This page last modified 1998 October 11
A Simple LED "red-dot" Finder
There are now a number of unit-power finders, which project the image of
either a dot or a reticle onto the sky. These finders are very easy to use and,
whilst there are times when the aperture of a conventional finder is useful, I
would not wish to be without my red-dot lash-up.
My finders are much simpler than any of the commercial offerings. The
essential components are:
A red Light Emitting Diode (LED)
A small convex lens with a focal length of about an inch (25mm)
A piece of tube in which to mount the optical components
A piece of aluminium baking foil for the pin-hole
A piece of plastic to seal one end of the tube
Two AA cells
A battery holder and connector clip
The principle of the device is simple: the LED illuminates a pin-hole at the
focus of the lens. The eye sees an image of the pinhole at infinity. You use the
finder with both eyes open, one eye looking into the lens, the other looking at
the sky. Your brain superimposes the image of one eye upon that of the other and
the visual effect is that you see a red dot placed on the sky.
I found a suitable tube and lens assembly in my scrap box – it had originally been an eyepiece in a "Long John Silver"-type telescope.
I needed to experiment to obtain a suitable pinhole. The method I have found to be most satisfactory is to place the foil on a metal surface and to use a very fine needle, which I have honed on a whetstone. I rotate the needle, like a drill, between finger and thumb and use very light pressure. I cut the foil with the pinhole into a disc slightly larger than the brass tube and simply stuffed it into the tube on the blunt end of a large twist-drill which is a close fit inside the tube. The foil is held in place by friction.
I made a plug for the LED end of the tube by heating the tube with a gas torch and using the heated tube to melt a disc out of the end of a black plastic 35mm film can. I made the holes for the LED wires with a heated panel pin.
The components are assembled as shown in the diagram:
The precise positioning of the pinhole is achieved by clamping the finder and sighting at the night sky. The pinhole is properly positioned when there is no observable parallax between the pinhole and the stars when you move your head. The twist-drill makes a suitable tool for pushing the foil back and forth in the tube. The position of the LED is not critical, but obviously it should be reasonably close to the foil.
I made a mounting bracket out of aluminium strip. I drilled this to accept a toggle switch which is wired in series with the LED and the battery. I secured the bracket to the telescope with a 'Jubilee' clip (pipe clamp), but all other parts are held together with 'Gaffer' ('Duck' or duct) tape.
The LED has such little current drain that I am still on my first set of
batteries after over 2 years. It is a simple matter to align the finder with the
telescope by bending the mounting bracket.
The first time I used this finder on a telescope, I was thrilled with the ease with which I could locate "old friends" in the sky. The toggle switch is simple enough to operate with gloved fingers. I was so pleased with this simple device that I immediately made another one, with which I can easily polar align my Scotch mount with great accuracy.
Apparently a few people cannot use these devices and find that the dot wanders all over the sky; however, for those of us that can use them, they are an inexpensive and simple route to increasing the ease and pleasure of observing.