I’m surprised how many surveyors i’ve spoken to over the years that aren’t aware of nodal prisms and the accuracy that can be gained by using them.
The principle behind a nodal prism is relatively simple: the optical and mechanical centre of the return wavelength of light coincide with each other. In other words – regardless of the prisms orientation in the holder ( tilted vertically or rotated horizontally) there are no offsets induced at the target.
Two of the most common offsets for nodal prisms are -17.5 (for 25-mm prisms) or -40 mm offset (for 62-mm prisms). They have aligned the mechanical and optical axis over the pivot point. The offset is derived from the diameter, thickness of the glass, position within the housing, the housing on the axis and the speed of light as it travels through the glass. Once these calculations are computed the offset is the result.
If you are using a zero offset target your are more than likely using the worst choice in offsets that will result in an induced error for any off-axis shots.