What are the Optical Tooling instruments?
Fundamentally, these are transits, levels, and alignment telescopes.
Let’s take a very quick look at each.
The alignment telescope is "just" a telescope, with a very straight
line of sight. It must be mounted in some sort of base to hold it
horizontally or vertically (we have all sorts of bases). Sometimes it has
built-in micrometers for making measurements, but not always. Sometimes it is
really just a telescope!
One step "up" from an alignment telescope is a level. The
most important thing about a level is that it is a telescope on a rotating
base, which allows the telescope to rotate back and forth in the azimuth
(horizontal) direction only. Further, this type of instrument is optimized for easy and quick
adjustment of the telescope so that the line of sight becomes perfectly level.
Everyone has seen a transit-type instrument at some point.
Surveyors use them outside to make measurements on roads and buildings. The most
important thing about a transit is that it has a telescope on a gimballing
mechanism, so it can rotate back and forth horizontally (azimuth direction), but
it can also rotate up and down (elevation direction).
However, it is worth explaining the general difference between
our transits and those used by surveyors. Optical Tooling transits have a
number of modifications to make them deadly accurate when shooting over
distances which, to a surveyor, would seem short (less than a hundred
feet). For example, our telescopes have an extremely straight line
of sight. This means that as you focus from near (2" from the objective)
to far, you are "moving" in a very, very straight line. Also, the vertical
and horizontal axes of our transits meet at exactly the same point in
space, along with the transit’s line of sight. Not only that, but
all of these lines and axes are at 90° to each other (mutually
orthogonal). This is one of the fundamental characteristics of our instruments
that allows us to evaluate geometric relationships of other things.
Finally, our 76-RH transit has an
infinity-focused telescope mounted in the horizontal cross-axis around
which the main telescope turns. We'll see why this is, later in our discussion.
Of course, there are a lot of other instruments and accessories
(collimators, mirrors, stands, bases, targets, scales, and so on), but the three
basic types of instruments above are the workhorses of Optical Tooling.
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What's a Reticle?
The telescope of each Optical Tooling instrument contains a reticle – or
crosshair – which defines the center of the line of sight for that telescope.
When you look into a telescope, you see the reticle
"superimposed" upon a distant image – whatever image you have the
telescope focused on. The reticle is split into two different halves; one
half has a single "wire", the other has a double
"wire". This configuration makes it much easier to visually
align the reticle with other images (ex., optical targets and reticle images
reflected from mirrors). [Interesting tidbit: in the old days, we used to
use threads from black widow spider egg nests to make our crosshairs. No lie!]
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Infinity
From an optical
standpoint, light rays coming from an "infinite distance" are all parallel
to each other, or collimated (see the next topic).
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Collimation
Parallel light rays are said to be
"collimated." When a telescope is "focused at infinity", it
means that only parallel light rays which are entering the telescope will be
clearly seen by an observer looking into the eyepiece. Remember that for our
purposes, objects that are farther than about 100’ will have light rays which
are sufficiently parallel to be in focus. When the telescope is focused in this
manner, images from closer than "infinity" are not viewable (out of focus).
And did you know that a telescope could work
backwards? No, we don’t mean looking into the big end so that your coworkers
look like ants. We mean that if you put a light near the eyepiece, you can shine
light rays back out the big (objective) end. This means that if you illuminate
the reticle of an instrument which is focused at infinity, it projects
collimated light rays out the front.
"So what?", you might say. Well, when
these collimated rays are viewed with a second telescope which is also set at
infinity focus, you can see an image of the illuminated reticle of the first
telescope. If the second telescope is aimed precisely at the "incoming
reticle image", you can exactly superimpose the first (projecting) telescope’s reticle
image over the actual reticle of the second telescope, and the lines of sight of
the two telescopes are established as parallel.
Remember that parallel doesn’t mean that the
lines of sight travel down the exact same path in space (if they did, they would
be collinear). The lines of sight are exactly parallel (which is very
useful for transferring reference lines) but they are not collinear – they are
more like railroad tracks. [Tidbit: if you want to make the lines of sight
collinear, you can do that too. All you need to do it is read on...]
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Collineation
Well, now that you have
made the lines of sight of two telescopes parallel by collimating
them, you can take it one step further if you'd like. Picture two
telescopes pointing at each other, both focused at infinity, and
collimated. As we've discussed, if you look into either telescope, you
should see two reticle images. One is in the near telescope (the one
you're looking into), and the other is in the far telescope. However,
if you were to focus both telescopes, say, on a piece of paper held up about
halfway between the instruments, the reticles would no longer be
superimposed - unless you were incredibly lucky, that is. Remember
that the lines of sight are parallel but not necessarily "superimposed" upon
each other. However, there is a procedure that you can use to superimpose
the reticles at this near shot while the instruments are still collimated
at infinity. (We call it a "near shot" because the telescopes are
focused closer than infinity.) After
performing this procedure, the lines of sight become collinear. They're not
just parallel any more - they actually exist along the exact same line in
space.
Why would you do this? Well, most of the time the transfer of a reference
line requires only collimation. But sometimes, if you can't see both sides of an
object, or if you are trying to transfer a particular horizontal or vertical
plane in an area where visibility is restricted, the technique of
collineation can be just the thing to use.
