more, stars. It has a rather deeply curved mirror and therefore is not suited to the tyro s maiden job, since deep mirrors are harder to make, but could be the amateur's second or third job. Foreign commentators say we Yankees "go" for anything biggest or smallest, longest or shortest, heaviest or lightest, or anything, so long as it ends in "est," and so there must now be hundreds of these Richest-Field telescopes in use, judging from reports we receive. Here are pictures of three of them. The first (Figure 1) is by E. H. Morse, 2401 Mar Vista Avenue, Altadena, California (see photo of his 15" reflector in "ATM," p. 355). It has a 5-1/2" mirror of 22" focal length. "It has exceeded my expectations in performance. " Morse writes. "In fact," he continues, 'I have had more fun with it in two hours than I have in many nights with my large telescope."

The "RFT" in Figure 2 was made by Howard P. Smith, Jr., 405 North Park Avenue, Tifton, Georgia, and has a 6" mirror of 20" f. l. Smith states that "the definition is superb." He puts in a claim to being the youngest to make an "RFT"-age 14. As very few youths under 18 ever make telescopes-for telescope making is essentially an adult hobby-we suspect he will retain this record easily enough.

The third "RFT" (Figure 3) is by B. L. Harrell, 114 South Street, Gadsden, 20-1/2" e.f.l. in a cedar tube, and is equatorially mounted on an old transit tripod.

HERE'S something that, until a few 11 days ago, was quite new to me, and I feel that the amateurs should know about it," Russell W. Porter says in sending the note which follows:

"I think I am safe in saying that the average mirror maker, when testing with the knife-edge at the mirror's focus (note, not at the center of curvature), takes it for granted that an even and uniform cut-off indicates a perfect paraboloid. However, this seems to be not precisely true, when the pinhole and image are sufficiently separated for convenient observation. The following note, made by Dr. John A. Anderson, executive officer in charge of the 200" telescope, will show clearly why the cut" off should not be uniform when testing a paraboloid at the focus."

'The other day,' Dr. Anderson states, 'one of our opticians called me in to see the test of an f/4.6 paraboloid of 26" aperture. The set-up was the regular one with a full-sized flat, and the conventional Foucault knife-edge. Pinhole and image were separated by a small distance-about 1/4" or less. The optician had found that the cutoff was not changed by rotation of the paraboloid or the flat. The small diagonal also was found to be free from suspicion.

'In the horizontal cut-off,' Dr. Anderson continues, 'the shadows came in from the left and right equally [see Porter's drawing, Figure 4.-Ed.] and in the vertical they came in from both ends of a diameter inclined about 45 degrees to the vertical.

"A reference to pages 100 and 101 of Dr. Gardner's "Optical Design,' or any other description of the formation of pure third order coma, will account for the above observations exactly. The interesting thing t remember is that the effect is so easy to observe even when the off-axis angle is only 3-1/2 minutes of arc."'

Supplementing this compact statement by Dr. Anderson, Porter writes, "It should be noted that the shadows referred to are somewhat fainter than the familiar shadows seen at the center of curvature. The diagrams may help to show what actually takes place. Figure 4 is the appearance of the shadow when the knife-edge moves in from the left. Figure 5 represents three zones, a, b, c, arbitrarily selected on the mirror, and the numbers 1 to 8 refer to octants discussed below. Figure 6 is a schematic diagram showing the three images formed by the zones a, b, c of the mirror. Now, if the knife-edge cuts in from the left in Figure 6, it collides with light rays coming from 1 and 5 on the mirror. Hence the shadow shown in Figure 4. If the knife-edge makes a vertical cut-off (say, from below) it extinguishes light reflected from 2 and 6 on the mirror, and hence the resultant shadows creep in at 45 degrees.

"The images," Porter continues, "produced by an off-axis pinhole and knife-edge are included in a 60 degree angle (regardless of the separation). This off-axis condition can be overcome by interposing a half-silvered diagonal on the mirror's axis, as in Figure 7. The diagonal must have a minimum thickness to avoid double reflections. One method, described in Strong's new work, "Procedures in Experimental Physics," page 75, is to half-silver a film of lacquer stretched over a frame."

"The full title of the book referred to by Dr. Anderson is 'Application of the Algebraic Equations to Optical Design,' by I. C. Gardner, May 1927, Superintendent of Documents, Government Printing Office, Washington, D. C."

