I am the proud owner of a new (old style) Brunton 16DLU with a sighting mirror. I tried it out as a signal mirror. I faced the sun and got the reflection on my extended arm with my thumb pointed up and aimed at a close object. I was shocked at how far off the reflection is from where the reflection was relative to the top of my thumb and the target. Good horizontal alignment is possible but the reflection is going to be high from where your thumb and eye aligns with the target.

So if you have a regular mirror (no hole) and you need to signal a plane; you may not be seen even though you have a bright sun and good reflection. Your reflection will be way above the plane.

i noticed that rather quickly, when i first learned to signal. A hole in the middle, or even a hole close to the edge, makes a big diffrence. Some compasses like the Recta DS-56 has a hole in the mirror, so i doesn't have problem with angles. Although the hole is made for a diffrent purpose.

I've had no problem signaling planes using a simple mirror and the two-handed aiming method. I think it comes down to the fact that when you wiggle the mirror even a little you are actually moving the signal a large distance out where the plane is (just consider the geometry) and in additon, it is not a laser and and the beam does expand a modest amount as you get a mile or more out. Again, the true test is the real world and it's worked fine in many tests with real airplanes.

It doesn't have to be one of the high class glass mirrors either, altho they are much better. I have signled more than one chopper in with an old "tin" mirror I always kept in my pinch book, It did have a hole that I drilled in it. It never failed to amaze me how difficult it can be for a chopper to spot you in some terrain, even if you have radio contact, but they never failed to spot the flash...

What is a "pinch book"?

That is the book, usually leather, usually black basketweave, that the mean ole highway cop pulls out to write you a ticket ("pinch you") when you have been doing bad...

No wonder this law abiding citizen doesn't know what that is. <img src="/images/graemlins/blush.gif" alt="" />

Good for you!!!

> Good horizontal alignment is possible

That's the crucial part. The idea (as I understand it) is to sweep the beam of light up and down, so everything in that vertical column gets a flash. You are not trying to focus the spot of light on a single point. That's partly because it is too hard, and partly because a brief flash is enough - the human eye is good at noticing them.

If you are trying to send morse code, then I guess you use the interval between flashes, rather than the duration of the flash itself.