Question

why is it necessary to take face left and face right observation?​

Answer 1

Explanation:

The purpose of measuring an angle on each of those two different telescope positions or faces was to cancel certain errors resulting from the fact that the tilting axis may not have been perfectly perpendicular to the turning axis of the instrument and that the line of sight of the telescope (its "axis of collimation" ...

Answer 2

The purpose of measuring an angle on each of those two different telescope positions or faces was to cancel certain errors resulting from the fact that the tilting axis may not have been perfectly perpendicular to the turning axis of the instrument and that the line of sight of the telescope (its "axis of collimation" ..