The 12" reflecting telescope shown in Figure 1 was made by an amateur, J. M.; Holcomb, 40 Clarke St., Burlington, Vt., for St. Michaels College in Burlington's, suburb, Winooski. Asked to describe it, Holcomb states:

"The mounting is compact and rigid, operates smoothly, and is convenient to use. The setting circles are divided to half degrees and are illuminated. The driving clock (Figure 2) is run by a synchronous motor and is adjusted to sidereal time. The mounting is also provided with slow motion controls, that in declination being manual that in right ascension motor driven.

"The building (Figure 3) consists of 12' by 26' classroom on ground level in a wooden framed, metal-sheathed dome 12 in diameter, 9' high, above. The telescope rests on a ten-ton reinforced concrete pier entirely free from the building, in order that no vibrations may be transmitted to the telescope.

"I believe the unit can be termed an amateur job, despite the fact that I did not actually lay the bricks, mix the concrete, or pour the iron castings for the mounting. I did design it all, also made the patterns supervised the masonry, and did the rest of the construction.

"The mounting contains a temporary 8'' mirror made by Father A. Rivard, S.S.E of St. Michaels College, but a 12" mirror being made."

The rugged proportions of Holcomb's mounting design are worth close study. This telescope will not shiver, as many do when a fly alights on it. A telescope magnifies its own vibrations just in the measure that it magnifies the stars, hence vibration that remain wholly invisible to the direct look cause the stars to dance wildly to the look through the telescope. This is why ordinary criteria for rigidity in machine supports, and so on, are much too weak for telescope mounting design. A telescope should be theoretically about as rigid and rugged as a cubic yard of cast iron. Holcomb's is.

FIFTH in a long line of eight Newtonians ranging from 4" to 12-1/2'', and from f/5 to f/10, and built over a period of four years, is shown in Figure 4, with it maker and owner, John J. Stoy 450 Hurt Building, Atlanta, Ga. In his letter Stoy styles himself "Director of the Smoky Hollow Freshair Observatory." The telescope is a 12-1/2" of f/8, and the total weight of telescope and pier is 3350 pounds. The design is conventional but unusually clear. "The polar axis counterweight," Stoy writes, "is open to argument as to necessity, but brings the center of mass to the center of the pier." This last idea seems excellent and in keeping with principles of mounting design stated by Porter in the treatise "Amateur Telescope Making." fourth edition.

Close inspection of the figure will reveal a very smooth, clean declination axis casting and union for tube attachment. These were cast in iron, from patterns made and donated by Fred Ferson, Biloxi, Miss., author of the chapter on molding and casting, in "ATMA," and a friend of Stoy's. The upper end of the tube rotates. The tube is ventilated through an oversized cell with 9-point suspension.

How well your scribe recalls the day when Stoy received and began working the 12-1/2" Pyrex disk for this telescope, which, it seems, was just a bit bubbly! Daily thereafter the mails brought generous samples of the most competent old-fashioned southern cussin', but finally Stoy learned to defeat the bubbles and the profanity imperceptibly dwindled until, at last, he actually came around almost to loving Pyrex. Even now, however, you must smile when you say "bubbles" within his hearing. (It is impossible to cast Pyrex disks without some bubbles-can't get it hot enough and fluid enough, because of its high melting point for all the bubbles to rise-but the makers throw out the poorer disks.) Stoy also refigured his mirror twice after most workers would have called it a day-we suspect merely in order to demonstrate to that- --hunk of glass that he, and not it, was boss. And then, becoming finally softened and sentimental, he named the telescope "Bessie."

THE following communication is from James G. Baker, of the Harvard College Observatory:

"In reading through the very interesting article by Hendrix and Christie in the August Scientific American the present writer has noticed some incompleteness that might cause ambitious amateurs a bit of grief. All the various types presented by the authors are certainly workable, but caution must be exercised in order that cameras constructed should not exceed in aperture that which is justifiable theoretically.

"The thick mirror type discussed (Figure 3, IX in the August article) is optically inferior to both the ordinary and the solid kinds of Schmidt camera, although still quite a good camera in itself. The authors have failed to point out that the position of the apparent center on the axis depends upon the angle that the incoming rays make with the optical axis, and that this type of camera does not possess the symmetry of the Schmidt arrangement. Just as in the case of the usual Schmidt the third order errors are zero, which fact insures good performance, but, optically speaking, the thick mirror is not aplanatic beyond the third order, whereas the Schmidt is aplanatic to all orders of accuracy. The practical effect of this lack of perfection is to limit the speed and usable field of the camera, as compared with the Schmidt in glass. By a suitable deformation of spherical mirror and intermediate refracting surface, one can obtain nearly Schmidt performance, but the required optical work is more involved. The Zeiss company has recently produced a thick mirror system involving only spherical surfaces that has a flat field. The authors of the August article have neglected to point out that, as in the case of the usual Schmidt, the thick mirror type has a curved focal surface, spherical and concentric with the spherical mirror face and should be made so. The authors state on page 119 that the speed of an f/0.66 camera can be obtained with the field and correction plate curvature of an f/1 of the usual kind, but this is not correct. The depth of the correcting surface is exactly n times deeper, zone for zone, than for the usual Schmidt of the same physical size, and the usable field about n times larger, angularly, in the case of the solid Schmidt, and about the same in the case of the thick mirror system, all compared with the usual Schmidt. The letter n used above stands for the index of refraction of the glass of the thick mirror. The index of refraction that appears in the denominator of the expressions for the correcting surface will be that for the glass of the correcting plate, in the above thick mirror system.

