Gently pull the mirror cell out. The mirror cell slips easily off the tube.
IF you have it on the tripod AND IF the tube is pointed up... the cell will
fall off when the last screw comes out.
This may result in 'rapid unscheduled disassembly' of the mirror.
If you break it you get to keep both pieces and cleaning of the mirror
requires no more than a dustpan and broom.

Apologies for the focus, holding the phone with one hand.