Unit-power finder

I first came across this type of device in the form of the commercially-made Telrad, and now I rarely use a conventional optical finder. The design shown here is simpler than the Telrad, but still very effective. The principle is similar: the finder produces a beam of collimated red light which appears to the eye as a small red dot. The other eye remains open facing the sky, and the brain superimposes the two images so that the dot appears projected onto the night sky. When aligned the unit power finder is easily capable of pointing to within a small fraction of a degree.

    Materials:

• Small convex lens with a focal length of around 25-50mm.

• Two short pieces of plastic tubing (nesting if possible).

• 5mm red LED.

• 1K resistor.

• Some baking foil.

• a new needle.

• 2 X AA batteries with a double battery holder and clip.

• Small toggle switch.

• 30cm insulated stranded wire.

• Small 1/2" self tapping screw.

• Insulating tape.

    Tools:

• Drill.

• Soldering iron (optional).

• Junior hacksaw.

    Construction:

        To make the pinhole, form a stack of five pieces of foil and place them on a hard surface. Then pierce the stack with the needle using gentle pressure and a twirling motion. Try and get a very small and circular pinhole (check against a bright light with a magnifier). The foil pinhole can then be glued on the end of the inner tube.

        Connect the resistor (all the wiring can be done by twisting together and wrapping with insulation tape, if you don't have a soldering iron) to the short lead of the LED and run wires to a small toggle switch and the battery pack. N.B. check which way the wires should go to the batteries, as LEDs are polarised. The LED can be mounted in a small card disk inside the tube.

        Mount the lens in the large tube in a card disk if necessary. If the small tube is too small, just build it up with paper or felt till it fits snugly inside the outer tube. Finally plug the end of the small tube with a card disk, punched with a hole just large enough to let the wires out.

Check out Stephen Tonkins version, or a more sophisticated unit.

To use the finder, keep both eyes open and adjust the position of the inner tube till the red dot appears sharp, then move your head slightly to check that the dot remains stationary. After alignment just the point the telescope and presto!