The beginner usually tends to crave high magnification and when the RFT with its low magnification is first described some amateurs hesitated to make it, knowing perhaps that their neighbors and friends would rate them low when they heard the answer to the inevitable question, "How much does it magnify? '' However, the RFT magnifying 15 or so diameters becomes popular and is used usually as an adjunct to the "regular" Newtonian magnifying 50 to 100 diameters-its compliment, in fact.

S. L. Walkden, of London, has described here in previous numbers several modifications of the RFT, and now comes his Herschelian RFT. The Herschelian type of telescope has had a bad reputation: perhaps it can partly reform. Walkden writes:

"Regarding RFT's: These instruments need not be of only the refractor and Newtonian types; there is another class of instrument, the Herschelian reflector, which very much claims attention. It does so chiefly on account of its brilliancy, due to the absence of losses of light by flat obstruction and by flat reflection. In the small sizes it is nearly as brilliant as a refractor, and in large sizes, more brilliant. Because it needs only a mirror and eyepiece its construction is easy; and is further made easy, compared with the Newtonian, because the mirrors of long focus have only shallow spherical curves to be ground, and need not be parabolized like the deep, short-focus mirrors and mirrors made for high magnifications. Of course, the notable weakness of the Herschelian has always been its defective definition, due to thc oblique view and the use of ordinary astronomical magnifying powers, but that can be minimized by giving the mirror a proper focal length. Then, with the very lowest possible powers (which are precisely those of the RFT's, only 3.5 per inch of aperture), the defect in definition-an occasional trace of cocked-hat shape in the star images-is must, in many opinions, be considered sufficiently nonobtrusive.

"In Figure 1, a graph of focal lengths for different apertures, a curve marked GDH (standing for "good-definition Herschelian") tells the focal length considered necessary for good definition, because it makes the oblique angle, like , or EJX in the corner squat diagram of Figure 2, not more than about 6 degrees. This angle has been judged in tests to be the best criterion of definition. [The squat diagram is not to scale. -Ed.] The formula used for the curve in Figure 1 is F=(7.5a²+15a)/(a - 1.5), but the curve, read to within a few inches, serves as well as the formula. If with this focal length the instrument is thought too long to manage, less focal length can be used-with less, however, to be expected in the way of definition; but less length than the PDH (poorer-definition Herschelian) curve of the same figure is not recommended. The formula for that curve is F=(4.5a²+ 9a)/(a-0.9), allowing the oblique angle to be as great as about 10 degrees.

"From 5" down to 3" aperture, the upper limit of focal length is at the thick straight line marked 3, unless an eyepiece wider than about 3" is not minded. From 3" down to 2-1/2" aperture the upper limit of focal length is at the thick straight line marked a- unless an eyepiece wider than the aperture is not minded! Actually, at less than 4" aperture, and certainly at less than 3" aperture, the refractor; for which local lengths are also indicated is the preferable type of RFT. For it, F=6a is assumed, but F=5a²/(a-1) shown in the 6 degree dotted curve, may help the realization of finer definition in these small instruments. Newtonians of greater than 4" aperture are rivals both for shortness and definition, but fade out in both these advantages at over 10" to 15" aperture and of course are harder to construct.

"When the F inches of focal length of the Herschelian of a inches aperture is settled, the focal ratio c=F/a and the width EK across the top of the instrument (see the squat diagram) may be calculated as 0.3c+a+3 inches. The top diameter of the main tube at the focal distance, or diameter DK, is then a + c/5 inches, and the diameter of the focal image, EL, is c/5 inches. The mirror has to be tilted toward the little cross midway between the points A and B, which points are at the centers of the main tube and the eyepiece tube. The distance AD, which will be found 3" after using the above rules, allows room for the side of thc observer's head, as be observes with his back to the sky and with the starlight coming into the main tube over his shoulder.

"The three black dots of Figure 1 are for general-purpose RFT's of the different types of telescope.

"The eyepiece is to be calculated by the simple rules already given in 'ATMA' for the other RFT's of the same aperture focal length. The little diagonal lines at the very top of Figure 1 indicate some eyepiece sizes, the abbreviation 'e.p.s.' meaning, as be quite obvious, eyepiece size. For similarly good definition all over the field eyepiece really needs directing at a point at about half way from C to P (squat diagram) but its best direction is easily found on test.

"It is a good plan to give a Herschelian, especially a large one, a focal length according to the heavy dashed curve for medium definition (drawn freehand in Figure 1, not calculated, but the formula is F=(5a²+15a)/(a-2) inches); as, for example, like the 10" instrument illustrated in Figure 2; and then, if the definition is not quite all that is wanted, move the eyepiece closer, so that bead intercepts a little light. The better definition may be considered ample compensation for the little loss of light not exceeding, perhaps, 10 percent. In larger and larger instruments, more and more of the observer may in this way intrude into the main tube till, at some aperture, perhaps out 100", the observer may entirely enter the tube. After that he may go to the center, observe from there, just as already mentioned for the 200'', and then have no obliquity of view to cause loss of definition.

