On Tue, 04 Oct 2005 17:27:46 -0500, Don Foreman
wrote:

On Mon, 03 Oct 2005 22:43:11 -0500, Don Foreman
wrote:

On Mon, 03 Oct 2005 18:19:35 -0500, Richard J Kinch
wrote:


This won't work with a simple beam. The laser spot will inevitably be many
times the size of the motion of the spot you are trying to measure.

(snip)

True, barring exotic lasers and optics. Now having dismissed what
won't work ..... let's devise what might work. Same idea, Eric
....optical leverage .... but without the laser. Tape a scale on
the distant wall. View the reflection of the scale in the mirror
using a powerful (spotting) scope, preferably one with a reticle.
The scope can be close to the mirror because it's focussed on the
distant scale via the mirror.

Your line of sight via the mirror will move 0.1" at range of 159
inches, as you noted. The distant scale will appear to move that
much when the mirror rotates. With a 20X scope, it will be as if
you are viewing the scale from a distance of about 8 inches. You can
easily discern 0.1" from that distance. At least I hope you can!

Your scale may need to be marked with lines .025" thick for clear
visibility in your scope -- depends on the scope. I can very easily
discern .025" lines at 100 feet thru my inexpensive spotting scope,
though it doesn't have a reticle. A good riflescope of 20X or more
magnification, or a transit or theodolite, would work well.

Oops. Range is 159 inches, not 159 feet. You should be able to see
an ordinary machinist's scale thru a 20X scope. Not all scopes will
focus that close (about 13 feet) but some do. Mine does.

I tried this. From 200 inches I can easily discern the 1/64"
gradations on the ruler of a machinist's square when looking at it
directly thru the scope at 20X. Resolution with the mirror I had
handy was not nearly as good. The mirror would have to be a good
quality first-surface mirror -- mine was a crummy inspection mirror.
Good first-surface mirrors are easily found at surplus stores for a
buck or two.

The "spot" from my $9.99 HF laser level is about .25" dia at about 17
feet -- but there is a central "bright spot" that is probably only
about .050" dia. So, with a good first-surface mirror, I think your
experiment would work OK with a laser. A small prism may also work
well.

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