Calibrating the Longitudinal Angle.

Determining the actual angle around the polar axis relies not only upon the panning motor position, but on the angle of the 45$^\circ$ mirror as well. We devised a simple experiment to determine both at the same time.

In a room, we aim the laser horizontally at a wall so it is roughly perpendicular, marking its position. We then move the scanning mirror over the pole 180$^\circ$, aiming at the opposite wall, and mark that position. We then pan the device 180$^\circ$, where it should coincide with the first mark. We mark that position, and again move the scanning mirror over the pole to point at the second mark. We mark its position. If all the hardware were perfect, the two marks on each wall would be coincident, but due to errors, they are not. From the separation of the points and knowing the distance from the rangefinder to the points, the panning error and mirror error are both determined. The values we found are 14,039 steps in 180$^\circ$, and 44.89$^\circ$ (the mirror error also affects the latitude). At a distance of 4m, this moves the data sample more than 3cm.