Main Components
Upper Plate:
It is the base on which the standards and vertical circle are placed. For the instrument to be in corrent adjustment it is necessary that the upper plate must be perpendicular to the alidade axis and parallel to the trunnion axis.
Telescope:
It has the same features as in a level graticule with eyepiece and internal focussing for the telescope itself.
Vertical Scale (Circle):
It is a full 400g scale. It is used to measure the angle between the line of sight (collimation axis) of the telescope and the vertical axis.
Vertical Clamp and Tangent Screw :
This allow free transiting of the telescope. When clamped, the telescope can be slowly transited using vertical tangent screw.
The Lower Plate:
It is the base of the whole instrument. It houses the foot screws and the bearing for the vertical axis. It is rigidly attached to the tripod mounting assembly and does not move.
Horizontal Scale (Circle):
It is a full 400g scale. It is often placed between the upper and lower plates It is capable of full independent rotation about the trunnion axis.
The Upper Horizontal Clamp and Tangent Screw:
used during a sequence or “round” of horizontal angle measurements.
The Lower Horizontal Clamp and Tangent Screw:
These must only be used at the start of horizontal angle measurements to set the first reading to zero
Circle Reading and Optical Micrometer:
The vertical and horizontal circles require illumination in order to read them. This is usually provided by small circular mirrors
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