SOMETHING NEW-DESTINED, many think, to become big in telescoptics-has suddenly burst forth like a nova: the Maksutov telescope.

First publication of design data concerning this intriguing invention appeared in the May number of the Journal of the Optical Society of America and at once this department began to hear from those of the more advanced amateurs who have access to that professional journal. They obviously were somewhat excited-plainly the Maksutov telescope "stirs up the animals." But, after all, why not? Thus far it looks stirring.

After a period for ingestion and digestion of this large, 15-page meal (just a bit hard going in spots for ordinary mortals) an invitation was extended to Norbert J. Schell, an engineer, 1019 Third Ave., Beaver Falls, Pa., widely known as the amateur who, with T. G. Beede, was responsible for the much-discussed off-side, or unobstructed reflector (this department April, 1939) and the off-axis and criss-cross off-axis (May, 1940), to "put the Maksutov into such plain, simple term that larger numbers of amateurs-the many who have made perhaps two or three mirrors but who are not yet optical design sharks-could take real hold it." Schell was known to be in sympathy with the honest, direct presentation of technical subjects and of minimizing rather than inflating confusion for sweet confusion's sake. It has long been suspected, and one perpetrator has willingly admitted the allegation, that a few whose brains function better than those of the rest of us in mathematics actually enjoy exploiting this advantage by keeping the suffering soul who isn't a genius as mystified and frustrated as possible; one adjunct to this kind of exhibitionism being the use of complicated, high-hat symbols and other slowing underbrush. In his article Schell substitutes for a possible flock of $64 symbols plain old-fashioned A, B, C, D, and so on, and avoids other minor impediments to the unhappy. The article by N. J. Schell:

THERE HAS just been released for publicity in this country the details of a new optical system invented in August, 1941, by D. D. Maksutov, of the State Optical Institute, Moscow, U.S.S.R. The inventor refers to it as 'Meniscus Catadioptric Systems" and describes its general application to telescope systems, although it is stated that the system is applicable also to other optical instruments. The release is given in a very friendly fashion, with special appeal and consideration for amateurs. It appears to the writer that the design will prove of very great interest to amateurs, due to its advantages.

The system consists of combining the action of a single meniscus lens of rather deep curvature and of nearly constant thickness, with a concave spherical mirror, for the purpose of compensating the aberrations of the latter. The location of the lens in the fundamental design is in the path of light preceding the mirror, but the system also includes a modification permitting the lens to be located in the converging cone of light from the mirror near the focal point. Only the fundamental design will be covered in this description. The drawing shows this design. [For Newtonian, add diagonal. -Ed.]

By way of prelude, it should be understood that a spherical concave mirror reflecting parallel light from a distant point, such as a star, does not form a sharp image, since rays reflected from the outer parts of the mirror come to a focus closer to the mirror than those from more central parts. This effect is known as "spherical aberration" (negative in this case). If we change the sphere to a paraboloid of revolution, this, effect can be eliminated, but such a mirror still will produce distorted images of points other than those confined to a more or less restricted field of view surrounding the axis of the paraboloid. This effect is known as coma, and is usually not troublesome in the fairly narrow fields meeting requirements of visual observations in focal lengths f/8 and longer, but is detrimental in the wider fields desired for visual observations and photography, in shorter focal-length instruments.

The meniscus lens, on the other hand, can be designed so that it will have very little power as a lens, but with sufficient positive spherical aberration to compensate for the negative spherical aberration of the spherical mirror. In addition, by suitably locating the meniscus, the coma is effectively eliminated for an adequate field.

The meniscus thus pre-conditions the light ahead of the mirror-much the same as in the case of the correcting plate of the Schmidt camera, but with the difference that the surfaces of the meniscus are spherical and much easier to produce than those required for the Schmidt correcting plate.

Also, the meniscus, acting as a very long focus negative lens, has very small chromatic aberrations (color), claimed by the inventor to be very much smaller than those of the usual doublet achromatic refractor objective; so that, even in short focal lengths, it gives images practically free from color on this account.

The important advantages claimed for the system as compared with refractors and reflectors, are as follows:

1-Images freer from color, also much shorter focal lengths than refractors, as well as a wide choice of glass for the lens, permitting photographs in ultraviolet light.

3-More exact and uniform correction due to the facility afforded by the deep spherical surfaces of the meniscus; particularly as compared with reflectors of short focal length, other aplanatic systems, and off-axis arrangements.

