John's Astronomy - ATM - Red Dot finder modification

This has proven to be a very good choice. When I'm hunting bright stuff, the red dot is sufficient to get me close enough to just move to the telescope eyepiece. When star-hopping, I can get on a starting star and then move to the finder.

I used the "wide angle" Crosman version of the red dot finder. There are several other versions with different names on them, but they're all basically the same thing.

All of these finders have two problems. First, the LED is designed for daytime use and so it's too bright. Second, again due to their intended purpose, the sighting glass is half-silvered.

I tried to do something about the half-silvering; I tried buffing it off, and I tried muriatic acid (hydrochloric acid, essentially). Couldn't touch it. I decided to just leave it alone, and I find it works fine, I just keep both eyes open and I can sight just fine.

Dimming the light - In order to dim the light, all you need to do is to get a resistor in line with it. This limits the current and voltage to the LED and produces a dimmer light.

I went to Radio Shack and purchased a 1K linear taper micro-sized potentiometer. This is a variable resistor like a volume control and varies from zero ohms (shorted, leaving the LED at full brightness) to 1000 ohms, which dims it nicely to my eyes.

First, use a phillips screwdriver and a pair of pliers to remove the screws that go through the casing for the windage and elevation controls. Be careful; the thumbscrew nuts have been locked to the screws with very stiff locktite (glue), and I broke my windage adjustment while taking it off. Once you get it off, you'll see the extremely simple wiring inside; a pair of wires going from the battery to the switch and to the LED, and a wire going from the LED to the switch. I chose to disconnect the wire that leads from the battery to the LED. You see that step here.

The next step is to drill three holes in the side of the case to insert the potentiometer. I chose to put it on the opposite side from the switch; I don't plan on adjusting it much and it won't get in the way there. Be sure to put it far enough back; towards the lens, there's a "keel" of plastic on the upper part that might hit the pot leads if you mount it too far forwards. When you get the holes drilled, press the pot firmly down and secure it with epoxy. WARNING - carefully apply the epoxy so as to glue the LEADS to the case as you see here. If you get epoxy underneath (between the pot and the case) or on the rotating part, you won't be able to turn the pot, and that would kind of defeat the purpose, right?

After allowing the epoxy to set a bit, do the wiring. Attach the wire you disconnected to the pot, in my case I put it on the center lead (wiper) of the pot. Then you'll need a tiny piece of wire maybe 3 cm long. Solder that between one of the pot leads and the LED lead you took the other wire off of. Finally, put it back together.

Ways to screw up the wiring: Don't attach to the two leads on one side. If you do the LED will be permanently dim and turning the pot won't do anything. You want one connection to the lead that's all alone on one side of the pot, and another to one of the leads on the other side. On the connection to the "two lead" side, you probably want to use the lead on the right hand side as you look at the pot from the outside; this means when you turn the pot to the right the LED will get brighter. This is the most intuitive situation. If you use the other lead, the LED will get brighter in the other direction. Either reconnect it on the other lead, or leave it like that if you like to be iconoclastic :-)