- carefully retract the grating, keeping it level as you do so (you may need to wriggle it slightly if it seems tight)
- take the grating that was in the instrument to the grating room on the NW side of the dome and replace it in its box
- select the desired grating from its box and take it out to the telescope
- back at the telescope, check that the grating is orientated correctly (2 bolts on 1 side, 1 on the other) before attempting to insert it
- keep the grating level as you insert it, applying firm but gentle pressure until it's flush with the edges of the grating holder frame