MAKSUTOV, pronounced Mack's-suitoff, is the name given to a new type of telescope originated by the Russian of that name. As soon as Maksutov published basic data for the design, this department facilitated the construction of Maksutov telescopes by inviting N. J. Schell, Beaver Falls, Pa., experienced amateur telescope maker, to interpret Maksutov's abstruse original article for other amateurs and by arranging with the Corning Glass Works to mold 24 of the special blanks needed for the construction, all of which are now in various amateur's hands.

First to furnish a description of a finished Maksutov telescope is G. Camilli, 155 Dawes Ave., Pittsfield, Mass., who enthusiastically reports the "Mak" a success. Some are still having difficulties with the extremely sensitive alinement after nearly finishing their Maks. Camilli writes:

"The optical system developed by D. D. Maksutov of the State Optical Institute, Moscow, U.S.S.R., is similar to the Schmidt camera and combines the action of a single meniscus lens with that of concave spherical mirror. The role of the meniscus is identical to that of the correcting plate of the Schmidt camera-the correction of spherical aberration and coma of the spherical mirror without introducing a noticeable chromatism.

"The meniscus shuts the tube and makes possible a sealed instrument free from convection currents and a mirror protected from dust and dew; in addition, secondary mirrors may be fastened to the meniscus to reduce their diffraction effects.

"The Maksutov system was described first by Maksutov in the Journal of the Optical Society of America, 1944, May, and subsequently by N. J. Schell in Scientific American, 1944, October and December.

"The optical system of the Mak completed by the writer is shown in Figure 1. In common with other members of the Mak group, the telescope has an aperture-focal ratio of 1:4. It can be used for visual and photographic purposes.

"The meniscus is made of crown glass (n_D = 1.5170 ± 0.0010; V = 645 ± 0.5) and was ground, polished, and figured against a tool of the same dimensions. Both surfaces of the meniscus are spherical. The convex surface of the meniscus needs no accurate figuring since the curve is very deep. This is one of the attractive features of the Mak design. Deep curves of this kind can be held truly spherical with reasonable care in the grinding and polishing; especially when it is known that a sphere is aimed for. A light polishing action has been found to be best.

"The spherical mirror is made with radius of curvature held as closely as possible to the dimensions shown in Figure 1. The success of this job depends very much upon holding radii very close to those called by the design theory. Unlike our ordinary telescope mirror jobs, where we get a certain radius and then correct to the extent called for by the actual radii obtained, this work is more like that of refractors where close adherence to radii as computed must be had if correct results are to be attained. For the above purpose a spherometer ('A.T.MA.,' 242) is almost a necessity. The meniscus must be optically centered by edge grinding.

"In the writer's optical assembly shown in Figure 2, the meniscus and the mirror are rigidly mounted in an aluminum frame. This method affords great simplicity in the mounting and adjustments of all the parts. Thus the cells for the meniscus and the mirror, as well as the spider that supports the diagonal, can be slid up and down on the rods of the frame. The frame will be found very useful during the final phase of the assembly. To obtain the best performance from the Mak system it is imperative that the optical axis of the meniscus and of the mirror coincide. This adjustment is of course done with the spider and the diagonal removed from the assembly. By providing three small bushings which are held against the cell of the meniscus, this and the spider can be removed and returned exactly to the original accurate position without injuring the optical alinement of the parts.

"A method of obtaining the alinement of the two optical axes (mirror and meniscus) is shown in Figure 3. Using a wax-pencil, a cross is marked on the convex (back) surface of the meniscus. Likewise a small round dot is marked on the exact center of the mirror. A light held slightly on side of the operator illuminates the front of the meniscus. Looking at the system from the meniscus and keeping his right eye in line with the center of the cross and the dot on the mirror, the operator will see also the reflection of the cross on the surface of the mirror. By careful adjustment of the mirror, the reflection of the cross can be made to coincide with both the original cross and the dot of the mirror. In making this adjustment it is of course assumed that the meniscus is squared on the mechanical axis of the frame.

