AXIS OF A THEODOLITE AND THEIR RELATION
FUNDAMENTAL AXIS OF THEODOLITE OF TRANSIT THEODOLITE
- The Vertical axis
- The Axis of plate Levels
- Line of collimation
- The Axis of telescope
- Horizontal axis or trunnion axis
- The axis of the altitude or azimuthal level
RELATIONSHIP BETWEEN FUNDAMENTAL AXIS OR LINES
When the theodolite is in perfect adjustment, the following relationship should exist in between all the fundamental lines:
- The axis of plate levels must be perpendicular to the vertical axis
- The line of collimation must be at right angles to the vertical axis
- The horizontal axis must be at perpendicular to vertical axis
- The axis of the altitude level must be parallel to the line of collimation
- The axis of the telescope must coincide with the line of collimation
ADJUSTMENT OF A THEODOLITE
There are kinds of fundamentals:
- Permanent Adjustment
- Temporary Adjustment
Permanent Adjustment:
- Adjustment of the axis of plate levels:
- Adjustment of plate level:
(ii)Necessity: The error due to vertical axis not being truly cannot be eliminated by taking observation with both faces, hence for the measurement of horizontal and vertical angles, this adjustment is essential.
(iii) Test
(a)Set up the instrument on the firm ground and level it by bringing the plate level parallel to two-foot screws. (The second plate level tube will be at right angles to the first and will be on the third foot screw) as in temporary adjustment
(b) Turn the instrument through 180 about the vertical axis, if the bubble remains central, the adjustment is correct
(iv) Adjustment.
(a)If not, note the deviation of the bubble say n divisions. This error is twice the actual error from the principle of reversal.
(b)Therefore, remove half the error by means of respective levelling screws and half error by means of capstan headed nuts provided at the end of the bubble tube.
(c)Repeat, till the bubbles remains central in all positions.
- Adjustment Of line of collimation:
The of line collimation should coincide with the axis of the telescope and should be at right angles to the horizontal axis.
(i) Object: The object of this adjustment is that the line of collimation should coincide with axis of the telescope i.e., to place the intersection of the cross hairs in the optical axis of the telescope. Thus, both horizontal and vertical hairs may need adjustment.
- Adjustment of horizontal Axis: The horizontal axis should be perpendicular to the vertical axis.
- Adjustment of horizontal hair
- Adjustment of vertical hair
- Adjustment Of vertical altitude Level:
The axis of the altitude bubble must be parallel to the line of collimation.
- Adjustment of vertical index frame:
The vertical circle Vernier must read zero when the line of sight is horizontal. For carrying out adjustment the theodolite should set up on fairly level ground, so that the length of sight on either side of the instrument should be 100 m approximately in the same straight line.
Temporary Adjustments:
Temporary adjustment are those adjustment which are carried out at each instrument station before starting taking observation over the station
- Setting up the theodolite over a station or centering
- Levelling
- Elimination of parallax
- Setting Up the theodolite over the station:
- Place the tripod over the station
- The tripod legs should be spread so that they make an angle of 600 with horizontal
- Take out the instrument from box
- Lift the instrument from the base and screw it firmly on the tripod head
- Adjust the height of the tripod so that the telescope is at a convenient height
- Suspend a plumb bob from the hook beneath the inner spindle
Levelling:
- The lower clamp is unclamped and turn the instrument so the longer plate level will be parallel to any pair of foot screw
- The other plate level is parallel to the line joining the Centre of the third foot screw and the mid-point of the line joining the centers of the first pair as shown fig .
- Then bring the longer plate level bubble in the center if not in the center by moving the two-foot screws equally in or out to which this plate level is parallel.
- Similarly bring the bubble of the other plate level in the center of its by means of third foot screw
- Repeat the process till the both bubbles exactly centered.
- Elimination of parallax:
It is carried out in two steps.
- Focusing the eye piece
- Focusing the object glass
- Focusing the eye piece:
To set the eye piece means a clear vision of the diaphragm wires. To do this point the telescope scope towards the sky or hold a sheet of white paper in front of the object and move the eye piece in and out with slight rotary motion until the diaphragm cross hair are clear visible.
- Focusing the object glass:
The object of focusing the object glass is to bring the image of the formed by the object glass in the plane of the cross hair. To do this, direct the telescope towards some object and move the eye up and down, if there is relative movement of the image with respect to the object, there exists parallax .To remove this turn the focusing screw till there is no relative movement of the object glass and the cross hair. The image so formed will be clear and sharp.
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