W. H. Keister, 415 California Ave. Oakmont, Pennsylvania, is the maker and he says that, when he at first simply attached casters to the tripod legs, they proved too small and did not roll well. The retractable running gear consists of rubber-tired wheels from a boy's coaster, mounted on a U-shaped frame and hinged to the base of the tripod. To shift the weight to the wheels the handle on the U is raised, which also locks the U in position. A tongue for towing the telescope about is attached to the third foot of the tripod. The latter rests at all times on a caster. The telescope is a 6" reflector, of clean, solid design, and it weighs about 200 pounds. The finder is a second-hand, Navy gun sight.

ONE more of Walkden's "Richest field" telescopes, an "RFT" of 6" aperture, made by Fred W. Forrester 252 Lemon Ave., Arcadia, California, is shown in Figure 2. It has a hexagonal tube made of 1/4" plywood. The eyepiece lenses are coated with fluorides, to eliminate the ghosts (optical ghosts, or ghost images). "I think the RFT is great," Forrester enthuses.

CLUB: Clyde W. Tombaugh, Lowell Observatory, Flagstaff, Arizona the amateur telescope maker who found Pluto, in order to obtain 16" Pyrex disks for other amateurs at a relatively low cost, offers to sponsor a "Sixteen-inch Club" which, if it obtains 20 members within six months will be able to purchase such disks at only $35 each. Regular price for this size, solid type, where single disks are purchased, has been more than $100 higher, because the high cost of producing single disks is chargeable mainly not to the glass or to the pouring, but to making the mold. Corning Glass Works, Corning, N. Y., has agreed with Tombaugh, the sponsor, that it will receive checks for $35 each and hold the funds six months or until 20 orders have come in then make the disks: failing this, refund the moneys. Three orders are lined up already.

Tombaugh has also ascertained that Pittsburgh Plate Glass Company, will supply 16" tool disks of 1 1/4" plate, boxed, f.o.b. factory at Ford City, Pennsylvania, at $7.80 each.

There's a lot of difference between a 16" and a 20", not alone in weight of the disk, where the 20", if worked without a machine, is a real backache, but in bulk and cost of the mounting, for the mass ratio between the two is almost as one is to two (calculating from the cubes of the respective diameters). The 16" size should afford a pretty telescope, big, impressive, powerful, yet not too big. For many, a 20" is.

STELLAFANE convention, Saturday, August 2.

ADVANCED amateurs and groups who are planning rather large reflectors will find a cleanly designed 24" described in Vol. VIII, No. 6 the Publications of the Observatory, University of Michigan. This publication, while not available for general distribution, may be consulted at astronomical observatory libraries. The 24" (Figure 3) was designed by Robert R. McMath and George H. Malesky, of the staff of the MeMath-Hulbert Observatory. The former is an amateur telescope maker and astronomer (motor car manufacturer) who turned professional and is widely known for his tower telescopes, his spectroheliokinematograph and other instruments, at Lake Angelus, Pontiac, Michigan, also for his educational motion pictures of the planets and of solar prominences. The new telescope will be used first for doing over these films, with the greater resolution it affords.

The skeleton tube is made of longitudinal struts of 13-gauge steel having box sections. The center ring is made of six pieces welded together in box section. The tube assembly has proved to be very rigid. The tube center section and cell are of 3/16" rolled steel with ring flanges, the assembly welded and thrice annealed.

The 24" Pyrex primary is an f/4 and there are two secondaries of 3 1/2" and 5 1/2" diameter, to give, respectively, f/25 and f/50. The primary was figured by Halley Mogey, of the Perkin-Elmer Corporation, and tested by Dr. Heber D. Curtis, who reported it an unusually fine surface. The two high-magnification secondaries presented real difficulties and E. L. McCarthy, now optical designer with the Perkin-Elmer Corporation but formerly an amateur (see "A.T.M.", page 389), feeling that the conventional tests were inadequate, proposed a new test which eliminates the combined testing of primary and secondary. This was used by Halley Mogey to test the secondaries, and proved to be an unqualified success, according to McMath.

This new "McCarthy test," as we suggest it be known, is described by McCarthy as "A New Test for Convex Surfaces," in the Journal of the Optical Society of America, February 1941, pages 107-108, as follows:

WHENEVER the object or the image point of a reflecting surface is virtual, as in the case of a Cassegrain secondary mirror, testing is more of a. problem than it is when both conjugates are real. Ideally, the test should permit a complete view of the surface at a convenient angular size; it should not involve large and expensive auxiliary pieces; it should not depend on slow measurement of zonal foci, but it should produce a simultaneous darkening over all parts of a perfect surface when a Foucault knife-edge is introduced.

