FOR OBSEBVING faint objects, such as aebulae and. the MiLky Way stars, t~esCopes of short focal ratio are the kst suited; they concentrate the light gatliered by the objective lens on a small area, well illuminated. On the other hand, for observing planets, ~here there is plenty of illumination, bigh magnification is possible and this ~ay be had by use of telescopes of long focal ratio which have a large j~age, sufficiently well illuminated. ,doSt ordinary telescopes are a compromise, and serve fairly well for both . purposes. .~.

The telescope described below has ~.n ~ry long focal ratio (1: 30) and is specialized on planetary purposes. Its hbe is nearly nine feet long, though ib.aperture is only 3~ inches. It was dedgned and built and the specifications for its objective lens are given below by Lieut.-Colonel Troy W. Lewis, an amateur telescope maker who both dedFns and constructs, and who lives at- 1812 Schiller Ave., Little Rock, kkansas.

0f co'urse, in optics, whenever you gam some advantage you must pay {or it with some kind of comprornise Qucky if only one) and thus this re6.actor frankly abandons the advantage of compactness. It is true that the Cassegrainian type of telescope enjoys both long focus and compactness but it does not enjoy the same simplicity &f construction as the Lewis refractor. Perhaps the following article, with the one on long-focus reflectors in the February number, wil1 help start a new trend in the long-focus direction. Colonel Lewis writes:

~1

THIs CO~NICATION deals with sevI ~ eral advantages found in designing, construcffng' and using an achromatic refractor with focal ratio double that of the usual f/15.

Splendid pioneer work was done by a*ronomers some generations ago who Used, through necessity, long-focus plano-convex single lenses made from inferior window or bottle glass. Per;bap5 some of the best work done by those pioneers was with telescopes from 12' to 33' long, with apertures between 2" and 3". Inherent spherical !115 and chromatic aberrations were rendered less obtrusive through the application of the well-known laws of diminishing curvature and the concealing of color errors into the image by diffraction. A maximum correction of aberrations under these laws occurred when the focal length approached 100 times the square of the aperture of the objective.

There were some monstrosities in telescopes in those days. Thus, the Huygens brothers made objectives up to 210' in length, with a 6" aperture, but their best work was done with a 23' telescope of 21/3" aperture. It was with this glass, power 100, that Christian Huygens solved the great Saturnigraphic mystery in 1659.

The achromat, invented by Chester More Hall in 1733, for many quite obvious reasons displaced the older longfocus single lenses in frail tubes or more unstable girders, but it could not wholly destroy the merits of properly designed long-focal-ratio lenses. Professor David P. Todd, as late as 1911, strongly recommended the construction of a long-focus open-air telescope in a steel box-girder, reinforced so as to be unyielding and rigid, thus to secure the many advantages of longfocus lenses under modern conditions

Most astronomers agree that the visual refractor is ideal in the study of fine detail in planetary images, but for critical work in selenography, saturnigraphy, or in areography, it must be admitted that the focal ratio of the objective should be very much higher than the standard type f/15 established by Fraunhofer many years ago.

The amateur astronomer who wants a modern~ long-focus telescope for planetographic work will usually have to make one to his own specifications, for such telescopes are rated by professional opticians as exceptional and expensive. A special lens design is called for and that, in itself, is no small item of expense.

Such special lens designing is, however, possible. An achromat for any desirable focal length may be achromatized for any two bands of the visual spectrum at the designer's option, such as B and F, C and F, D and E, D and F, or, fcr photography, D and G'; and where the amateur makes his own ob" jective an effort should be made to achromatize for his particular vision if he is allergic to either red or blue Vision differs widely with individuals.

It had long been the desire of the writer to design and construct a modern 23' achromatic objective of 4~ aperture, and a glass of that size was under actual construction when the war called him to active duty with the armed forces, thus suspending further activities in the optical field for the duration and six months. Fortunately, two smaUer long-focus telescopes had been made, in 1939, as stepping stones to the ultimate f/69. One of those, Figures 1 and 2, is here described.

This 3Vin achromatic plano-convex objective, f/30' was made from optical glass readily obtainable on the American market. It is shown (Figure 1) in its first temporary altazimuth mount. Should the amateur wish to make this telescope for his own use let him do so with complete confidence. He wiU find, provided his vision is entirely normal, that the objective has many of the characteristics of an apochromat. Such trying objects as Venus will be seen without noticeable secondary spectrum-bright and colorless. Fine details on the Moon are remarkably clear and distinct. It will resolve to the full theoretical Dawes' limit and, as an enthusiastic friend has commented, "a leetle more!" Its performance really has to be experienced to be fully appreciated.

This lens is a thin plano-convex achromat, cemented, of 3~" aperture and 105" equivalent focal length. It was designed from Bausch & Lomb glass, Crown, BSC-2; Flint, DBF-1. It was achromatized for the C and F lines. The following is a first approximation design:

Let f = focal length of the crown glass f' = focal length of the flint glass Then 1/f + 1/f' = -1/105

(1) For the crown glass:

Mean refractive index, = (1.51461 + 1.52262)/2 = 1.51861 Dispersive power,

= (1.52262 - 1.51461) /0.51861 = 0.01544.

(2) For the flint glass:

Mean refractive index = (1.61242 + 1.62843) j2 = 1.62042 Dispersive power, = (1.62842 - 1.61242l /0.62042 = 0.02585.

