For additional information, select: Note #1 by Bruce Johnston
Step 4. This step will determine the point on the wall where the laser beam intersects the plane of the wall at right angles (perpendicular). This point will be where you will place the point of intersection of the two perpendicular lines on the target. This point is critical to the procedure. The reason for this is because you will be sweeping the laser beam via a rotational motion against a flat surface. If you place the target intersection point above or below this point the beam will sweep a curve instead of a straight line when checking the RA axis against the horizontal target line. If the target intersection point is left or right of this point the beam will also sweep a curve instead of a straight line when checking the DEC axis against the vertical target line.
Adjust the scope RA and DEC to point the laser to where you think the beam intercepts the plane of the wall at a right angle. Then tape the paper with the straight line to the wall (horizontal) so that the beam is on the line halfway from either end of the paper. With the DEC locked rotate the scope in RA so the beam sweeps from one end of the line to the other. Adjust each end of the paper so that the beam is on the line when pointed to that end of the line.
Starting at one end of the line with the beam on the line slowly sweep the beam to the other end of the line. If the beam strays high near the center of the line then the line is too low with respect to the point where you want the beam to intersect the plane of the wall. If the beam strays low near the center of the line then the line is too high.
Adjust the elevation of the line and repeat this procedure until the beam sweeps from one end of the line to the other without straying from the line. When this is accomplished you have established the proper elevation at which the beam intersects the plane of the wall at right angles.
Figure 5 - Tape horizontal line on wall with beam in centerpaper
Figure 6 - Lock Dec and move left side of so beam strikes line
Figure 7 - Right side of line is too low
Figure 8 - Raise right end of paper so beam strikes line