"The performance of the Schmidt in glass is truly remarkable. The solid glass combination was discovered independently and was investigated through its fifth order image errors in May, 1938, by the present writer, and discussed at the October, 1938, meeting of the Optical Society of America. The solid Schmidt, I have heard indirectly from Bergedorf [Bergedorf Observatory, in Germany, where Schmidt worked.-Ed.] workers, was contemplated by Schmidt himself, some years ago. Sinclair Smith seems to have been the successor to Schmidt in coming upon the solid Schmidt, but the publication was delayed by his untimely death in May, 1938. The Bergedorf workers unfortunately concluded that the thick mirror type constitutes the only practical way of, getting to the focal surface, as Hendrix did also in the early stages of his work. Hendrix, however, has now provided an ingenious solution in the form of the folded type. An off-axis type provides another.

"Let us compare the solid Schmidt with the ordinary Schmidt of the same physical size, that is, with the same aperture and radius of the mirror surface. The focal length is 1/n that of the usual Schmidt, the speed n² greater, the depth of the correcting surface n times deeper, the size of the chromatic aberration disk n times smaller at each wavelength, and the curvature of the field the same. The third order astigmatism, re-introduced by the central hump in the correcting plate, is slightly smaller than that for the usual Schmidt, and for both is numerically of the fifth order. The first error of importance in either the solid or the ordinary Schmidt is the variation of spherical aberration with angle. This defect is several times smaller for the solid Schmidt than for the usual kind. The variations of third order defects with color are all zero with exception of spherical aberration, as in the case of the usual Schmidt. The front surface of the solid Schmidt behaves as a single prism face, so that, for large field angles, a star photographs as a very short spectrum. This defect is of no importance in spectroscopy, for which such a glass camera is most useful. The Harvard Observatory has under construction a solid off-axis Schmidt, for which the focal ratio has been pushed to the extreme of f/0.30. The focal length is 15mm, and the usable field 3mm linearly, or about 11 degrees. The field is still larger for reduced aperture.

"A variation of the Schmidt in glass is offered by converting the spherical mirror into a spherical lens surface, with a corresponding change in the depth of the correcting surface. Because of the small power of a lens surface of the same radius as a mirror surface, as compared with that mirror, the aperture-ratio of this type camera is limited to f/1.5 or slower. It is, nevertheless, a true Schmidt camera. The focal surface is spherical and concentric with the lens surface.

"In the article by Hendrix and Christie, even for the folded type, they mention a plane focal surface, and count it among the seven plane surfaces to be made. This is inaccurate, for the focal surface, as in the usual Schmidt, is spherical and concentric with the mirror, and should be made so.

"In the discussion of the Wright type, which has a flat field, the authors state that their f/1 was unsatisfactory because of higher order aberrations. This is not altogether the complete story. The Wright type, in addition to having double the chromatic trouble of the Schmidt of the same focal length and aperture, has astigmatism of the third order, that seriously limits the field at high apertures. As the authors state, cameras of f/3 and f/4 are satisfactory for limited fields, but then there is no need to make one in glass. For spectrographic purposes, the Wright type, called specifically the "short" type to distinguish it as one of a family presented by Wright, also has a curved field, and on that surface third order astigmatism along the spectrum is zero.

"The present writer would like to caution the amateur, who is contemplating a very fast camera, to compute his correcting surface from the more accurate formulas given by Wright in 'ATMA,' and to try contacting some person who has made a Schmidt to check the computation. If the amateur makes a solid Schmidt, he should use the ordinary formulas for a correcting surface, but with a factor of n, the index of refraction, throughout, so that the correcting plate is n times deeper, zone for zone, than is the case for the usual Schmidt. Moreover, the position of the focal surface must be carefully computed.

"Hendrix and Christie mention that the easiest form of correcting plate to make is one for which center and edge are at the same height; that certainly must be correct from their wealth of experience and practice, but I would like to point out that the departure from the nearest sphere (one through center and edge) is nearly independent of their parameter k and, therefore, that the amount of aspherical figuring is likewise nearly independent of k.

"The above is in friendly criticism, with an eye toward aid to the amateur, and in no way should detract from the excellence and usefulness of the article as a whole."

In his letter of transmittal, Baker states that he has the entire set of quantitative formulas that bear on the Hendrix and Christie designs and will be willing to cooperate with any amateur desiring information, who is really serious about making some of these telescopes. In each case, he states, the many factors should be carefully balanced before the maker starts precipitously on a part not justifiable theoretically. He will give information to anyone wishing to know about the performance of a Schmidt of given specifications, or will recommend specifications to fit a given need. He has also designed a family of flat-field cameras, equivalent in performance to the Schmidt camera, which he described before the American Astronomical Society last summer.

Priority on (1) the fact that the correcting lens of a Schmidt should be made larger than needed and masked out, to facilitate the work, (2) the use of a liquid on the correcting lens while it is ground to shape, also use of a pattern of straight lines at the plate holder, and (3) the interpretation of the zones by the method just described, all described in the Hendrix and Christie article of last August, is claimed by Arthur De Vany, 727 Sylvan Court, Davenport, Iowa. In letters offered this department for publication long ago, De Vany describe these methods. Owing to your scribe's procrastination in publishing them they were offered to Popular Astronomy, which published them a year or so ago, and it is not desired to mention the matter in order as far as possible to make amends.

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