Visible crew/equipment: When Rimmer is being questioned by The Inquisitor, a full boom mic and arm can be seen rising out of shot by the three candles on the right of the screen. (00:08:35)
Visible crew/equipment: A boom mic shadow appears on Lister's head just after he opens the door with the severed hand. (00:19:20)
Visible crew/equipment: At the start of the episode, when Lister and Kryten talk about Lister's comic, a boom mic shadow can be seen on Kryten's head. (00:02:10)
Visible crew/equipment: When Lister is being taken over by The Inquisitor, a boom mic shadow can be seen moving on Kryten's chest. (00:04:15)
Answer: The cello is a large four-stringed instrument, which, when it is played, stands vertically on the floor between the player's legs (assuming they are seated). If it is to grow as large as the other instuments mentioned will, it would require a rather unseemly lack of femininity to be able to encompass it with the legs.
Rooster of Doom