Of course, in optics, whenever you gain some advantage you must pay for it with some kind of comprornise (lucky if only one) and thus this refractor 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 and construction as the Lewis refractor. Perhaps the following article, with the one on long-focus reflectors in the February number, will help start a new trend in the long-focus direction. Colonel Lewis writes:

THIS COMMUNICATION deals with several advantages found in designing, constructing and using an achromatic refractor with focal ratio double that of the usual f/15.

Splendid pioneer work was done by astronomers some generations ago who used, through necessity, long-focus plano-convex single lenses made from inferior window or bottle glass. Peraps 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 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 2-1/3" 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 long-focus 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 long-focus 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, for photography, D and G'; and where the amateur makes his own objective 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 smaller 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 3-1/2" 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 will 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 3-1/2" 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 = 1.5186. Dispersive power, = (1.52262 - 1.51461) /0.51861 = 0.01544.

(2) For the flint glass: Mean refractive index, = (1.61242 + 1.62843) / 2 = 1.62042. Dispersive power, = (1.62842 - 1.61242) / 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.

The 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 = - 1544/2585 X 70.79310 = 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. Let the free surface of the convergent lens, therefore, have a radius of curvature denoted by r.

Then 1/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:

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 1/4" 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 R, R, R, 44.40" and R, 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).

Incidentally, a Fraunhofer type long focus objective, with small air spaces between lenses, may be made from B & L BSC-2 and DF-2 with this set of curves: R, + 63.87; R, -37.21 R, -3759;R, -155.82. The writer much prefers the cemented crossed crown lens for his long-focus objective because of its beautiful flatness and wide field. The crossed crown is not, however, the 2:3 structure mentioned in "A.TM.", p. 110, by Rev erend Ellison.

With the explicit information on first approximation design given in above notes the amateur should be able to design his own objective for any part of the spectrum as he may choose if C and F be deemed undesirable.

Since Hastings has shown ("A. M.", p. 179), that it is no more difficult to make a flat for rays normal to the surface than to make a true curve, and since Driscoll has shown (Scientifi American, March-April, 1945) how easy it is to grind, polish, test and figure a set of curves, it is believed that the amateur will no longer hesitate to indulge in making for himself a highly desirable long-focus plano-convex refractor for planetary work. Here is how, and luck to all who try.

ANY 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 of optics has omitted to write in and crow 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 department isn't the actual papa of the things it publicizes but it enjoys playing uncle, so crow.

STELLAFANE convention of 1946, Saturday 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, is the man to write to if you have any problems connected with the meeting.

Stellafane's first convention was held 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 (its 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 were valued.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.