Target axis

The sighting axis or line of sight of a measuring telescope is the straight line with which terrestrial points or celestial bodies are aimed at when measuring angles . When the telescope is precisely adjusted, it falls on its optical axis . Technically, it is realized by the objective axis and the crosshairs of the eyepiece.

Since such an adjustment never remains completely unchanged for mechanical and thermal reasons, the target axis is de facto defined by two points:

• the optical center of the lens and
• the crosshairs or the center of the thread network , which is located in the inner focus of the eyepiece .

For the observer on the telescope, the sighting axis points to that point in the environment or on the sphere (celestial sphere) on which the crosshair is projected when the focus is exact .

In order for a theodolite or total station to provide exact results, the following conditions must be approximately fulfilled:

1. The sighting axis coincides with the optical axis.
2. The target axis should be perpendicular to the tilt axis , i.e. it should enclose an angle of 90 °. The deviation is called a collimation or sighting axis error .
3. The tilt axis should be perpendicular to the vertical or standing axis . The deviation from 90 ° is called the tilt axis error .
4. The vertical axis should coincide with the perpendicular direction . This will help in drawing up the tripod by dragonflies or Lot - sensors accomplished.
5. When aiming at the horizon , the read zenith distance should be 90 ° (or 100 gon ). The difference is called the index error .

For adjustment or arithmetical elimination of these influences, see instrument errors .