FLATS are sometimes as temperamental as prima donnas. H. E. Dall, 166 Stockingstone Road, Luton, Bedfordshire, England, relates in a private communication, how one misbehaved. "I had an experience with a flat which I made for A. P. Norton (of star atlas renown) which has rubbed home its lesson. The flat was for a Newtonian and was a properly shaped oval of 2-1/4" minor axis, 11/32" thick. I started by making a circular flat large enough to cut out two ovals, using polished plate completely free from strain, one side to be ground and worked to a flat. I succeeded in getting flatness within 1/20 wave in four or five hours' total work. Now comes the rub! I cut it in half, lathe ground an oval from one half, ground the back and, on testing it before silvering, found it was no longer flat. The error was a regular bend of a few millionths of an inch-far more than I could tolerate for a first class flat, although even then it would take a trained eye to detect its effect in the telescope. No alternative but to cut another flat, using the remaining half. This time I didn't grind the polished back and on test, when finished, it still was flat! The apparent explanation is in line with some earlier experiences of the surface tension effects of polished glass, and the moral is: 'don't grind the polished back of a flat mirror after finishing the front."'

JUST as we go to press, the November number of the Journal of the Optical Society of America comes out, containing an article entitled "A New Method for Testing Cassegrain Mirrors," by Dr. E. Gaviola, of the Observatorio Nacional at Cordoba, Argentina. Too long (4 pp.) to reprint here at present, the gist of it may be gathered from the following:

Dr. Gaviola (whose photograph, by the way, appears in "ATMA" at p. 474 and who has done much research at Pasadena-see also p. 76, "ATMA,"-and who is especially keen on mirror refinement) was confronted with the problem of testing and correcting the Cassegrain secondary of the 32" telescope at the La Plata Observatory, but had no flat of that size and no Hindle sphere. So he thought up the new test. Directly in front of the convex he put a mask-typical kind. Close to that he put a convergent lens-a bad lens serves as well as a good one-a trifle larger than the secondary and with f. 1. shorter than the r. c. of the convex. Using monochromatic light and a test rig in which k-e and slit were virtually coincident, he determined the position of the image of the various zones. Next, he removed the convex from the set-up and put a small flat at proper distance beyond the lens, so that the new image would return to the k-e end, and once more measured each zone. Amateurs who want to try it may obtain the November number of the "J.O.S.A." from the offices of the Optical Society of America (American Institute of Physics), 175 Fifth Avenue, New York, for 60 cents.

In the same number of the same journal there is a 15-page paper by Ricardo Platzeck and Dr. Gaviola, entitled "On The Errors of Testing and a New Method for Surveying Optical Surfaces and Systems." Fortunately, it involves precision in workmanship not ordinarily reached by the beginner. The paper shows that the regular method of testing is not accurate enough to work to a close degree of precision. The new method consists of determining the caustic or line of the centers of curvature of the surface elements. The method may be used in the Cassegrain secondary test described above, also to determine astigmatism' to study Schmidt correcting plates, single lenses and lens systems. A wire is used for k-e, to give higher precision. Note from Porter, Pasadena: "Gaviola has just been here and told us of a new way of making k-e test at c. of c. Dr. Anderson says it's O.K. If so, then all mirrors thus figured are over-parabolized." Evidently the same test. And how Ellison, if he were alive, would chuckle! An indication for his under" correction argument.

As stated above, these two papers came out on the day when the present notes had to be sent to press, so there was time to read only the first one better than sketchily. The second is illustrated with interesting focograms o£ secondary zones too subtle to be seen in ordinary testing-also, unlike the first, is plagued with several acres of higher mathematics. An effort will be made to have the low-down on it for later publication. In the meantime, let none but advanced workers worry about their mirrors now being all wrong.

SUN telescopes, described in Porter's part of "ATM," seemingly seldom are made. Just why amateurs pass up the only star whose details any telescope can reveal at all and whose details are varied and ever-changing as well as conspicuous, is something of a puzzle. Anyway, E. H. Morse, 2401 Mar Vista Ave., Altadena, Calif., has made a 6" sun telescope whose coelostat is shown in Figure 8. "It has 15' f. 1." he writes, "and is mounted in my workshop on a plan very similar to that of the Snow telescope on Mt. Wilson. All three mirrors are unsilvered, which brings the light down almost sufficiently for the eye, although I have a selenium glass wedge to reduce glare when necessary. Observations of the ever-changing features of the Sun are even more interesting," Morse states, "than the sameness of double stars, clusters and so on."

In your copy of "ATM," unless you have the recent and corrected printing of the fourth edition (that of 1935-the most recent edition) we suggest that you rearrange page sequence as follows: Change p. 37 to 42, 38 to 39, 39 to 40, 40 to 41, 41 to 37 and 42 to 38, and thereafter read in that sequence. How these pages ever got out of sequence, and stayed so many years thus without being complained of by any reader, is a second puzzle. How the book's editor also failed to discover this is more easily explained: he just hadn't, until recently, ever sat down and read the book straight through-too much like a busman's holiday!