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Published on Jan 24, 2012
I really recommend using a real signal mirror over using a CD: even the small 2"x3", 0.32 oz "SOL" or "Rescue Flash" signal mirror (available at REI and elsewhere) is MUCH brighter, more compact, and a lot easier to aim than a CD. The beam from a mirror is deceptively wide at close range - at range, the angular diameter of the beam is the size of the full moon - hold your "Vee-fingers" up against the full moon some time and see what a small target that is. Sure, the CD is probably free, and the real signal mirror is $9.50 - but what's your life worth?
A CD can do the job (though it isn't anywhere near as easy as you may think) as I show here with a 4.6" diameter CD at a range of 11.1 miles (though note my wife's spontaneous comments about how dim it is - and we'd tested a 2"x3" Rescue Flash mirror 5 minutes prior- you can view that here: https://www.youtube.com/watch?v=Qu4iJ... )
The secret to aiming the reflected light from a CD accurately is to auger a countersunk aiming hole from the rear that is about 1/4" diameter (I used the tip of my penknife), centered between the edge of the center hole and the edge of the CD. Then you look through that small hole (not the big one in the center) and reflect the light of the mirror on your free hand, looking for the small shadow cast by your small hole. You want to line up your eye, the small hole, that shadow, and your target. I generally hold my forefinger and middle fingers outstretched and touching in a "cub scout salute", then line up the bottom of the "vee" where my fingers touch with my target, then scan the small shadow up to center it on the bottom of the "vee". Note that you want the very center of the hole, the very bottom of the vee, and the very center of the shadow, lined up to hit the target. To minimize the blockage of the mirror beam by your fingers and hand, put your "vee" below the target, and rotate your CD so that the hole is at the 6 o'clock position.
Also, pick through your CDs to find the one with the brightest, smallest reflected disk of light. Also, a thin disc case to protect the surface from getting scuffed or dirty helps, too. The reason you want the hole countersunk is to help you see through it at an angle while reflecting obliquely.