Article Index

1. Instrument axes

a. Identification

Besides the Vertical Axis (VA), Axis of the Bubble Tube (ABT) and Line of Sight (LoS) defined in Basic Principles Chapter E, a TSI has these additional axes, Figure B-1:

Horizontal axis (HA). Axis about which the telescope rotates in a vertical plane.

Optical plummet axis (OPA). This is also a LoS except it is bent. Like the LoS, the OPA is defined by the plummet's sighting mark and optics. Along its path it intersects a prism which deflects it 90°, Figure B-2. 

Figure B-1
TSI Axes


Figure B-2
Optical Plummet Axes

b. Relationships

On a perfectly adjusted TSI:

  • HA and ABT are parallel and both perpendicular to the VA, and,
  • LoS is perpendicular to the HA, and,
  • OPA coincides with the VA


  • The vertical circle is centered on and perpendicular to the HA
  • The horizontal cirlce is centered on and perpendicular to the VA

When the TSI is level, then theoretically:

  • HA, ABT, and initial part of the OPA are horizontal
  • VA and deflected OPA are vertical
  • LoS rotates in a vertical plane
  • Vertical circle is vertical
  • Horizontal circle is horizontal.

In reality, because these axes represent mechanical aspects of the TSI subject to wear and rough handling, relationships and orientations may be off. Any maladjustment represents a systematic measurement error. As we describe TSI use, we will address these errors, their effect on angle measurements, and how they can be overcome.