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Alignment Advice (for a barndoor tracker)

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#1 sternenhimmel

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Posted 07 July 2013 - 08:05 PM

I'm hoping someone here has a tip or two on how I might be able to accurately align a laser with the hinge of my barndoor tracker. I've been attempting the iterative method of:

1. Roughly align laser to hinge axis.
2. Point hinge at distant object
3. Turn on laser and rotate platform about the hinge axis.
4. Make adjustments to laser axis.
5. Repeat 3 & 4 until change in laser position (on target) is at its minimum.

This method probably works a lot better for finder scopes than it does for lasers. But then again, maybe I just need practice? I'm using a home built 2-ring holder with adjustment screws for the laser. In the current design, I don't have a spring loaded pin, so adjustments are also a little tedious.

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#2 Gordon Rayner

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Posted 08 July 2013 - 12:28 PM

This is similar to making the line of sight of half of a binocular to be parallel to the hinge axis.

With the target at a long distance, one has
an isosceles triangle ("equal legged", not equilateral "equal sided", except as a special case, which confusingly happens to be close to the triangle which results from the usual "swing" of a binocular hinge from about 58mm interpupillary distance to around 72mm. IPD).

The hinge axis is the apex. The two other corners of the triangle are the pre-swing and the post-swing positions of the laser beam on the target. The included angle of the triangle at the apex is the angle by which the hinge has been swung.

The adjustor can graphically, and with practice mentally, find the hinge axis ( the apex) and adjust the laser toward that apex. This shrinks the triangle. Repeat until the triangle has shrunk to nearly vanishing. The laser beam and the hinge axis are now parallel.

Hanna in ATM III, or now in ATM II from Willmann-Bell, describes binocular adjustment. The above is similar to his second method. He did not describe the isosceles triangle, but that is the geometric figure of his second method.

He was writing pre-barndoor tracker and pre-laser.

Addendum/Revision :

The Hanna second method requires that the hinge axis point at the target center , before any adjustments are made. For that, and other considerations, I may have carried the binocular adjustment analogy too far.

A workable scheme for a close target may be:
Using a compass, draw an arc whose radius is the distance from the hinge to the laser beam exit.
Mark the center of the arc, and aim the hinge to closely point it at that center, by sighting along the hinge. Use a tripod head or other device with two angular adjustments.

Aim the beam at one end of the arc, using the laser adjustments . Next, swing the hinge. Then, using the adjustable fixture, move the beam to strike that portion of the arc. Swing back to the start of the arc. Move the beam again onto the arc, using the laser adjustments. Swing again to the other end of the arc, and again move the beam onto that part of the arc, via the 2-axis fixture. Swing back, move beam onto arc , using laser adjustments. Swing to end of the arc, and move beam to arc with the fixture adjustments, ........

After a few iterations, the laser beam should be parallel to the hinge.

Comments/suggestions?

#3 sternenhimmel

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Posted 09 July 2013 - 04:08 AM

Thanks for the detailed reply Gordon!

I gave it some more thought, considering the point you made. Which, if I understand correctly goes like: the laser
will map out an arc whose end points are two vertices of an isosceles triangle, and the idea would be to visualize where that apex would be and move to this point.

But then I got to thinking, why not do a 180 degree rotation and move towards the midpoint? The midpoint would be much more intuitive to locate, and as you got closer, the line would become smaller and smaller.

I suppose this really depends on the configuration though. Being able to sweep a full 180 degrees would be an important requirement ;)

#4 Gordon Rayner

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Posted 09 July 2013 - 11:03 PM

Addenda added to my previous post.

#5 sternenhimmel

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Posted 16 July 2013 - 01:02 AM

Thought I'd give a quick update!

This method seems to work pretty well. I gave it a shot tonight just by aiming it roughly to polaris and iterating through the 180 degree technique. Eventually I got it down to the point where it would point at the same spot throughout the swing.

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However, due to some unrelated technical difficulties, I was unable to track and shoot tonight. Just took a still shot of the laser beam aligned... well, not really, but that's a different problem.






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