Re: Will this laser idea work? On topic
- From: Eric R Snow <etpm@xxxxxxxxxxx>
- Date: Tue, 04 Oct 2005 18:28:17 -0700
On Tue, 04 Oct 2005 17:27:46 -0500, Don Foreman
<dforeman@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Mon, 03 Oct 2005 22:43:11 -0500, Don Foreman
><dforeman@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>>On Mon, 03 Oct 2005 18:19:35 -0500, Richard J Kinch
>><kinch@xxxxxxxxxxx> 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
.
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