Once a grating is removed as described in the previous section a different one can be mounted by reversing the procedure. As with the slit, before tightening the three holding screws ensure that the base is rotated anti-clockwise to contact the tangent screw against its stop. The grating angle can be set according to the observational requirements by reading the main scale (divided into whole degrees) and the control knob scale (which produces 2grating rotation per one revolution and can be set to a precision of a quarter degree).