Warning: for corrective information, select: Note #2 by Bruce Johnston
Step 5. Find the point on the horizontal line where you want the beam to intersect the plane of the wall. Basically repeat the same procedure you used for positioning the horizontal target line, but now you will do this procedure for a vertical line.
Place the beam on the horizontal target line where you think the beam intersects the plane of the wall at right angles. Tape the second large sheet of paper to the wall vertically so that the beam is on the line half way between the ends of the line. Lock the RA and loosen the DEC and raise the beam to the top end of the line and reposition the top end of the sheet of paper so that the beam is on the line. Sweep the beam down to the bottom end of the line and adjust the paper so the beam is on the bottom end of the line.
Slowly sweep the beam from one end of the vertical line to the other. If the beam strays to the right near the center of the vertical line then the vertical line needs to be moved to the left. If it strays to the left the line needs to be moved to the right. Move the line and repeat this procedure until the beam can be swept along the line from one end to the other without straying from the line.
Fig. 9 - Tape vertical line to wall where you think it should intersect the horizontal line
Figure 10 - Lock RA and move
top of paper so beam strikes line
Figure 11 - Bottom of
vertical line is too far left
Figure 12 - Move bottom
of paper so beam strikes line
Step 6. You should be able to see through the sheet of paper with the vertical line to see where the vertical and horizontal lines intersect. Point the laser beam to this location of intersection. Lock the DEC and RA and remove the paper with the vertical line. The beam should still be pointing to the location of intersection on the horizontal line. Mark this location with a pen. This is the point at which the beam intersects the plane of the wall at right angles.