"After this adjustment has been completed, the spider is returned to place and the assembly is inserted in a protecting tube as shown in Figure 4. The adjustment of the diagonal with respect to the mirror is done in accordance with one of the schemes described in 'A.TMA.,' 272.

"The performance of the Mak well repays all the work which the writer has put into it-the definition is much superior to that obtained by a simple reflector. The images of stars are very small and sharp, free from coma and color. The writer expects to run a series of comparative tests between the Mak and an f/17, 6-1/2" reflector, the mirror of which is made of fused quartz, mounted in tandem with the Mak.

"The mounting for this telescope (Figures 4 and 5) is similar to Everest's 'Old Town Pump' described in 'A.T.M.A.,' 514. An Alnico motor (75 r.p.m ), through an additional gear-reduction, drives the final worm gear of the telescope (Figure 5). The same worm gear can be driven at very high speed by a capacitor type reversible motor Ordinarily the Alnico motor drives the right ascension shaft at sidereal time. When it is necessary to advance or retard on the normal speed, a push button arrangement brings the capacitor motor into operation. After the correction has been made the A1nico motor immediately goes back into service without the mediation of any mechanical clutch."

CONFUSION exists in some minds regarding the type of Pyrex Brand glass in standard molded telescope blanks. Are these regular products or some "off" variety?

Such a rumor probably started from the fact that Corning developed for the 200" mirror a special glass having only two thirds the expansion co-efficient of the standard Pyrex. While this special glass is available for to order jobs, it is much more expensive. The regular type in standard molded mirror blanks, 4-1/4" to 12-1/2", also the 16" blanks recently made for special price club groups, is the same as used in Corning's huge annual tonnage of Pyrex Brand laboratory glass, cooking ware, insulators, and other industrial and consumer ware. It is drawn from the same tank furnaces and from the same melts in them. If a farmer, milking a cow, directs a squirt into a cat's mouth, the milk is from the same cow, same bag, same batch, and has same composition, and Corning mirror blanks are as completely regular material as the milk squirt is the cow's regular product. This also explains why Corning molded telescope blanks can be as inexpensive as they are, since the regular glass used in immense quantities for products having large sale, is always in continuous melt.

Not all may even yet know how large an industry Corning Glass Works is. They have 10 plants, make more than 37,000 different items of glass including 45 percent of all the glass bulbs for incandescent lamps made by others in the United States, and do a $50, 000,000 in annual business of which telescope blanks is fractional.

Let's look at the two kinds of Pyrex Brand glass mentioned above. Wide awake telescope makers will learn their numbers.

Why, you may now wish to ask, couldn't Pyrex Brand No. 716, the lower expansion brand, be just as easily used in telescope blanks as the 774? Be-cause it isn't in continuous melt. Much of it is in stock but it is cold. Obviously the telescope demand wouldn't begin to justify keeping it in continuous melt, and melting it is expensive in more practical ways than the one that is most obvious from the table- that is, higher softening point (more fuel). This softening point is only 12 degrees Centigrade, above that for Pyrex 774 but at the top temperatures involved this increment is appreciable. It took five weeks to melt 60 tons of 716 from which to dip enough to fill the mold for the 200" mirror.

When an order for a blank of Brand 716 is filled, a special ceramic mold is built up, large irregular pieces of 716 are laid in it, cold, and melted down to shape of mold. In practical ways this is expensive. For example a single blank of 716, 16" by 3 1/3", was priced as of last spring, $198, and a 20" by 3 1/3" was priced $306. In club group of 24 blanks molded at on time the 16" blanks of 774 sell for $37.50 net.

This department has now been urge to start a club of purchasers of 20" mirror blanks, as it did with I6" blanks. Investigation revealed that the economics in the two cases were dissimilar, with heavy accent on the dis, and no such club is in the offing.

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