"These conditions are reasonably well met by the system shown in Figure 4. Light from an illuminated pinhole passes obliquely through two plane-parallel glass plates to a silvered spherical mirror. Reflected first from the sphere and then from the large plate, the light converges toward the virtual focus of the secondary mirror, which returns it through the plate to a real focus at the knife-edge. These two foci, of course, represent the conjugates for which the secondary mirror is to be free of spherical aberration in actual use.

"Since the large plate has a considerable thickness, it introduces serious aberration into the beams of light diverging and converging through it. It is the function of the two small plates to cancel the unsymmetrical aberrations such as lateral color and coma. To effect this compensation, all three plates are of the same thickness, and the small ones are set at t he same angle to the axis as the large one, but in the sense which restores the displaced axis. The longitudinal aberrations, such as astigmatism and spherical aberration, are augmented by the small plates and must be removed in a different manner. If the pinhole is displaced from the center of curvature in a radial direction, either toward or away from the spherical mirror, aberration is introduced of the proper sign to cancel the spherical aberration of the plates. Astigmatism is compensated by displacing the pinhole laterally with respect to an axis of the sphere, since the astigmatism in the field of a concave sphere is of the proper sign to cancel that of the plates.

"Obviously the conditions will be different from one case to another, depending upon the character of the mirror to be tested, the thickness of the plates, and the axial obliquity through the plates. It is therefore desirable to calculate the correct arrangement for each case by ray tracing, and to place the pieces in accordance with this calculation. If the second conjugate distance is large compared to the aperture of the test 5 piece and the plate thickness, as it usually is in a Cassegrain, no calculation is necessary. Instead, the adjustments may be made by trial until the beam of convergent light reflected from the large plate is shown by the knife-edge test to be homocentric. In such a case, of course, the secondary to be tested is temporarily removed in order not to interrupt the beam. This method of adjustment assumes that the final passage through the large plate will introduce no appreciable aberration, and it is justifiable in the case of a slender emergent beam. Whether it is good enough for a particular case may be tested by examining the effect of the final small plate. If its presence or absence makes no observable change in the character of the final image, other than a displacement, the indication is quite definite that the trial and error method of adjustment is suited to the particular case, inasmuch as the aberrations of a plane-parallel plate are constant, no matter where it happens to be along the axis.

"An illustration is provided by two Cassegrain secondaries made for the McMath-Hulbert 24" telescope. Primary and secondary mirrors were made by Mr. Halley Mogey, who also introduced improvements over the original idea of the test. The primary is f/4 with a central hole 4.5" in diameter. The secondaries are 3.5" and 5.5" in diameter, and give magnifications of 12.5 and 6.5 respectively. The combination of large central hole, high magnification, and large unvignetted field made it impossible to see any error on the secondary mirrors when tested by the Ritchey method, even before any figuring at all had been done.

"The test pieces, also made by Mr. Mogey, were a spherical mirror of 9" aperture and 24" radius of curvature, a plane-parallel plate of 12" diameter and 0.5" thickness, and two small plates 0.5" thick. The figure of the spherical mirror is required to be excellent; both sides of the large plate may depart from flatness by several waves, but should be parallel within a few minutes of arc; the small plates do not need a good figure. There are two unsilvered reflections, so that a fairly bright light source is necessary. Either a mercury or a ribbon-filament incandescent lamp has been quite satisfactory.

"To make a Cassegrain secondary accurate to one tenth of a wavelength does not require tenth-wave flatness in the large plate. If its figure is roughly spherical, of very long radius, the astigmatism introduced by oblique reflection and transmission may be compensated in the manner already described for the normal astigmatism arising from oblique passage through a plane-parallel plate. If the figure is generally flat with residual zonal errors, such errors are readily separated from those occurring on the secondary mirror, since the former appear elliptical and the latter circular.

"In the present case, the first small plate was placed 1.5" from the pinhole and at an angle of 45° to the direction chosen as the optical axis. Elimination of astigmatism required an angle of 8.76° between the axial directions before and after reflection from the spherical mirror. A longitudinal separation of 2" between the center of curvature and the pinhole removed the slight spherical aberration. It was found that the last small plate was not required for either of these secondaries.

"In testing larger Cassegrain mirrors, some economies could be introduced. For instance, in the case just described, the test equipment is large enough to show the complete secondary. An arrangement permitting an unsymmetrical view of about two thirds of the surface would involve test pieces scarcely larger than the secondary itself."

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