On Wed, 05 Oct 2005 12:26:16 -0400, Brian Lawson
wrote:

On Wed, 05 Oct 2005 08:18:11 -0700, Eric R Snow
wrote:

On Wed, 05 Oct 2005 08:01:13 -0500, don holly
wrote:

Eric,

I think you forgot a factor of two again -- the spot should move 1.13
inches if the mirror rotates 1/10,000 of a revolution. No problem seeing
that with a 1.5 inch spot.

I like the spotting scope idea too -- tape a scale to the side of a
spotting scope which has a reticle and view the scale through the
rotating mirror, and you've got a cheap autoreflection alignment scope.
But to get a clear bright image with the scope, you need a good mirror
that is about the size of the scope objective. The laser method has
lower resolution but can use a much smaller mirror.

Don Holly

OOPS! You're right of course. Shows what happens when you can't keep
all your attention on one thing.
Eric

Hey again Eric,

Ummmmmm.... well, I'm no physics major here, but if I'm not
mistaken, it is the "angle of incidence" that "doubles". So every time
the mirror rotates at all, one "step" or more, unless the laser
position goes "with it" (so that angle DOES NOT change), then the
angle the laser beam strikes the mirrored surface the will change, and
it is THAT angle that "doubles" the laser-beam deflection. That would
mean some relatively complicated method of figuring the amount the
angular change of the mirror, and how much the "spot" should move at
the 132 feet distant surface. Wow!!
Brian,
I just drew the setup. With the laser hitting the center of the
mirror. I drew it with the laser origin the same and just rotated the
mirror. So the laser is stationary but the mirror was drawn at two
angles. The difference beteween the laser included angles appears to
be twice the angular rotation of the mirror. Try it and see if you get
the same thing. Since the laser is shining on a flat target there will
be some error because the radius will be changing with rotation. But
since the measurement is only going to be, at the most, 1/1000 of a
full rotation, or .36 degrees, it won't matter much.
Eric
Try it quickly, just for fun....mount the mirror fixed to something,
and fasten the laser to a moveable/swingable "radius bar" with the
"centre" of that radius at the mirror, and with the laser aligned down
the radius bar to strike the mirror. Create a surface at approx the
same distance as the radius of the laser, and starting at a point
roughly perpendicular to the mirror/ turn on the laser and note the
point it strikes the "wall". Move/swing the radius bar a certain
distance, and note the position it intersects the wall. Move the
radius bar again the exact same distance, and I believe you will find
that the point of intersection distance "doubled" the distance of the
first move, and the next move would double the second.

Even just thinking about this.... imagine that you shone a light at a
mirrored surface directly in front of you and perpendicular to you.
The reflected light would come right back at you. Rotate the mirror
only 45 degrees, and the light will reflect a right angle. Rotate
further to almost 90 degrees, and the light will be reflected to
almost dead away from you, or near 180 degrees. Pool ball off the
cushion idea.

Take care.

Brian Lawson.




All I've had time for today is to make a quick measurement to a closer
target. At 75 feet the spot is 1.5". And I calculated that the spot
will move .568" at 75 feet. So, the spot is about 3 times as big as
the movement of .0001". But, the edge is pretty well defined, so I can
see the edges move. If I draw lines at the edges I'll be able to see
how much things have moved. I have a couple lenses that I used on a
laser pointer to get a smaller spot at 30 feet. They worked pretty
well. But laser pointer spots are not round and so any improvement
really helps. I was surprised how much the beam expanded. As for the
other ideas posted here, they have merit and if I can borrow a 20 or
so times scope these will be explored too. It sure is fun to learn new
stuff.
Thanks,
Eric