Final answer:
The Assistant Gunner turns the elevation knob to align the M138A1 Elbow Telescope's elevation line with the DAP.
Step-by-step explanation:
When boresighting the M138A1 Elbow Telescope, if the elevation line is not aligned with the DAP (Director of Aiming Point), the Assistant Gunner turns the elevation knob.
Theodolites used by surveyors and military personnel, such as those in the M138A1 Elbow Telescope, have an azimuth rotation allowing for 360-degree horizontal scanning and an elevation angle adjustment for targeting points at various vertical angles. While azimuth measures angles clockwise from north, elevation adjustments involve aligning the telescope vertically to match specific reference points. If these components are not properly aligned during the boresighting process, precise adjustments are necessary to ensure accuracy. The Assistant Gunner makes the needed elevation adjustments to properly align the sight with reference points such as the DAP. This is critical for accurate aiming and targeting in military operations where precision is paramount.