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Projection
Remember what we said about a telescope being
used backwards? Another thing that you can do when you illuminate the reticle is
to put a telescope in projection mode, which allows you to project an
image of the reticle onto some surface at which the telescope is pointed. This
time, you don’t work at infinity focus, but instead you must focus at the
surface on which you want the reticle image to appear. So once you establish the
desired reference line, turn the ambient lighting down a little and project an
image of the crosshairs on the surface – this will give you a clear visual
indicator of your reference line for use in building, positioning, estimating,
etc.
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Reflection
OK, this was a trick one that you already know. A reflection is just
what you see when you look into a mirror. But this brings us
to our next definition…
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Autoreflection
Suppose you were to look through a telescope at
a mirror. If you pointed the telescope directly at the mirror, so that the line
of sight was perpendiular (90°) to the mirror, you would see the end
of your own telescope. Now suppose you put a target on the end of the telescope
itself, like a nice black crosshair painted on the objective lens (don’t
actually do this – we have special targets for you to use). Since you can see
the objective lens of your own telescope (and the nice target on it) in the
mirror, you would be able to position your own instrument’s internal reticle over that
reflected image. This would give you a very accurate way to make sure that your telescope
is perpendicular to the mirror.
"Then what?", you might say. Well, suppose that the mirror is on the end of a
rotating shaft from which you need to extend an axial reference line. Not so strange after
all, is it? Especially when you remember that you
can do other interesting things with the mirror, like turning the shaft on which
it is mounted and observing the target’s reflection move around, letting you
know whether the mirror is truly perpendicular to the center of rotation of
the shaft or not.
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Autocollimation
When you really want the ultimate accuracy in angular control, autocollimation
is the technique to use. Autocollimation is more accurate than autoreflection,
but often, autoreflection is performed first, making autocollimation easier to achieve.
Remember that in autoreflection, the line of sight is aimed at the reflection of a
target which is mounted on the objective
end of the telescope through which the observer is looking. But in
autocollimation, the line of sight is aimed at a reflection of the crosshairs of
the telescope itself.
Remember again what we've said about using a telescope "backwards".
To autocollimate an instrument, it is
necessary to direct a small amount of light on the reticle. This light will then
shine out the front end of the telescope. Before you start drilling holes in
your telescope to shine a flashlight in there, we should tell you that we make
special things to light up your telescope’s reticle. Anyway, when the reticle
is lit up and the telescope is focused at infinity, parallel light rays will go out
the objective end. Then, if you then look through that same telescope, since
it is pointed at a mirror (just like with autoreflection), the parallel
light rays bounce back from the mirror, and you can see a reflection of your own
crosshairs in your own instrument. Stay with us - this is not nearly as
difficult as those time-travel paradoxes.
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This is what the process of autocollimation looks like when looking through the
telescope. After the reflected image is lined up exactly on the original reticle
(in the illustration, blue lines directly over black lines), the telescope is
precisely perpendicular to the mirror that is producing the reflection. |
When the mirror (or the instrument) is adjusted so that the reflection of the
reticle falls exactly on the original reticle itself, the telescope has been very
accurately positioned perpendicular to the mirror.
"Why do this?", you ask. Well, as it turns out, this is an extremely
accurate way to establish a right angle with respect to an optical reference line. Our
transits have an infinity-focused telescope on the cross-axis, which happens to be
just perfect for use in autocollimation.
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Measuring with Micrometers
"So far", you might say, "we
haven’t actually measured anything". Of course, you are right.
How do we measure things with optical tooling? That’s where micrometers and scales come
into play.
Suppose that you are suspicious that a large table is not level. Not being the
type that is normally paranoid, you want
to take a profile of the table to see for sure. You have set up an optical
instrument called a "level" overlooking the table. The telescope
of the level has been easily adjusted so that the line of sight is perpendicular
to gravity, and the line of sight sweeps across the table, in a plane which is
pretty much parallel to the table top. Now you want to take some measurements.
So you pick up an Optical Tooling scale and scale holder, and set it up somewhere on the
table. Optical Tooling scales are just like the wooden rulers that you used in
school, except that they are not wooden, and they are a lot more accurate.
The scale is oriented so that it sticks straight up vertically from the flat table. Now you
look through the level's telescope at the scale, and you notice that the
crosshairs are right in between the 3.2 and 3.3 markers. Hmmm...
Don’t worry. There is a micrometer on the front of your telescope. This is
an optical micrometer which contains a block of glass that when rotated, moves
the incoming image up and down by known amounts. It’s one of those really cool things
from physics. Anyway, all you have to do is turn the graduated drum on the
micrometer, which offsets the image and makes it appear to move up and down, until
the crosshairs appear to be right in line with the 3.2 mark on the scale. Then
you look at the graduated drum on the micrometer to see how much you moved the image.
If you moved it, say, 0.036", then you know that the line of sight is
precisely 3.2 (as read from the scale) + 0.036 (as read from the micrometer) = 3.236"
above the table. Cool!
Now move the scale somewhere else on the table and take another reading. This
time, using the same process, you take a reading of 3.276", which you
can immediately calculate is a point 0.040" below the first reading (yes, below,
because the scale had to go down by 0.040" to get a reading of 3.276 –
the line of sight stayed level). You can take any number of additional points
to determine whether the table is level – or whether it’s even flat, which
is a different question. This process is essentially how all measurements are
made using Optical Tooling instruments, whether they’re made up, down, or
sideways.
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Go on to Applications of Optical Tooling
