"It has occurred to me that some further remarks concerning the instrument-which has now been in regular use for two years-might be of some interest to your readers. As shown in your photo, the triangle which forms the base of the mounting stands clear of the ground; this was soon found to cause some inconvenience, as well as to make the eyepiece rather inaccessible in some positions; so the triangle has been sunk 6" in the ground with great advantage. At the greatest northern declination (about 43 degrees north) which permits of the telescope passing the meridian without reversal, the lower end of the tube just swings clear of the ground, while the eyepiece is always readily accessible. In respect of convenience of working, the instrument equals, if it does not surpass, a reflector with rotating tube; and the optical parts, once adjusted, are so stable that finding objects by the circles is an easy matter. This is just as well, since it has not, as yet, been found easy to mount the finder so as to be easily accessible in all positions; the arrangement shown in the photo is probably about the best, but is not very satisfactory; however, since the finder is hardly ever used, this does not matter very much. The 'swiveling' arrangement for the eyetube, giving a side-to-side range of some 225 degrees, has been found very advantageous- especially for work on variable stars-as the orientation of the field of view is always under control.

"Of course, with two reflections between the object glass and the eyepiece, some loss of light is bound to occur; assuming 87 percent for the reflectivity of the aluminized 6" flat, and the same figure for the prism, the effective aperture of the object glass is about 8.4" x 0.87, or about 7.3"; this gives ample light for such powers as can usefully be employed on Jupiter; while I find that my not very sensitive eye can just see steadily a star of magnitude 13.7 on a good night, so that there is sufficient light for observation of pretty faint variables.

"The circles, which are divided only to half degrees and read by simple pointers, bring any desired object into the field of a low power eyepiece giving x 60 and a field of 43'; and the remarkable stability of the whole stand goes a long way toward making 'setting' easy and certain; repeated tests on the Pole Star have failed to show any appreciable movement in the polar axis.

"I venture to think that anyone possessing an object glass of similar, or even greater aperture, might do worse than consider the possibilities of this form of mounting, especially for prolonged observation of such an object as a planet."

The photograph reveals less distinctly than the drawing the position of the eyepiece, the latter being foreshortened in the photograph. Commenting on this, Horace E. Dall, of Luton, Beds., England, who made the optics (the mounting was made by Perry, also shown in the photograph) states:

"The whole point of that extra reflection is accessibility of eyepiece and orientation of image. The Captain is keen on planetary work, in estimating position angles of Jovian spots, and so on. He prefers to set the belts horizontal, which is easily done by swinging the short, projecting eyepiece tube 'round,"

So far as is known, no American amateur has yet made a reflex-or "euphonium."

BECAUSE glass, such as that from which mirror blanks for reflecting telescopes are made, is cooled rapidly on the outside after it is poured, stresses due to unequal contraction are set up in it. Annealing in a lehr is supposed to ease off these internal stresses but does not always do so. Later, when the telescoptician removes glass from parts of the disk, these stresses (for some unknown reason called "strain" by the optical industry) are able to warp the disk out of shape, and then there is grating and gnashing of teeth, for the disk is probably a candidate for the discard.

Last April, H. E. Dall, of England told in these columns how he had designed and built an inexpensive tester for strain in glass, making use of the new polarizing material, Polaroid. This tester, he stated, consisted of ( 1 ) a lamp in a blackened box having an opening covered with Polaroid; (2) a ground glass screen; (3) Polaroid goggles worn by the viewer. He added that this simple equipment "showed up strains so brilliantly that there was no need even to test in a darkened room, subdued light sufficing to permit the indications to reveal themselves."

F. M. Garland, 1006 Davis Ave., Pittsburgh, Pa., read this note, obtained a few more details from England and built a Dall tester. He now writes: "Scanlon and I tested all the glass within reach at the Valley View Observatory with it and the set-up really works." Therefore we have obtained from Garland some of the details.

