The Amateur Telescope Maker’s Page (Jul, 1956)
There now some slightly bigger telescopes in the Pacific area.
The Amateur Telescope Maker’s Page
AT a cash outlay of $300, boys at a Hawaiian school built a 20-inch reflecting telescope which has been valued at $20,000. It is said to be one of the largest telescopes in the Pacific area. With the exception of the grinding of the mirror, all the work was done by the students of the Kamehameha school, a private grammar school named after Hawaii’s greatest king. The f-6 mirror was donated by a government employee who ground it himself, taking six months for the job.
The scope is of all-steel construction and weighs 3,000 pounds. The mirror alone weighs 125 pounds. The mounting is German-type equatorial. At present it is manually operated but the ambitious young astronomers are planning to add a clock mechanism.
A good deal of the material used in the construction consists of spare parts and pieces found lying around the Kamehameha machine shop, according to Ardean Sveum, shop instructor, who directed work on the project. An observatory site has been selected and plans are proceeding for the early construction of a permanent building near the school.
Telescope Mirror Grinding Tool
Many an amateur astronomer has found himself in the position of owning a glass or pyrex disk suitable for fashioning into a telescope mirror but without a suitable tool. A tool, of course, can be purchased from one of the numerous telescope supply houses. However, if the mirror is a large one, this is expensive, or if the disk is not of a standard diameter, it might not be possible to find a suitable tool. By following the procedure outlined, a tool can be made for any size disk, inexpensively, and with very little effort.
You’ll need a number of the small hexagonal tiles used for bathroom floors and a matrix. The matrix can be cement or any plaster-type material that sets hard. The tool illustrated was made of dental stone, a powder used in dental work to make casts for bridges or plates. This material is readily available at any dental supply house, is inexpensive, and sets extremely hard. This tool is 8-in. in diameter and 1-1/2 in. thick. It required three pounds of dental stone and 42 tiles.
The first step is to fashion a stiff collar around the circumference of the disk. Cut strips of paper, newspaper will do, somewhat longer than the circumference of the disk and 1/4 in. wider than the thickness of the disk plus the thickness of the desired tool. Place these strips around the disk and secure the end with masking tape, Scotch tape or string.
Place as many of the bathroom tiles on the surface of the disk and within the surrounding paper collar as will fit. Edges of the outer tiles should touch the collar. Place the tiles close together so that points of tiles touch tile edges, but try to minimize the number of edges touching edges as this decreases the tile area secured by the matrix.
Mix the matrix material with cold water until it is about the consistency of thick cream. Be sure it is mixed thoroughly so that no lumps of dry powder are left to weaken the tool. Carefully spoon the mixture onto the tiles so as to avoid disarranging the pattern. Then, when the tiles are covered, pour out the rest of the mixture to within 1/4 inch of the top of the collar.
Allow ample time for setting; at least one-half hour if using dental stone. Slide the cast from the mirror (used as a guide) and strip off the paper collar.
Allow the tool to set overnight, then smooth off the rough or raised edges of matrix material, using a fine file. Be careful not to file tile edges lest they be damaged. After the first few minutes of using the tool to rough grind the mirror, the surface of the tool will have smoothed off even with the inset tiles. • —R. W. Ferguson