Pinhole LED Finder

The LED pinhole finder costs less than a £1 to make and is a better solution than looking along the length of your telescope tube to Centre your telescope.

The finder is mounted on the telescope, just like any other finder, the lens points into one of the observers eyes. Unlike other finders both eyes stay open, one eye focused on the sky/stars the other looking at the red dot produced by the finder. By focusing the lens to infinity, both stars and red dot are in focus at the same time making for comfortable viewing. (You can test the red dot is focused at infinity by moving your head slightly, if the red dot moves in relation to the stars then it is not focused correctly - you are experiencing parallax)

In operation you simply look at the point of light with one eye and the sky with the other, the brain combines the two images and the red dot appears in the sky. Some adjustment will be required to position the finder so that the dot points to the centre of the telescope field.

Modification:

Replace the pin hole with a pattern of concentric circles and you have the back yard version of a Telrad finder.

I have experimented with printing this on transparency using an ink jet printer with reasonable success, print the black area leaving the red area clear for the LED to shine through, a couple of layers may be needed (the results work but are a bit crude).

Parts List:

Small convex lens - with a focal length of about 30mm - Quality is not that important a cheep plastic lens from a Christmas Cracker will probably work!

Red LED
Resister - see below how to select the best resister.

A power source
Switch
Tubing - Anything handy as long as it is sturdy enough to mount and dew proof. I have used a 35mm film canister.

LED Circuit:

LED's are simple components but a small amount of care is needed.

Never connect an LED directly to a power source, always use a resister in series, a 10mA supply is all that is needed.

Recommended resister values. 3v, 180 ohm; 6v, 390 ohm; 12v, 1k ohm; 24v 2.2k ohm

LED's are polarity sensitive always check you connect the wires the right way round. The cathode (-ve) is the shorter of the two wires, and usually has a flat edge on the casing closest to the cathode.