Then, from the algebraic formula for achromatization, it is seen that 0.01544/f + 0.02585/f' = 0. The linear spectrum formed along the axis will be folded on itself, bringing the red and blue rays into coincidence.

l~e design was achromatized to bring the minimum focal point to the brightest portion of the spectrum as viewed by the designer's eye, leaving any outstanding blue and red rays equally pushed away from the axis where they can do no harm, since in an objective of this focal ratio both spherical and chromatic aberrations, chromatic residuals, if any, would focus both absolutely and relatively beyond the image plane, as was first observed in ancient long-focus single lenses.

To determine what must be the focal length of each of the two lenses: Let 1/f = -2585/1544 X 1/f', Then, ( - 2585/1544 + 1) X 1/f' ~ -1/105 Therefore, f, = 70.79310". Also

f = - 1544/2585X70.79310 = - 42.28841". Since one of the surfaces of the flint lens composing this achromat is to be plane, the radius of curvature of the other surface will be identified as R, then, 1/f' = (~, -1) /R,

therefore, R = (~.'-1) X f, = 43.92145~. The intention being to cement this objective with Canada balsam it follows that the crown lens also must have one radius of curvature coincident with the divergent lens, R = 43.92145. I:`et the free surface of the convergent lens, therefore, have a radius of curvature denoted by r.

Then l/f = (~-1) (1/r-1/R) and 1/r = 1/(~-1) f + 1/R

= - 1/ (.51861 X 42.28841) + 1/43.92145 = -232396. Therefore, r = -43.03".

This completes the first approximation design. Specifications follow:

Crowo lens 1st surface-43.03" 2nd sur(ace + 43.92" Fliot lens 3rd surface + 43.92-

4th surface oo flat
Cro~o rOcus 42.28-
Fllot rOcus 70.78" Combloedrocus = 105"

To afford full benefit of light traps or stops this objective should be mounted in 4" or 4Y2" I.D. steel or brass tube and to obtain necessary stiffness this tubing should have at least Y4" wall thickness. The objective should be housed in a heavy brass push-pull cell (Figure 2) for squaring on. Haviland's remarks on tube structure, page 215, "A.T.MA.", should be read and his advice followed when calculating the size of the holes in the diaphragms and eye tube. At least six stops should be used; eight would be better. The Moon's image in this telescope is approximately 1" in diameter and very bright.

A battery of at least five Huygenian eyepieces is recommended. These should be a 3", 1.5" and 1" for low power and a .5" and .25" for high power. The 3" and 1.5" eyepieces should be especially made to avoid stopping off part of the light. The Y4" eyepiece will give 420 diameters for resolving fine detail during moments of fine seeing.

There are several ways of desigmng a long-focus objective from this or other selected glass.

Thus, the Littrow type (cemented) may make a fair to middling objective with Chance Brothers 1.5115/60.8 Crown and 1.620/36 Flint, all radii of curves to be 42.00" except 4, which will be nearly plane. The Haviland first approximation formula (and it is a good one) indicates R1, R2, R3, 44.40" and R4, 671.72" convex (cemented) when using 1.516/63.5 Crown and 1.604/37.5 Flint to be had from Leo D. Keller, 2438 N. l9th St., Philadelphia, in 60 mm pressed lens blanks (see "A.T.M. A.", p. 226).

IncidentaUy, a Fraunhofer type long focus objective, with ~ air Spa, between lenses, may be made frok B & L BSC-2 and DF-2 with this s'' of curves: Rl, + 63.87; R3, -37.21 R3, -3759; R4, -155.82. The vvriter much prefers the cemented CrCSsed crown lens for his long-foc~ objec. tive because of its beautiful flatne55 and wide field. The crossed crovn1 not, however, the 2:3 structure men tioned in "A.TM.", p. 110, by Rev erend EUison.

With the explicit information on firs approxunation design given in above notes the amateur should be abl to design his own objective for an part of the spectrum as he may choo if C and F be deemed undesirable.

Since Hastings has shown ("A. M.", p. 179), that it is no more diffi cult to make a flat for rays normal the surface than to make a true curv and since Driscoll has shown (Scientifi American, March-April, 1945) ho, easy it is to grind, polish, test an figure a set of curves, it is believ that the amateur will no longer hesi tate to indulge in making for hims a highly desirable long-focus plano convex refractor for planetary worh Here is how, and luck to aU who try.

ANlr READER who undertakes the type of telescope described above is urged to keep in touch with Colonel Lewis or with this department or both. though reports to either one are likely to be seen by the other since this department keeps in direct-mail touch with numerous readers everywhere.

It has happened numerous times that the maker of some unusual piece ol optics has omitted to write in and crov' about it, either from modesty or from the supposition that editors are fed up and probably wouldn't care. It is true that the lower orders of animals lay their eggs and thereafter forget their progeny but the higher mammals. including editors (provided you do) maintain post-partum and post-parental intereSt Of course, this departnent isn't the actual papa of the things it publicizes but it enjoys playing uncle. so crow.

CTE~rAFANE convention of 1946, Satur

day August third, Sunday August fourth! This promises to be a mammoth meeting. Begin planning now. Porter is coming East-"going to Stellafane in August, shake hands on it," he writes. John W. Lovely, 27 Pearl St. Springfield, Vermont, secretary of the Springfield Telescope Makers, i~s the man to write to if you have any problems connected with the meeting.

Stellafane's first convention was helt in 1926. Conventions were thereafter held annually up to and including 1941, that being the 17th of the series. Four conventions, those of '42 '43 '44, and '45, were skipped because Springfield was too war-busy to entertain (it5 more work than some of us maY realize). During those four years manY amateurs, with tones of fear and regret, have inquired whether theSe elassics were ended forever. They ier valued.