"The two 5" RFT's illustrated in Figure 2 show how small instruments of identical aperture compare in length and size of eyepiece, one telescope made according to the GDH curve and the other according to the PDH curve. The 3-1/4" represents the important little general purpose Herschelian RRFT-superior, at least in illumination, to the Newtonian general-purpose RRFT. It is not according to the GDH curve, but it cannot be made longer without becoming unwieldy and requiring a dreadfully large eyepiece. A small number may prefer this Herschelian to a refractor of half its length on account of the comfortable downward view, and some may like to bring the eyepiece closer to the main tube, even though the head does then intercept a little light. One idea for keeping the head out of the light when the eyepiece is thus put closer' is to incorporate with the eyepiece a compound achromatic prism, bending the light outward through about a dozen degrees and itself absorbing very little light. Some such prism can be balsamed to the eyepiece lens. A more drastic plan is to use a total-reflection right-angled prism or a diagonal, but that may have more drawbacks; besides, it half turns the instrument back into a Newtonian, although one superior in illumination. There is much scope here for experimentalists to find toward the arrangement they prefer.

"Shorter Herschelians of good definition may possibly be made by those who specially figure the mirrors, but since parabolization has apparently to be done eccentrically, with reference to an axis nearer AP than to XC (in the squat diagram), only the heroic few like the makers of Schmidt cameras, may ever attempt that figuring-or perhaps devise ingenious correcting plates.

"So long as the insides of the instruments are of the shapes indicated, allowing free passage for the light, the outside shapes may of course be different, according to fancy or convenience. In the limit nothing is absolutely necessary, except a beam or structure rigidly holding the mirror and the eyepiece in their proper relationship, but it seems better to have something like a tube.

"This is written with the awareness that there are some who can never be indulgent toward the definition of a telescope, and who would want to boil in oil one who persuaded them to make a Herschelian, but many more will consider they have found the very thing for brilliantly joy-riding with low power around the glorious celestial scenery of the Milky Way. The first kind are recommended to solace themselves by converting their in instruments to high-power Newtonians, for the more sober joys of planetary and double-star observation, for which purpose the long focus mirrors are very suitable.'

IN advance of publication of the above has come a testimonial on the Herschelian RFT, part of a letter from Clyde W. Tombaugh of Lowell Observatory-the amateur telescope maker who discovered the first tangible evidence of the planet Pluto. He writes:

"Some time ago, Mr. Walkden suggested to me that I try out my f/15 long-focus 5 reflector [the one shown in 'ATMA' at page 639.-Ed.] in Herschelian form as an RFT. So I remounted the mirror on a wooden beam, offsetting the incident pencil 2-1/2" from the principal axis on one side and the center of the eyepiece 2-1/2" on the opposite side, and used an eyepiece of 3-1/2" e.f.l. an l-7/8" field lens diameter. This gives 33X still too high. Nevertheless, the star cloud and dark holes in Cygnus, Sagittarius, and so on were very beautiful. The definition is good over the whole field, 75 degrees of arc in diameter. The off-axis effect is only slightly noticeable on the far side of the field."

Tombaugh adds that his eyepiece was 45-cent, mail-order catalog 3X hand magnifier 1-7/8" in net aperture, plus a simple lens of 2" f.l. taken from another eyepiece, the two separated 2.43" as a compromise on field and power. This gave an e.f.l. of 2.3". As no out-size eyepieces, like some of those shown by Walkden, are known to be purchasable, the amateur may have to design his own, in the meantime possibly experimenting with some simple makeshifts like Tombaugh's. For a 3" (focal length) eyepiece, Walkden offers the following specifications: f/1. of each lens, 4". Distance of lenses apart, 2.67". Diameter of field lens, 2.31", of eye lens. 1.31".

IN a private communication bearing on the Herschelian RFT, Walkden remarks that "perhaps some of the detail improvers will now get to work on special figurings of the RFT, eccentric and the like, to equip it for high-power magnification and a little less length." ''Well, that is just what two American amateurs have been doing already- making an offside paraboloid. But we'll tell about this in a later number.

THE telescope shown in Figure 3 is a 4" f/5 RFT, ordinary Newtonian type, made by R. B. Rice, 17 Maple St., Saugus, Mass., who says "the results with this telescope are very fine." But when it comes to enthusing over the RFT, as many have, George E. Dunn, 6906 Bingham Ave., Dearborn, Mich., whose 8" f/4.5 RFT is shown at the left in Figure 4, says: "Mine was disappointing at first and continued to be so until perfect optical line-up was attained. Everest's 'diagonalology' published in your September 1938 number, brings out this matter but it cannot be stressed too strongly for these short-focus telescopes. I found it necessary to redesign the diagonal holder to get closer adjustment and more rigid construction before calling it a job. Even then it was not particularly impressive until one night when it was chucked into the rear of the car and taken into the country. That was my first view of the heavens away from the city haze. My vocabulary does not include much in the way of poetic expression, but no one can describe those clouds in Cygnus as seen through an RFT. This goes also of the double cluster near Perseus and countless other regions. Diffraction effects are bad when first or second magnitude stars are viewed, and the RFT is useless on planets. On the other hand it out-performs anything I've looked through for terrestrial work."

No sooner does the telescope owner begin studying the Moon's map than his curiosity is aroused by the odd names given formations, mostly names of persons. Who's Who in the Moon' is a newly published, 130-page memoir of the British Astronomical Association in which these names explained-some 600 of them-and their owners' personal histories told.

TIME budget of the average TN, as worked out by a telescope widow, Mrs. Howard Morehouse, Dearborn, Mich., assisted by a very accurate stopwatch and two checkers:

Making telescopes 90 percent

Talking about them 9.9 percent

Using them You figure it out

INNOCENT literary agent in New York asks: "Would you people be interested in an article on 'How to Make a Telescope'"?

CUSSEGRAIN is what S. S. Weisiger, one of the Pittsburgh amateurs, calls a Cassegrain, no doubt after collimating one.

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