HDD signaling mirror?

[blackdog]

Tried a platter from the guts of an old (and crashed) hard disk drive as a signaling mirror the other day. The HDD actually yielded two disks. I put one of them into a ziplock bag and into my daypack. The other one has been abused with sand, fire and chemicals. The surface is only slightly scratched now. Seems to be a really durable material and it doesn't bend easily. The disk is polished like a mirror, uncoloured, has a 3.5" outer diameter and a 1" hole. ...and the price was right.

[Sgathak]

Sounds good... IF you can do the ONE thing a signal mirror MUST be able to do.

direct a focused beam of light to a location specified by the handler.

Its no good if your aiming at a plane and the reflection isnt going where it needs to

[blackdog]

That is so true.

...So we tried the two HDD mirrors across a small lake today (the distance was roughly 1.6km/1mile according to the gps). With a half-clouded sky we were able to "point the dots" at eachother quite well.

I looked through the hole in the disk while holding it with my right hand and used the two fingers of my extended left hand (as I've seen on some site on the net) to see where the beam was headed. I have no idea how my collegues did, but I could clearly see their signals too.

The unscratched mirror stays in my daypack from now on.

[Kevbo]

Sounds like the HDD disk would work well with the sighting instructions for the old boyscout signal mirror.

This aiming methode is hard to explain with words, but will work if the disk is reflective on both sides and has a hole in the middle.
As long as the front and back surfaces are paralell, the methode is very accurate.

Another advantage is that it requires nothing in front of the mirror to partially block the beam. You can also use your now free hand to shutter the mirror....not there is much chance your target will know morse code even if you do. (and almost everyone who does know the code is radio trained and so can copy by ear, but NOT by eye!) However, rapid flashing may attract the attention of searchers more than a steady glare point, especially if the target is an aircraft.

1) Hold the mirror in a position where the shadow of the mirror falls on some screen-like surface. The tee-shirt you are wearing will work fine, as will your companion's...The ground might work if it is not covered with leaves or pine needles...or you can use your hand or a nearby light colored rock....what works best depends on the angle between the sun and your target.

2) Hold the mirror at about half arms length between your sighting eye and your target....not right up to your eye, and not at arms length. You need to accomplish this and point 1 at the same time.

3) View the shadow of the mirror by looking at the reflection off the back side of the mirror. Tip the mirror so that the hole in the middle of the shadow is at the hole in the mirror. Try not to move the mirror, just tip it one way or another This is the difficult to convey step.

4)The beam is now aimed at the distant point you see through the hole in the mirror.

...Which is probably not your target because you moved the mirror or your eye to accomplish point 3....With just a little practice you will learn to keep the target in the hole while tipping the mirror to get the alignment of the shadow correct.


This methode works because the sun is one one side of the hole, the shadow is exactly 180 degrees opposite...and the same geometry applies to your eye and the target. By using the back side of the mirror, (180 degrees opposite the front) you can bring all into alignment at once.

Note that any methode works best when the angle between the target and the sun is small, and of course none work when the sun is directly behind you. Two way signaling works best when the sun is 90 degrees to your target.