Purpose of adjustment of telescope bubble tube to make the axis of the bubble tube parallel to the line of sight.

Explanation:

- In surveying, the bubble tube is an important component of the level instrument. It contains a liquid (usually alcohol or ether) with a small bubble that moves within it.
- The purpose of the bubble tube is to indicate whether the level instrument is horizontal or not. When the bubble is centred, it indicates that the level instrument is horizontal, and adjustments can be made accordingly.
- However, it is important to note that the bubble tube does not indicate the direction of the horizontal plane. In other words, it only indicates whether the level instrument is level, but not whether it is aligned with a specific reference.
- To align the level instrument with a specific reference, such as a benchmark or a known elevation, adjustments must be made to the level instrument using the leveling screws or knobs.
- One of the important adjustments that must be made is the adjustment of the line of sight. The line of sight is the imaginary line that connects the eyepiece of the level instrument to the target or object being observed.
- When the bubble is centred, the axis of the bubble tube becomes parallel to the line of sight. This means that the line of sight is horizontal and parallel to the level instrument.
- However, it is important to note that the line of sight does not necessarily coincide with the line of collimation or the optical axis of the telescope. The line of collimation is the imaginary line that passes through the center of the objective lens, the center of the crosshairs or reticle, and the center of the eyepiece. The optical axis of the telescope is the imaginary line that passes through the center of the objective lens and the center of the eyepiece.
- Therefore, while the bubble tube indicates that the level instrument is level and the line of sight is horizontal, adjustments must still be made to ensure that the line of collimation or the optical axis of the telescope is aligned with the reference being used.
- In summary, when the bubble is centred, the axis of the bubble tube becomes parallel to the line of sight, indicating that the level instrument is level and the line of sight is horizontal. However, adjustments must still be made to ensure that the line of collimation or the optical axis of the telescope is aligned with the reference being used.