Starting at the lower, left-hand corner of the photograph (Figure 2) the following elements of the set-up are seen: (1) A wooden box, hinged beneath for adjustment in altitude and containing an ordinary 25-watt lamp. Fastened over the hole in its front this hole being made as large as the polarizing material will permit, is one half of a pair of Polaroid goggles of the kind now obtainable at many filling stations, with their normally horizontal dimension vertical. (2) An easel with a pane of common window glass frosted on one side, to provide a fairly uniformly illuminated area of polarized light large enough to permit the examination of a moderate sized disk against it. (3) The glass under test, held in the hands. If the glass is not strained, the expansion due to the heat of the hands will give it a strain, hence it should be insulated with a cloth or felt; but if the desire is to study the effect of heat in causing temporary strains, no insulator should be used. (4) A pair of Polaroid goggles worn in the normal manner, which will use the horizontal dimension of the lenses to cross that of the Polaroid in the lamp box and thus be at right angles to one another. (5) The eyes of the tester-in Figure 2 these are the property of F. M. Garland. The photograph was taken at our request.

"When the glass is badly strained." Garland points out, "dark and light streaks are seen strongly contrasted against the fairly dark screen. Faint shadings are inconsequential; but if there are strong bands it is best not to work such a disk. If no bands can be seen it is interesting to make some, simply by warming the mirror with the hands."

The above paragraphs may supersede the description of apparatus in ATM, page 461. When that description was prepared, only a few years ago, no such low-cost, large-area polarizing material as Polaroid was anywhere available.

PORTABILITY is a quality many seek in telescopes which they wish to carry about in a car. Figure 3 shows a little 1-7/8" refractor with 1/2" eyepiece, giving about 30 diameters magnification, owned by the F. M. Garland mentioned above. The mounting is a bar with a bend equalling the latitude and the fork is attached to it by means of a quarter-inch removable pin, as shown. By giving the bar a curve instead of an angle such a telescope would be adjustable for any latitude and be suitable for the wandering type of telescope user.

THAT the Republica de Argentina is very much on the telescoptical map is evidenced by a letter from a member of the "Amigos de la Astronomia," in Buenos Aires Carlos Luis M. Segers, Calle Jose Bonifacio 1488 the address of the library of the Association. Segers, who writes better English than some of us Yanks, says many amateurs have made and are now making telescopes in the Argentine Republic and that ATM and ATMA are thoroughly known in those latitudes, They have published in Spanish an excellent booklet on telescope making and they regularly publish the Revista Astronomica. This is an amateur journal, but is quite professional in appearance, judging by the samples sent to your scribe.

IN a chapter in ATMA, Hindle of England tells how to make a diagonal for a Newtonian and states that, while the use of a totally reflecting prism is permissible, we must use an optical plane of elliptical contour if we desire the very best results. Commenting on this in the present department some time ago, Wates of Canada showed the very small losses of light caused by other types of diagonal.

Replying to Wates, Hindle now adds the further fact that "the use of an elliptical flat insures that the black spot [the spot at the center of the diffraction pattern, which is the shadow of the diagonal.-Ed.] is perfectly circular, and one can easily judge the perfection of the instrument by the out-of focus images of a star. Not only the permanent characteristics of the telescope but the varying characteristics due to temperature changes can be immediately determined." He quotes from a letter by W. H. Steavenson, prominent English amateur astronomer:

"Other things being equal, I think there is no doubt that the ellipse is the shape of choice, since it combines circularity of projected outline with a minimum of obstruction. The objection to any other shape necessarily presenting a larger area by projection) is not, to my mind, concerned chiefly with reduction of light grasp; because, as Wates has shown, the loss in ordinary cases is negligible from a practical point of view. The drawback consists rather in the production of increased or unsymmetrical diffraction effects or both. Any central obstruction tends to throw a certain amount of the light from a star into the diffraction system surrounding it, which is objectionable from certain points of view, though inevitable in all practical forms of the reflector. The most we can do is to keep the obstruction down to a minimum, as is done by adopting the beveled ellipse. Any increase in size of obstructed area increases the amount of diffraction, as may be seen by taking the extreme case of a flat so large as to leave only a narrow annulus of main mirror exposed. In this case you get a slightly reduced central image, but an enormously enhanced ring system, which would blot out any close faint comparisons and also destroy contrast in planetary detail.

"In any case," Steavenson continues "I should always, even at the expense of slight extra loss of light, fit a circular disk as a mask over the flat mount, since any other shape, square or elliptical in projected outline, will produce unsymmetrical effects. The square gives four rays, and the ellipse two, though these may be so oriented as to coincide with those produced by the supports.