The elements of the system may be computed by the usual methods, of course taking into account the characteristics of the glass from which the lens is to be made. A complete mathematical treatment will not be attempted here but, instead, we may take the dimensions from certain empirical formulas which the inventor has supplied, to indicate a typical design. These dimensions are sufficient for practical purposes if for a chosen aperture the theoretically greatest ratio of aperture to focal length is not approached too closely, thus allowing for usual variations in different melts of the type of glass indicated. These dimensions apply to optical glass for the meniscus having a refractive index 1.5163 and medium dispersion V 64.1, and with the stipulation that the meniscus thickness at center is 1/10 of its active theoretical aperture.

In the drawing the following may be identified:

The figures in the table give factors, which, in each case, when multiplied by the chosen aperture in inches, give the dimensions applicable for a given ratio of aperture to focal length (A/F). These are tabulated from A/F of 1:3 to 1:5, also 1:8. The table also gives the maximum theoretical apertures for visual use in which all aberrations are within the Rayleigh limit, according to the inventor. For photography, these limits can be greatly extended, also A/F ratios greatly increased.

Taking as an example, a design of 8" aperture and 32" focal length-that is, an aperture-focal ratio of l:4-we find the dimensions as follows:

In this case S will be approximately 33.6", as this figure is not included in the empirical formula and must be calculated.

From the above example we find that the depth of the concave surface of the meniscus is .673", which, added to T, gives 1.473", the minimum thickness of glass required for the lens before shaping. In actual practice this should be upward of 1-1/2'', to allow for working, edging, and so on. As a comparison, the following are dimensions for two of the more familiar f/8 (1:8) Newtonian design, usually favored by amateurs:

The light diverges within and after leaving the lens, so that the active aperture of the mirror is slightly larger than the lens. The diameter of the mirror should be such as to take this into account, plus an allowance for full illumination of the field of view desired in any particular case. A trace of a ray (exaggerated) near the periphery is shown in the drawing, also a dotted line indicating the path it would follow if the lens were not used. The point of intersection of this dotted line with the ray reflected from the mirror determines the focal length, and explains the difference between dimension S and the focal length. Note also that, since all three surfaces are concave to incident light in this fundamental design, the radii would be indicated as minus (negative) in design calculations.

To avoid possible misunderstanding, it is emphasized that the above table and examples apply only to the particular conditions specified, and that almost any desired modification in sign can be made-for which, of course, the elements must be specially calculated by trigonometrical methods. Regarding the glass for the lens, whether for the given example or not, the refractive index is more important than the dispersion, as it is the former which governs the accuracy of the correction for spherical aberration and coma. The dispersion is not critical; it being desirable only to keep it as low as possible (greater V value)-preferably a type of crown, as indicated, for visual purposes-thus ensuring the most color free images. The mirror, of course, may be made of any kind of glass desired.

The system can be extended beyond the limits imposed by spherical surfaces, even though these seem adequate, by slightly altering or retouching the surfaces.

While this short description covers only the fundamental principle involved in image formation, it is obvious that practically any of the familiar arrangements of reflecting telescopes can be used in connection with it, and the inventor has in fact given most of these as examples, along with several innovations, not the least of which, in the writer's opinion, is an off-axis application in which all surfaces are spherical. There is also a suggested method of reducing the size of the central obstruction in Newtonian types.

Perhaps the feature having greatest appeal for amateurs is that of closed-tube construction, which should give much steadier images than reflectors, and, at the same time provide an instrument practically as permanent and foolproof as the usual refractor, with the advantage of much shorter length.

It is anticipated that an extended discussion of this interesting improvement will be forthcoming, in which the various arrangements and technical details will be fully developed. In the meantime, it seems that it would be appropriate if we referred to this design as the 'Maksutov Telescope."

END of Schell's contribution.

The name Maksutov is pronounced Mack-soo-tof, stress on the soo.

As Schell hints, other articles on the Maksutov will be forthcoming, short of the unforeseen.

KNOWING of the recent tight situation on optical glass, and to get things started, your scribe scurried around and dug up promise of some 7.9" x 1-5/8'' disks and then advised a few advanced amateurs. Ten ordered the disks, and in this "Mak Club" are: Schell Broadhead and Paul of western New York State; and the following members of the Long Island Astronomical Society: King, Cristman, Luechinger, Franklin, Cameron, Thorne, Rekouski. After a few months their findings should become available for the benefit of a later wave of aspirants, and by then it is hoped that the glass situation will not be so tight as it is at present.

However, this new Maksutov thing isn't going to run away, and a slow level-headed evolution may prove more desirable than a big sudden blaze that cannot long endure.

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.