8
4.1 Instrument
The base plate (1) has a standard thread enabling the
ESL2
to be used on
any of our tripods which are sufficiently stable (and on any other tripod
having a 5/8
″
thread fixing screw). The rotatable upper part of the
instrument consists chiefly of the telescope with an optical mechanical
compensator. The compensating element is essentially a pendulum,
supported by four suspension tapes and carrying a prism. The rapes are of a
special alloy to guarantee correct compensator functioning even at extreme
temperatures (-30
℃
to +50
℃
). Provided the circular bubble(8) (sensitivity 8
′
per 2mm) has been set in its setting circle by turning the three
footscrews(2). The line of sight will be automatically horizontal as the
pendulum will be well within its working range of ±14
′
. The movement of
the pendulum is efficiently air damped.
The
ESL2
is equipped with a press button(7) to check the functioning of the
compensator. When the button is pressed, before reading the staff, the
pendulum is given a slight tap and, as a result of the pendulum’s swing, the
staff image swings smoothly away and then floats gently back to its original
position with respect to the horizontal cross hair. The observer is then
certain that the compensator is working and that the line of sight is