"But actually, as I said at first, none of these effects is very important in its bearing on practical performance, and, to quote Bell they 'affect the observer's feelings more than his images.' The matter only becomes important with small f ratios, when the flat is necessarily already on the large side in comparison with the main mirror. Thus for f/4 the change from the elliptical type with circular projection to the circular flat of same minimum projection, or to a type with projection filled out round by a mask, might be really serious, though fortunately the work done by such mirrors is generally photographic, where definition under high powers does not enter into the question. For the ordinary amateur's visual work, at say f/6 to f/8, I don't think much harm would result from a projection of any reasonable size or shape."

Thus the beginner needn't worry very much about the shape of his diagonal. Figure 4, drawn by Wates, provides an organized picture of the ins and outs of it. Wates' comment on the statements by Hindle and others is as follows: "I should imagine it would be just as easy to get used to an elliptical extra-focal spot as to a circular one; that is, as a standard of performance. My whole object was to point out that, while the beveled ellipse is undoubtedly the ideal shape, there is no serious objection to an unbeveled, circular disk which does no apply equally to a prism. The advantage of the circular disk is the ease with which flats of this shape can be made. All this is, for the 12-incher, not for the 6-incher who, of course uses a prism, or the lordly 20- incher who doesn't give a whoop for trouble or expense."

A good many letters received from amateurs are passed round by your scribe among other amateurs, much like family letters passed to relatives, and when Everest saw the collection reproduced above he added the following on a cognate subject:

"I agree that the elliptical diagonal at 45 degrees is the best. Another thing is that, for uniform diffraction effects, the projected circle must be concentric with the mirror's axis, which will make it appear off-axis when viewed from the focal point at the side of the tube. Most tyros don't understand this and try to correct for the apparent displacement, but a simple geometrical diagram will show why it can't be done. Also, theoretically, this means adding slightly to the size of the diagonal as usually calculated for complete coverage of the field lens of the eyepiece, but it's not important."

Everest then proceeds with the following train of logic:

"1. The whole plea for the elliptical diagonal is to project a circular shadow on the mirror, so that its figure can be interpreted from the extra-focal image of a star, also to produce uniform diffraction around the image at focus. The light gain over the rectangular diagonal is insignificant.

"2. To produce concentric rings in the extra-focal image, as shown at A, Figure 5, or uniform diffraction effects around the in focus image, this circular shadow must be concentric with the axis and rim of the mirror. If this condition does not exist, the extra-focal image will be as at B, with rings not concentric, while the in-focus image will be cockeyed to the critical observer.

"3. For freedom from astigmatism, the axis of the cone of light reflected from the mirror must coincide with the axis of the mirror.

"4. For the same reason, the axis of the cone of light reflected from the diagonal coincide with the axis of the ocular.

"5. For 3 and 4 to be possible the intersection of the axes of mirror and ocular must lie in the plane of the diagonal.

"6. If you agree with 1, 2, 3, 4 and 5, you must also agree to the construction shown in Figure 5.

"7. If you agree to 1, 2, 3, 4, 5, and 6, you must also agree that, when viewed from the focal point, you will see something like C.

"8. If you have followed thus far, you will see that the area of the diagonal actually used in reflection will be as shown in D.

"9. If you cut down the diagonal to the area actually used in reflection, the projected circle will not be concentric with the mirror's axis."

10. If you push the diagonal toward the mirror (dotted line), as most tyros attempt to do, the cone of light between diagonal and ocular will be off the ocular axis, resulting in astigmatism.

"And there you are.

"The extra size of diagonal needed to accommodate the field lens of the lowest power ocular used has not been considered in the diagram, as this would complicate the explanation.

"The amateur who is interested only in seeing the splendors of the heavens through his telescope needn't worry much about all this theory-as it is not important from a practical standpoint. This is directed at the armchair telescope makers with the hope that it will cause them no end of worry."

TOO convenient is a common belief that a poor flat will do for a diagonal "because it is used close to the focus." Due to frequent repetition without examination his error has acquired sanctity. But F. J. Hargreaves has analyzed the question, in the Journal B.A.A. If a large surface having uniform departure from flatness were used, so that the reflected beam filled the whole surface, the error would equal that of this surface. If less of the same surface were used, the error would be reduced (the linear error being reduced in the ratio of the squares of the diameters taken), and perhaps that part of the uniformly curved surface might be near enough to flat to be harmless-not, however, directly because of its nearness to focus. Distance, as such, has no effect In other words, as Hargreaves puts it, "A small part of a large bad mirror may be good, but it does not follow that a small bad mirror is a good one."

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