While in high school I developed an interest in astronomy. I built an 8″ telescope, as described below.
DESCRIPTION OF THE EIGHT INCH TELESCOPE
(written at the time)
The telescope is an eight inch F/8 Newtonian, with attachments for photographing at the prime focus. The mirror is mounted in a cast aluminum cell at the base of the frame, which is made of aluminum angles and 9” x 9” x 1/8” aluminum plates. The light shield at the top end is Masonite. A ¼” aluminum plate on the side has four holes corresponding to four bolts silver-soldered to the mounting. It takes about one minute to carry the telescope out, set it on the mounting, and screw down the four wing nuts.
The mounting is an “AAFI (Amateur Astronomers of the Franklin Institute [Philadelphia]) Pillow Block Mounting”. It is made of cold rolled steel with 1 ½” steel axes.
There are setting circles in right ascension and declination.
The original drive was a large gear cut in aluminum with a ¼-20 tap. The worm was turned by a ½ RPM motor. The system suffered two problems – it was shy about ten teeth and ran too fast, and it had an oscillation due to an uneven worm. Although it was excellent for visual work, it was useless for photographic work.
A new system was devised, using a 4 RPM heavy duty Telechron motor driving a 60:1 worm and an 8:1 spur. These in turn drive a friction coupled brass pulley with a small groove accurately cut in it. A fine steel wire connects the pulley with a large bakelite pulley on the polar axis. The ratio is 120:1436.0681833.
Instead of a conventional eyepiece holder, a camera attachment is at the prime focus. It is simply a flat metal plate with a 2 ½ x 3 ½ inch hole The plate can be moved by four screws to make it perpendicular to the optical axis, and to place it exactly at the flocal plane. A Graphic cut film holder is clamped over the hole.
The purpose of the bellows is to keep out stray light.
For visual work, a metal plate containing the eyepiece is clamped in place of the film holder at the prime focus. In a similar manner, a spectroscope or photometer might be clamped at the prime focus.
On the inside of the telescope, a sheet of thin aluminum with a 2 ½ x 3 ½ inch hole slides in a track past the opening to the bellows. This serves as a focal plane shutter for both instant and long exposures. When a small slot is used, very high speeds can be obtained by snapping the slot past the opening.
A HISTORY OF THE EIGHT INCH REFLECTOR
(written ca 1952)
One evening in November of 1949, while working at the Franklin Institute on a six inch mirror, Mr. Munshower – also a member of the Franklin Institute Amateur Telescope Makers, gave to me his ground and polished eight-inch mirror.
During the winter I did little with it, but during the following summer, I began working on it, and it was completed in July 1950.
Later in the same month, I obtained the aluminum for the tube from Mr. Kirk Wyatt – Turbo Machine Co., Lansdale PA. With the help of my Father, an aluminum cell was cast, and the telescope assembled. On August 27 the telescope, with the mirror still un-aluminized, was point to the Sun, Moon, and Jupiter. The results were gratifying. Four of Jupiter’s satellites (magnitudes 5-6) were visible.
The mounting had been built during the winter of 1948-9. It is an “AAFI Pillow Block Mounting” for 6” and 8” telescopes. It is constructed of cold rolled steel with 1 ¼” steel bearings.
The mirror was aluminized at RCA and again placed in the cell on August 31. Jupiter, and some clusters in the Milky Way were observed with great satisfaction.
I bought a 1” FL eyepiece, and did much visual work, becoming acquainted with the telescope.
The mirror, when aluminized, revealed a smoky ring from the edge inward about 1”, and two scratches.
In January 1951, I built the camera attachment, and made some star trails. The following month some fairly successful exposures of the Moon were made.
After an unsuccessful attempt to cast a brass gear blank, I got an aluminum blank through my Father, and in March the gear was hobbed with a ½-20 tap by Mr. Yingst, an amateur astronomer who works at RCA and lives in Ashland, N.J.
It lacked about ten teeth, but this was ignored, since it was very small.
The worm was mounted and set up so that the gear could be driven by hand. A couple of exposures were made by turning the gear by hand while watching a watch. They were fairly successful.
In July I bought a ½ RPM motor. The exposures showed that the worm, which was made with a die, was off center, and produced an oscillation which made it worthless for photographic work. A new worm was cut on a lathe, and the motion was reduced. The error in number of teeth of the large gear was too obvious, and the system was useless for photographic work as it was then.
Various suggestions were made, the most promising being an electronic oscillator which could be varied in frequency, and would thereby vary the motor speed. Another system suggested for slowing the motor was an interrupter which opened and closed the circuit to the motor quickly. But it was found that it either didn’t’ slow it a all, or would stop it completely. As an alternative to these methods, which were to remedy the poor operation of the worm drive, other drives were considered – gear trains, and steel wire drive. The gear trains proved too expensive and at last the steel wire drive was considered.
About the time that the worm drive was completed, I made a two inch F/3.5 camera, using a war surplus lens. I mounted it on the side of the telescope, and used it as a patrol camera. I found that the lens was in the cell backward, and the field was curved. After turning it around, the field was flat, but reflections were present.
The winter of 1951-2 found another lull in activity mainly because I was discouraged with the poor operation of the drive system.
The following spring I completely dismantled the mounting, removed all the paint and rust, and rebuilt it. I realigned the bearings, and lapped the polar axis to the bearing. I also put setting circles on the shafts.
The new drive system consisted of a 4 RPM motor and a drive box which contained a 60:1 worm and an 8:1 spur, with a resultant speed of one round per two hours. This drives a friction coupled brass pulley with a small groove accurately cut in it. A fine steel wire connects the pulley on the polar axis. The ratio of the pulleys is 120:1436.0681333…
At the same time, the telescope was in need of some revisions, and I rebuilt it. The cardboard light shield I replaced with masonite, and repainted the entire telescope.
The gearbox was not completed until December of 1953, which school, work, and a move to Minneapolis, Minn made progress impossible At that time, it was resumed in anticipation of the total eclipse of the sun in June of 1954.