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Tribrach and Prism Pole Adjustments

If the circular level in a tribrach is out of adjustment it will become apparent when you place an instrument in it and then have to move the foot screws quite a bit to get accurately level. This, in turn, will pull the optical plummet line off the point, forcing you to release the tribrach clamp and slide it into the correct position. Once a tribrach has been properly leveled with an instrument it is a simple matter to adjust the circular bubble. There will normally be three screws under tension holding the bubble vial down on a pivot point. Loosen the screw that is farthest from the bubble to allow the low side of the vial to rise while you bring the bubble back into the center of the vial by gently tightening the other two screws and then retightening the first screw. You must have your eyes directly above the level vial when you center the bubble or our old enemy parallax will get you. Because the center circle is marked or etched on the outside of the vial while the bubble is on the inside, the thickness of the glass causes the parallax. When viewed at a very flat angle the bubble will appear to be centered in the bull's-eye, when in fact it is quite a bit out of center.

Precisely machined rings or spacers are used to check and adjust the optical plummets in tribrachs. Adjusting rings come both with and without a built-in level vial. If they have a level vial they can also be used to check and adjust the tribrach level vials.

Tribrach adjustments are totally independent; changing the adjustment of the level vial has no effect whatever on the adjustment of the optical plummet. The level vial is adjusted so that the bubble is centered when the plane of the tribrach is level. The optical plummet is adjusted to make its line of sight perpendicular to the plane of the tribrach. In theory the plane of the tribrach is the flat surface on which base of the instrument rests. In actual use the plane is an imaginary flat surface that touches the surface of three equally spaced small flat support points.

To check and adjust optical plummets with an adjusting ring you need an area with a high ceiling and a concrete or equally solid floor. Tape, tack, or otherwise attach a good precise sight such as a plumb bob target to the ceiling. Set up a tripod and tribrach underneath the ceiling target. You do not have to be exactly on a plumb line from the ceiling target and the tribrach does not need to be leveled. Place the adjusting ring in the tribrach and place the tribrach you want to check on top of the adjusting ring in an upside-down position so that the optical plummet is facing the ceiling target. While you look through the optical plummet use the foot screws of the tribrach that is clamped to the tripod to bring the optical plummet onto the ceiling target.

Now all you have to do is rotate the upper tribrach to different positions to see if the optical plummet sight line is still on the ceiling target. If it is not staying on the target it will be scribing a small circle. Use the lower tribrach foot screws to put the ceiling target in the center of this small circle. Next, just as you would adjust the cross hairs on an instrument telescope, you can bring the cross hair ring of the optical plummet back onto the ceiling target using the capstan head adjusting screws. And as always, be sure you know what you are doing. Different manufacturers use different procedures to obtain the same result. One popular model uses a spring in place of one of the adjusting screws and a lock nut on the opposing screw.

Prism poles are easy to check. Just pick a doorframe in an out-of- the-way location. Drill a small hole, about 1/8th inch in diameter, in the top of the door frame. Then, using a plumb bob, find a point in the doorway saddle that is directly below the first hole and drill a second small hole at this point. Set the point of the prism pole in the bottom hole and attach the point from a sight pole to the top of the prism pole. Now you can extend the prism pole until this second point enters the top hole, then lock it. If the level bubble is not centered you can adjust it. Then rotate the pole to make sure it stays in the center.

A quality precision ring spacer for tribrach adjustments and a wall mounted wedge device for adjustment of rod levels  are available from Lo-Ink Specialties at www.loink.com web site.

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