"Your letter," he replied, "puffed me up a bit and, if I do any bragging, it's your responsibility. I started telescope building about three years ago, using a borrowed copy of the first edition of 'Amateur Telescope Making.'

"That's when my trouble started, and haven't been the same since. After making the two smaller instruments I obtained a 21'' Pyrex disk weighing 100 pounds and, at the end of about 40 hours work, I had it roughed out to f/5. Fine grinding took six hours polishing nine. This was all done by hand, the 100-pound mirror face down over a full-sized tool and polisher, and I would not advise any one to attempt this unless he has an elephantine physique. [A. W. Everest also did this.-Ed.]

"Figuring was done with small tools, mirror face-up. The correction is within 1.5 percent of a full paraboloid and the figure is smooth and regular, the performance good.

"Both the convex secondary and the diagonal flat are of Pyrex, I turned down 8" disks to 6-l/2" to gain extra thickness. In correcting the convex I used Hindle's method, and a sphere to figure the flat.

"Hindle's 18-point flotation system (Figure 4) was also used to support the mirror, though I doubled its number of inner edge supports. Figure 4 also shows the outer edge support shoes. The cell is a combination of steel plate and castings, welded, as is the lower part of the tube.

"The lattice-type tube (Figures 1 2, 3) is bolted to the solid part. Its eight pieces of tubing are of 1-1/4" diameter, 20-gage wall, cold-rolled steel. The 3/16" tie rods and the perforated octagon hoops also are cold-rolled steel, the latter (Figure 3) being made of 16-gage channels. A 1-1/4" washer was brazed into each corner of the octagon and the tubes were then pressed through these holes and brazed, making a very stiff job regardless of the angle at which the tube rests. Figure 2 best shows the pairs of additional tube braces stretched over the squared ring at the top of the solid part of the tube. These give added rigidity. I can detect no flexure by visual inspection of the mirror reflections.

"For the spiders (Figures 3 and 5) to support the secondary mirrors I used saw blade steel.

"The declination axis trunnions (Figures 1, 6, 7) are castings of special material, (cast iron, nickel steel), as are the four other large parts of the mounting. All were normalized before machining. I made my own patterns.

"As I own no lathe or drill press I was forced to have the lathe and similar work done outside.

"The declination bearings are used truck ball bearings (radial thrust, 5" O.D.) They had to be large because the trunnions were to be hollow to allow the reflected rays to pass through to the eyepieces.

"I placed the declination circle with its shielded light (Figure 6) on the west side. It is divided into half degrees. The index consists of a bright copper wire. On the east side is a reversible slow-motion motor with gearing (Figure 7), mounted on rubber. A small clutch allows this axis also to be moved manually without adjustment or disengagement of gears.

"For the south polar axis I used a radial thrust, self-alining type of bearing (Figure 8). The bearings under the split ring (Figures 2, 3, 9) are used auto parts. The inner trace is stationary, while the outer race rotates under the big ring. This gives a four-point support, spread over a large part of the ring. Each pair of rollers is pivoted, and this tends to cancel out possible irregularities on the bearing face of the ring.

"The R.A. setting circle (Figure 9), is divided into minutes.

"The principal gear train for the drive in R.A. has a 1/8 H.P. synchronous motor (Figure 8), mounted on sponge rubber. Slow motion in R.A. is obtained by a reversible motor operating through a clutch, as in the declination axis drive. This is mounted on the underside of the large worm wheel beneath housing. The large circular object in Figure 8, near the worm wheel, is a guard to protect the gear, also the clothing.

"A 2-1/2 prism, shown dimly beneath it cell ring and above the spider in Figure 5, is used to divert the image to either of the eyepiece at the respective ends of the declination axis, simply by turning the prism east or west one half turn. Very comfortable observing position is thus made possible.

"As a finder I use a 6", f/6.5 reflector with rotatable tube (Figure 1).

"My observatory (Figures 10, 11) was made by adding 8' to my garage. The telescope rests on piers (Figure 12), with footings 6' deep. The observatory floor is 6-1/2' above the garage floor and is 11' square, giving ample room. The housing for the telescope rolls off on a gantry (Figure 10), and is so compact that, when closed, it is scarcely noticeable from the street.

"The sidereal clock (Figure 13) was made from an ordinary pendulum clock. For the pendulum I substituted a longer and compensated pendulum, thus reducing the speed of the hour hand to one revolution per 24 hours. On the dial face is a star map, and stars at the zenith are indicated at all times by the hour hand.

"For the benefit of any faint-hearted aspirant for a 20", you may say that I am not an engineer, neither am I a mathematician: my school training was very elementary.

"Our local Toledo Astronomy Club is an active one. Professor Winslow, of the University of Toledo, is our skipper and we meet monthly at the University."

LETTER from an author:

"In our article, "Some Applications of the Schmidt Principle in Optical Design," by D. O. Hendrix and myself, in the August number, it was certainly far from our intention to deny credit to anyone to whom credit is due. The meaning of our offending sentence would have been clearer had it been phrased to read '. . . little that was not known to Schmidt' instead of 'little that is new.'

"Through private sources we know that Schmidt himself had derived the fundamental equation and had considered various modifications of the principle to meet the requirements of special problems. Had we been concerned with the history of the matter, which, as our title suggests, was not the case, a complete bibliography naturally would have been included. The methods of polishing and testing used at Mount Wilson for several years were developed independently by Hendrix and described by him in a public lecture in 1933."

-William H. Christie.

SURVEYING the durability of aluminized and chrome-aluminized mirrors in the June, 1939, Astrophysical Journal, Robley C. Williams of the University of Michigan, after questionnairing 16 observatories, states in an 11-page article that "the most probable useful life of a coating of either of the films is from 2-1/2, to upward of five years, depending upon the excellence of the coating, its care, and the degree to which it is subjected to condensed moisture and dirt particles."

In the July number of the same journal-a professional astronomer's journal than which there certainly is no whicher-occurs the statement that the new Skilling and Richardson "Astronomy", reviewed in Scientific American for August, is "the best single book available for teaching a large amount of astronomy to the uninitiated student," is flattering agreement with our own review.

Dave Woodbury, a T.N. whose book, "The Glass Giant of Palomar," on the 200", is reviewed elsewhere in the present number and who lives in Ogunquit, Maine, is now lecture-touring here and there among clubs of amateurs, using a heavy assortment of lantern- slides. He has also lectured before Rotary Clubs and even Women's Clubs, but here he tempers his technicalities to the shorn lambs.

WHO can excel the record of C. P. W Dayton, Lyford, Tex., for telescope pier? His is 2200' deep! Because his locality is underlain by a 40' stratum of quicksand, trains moving within five miles of the telescope caused serious shimmying. An 8' concrete pier made it worse. He felt whipped. Then came an oil company, drilled a 9000' well on his land. Dry hole. But it wasn't a total loss, for Dayton found that, when he set his telescope on the 2200' of 10y4" casing the drillers had left cemented in the bowels of the earth, the telescope had the stability of Palomar. Seismoscope becomes telescope.

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.