THE POOR MAN’S TELESCOPE (May, 1962)
THE POOR MAN’S TELESCOPE
AS EVERY astronomer knows, a steady mounting is a must when using high magnification. Generally, to obtain the required steadiness, it has been considered necessary to build a strong, heavy instrument, made with high precision, often mounted on concrete piers. The disadvantage of such instruments, in their lack of portability, has led us to develop the six-inch reflecting telescope and mounting shown here. We feel it combines features especially suited to the needs of the amateur. With a 6-inch diameter and 54-inch focal length the whole assembly including mount weighs only 9-1/2 lbs. As the beam of the mounting is supported in two places instead of one, powers of up to 360 can be used with the same steadiness of image normally found in a telescope of this size when using only 50 power. Backlash has been practically eliminated and eyepieces may be changed without losing the object under observation. Fine planetary detail and moon craters as small as half-a-mile in diameter may be seen if the optical elements are of good quality. Above all it is easy to carry and to set up, steady even in a wind and can be built with a minimum of accurate machine work.
Construction begins with the tripod. Obtain a 3/4 -inch pipe 12 inches long with one end threaded. Cut the tripod legs from pine and the two circular plates from plywood as shown. Drill a hole one inch in diameter in the center of the larger plate and screw the pipe into it so that it is perpendicular. Draw three radial lines 120Â° apart on the bottom of the plate and on each line drill a 3/16 diameter hole 1/2 and 1-3/4 inches from the pipe. Position legs one at a time, butted against the pipe, and mark, drill and screw onto the large plate. Bore a 1-1/16-inch diameter hole in the center of the other plate so that it fits snugly onto the pipe. Slide it down the pipe, mark, drill and screw it onto the legs. Drill and tap two holes in the pipe for 1-inch 6-32 screws 1/4 and 3 inches from the top. Bend the screws slightly so they can be turned by hand. Insert conduit into the pipe and lock. Finally, to complete the tripod turn the shouldered plug on a lathe from bar stock with top threaded for 3/8-16 and provide relief for the lock screw to retain it in the conduit.
Next cut the mirror cell out of 3/4-inch plywood (see detail A) and tracing its outer contour onto 1/4-inch plywood cut out the bottom. Glue and nail the bottom to the cell. For the mirror support bend the piano wire as shown with each apex bent up half-an-inch for spring action. Using galvanized clips screw the wire to the inside bottom of the cell. Cut out the mirror hold and drill three 5/32-inch clearance holes 120Â° apart. Drill corresponding holes in the cell 3/32 inches in diameter and screw in the 6-32 hold-down and collimating screws. Drill for and cement in the dowel plugs which take the screws anchoring the cell to the beam.
Mark out the beam on a piece of pine or fir and leave it long at the upper end until the exact position of the eyepiece has been determined. If the beam is cut out with a circular saw the slot for the steadier-clip can be cut exactly parallel to the beam edge. Alternatively a separate piece, 5/16 x 1-/14 inches may be screwed to the beam bottom and the slot eliminated. The extreme upper beam, carrying eyepiece, prism and finder is a separate piece held to the main beam with four screws through slots allowing a V2-inch adjustment. Cut the 1-5/16 inch eyepiece hole and line it with felt to fit a 1-1/4-inch eyepiece tube. Make a similar eyepiece storage hole in the lower beam. A surplus prism is fastened to a 3/4-inch tubing by two .040-inch dural clips held to the tube by 6-32 screws tapped in the tube. The other end of the tube is press-fitted into a hole in the plywood piece screwed opposite the eyepiece.
After the telescope is assembled the lengths of the dowels for the steadier can be determined. Cut to length, taper the ends and drill ends with a 3/32-inch drill. Bend the wire rest and feet as shown, file flat and press into dowels with glue. Bind together at the top and cement to insure triangulation. Drill tripod legs to take the steadier’s feet making sure the holes line up with each other and fit the feet snugly. A .045-inch piano wire clip fastens the steadier-rest to the beam. The clip should be exactly the beam’s width and when wrapped with a layer of rug thread, should fit the steadier rest without play.
The finder uses a surplus achromatic lens of 1 to 2 inches diameter, 5 to 8 inches F.L., with matching aluminum tube. Bind the lens to the front of the tube with tape. The finder eyepiece is a surplus, simple lens of 1/2 to 1 inch F.L. that fits the other end of the tube by means of the wooden adapter. The point of a straight pin, placed at eye lens focus serves as crosshairs. The aluminum strap which fastens the finder to the upper beam is held to the tube by a 6-32 screw as shown on the drawing.
The prism is mounted so that one small face is at right angles to the eyepiece cen-terline and the other at right angles to a line from prism center to mirror center. Fasten the mirror cell to the beam so that the distance from mirror surface to prism plus the distance from prism to beam center equals the mirror’s focal length. To find the latter project the sun’s image on a piece of paper until the smallest image is obtained, then measure the distance from paper to mirror surface. Mirror surface must be at right angles to a line from its center to the prism center. Removing the eyepiece, look into the hole and adjust the collimating screws until the prism appears in the center of the mirror.
In use a telescope with a steadier must first be pointed in the general direction of the object to be viewed. The beam can be moved nearly two inches either way across the steadier and up or down until the object is found. The prism and the low power eyepiece (1 inch F.L.) used for star clusters and nebulae were bought surplus from Edmunds Scientific Co., Barrington, N. J. For lunar and planetary work I use an 8mm eyepiece giving 180 power and on especially good nights one of 4mm giving 360 power.â€”William T. Thomas, Jr.